CAUSES, MECHANISMS AND PREDICTION
LANDSLIDING IN SEATTLE
by DONALD WILLIS TUBBS
A dissertation submitted in partial fulfillment of the requirements for the degree of
DOCTOR OF PHILOSOPHY
UNIVERSITY OF WASHINGTON
DONALD WILLIS TUBBS
University of Washington
CAUSES, MECHANISMS, AND PREDICTION
OF LANDSLIDING IN SEATTLE
by DONALD WILLIS TUBBS
of the Supervisory Committee: Professor A. L. Washburn
Department of Geological Sciences and Quaternary Research Center
Quaternary events in the Puget Lowland have produced materials and landforms that are highly susceptible to landsliding. During the winter of 1971-1972 a large number of landslides occurred in Seattle, causing extensive property damage. Information on 47 of the landslides is available from Federal disaster assistance records for that period. These slides were studied to determine their relationship to geologic, climatic, and human factors.
The landslides were of three general types described by Varnes (1958): debris slides, slumps, and debris avalanches. More than three quarters of the landslides originated as debris slides and they can be modeled as infinite-slope failures. A generalized stability analysis suggests that such slides should not be common in areas sloping <13 percent, and the locations of the studied landslides corroborate this conclusion. Over three quarters of the landslides occurred in areas directly underlain by either the Lawton Clay or pre-Vashon sediments, and nearly half of the slides occurred along the trace of the contact between one of these units and the overlying Esperance Sand. On the basis of such geologic considerations a slope-stability map was constructed that predicts the spatial distribution of landsliding in Seattle.
Nearly 90 percent of the studied landslides occurred on three of the four days during the winter of 1971-1972 having over an inch of precipitation. To determine the relationship between landsliding and various climatic factors over a sufficiently long record to be representative of Seattle's climate, dates of landslides reported in The Seattle Times from 1932 to 1972 were compared to annual, cumulative, and short-term precipitation, and to intervals of sub-freezing temperature. It appears that the primary control on the temporal distribution of landsliding in Seattle is short-term precipitation, especially precipitation during periods of one or two days.
Human factors that may have contributed to landsliding were recognized in over 80 percent of the landslides of 1971-1972. Human influences included the diversion of water onto slopes, the steepening of slopes by excavation, the placing of fill on slopes, and the failure of retaining walls. Human factors affect both the spatial and temporal distribution of landsliding, and cause occasional departures from the predictions that can be made on the basis of geologic and climatic considerations.
TABLE OF CONTENTS
History of the study
DESCRIPTION OF THE LANDSLIDES
Relationship of landsliding to topography
Relationship of landsliding to stratigraphy
Influence on landsliding during 1971-1972
Influence on landsliding during 1932-1972
APPENDIX 1 - LANDSLIDES DURING 1971-1972
APPENDIX 2 - LANDSLIDES DURING 1932-1972
1. Location map
2. Stratigraphy of late Olympia Interglacial and Fraser Glacial sediments in Seattle
3. Olympia Interglacial sediments
4. Lawton Clay
5. Esperance Sand
6. Intercalated sand and silt in the Esperance Sand-Lawton Clay transition zone
7. Vashon advance outwash overlying Esperance Sand
8. Vashon till
9. Debris slide
10. Debris slide along Culpepper Ct. NW, 10600 block
11. Slump along 33rd Ave. SW, 3400 block
13. Debris slide -> debris avalanche
14. Debris slide -> debris avalanche along Perkins Lane W, 1700 block
15. Slump -> debris avalanche along Perkins Lane W, 1800 block
16. Slumps -> debris avalanche
17. Slumps -> debris avalanche involving intercalated sand and clay
18. Infinite-slope model
19. Relationship between stability, slope inclination, and fraction of regolith below the water table
20. Number of landslides occurring in various categories of slope inclination during 1971-1972
21. Aspect of the slopes on which landslides occurred during 1971-1972
22. Daily precipitation and landsliding during 1971-1972
23. 20-day running mean daily precipitation, 1932-1972
24. Annual precipitation and number of landslides, 1932-1972
25. Relationship between number of landslides and annual precipitation, 1932-1972
26. Maximum number of reported landslides for all combinations of one-day and cumulative precipitation, 1932-1972
27. Maximum number of reported landslides for all combinations of two-day and cumulative precipitation, 1932-1972
28. Maximum number of reported landslides for all combinations of three-day and cumulative precipitation, 1932-1972
29. Maximum number of reported landslides for all combinations of four-day and cumulative precipitation, 1932-1972
30. Maximum number of reported landslides for all combinations of five-day and cumulative precipitation, 1932-1972
31. Relationship between average number of reported landslides per day and one-day precipitation, 1932-1972
32. Relationship between average number of reported landslides per day and two-day precipitation, 1932-1972
33. Relationship between average number of reported landslides per day and three-day precipitation, 1932-1972
34. Relationship between average number of reported landslides per day and four-day precipitation, 1932-1972
35. Relationship between average number of reported landslides per day and five-day precipitation, 1932-1972
36. Expected number of reported landslides per day as a function of short-term precipitation
37. Cumulative number of landslides following intervals of sub-freezing temperature, 1932-1972
1. Types of landslides in Seattle during 1971-1972
2. Stratigraphic relationships of landslides in Seattle during 1971-1972
3. Comparison of the hazard associated with each of the slope-stability classes represented in Plate I
4. Some human causes of landslides in Seattle during 1971-1972
I. Slope-stability map of Seattle
History of the study
During the winter of 1971-1972 a large number of landslides occurred in the Puget Lowland. The slides caused extensive property damage and greatly increased public awareness of landsliding as a geologic hazard. In response to the demand from public officials and private citizens for information concerning landslides, the U.S. Geological Survey and the Washington State Division of Geology and Earth Resources began a cooperative program of landslide studies in the Puget Lowland. As part of this program the U.S.G.S. granted the Division of Geology and Earth Resources funds to support student research on landsliding.
Observations of several landslides shortly after they occurred and consideration of the timing of the slides suggested that landsliding in the Seattle area is related to certain geologic and climatic factors. This dissertation considers these relationships and their implications for predicting the spatial and temporal distribution of future landsliding. The study began during 1972 with the compilation of the locations of landslides known to have occurred in Seattle during the winter of 1971-1972. Each of the landslides was examined in the field and many of the property owners and neighbors were interviewed.
In 1973 the U.S.G.S., as a participant in the Department of the Interior Resource and Land Inventory (R.A.L.I.) program, supported several studies in part of west-central King County. The R.A.L.I. project area was mostly south of Seattle, but included a small part of the southwest comer of the city. The landslides that had occurred in the R.A.L.I. project area during the winter of 1971-1972 were examined during early 1973 (Tubbs, 1974a). On the basis of experience gained while working on the R.A.L.I. project, many of the landslides that had occurred in Seattle were revisited and reinterpreted during late 1973 (Tubbs, 1974b).
In early 1974 landslide processes at Discovery Park in Seattle were studied to determine the relationship between landsliding and certain stratigraphic units at their type section (Tubbs et al., 1974), and techniques for evaluating slope stability in the Seattle area were reviewed (Tubbs and Frederick, 1974). During late 1974 one such technique was utilized in a slope stability mapping project funded by King County, involving nine 7-1/2-minute quadrangles in the vicinity of Seattle.
In early 1975 information on landsliding in Seattle from 1932 to 1972 was compiled from newspaper accounts, and computer programs were developed for the analysis of relationships between landsliding and various climatic factors.
Much of this study was supported by the Washington State Division of Geology and Earth Resources, with funds from the U.S. Geological Survey. The assistance of both organizations is gratefully acknowledged. Donald R. Nichols and Robert D. Miller of the U.S. Geological Survey, and Gerald Thorsen of the Division of Geology and Earth Resources were particularly helpful in criticizing several of the previously cited papers, thereby benefiting this dissertation.
Each of the dissertation committee members contributed ideas from their various fields of interest that have been useful in this study. Professor A. L. Washburn deserves special thanks as a thoughtful advisor, a persistent committee chairman, and a demanding editor.
Seattle is located near the center of the Puget Lowland of western Washington (figure 1). As is generally the case throughout the Puget Lowland, most of Seattle is underlain by a considerable thickness of sediment, and relatively little bedrock is exposed at the surface. Bedrock is present at or near the surface in the area between Seward Park and the Duwamish Valley, and also in the vicinity of Aiki Point (Waldron et al., 1962). All bedrock exposed within the city belongs to the Blakeley formation. The rocks include marine, tuffaceous sandstone, siltstone and shale, and range in age from middle Oligocene to early Miocene (Liesch et al., 1963, p. 13).
Most outcrops within Seattle, and all outcrops in the northern three-quarters of the city, expose sediments of late Quaternary age. Depth to bedrock in the northern part of the city ranges from two to four thousand feet (Hall and Othberg, 1974). It seems likely that sediments of earlier Quaternary and Pliocene age are present at depth, but such materials are nowhere exposed in the city.
Relatively little evidence exists within Seattle for glacial and interglacial events prior to late Olympia Interglacial time (figure 2). Most of the earlier sediments are covered by subsequent deposits or have been destroyed by erosion. The remaining material is visible only in scattered outcrops from which it is difficult to interpret earlier events. Where these older sediments have been studied, one or possibly two older glaciations have been recognized (Mackin et al., 1950; Stark and Mullineaux, 1950, p. 23; Waldron, 1967). Studies elsewhere in the Puget Lowland have demonstrated at least four or five glaciations (the actual number depending on the correlations assumed), separated by interglaciations (Crandell et al., 1958; Easterbrook et al., 1967; Mullineaux, 1961, p. 71).
The interglaciation immediately preceeding the most recent major glaciation was named the Olympia Interglaciation by Armstrong et al. (1965). During the Olympia Interglaciation the Puget Lowland probably looked much like it does today, except perhaps for the absence of the marine inlets that presently comprise Puget Sound. Hills that were roughly 200 to 300 feet high, with steep slopes and relatively flat tops, existed in some of the same positions as Seattle's present hills; they were separated by flood plains where fluvial and lacustrine deposits accumulated (figure 3). The hills were sites of weathering and erosion during Olympia time; a zone of montmorillonitic-weathered clay has been recognized on one of the hills that stood above the Olympia flood plain (Mullineaux et al., 1964).
Approximately 25,000 years ago alpine glaciers began to form and advance in the mountains of western Washington and an ice sheet was developing in the mountains of western British Columbia; this marks the beginning of the Fraser Glaciation (Armstrong et al., 1965). The initial phase, the Evans Creek Stade, involved the expansion of alpine glaciers in the mountains adjacent to the Puget Lowland. South of the latitude of Seattle the alpine glaciers did not generally reach the Lowland; further north their terminal moraines are obscured by subsequent deposits from the Cordilleran ice (Crandell, 1965, p. 346). Since alpine ice did not advance into the central Puget Lowland, the Evans Creek Stade is not recognized in the Seattle area. Pollen evidence, however, indicates that while the late Olympia Interglacial sediments were being deposited in the Seattle area the climate was cooler and wetter than at present (Mullineaux et al., 1965). Following the Evans Creek Stade the alpine glaciers in western Washington retreated, but the ice sheet in the mountains of western British Columbia continued to expand into the lowlands of southwestern British Columbia and northwestern Washington during the main phase of the Eraser Glaciation, the Vashon Stade (Armstrong et al., 1965).
Approximately 15,000 years ago a lobe of Cordilleran ice, the Puget Lobe, pushed south into the Puget Lowland far enough to block the northward-flowing drainage to the Strait of Juan de Fuca. This resulted in a large proglacial lake which drained southward into Grays Harbor via the lower Chehalis Valley. Water and sediment entered the lake from the glacier, which constituted its northern boundary, and from the highlands on both sides. The coarser sediment carried by the water was dropped as the streams entered the lake, while the silt- and clay-size particles settled to the bottom in the quieter water at some distance from the ice margin. A widespread deposit of silt and clay was thus created, which constitutes the Lawton Clay Member of the Vashon Drift (Mullineaux, et al., 1965) (figure 4).
As the Puget Lobe advanced farther south a thick unit of proglacial fluvial and lacustrine sand was deposited. This unit, the Esperance Sand Member of the Vashon Drift (figure 5), spread over not only the Lawton Clay but also the hills of older material that were protruding through the Lawton Clay.
The contact between the Lawton Clay and the Experance Sand is not generally an abrupt change from silt and clay below to sand above. There often exists a zone, up to several tens of feet in thickness, in which beds of sand alternate with beds of silt and clay (figure 6). In describing the type section of the Lawton Clay Member and the Esperance Sand Member of the Vashon Drift, Mullineaux, et al. (1965, p. 04) stated:
...these beds, which record a transition between the Lawton Clay Member and the relatively uniform medium sand above, are arbitrarily assigned to the Esperance Sand Member.
Article 5 of the Code of Stratigraphic Nomenclature (American Commission on Stratigraphic Nomenclature, 1961, p. 650) requires that:
Boundaries of rock-stratigraphic units are placed at positions of lithologic change. Boundaries are placed at sharp contacts or may be fixed arbitrarily within zones of graduation. Both vertical and lateral boundaries are based on the lithologic criteria that provide the greatest unity and practical utility.
In regard to the last sentence, it is notable that the Code remarks (Article 5, paragraph a):
Because of creep, it is generally best to define such arbitrary boundaries by the highest occurrence of a particular lithologic type, rather than the lowest.
Although the arbitrary assignment of the transition zone to the Esperance Sand Member is in accordance with the requirements of the Code of Stratigraphic Nomenclature, it is more expedient for mapping purposes to include the transition zone in the Lawton Clay Member, as the Code suggests. Such a reassignment is also useful in identifying certain areas of relatively low slope stability and in describing the mechanisms of the landslides. Therefore, as used in this dissertation, the boundary between the Lawton Clay and the Esperance Sand is informally redefined as the top of the uppermost laterally extensive silt and clay bed within the transition zone described by Mullineaux et al. (1965) (figure 2).
The Esperance Sand in some places becomes coarser and more pebbly near its top, grading into the Vashon advance outwash. In other places the Vashon advance outwash was deposited in stream channels cut into the upper part of the Esperance Sand; there the change in character is more abrupt (figure 7).
The front of the Puget Lobe continued to advance southward to about 15 miles south of Olympia (figure 1). At its maximum, the ice thickness in the vicinity of Seattle may have been 4,000 feet. The ice scoured out the proglacial sediments more readily than the older deposits, and thus eroded troughs where there had previously been valleys, leaving hills with cores of older deposits (Crandell et al., 1965). Some of the material eroded by the glacier was redeposited further south as advance outwash; the remainder was incorporated into the Vashon till (figure 8).
The recession of the Puget Lobe was extremely rapid. By approximately 13,500 years ago (Mullineaux et al., 1965) the ice had retreated to a latitude north of Seattle and by 11,000 years ago the ice front had retreated up the Fraser Valley. As the ice retreated it uncovered a glacially sculptured landscape of uplands and intervening valleys. In front of the melting ice, and coursing across the uplands, were meltwater streams that connected proglacial lakes in the valleys. These streams often cut large channels and, especially where they emptied into the lakes, locally deposited Vashon recessional outwash.
A final readvance of the glacier, called the Sumas Stade of the Fraser Glaciation, occurred approximately 11,000 years ago (Armstrong et al., 1965). The ice advanced only a few miles south of the Canada-United States boundary before it again began to retreat; the Sumas Stade is not recognized in the Seattle area.
Accompanying worldwide deglaciation, sea level rapidly rose and marine water invaded the glacially carved troughs to form the inlets of Puget Sound. Most of the rise in sea level had taken place by about 7,000 years ago, but since then there has been a slow rise of relative sea level in the Puget Lowland amounting to more than 30 feet (Biederman, 1967, p. 16).
The postglacial history of the Seattle area primarily involves weather-ing and erosion of the uplands and infilling of the intervening valleys and inlets. The steep slopes surrounding many of the upland areas were left in a relatively unstable condition by the retreating ice, and the instability of some slopes has been maintained or increased by shoreline erosion during the past few thousand years.
Widespread flooding and landsliding in western Washington during the winter of 1971-1972 caused the President to designate part of the Puget Lowland as a natural disaster area for the periods January 19-20 and February 13 - March 10, 1972. This action made Federal disaster assistance available and allowed both public and private property owners to apply for funds to aid reconstruction. Federal funds compensated for much of the economic loss due to landsliding and the resulting disaster assistance records include information on the location and the amount of damage due to landslides during this period. Both the Federal Disaster Assistance Administration and the Small Business Administration provided information from those records for the purposes of this study.
Federal disaster assistance records include 47 landslides that occurred in Seattle during the winter of 1971-1972. These slides caused a total of $456,000 in damages, 64 percent to public property and 36 percent to private property. This damage estimate should be considered a minimum amount, representing only those losses that can be readily expressed in monetary terms. Losses such as traffic delays and personal inconvenience cannot be as easily measured as the cost of engineering studies, reconstruction, and preventative measures, and are excluded from the estimate. Nevertheless, such indirect costs are a significant part of the total impact of the landslides.
Other possible sources of information on landslides occurring in Seattle include news accounts and city records. The Seattle Times reported nine landslides in the city during the winter of 1971-1972, seven of which are included in the Federal disaster assistance records. Seattle Engineering Department records include references to 57 landslides that occurred in Seattle during the winter of 1971-1972, all but two of which affected city property. Information on 31 of those slides is included in the Federal disaster assistance records.
A total of 75 landslides were noted in Seattle during the winter of 1971-1972 by one or more of the three above-mentioned sources. Only one of these slides is known to have occurred outside the periods eligible for Federal disaster assistance. Six other landslides known to have occurred during the winter of 1971-1972 were found during fieldwork for this study, and other apparently recent slides were observed. Undoubtedly, still other slides occurred during the winter of 1971-1972 but were not found. A total of at least a hundred landslides probably occurred in Seattle during the winter of 1971-1972.
The sections of this dissertation dealing with landsliding in Seattle during the winter of 1971-1972 are based on the 47 landslides included in the Federal disaster assistance records. This restriction focuses the study on the best documented landslides. Damage estimates are available for all of the landslides included in the Federal disaster assistance records, but for only a few of the slides not included. Dates of movement are also known for a larger proportion of the landslides included in the Federal disaster assistance records.
By restricting consideration to landslides included in the Federal disaster assistance records the total number of slides considered is decreased and the average damage per slide is increased. The landslides that are included in the Federal disaster assistance records were generally larger and more destructive than those that were not included. Many slides affecting public property involved only a few cubic yards of material that slid into a ditch or onto a right of way; such minor slides were considered by the Federal government to be normal maintenance problems and were not compensated for by disaster assistance. Private property owners did not seek Federal disaster assistance unless significant damage occurred, because the assistance covered only the reconstruction costs. The proportion of slides in which human activities were a contributing factor may also be increased, since undeveloped sites were not eligible for funds.
Each of the landslides that occurred in Seattle during the winter of 1971-1972 and that are included in the Federal disaster assistance records was examined in the field to verify the location, to determine the type of slide, and to evaluate possible geologic and human influences contributing to the slide. Whenever possible, the owner of the property on which the slide occurred was interviewed to obtain information concerning conditions no longer apparent and to determine as closely as possible the dates of sliding. Information on each of these landslides is tabulated in Appendix 1.
In order to discern predictive relationships between landsliding and climatic variables it is necessary to consider a record that is sufficiently long to be representative of the variety of possible climatic conditions. Weather observations in Seattle began at the U.S. Weather Bureau City Office in 1893 and continued through October of 1972, when the City Office was closed (Church, 1974). The recording of landslides in Seattle has not been as systematic. Newspaper accounts represent the longest record: The Seattle Times research library has clippings and microfiche of articles concerning landslides since 1926. Only three such articles predate 1932, but later accounts are relatively numerous.
Newspaper accounts published in The Seattle Times for the period 1932-1972 were reviewed for the purpose of comparing the dates of landsliding to climatological data for the same period. The dates for which landslides were reported and the number of landslides known to have occurred on those dates are listed in Appendix 2. One hundred-sixty landslides are listed in Appendix 2; seven of these are also included in Appendix 1, so the total number of landslides in the appendices is 200.
Newspaper accounts probably contain the same bias as the Federal disaster assistance records in that there is a selection toward the larger and more destructive slides. Since there is usually no economic motivation for a property owner to inform the press of a landslide on his property, and since slides which do not cause significant damage or inconvenience are unlikely to be reported, the number of landslides reported in newspaper accounts is much smaller than the number of landslides that actually occur. During the winter of 1971-1972, for instance. The Seattle Times reported only about one fifth as many landslides as were included in the Federal disaster assistance records, and probably not more than one tenth the total number of slides occurring in Seattle. The proportion of all landslides that are reported probably varies from year to year as a result of changes in editorial policy and the abundance of other news.
DESCRIPTION OF THE LANDSLIDES
The landslides included in Appendix 1 are of three general types described by Vames (1958) : Debris slides, debris avalanches and slumps (table 1). Landslides south of Seattle of the same general nature as the first two types were collectively called earthflows by Miller (1973) on the basis of the fluidlike movement of some of the slides, and this terminology was followed by Tubbs (1974a) in a subsequent map for the same U.S.G.S. Folio. In this paper, however, those slides of the sort called "earthflows" by Miller that exhibited flowage are classified as debris avalanches and those that exhibited less internal deformation are classified as debris slides.
Debris was defined by Vames (1958, plate 1) to mean "natural soil and rock detritus", and used to indicate all materials overlying "bedrock" excepting artificial fill. As discussed under Geologic setting, bedrock in the sense of lithified material is relatively scarce in Seattle. However, all sediments of Vashon age and older (excepting minor amounts of Vashon recessional sediment) have been overridden by several thousand feet of glacial ice, and as a result are quite consolidated relative to the overlying material, for which the term "regolith" is used in this paper. "Surficial debris" or "debris", and "artificial fill" or "fill" are used to indicate, respectively, the natural and anthropogenic components of the regolith, and "substrate" is used to indicate the stratigraphic units, whether or not lithified, immediately underlying the regolith.
The debris slides involved the movement of the regolith only (figure 9). The position of the failure surface of nearly all the debris slides was controlled by the contact between the regolith and the substrate. Where discernible, the depth to the failure surface was only a few feet, and the downslope and lateral dimensions were generally several tens of feet. The center of mass of debris slides usually moved downslope a distance less than approximately half the crown-to-foot length of the slide, although in one instance it moved considerably farther (figure 10). Of the 47 landslides listed in Appendix 1, 37 began as debris slides; 15 of these continued as debris slides until cessation of movement and 22 evolved into debris avalanches (table 1).
The slumps differed from debris slides in that the failure surface cut the underlying substrate, a significant quantity of which was included in the slide mass (figures 11 and 12). The position of at least part of the failure surface was controlled by certain beds within an underlying stratigraphic unit or by the contact between two such units. Slumps were generally somewhat larger than debris slides, but even the largest did not exceed a few hundred feet in maximum dimensions. The distance moved by the center of mass of slumps was usually less than approximately half the crown-to-foot length of the slide. Ten of the landslides listed in Appendix 1 began as slumps, five of which subsequently evolved into debris avalanches.
Where they occurred on slopes of sufficient steepness and height, both debris slides and slumps evolved into debris avalanches. In these slides, rapid differential internal movements resulted in an overall fluidlike motion of the slide mass. As previously mentioned, this fluidity caused Miller (1973) to call such slides earthflows; however, two important differences exist between these slides and the type of earthflow defined by Vames (1958) and referred to by Miller. As defined by Varnes (1958, plate 1), such earthflows involve "mostly plastic" material or fine-grained, non-plastic material and move at a "slow to rapid" rate (five feet per day to one foot per minute), whereas debris avalanches involve "mixed rocks, soil, clay, etc." and move at a "very rapid to extremely rapid" rate (more than one foot per minute). In both these respects the landslides referred to in this report as debris avalanches fit the definition of debris avalanches rather than that of earthflows.
All the debris avalanches originated as either debris slides (figures 13 and 14) or slumps (figures 15, 16, and 17). Debris avalanches were often elongated in the downslope direction, extending to the base of the slope or to a decrease in slope inclination. The width of the debris avalanche scar was generally proportional to the width of the originating debris slide or slump. The depth of scour by the moving debris below the foot of the landslide was quite shallow, often not even penetrating the regolith. The center of mass of the debris avalanches usually moved a distance greater than half the crown-to-foot distance, and often much further. Of the 27 debris avalanches listed in Appendix 1, 22 originated as debris slides and five originated as slumps.
Several other types of landslides described by Varnes (1958) are known to occur in Seattle but are not represented in Appendix 1. Slow earthflows, previously discussed and contrasted with debris avalanches, only rarely occur. Debris flows occasionally develop from debris slides and slumps; they differ from debris avalanches by having a higher water content and greater mobility. Soilfalls frequently occur along the sea cliffs adjacent to Puget Sound and, although usually quite small, are locally responsible for considerable cliff retreat.
Over three quarters of the landslides included in Appendix 1 originated as debris slides. They may be considered infinite slope failures as analyzed by Taylor (1948, p. 418-431) inasmuch as they occur on slopes of nearly constant inclination, are shallow relative to their areal extent, and involve a fairly uniform thickness of regolith. If the soil properties do not vary areally, the stability analysis of a potential slide mass is reduced to an analysis of the stresses in a column of soil extending from the ground surface to the substrate (figure 18). The vertical stress at the base of the regolith is
W cos i = z cos i
where W is the weight of the column of unit width, is the unit weight of the soil, z is the thickness of the regolith (measured vertically) and i is the inclination of the slope. The shear stress on the potential failure surface at the base of the regolith and parallel to the slope is thus
= z cos i sin i
and the normal stress is
= z cos i.
The shear strength at the base of the column of soil is
s = c' + ( - u) tan '
where c' is the effective cohesion of the soil, ' is the effective angle of internal friction, and u is the pore water pressure at the base of the column. As will be shown under Relationship of landsliding to stratigraphy, the debris slides were generally underlain by stratigraphic units of extremely low permeability, such as the Lawton Clay and most of the pre-Vashon sediments. This causes the movement of the groundwater within the regolith to be approximately parallel to the surface of the slope. The elevation of the water table above the substrate is shown as distance h in figure 18. Under these conditions the pore pressure at the base of the regolith is
u = (w) h cos i
where (w) is the unit weight of water. Combining this relationship with the previous two equations yields
s = c' + z cos i (1 - (((w) h)/( z))) tan '.
Failure occurs when the shear stress becomes equal to the shear strength, i.e. when
z cos i sin i = c' + z cos i (1 - (((w) h)/( z))) tan '.
In a study of slumps involving the Lawton Clay that occurred during the construction of Interstate 5 in Seattle, Palladino (1971, p. 82) found that, although the peak strength parameters for the Lawton Clay were
c' = 0.65 tons/ft and ' = 35°,
the field strength of the Lawton Clay was more closely related to the residual strength parameters, which were approximately
c'(r) = 0 and '(r) = 15°
where c'(r) and '(r) are the residual effective cohesion and the residual angle of internal friction, respectively.
The lower portion of the regolith involved in the debris slides considered in this paper generally consisted of material derived from the under-lying substrate. For the regolith overlying the Lawton Clay and some of the pre-Vashon sediments, this material formed largely by the slow hydration and slaking of the silt and clay. The failure surfaces of the debris slides were usually located along the interface between the underlying, undisturbed clay and the overlying regolith. Below this interface the shearing resistance of the substrate increased to its peak strength, and above this interface the increasing density of plant roots probably caused an apparent cohesion that inhibited failure.
Assuming the above residual strength parameters for the shearing resistance along the interface between the substrate and the regolith, and a unit weight of 120 pounds/ft for the regolith, the previous equation reduces to
h/z = 1.925 - 7.18 tan i.
Thus the distance that the water table must rise above the substrate (expressed as a fraction of the total thickness of the regolith) in order to induce a debris slide is a function of the slope inclination (figure 19).
Figure 19 suggests that debris slides should not occur in areas sloping less than approximately 13 percent and underlain by substrate having residual strength parameters equal to or exceeding those of the Lawton Clay. This conclusion is supported by figure 20, which indicates the number of landslides occurring in various categories of slope inclination, and which will be further discussed in the next section. In the present context, however, it is useful to note that only three of the 37 landslides originating as debris slides occurred on slopes of <15 percent inclination, and that none occurred on slopes inclined <10 percent. Figure 20 also shows that a large number of debris slides occurred on slopes of >30 percent inclination, whereas figure 19 would seem to indicate that significant quantities of regolith cannot accumulate on slopes inclined more than approximately 27 percent. The accumulation of some regolith is a prerequisite for debris slides, and is explained by at least two departures from the assumptions upon which figure 19 is based. Many of the debris slides represented in figure 20 were not underlain by the Lawton Clay, but by other materials having greater residual strength. Also significant is the role of vegetation in establishing a zone of apparent cohesion that partially overlaps the underlying region of higher (peak) strength, thus allowing a finite thickness of regolith to accumulate in areas where the residual strength of the soil alone would be insufficient. The amount of apparent cohesion generated by root systems and the rate of weathering of the underlying substrate have not been investigated in this study, but might be useful subjects for future research, offering a theoretical approach to the problem of estimating recurrence intervals of debris slides.
Ten of the landslides included in Appendix 1 originated as slumps rather than as debris slides. The slumps are more difficult to describe with generalized analyses because they often involved more than one type of material, each type having its own strength parameters. Furthermore, the slumps were generally associated with ground-water conditions that were stratigraphically controlled, thus complicating the calculation of effective stresses. Generalized analyses are available for relatively simple models involving sand overlying clay or sand beds within clay (Henkel, 1967), but the application of such models requires more information about pore-water pressures than is normally available in reconnaissance work, where such models are of greatest utility. Furthermore, analyses based on seepage along certain contacts or beds appear unrealistic considering the relationships between landsliding and precipitation that are described under CLIMATIC FACTORS. The rapidity with which both slumps and debris slides responded to precipitation suggests that the slides were related to changes in pore pressures within a few feet of the surface. This, in turn, suggests that many of the slumps involved retrogressive failure triggered by debris slides or by localized failures near the contact between an overlying impermeable unit (e.g. the Esperance Sand) and an underlying impermeable unit (e.g. the Lawton Clay), where a local steepening of the water table can be expected immediately following a period of intense precipitation.
Subsequent changes in style of movement transformed 27 of the debris slides and slumps into debris avalanches. The conditions under which debris avalanches develop and continue to move are not predicted by Mohr-Coulomb failure criteria since failure has already occurred. The movement of debris avalanches is a rheological problem that has not yet been fully solved, but probably involves factors common to both debris flows (Johnson, 1970, p. 495-519) and rock fragment flows (Hsu, 1975) as defined by Varnes (1958).
Relationship of landsliding to topography
Slope inclination was used by Miller (1973) as one criterion for evaluating relative slope stability in an area south of Seattle. By measuring the distance between contour lines on topographic maps the area of that study (except for land extensively modified by human activities) was divided into two categories: areas sloping <15 percent and areas sloping >15 percent. Areas sloping <15 percent were considered relatively stable; areas sloping >15 percent were considered less stable and were further subdivided according to the nature of the underlying material. Selection of 15 percent as the boundary used for slope mapping was based on previous experience with landslides in the area, although figure 19 suggests that it is also justifiable on the basis of soil mechanics.
The slope inclination for each of the 47 landslides included in Appendix 1 was determined from topographic maps in a manner similar to that described above, and the results are expressed in figure 20. All but three of the landslides occurred in areas sloping >15 percent, and those three slides occurred in areas sloping >10 percent. The use of 10 percent as the boundary for slope mapping would include the latter slides but would also increase the total area outlined by approximately 50 percent (Tubbs and Frederick, 1974). Increasing the boundary value to 20 percent would exclude a total of four slides, but would not significantly decrease the area included from that outlined by the 15-percent criterion. Thus, for empirical as well as theoretical reasons, 15-percent slope appears to be a useful criterion for constructing maps of relative slope stability in the Seattle area.
Another topographic factor that might affect slope stability is the aspect of the hillside. Winter storms in Seattle generally come from the southwest, so precipitation might be greater on the southwest than on the northeast sides of hills; also, due to differences in insolation, north-facing slopes might be wetter than otherwise similar south-facing slopes. The aspects of the hillsides in the immediate vicinity of the 47 landslides included in Appendix 1 were therefore measured from topographic maps and are plotted as figure 21.
Although a large number of landslides did occur on slopes facing slightly south of west, a comparable number occurred on slopes facing the opposite direction. The symmetrical pattern reflects the fact that most hillsides in Seattle face one of these two directions, because glaciation produced hills and linear uplands that are generally aligned in a direction from slightly west of north to slightly east of south. Slopes with northern aspects experienced a somewhat larger number of landslides than did slopes with southern aspects. However, it is not clear that this difference is due to microclimatic effects, because there are more steep north-facing than south-facing slopes in Seattle.
Relationship of landsliding to stratigraphy
Slope stability in Seattle is strongly influenced by stratigraphic factors. Table 2 summarizes the stratigraphic relationships of the 47 landslides included in Appendix 1. Landslides immediately underlain by more than one stratigraphic unit (e.g. landslides occurring along stratigraphic contacts) are attributed to the stratigraphically lowest unit. Table 2 demonstrates a strong correlation between landsliding and the presence of either the Lawton Clay or pre-Vashon sediments. Although these materials together immediately underlie less than 10 percent of Seattle, they were present beneath nearly 80 percent of the landslides. This relationship is partly due to the generally very low permeability of the Lawton Clay and pre-Vashon sediments as compared to other stratigraphic units in Seattle. The presence of a relatively impermeable substrate is conducive to the development of seepage forces within the overlying material, and is an important factor affecting slope stability in the Seattle area.
Miller (1973) used the type of substrate as a criterion for evaluating slope stability in an area south of Seattle. Excepting land extensively modified by human activities, areas sloping <15 percent were considered relatively stable, areas sloping >15 percent and underlain by stratigraphic units containing "tight silt or clay" were considered relatively unstable, and areas sloping >15 percent but underlain by other materials were considered of intermediate stability. Tubbs (1974a) demonstrated that there was a good agreement between the locations of landslides known to have occurred during early 1972 and the relative slope stability as evaluated by Miller.
Table 2 also shows a strong correlation between landsliding and the presence of the contacts between the Esperance Sand and either the Lawton Clay or pre-Vashon sediments. This is primarily an effect of the contrasting permeabilities of the stratigraphic units. Where the Esperance Sand overlies the Lawton Clay the movement of ground water within the Esperance Sand is largely controlled by the uppermost laterally extensive silt and clay bed within the Lawton Clay. The trace of the contact between the Esperance Sand and the Lawton Clay is therefore typically the site of considerable seepage, which decreases the stability of the Esperance Sand in the immediate vicinity of its lower contact, and contributes to the saturation of the regolith along that contact. Similar conditions prevail where the Esperance Sand overlies pre-Vashon sediments.
A slight variation of this scheme may occur where sand is intercalated within the upper part of the Lawton Clay. Under such circumstances water may enter the sand beds at some distance from the hillside, resulting in seepage and the development of significant pore-water pressure within the sand beds. Where the Esperance Sand overlies pre-Vashon sediments this variation is not applicable; however, weathering during the Olympia Interglaciation produced clays with undesirable engineering properties that locally may contribute to instability along the contact between the Esperance Sand and pre-Vashon sediments.
The contacts between the Esperance Sand and either the Lawton Clay or pre-Vashon sediments appear to affect slope stability even where the contacts are covered by the Vashon till. Four of the seven slides underlain by Vashon till occurred at or near the inferred local elevation of one of these contacts. Water derived from the Esperance Sand behind the till appears to contribute to the saturation of the regolith in areas where the till is relatively thin. The thickness of till necessary to prevent the influence of such water has not been evaluated, but is expected to be primarily a function of the permeability of the till and the hydraulic head within the Esperance Sand.
On the basis of the relationships between landsliding and the previously discussed topographic and stratigraphic factors, a slope-stability map of Seattle was constructed in the following manner. Areas mapped as artificial fill or modified land by Waldron et al. (1962) were outlined and designated modified land; these areas were excluded from slope-stability classification because their stability is largely dependent upon the design of the grading project. The traces of the contacts between the Esperance Sand and either the Lawton Clay or pre-Vashon sediments were plotted on the map, using information from Waldron et al. (1962), Liesch et al. (1963), Waldron (1967), and field data collected as part of this study. A zone representing a strip of land approximately 200 feet wide was drawn along those traces and designated class 4. This width approximates the horizontal extent of a 35-foot thick stratigraphic interval outcropping on a 15-percent slope. Thirty-five feet is the thickness of the intercalated zone within the top of the Lawton Clay at its type section (Mullineaux et al., 1965), and appears to be the maximum depth within the Lawton Clay at which sand beds are commonly involved in landsliding; it exceeds the depth of the weathered zone locally developed upon pre-Vashon sediments (Mullineaux et al., 1964). Class 4 areas are considered relatively unstable.
The remainder of the land area of the city was divided into two categories: Areas sloping <15 percent and areas sloping >15 percent. Areas sloping <15 percent (excepting areas previously designated class 4) were designated class 1, and are considered relatively stable. Areas sloping >15 percent (excepting areas previously designated class 4) were subdivided into class 2 and class 3 on the basis of the underlying substrate. Class 3 areas are directly underlain by either the Lawton Clay or pre-Vashon sediments, whereas class 2 areas are underlain by the Esperance Sand or younger stratigraphic units. Class 3 areas are considered less stable than class 2 areas; both class 2 and class 3 areas are considered intermediate in stability between class 1 and class 4 areas.
Plate I. Slope-stability map of Seattle
(Click on map to enlarge)
The resulting map is included as plate I. The capability of this map to predict the spatial distribution of landsliding and the relative hazard associated with each of the four classes indicated on the map has been assessed by comparing the locations of the landslides included in Appendix 1 to the classification of those locations as indicated on the map. In the comparison all landslides occurring on the boundary between two slope stability classes were attributed to the less stable class. As shown in table 3, progressively larger numbers of landslides occurred in progressively less stable classes. When the amount of land included within the various stability classes is considered and a comparison made of the landslide density (the number of landslides per unit area) this relationship is accentuated.
Some of the landslides are shown as having occurred in more stable classes than their topographic and stratigraphic relationships would merit, because of the scale of topographic and geologic mapping in the Seattle area. The two slides attributed to class 1 areas actually occurred on slopes of >15 percent inclination, but of insufficient height to be shown with a 25-foot contour interval. Likewise, at least two of the landslides that are shown as having occurred in class 2 areas were underlain by pre-Vashon sediments that were not indicated on existing geologic maps. Although minor adjustments to the location of contacts shown on existing maps were made on the basis of field observations, no attempt was made to totally remap the city. Despite these sources of inaccuracy the agreement between the locations of landslides and the slope-stability classes shown on plate I is excellent, with a landslide density for class 4 areas over two orders of magnitude larger than for class 1 areas.
The relationship between landslide-damage density and the slope-stability classes shown on plate I is even stronger, the difference between class 4 and class 1 areas being over three orders of magnitude. It should be noted that table 3 depicts the hazard associated with the various slope-stability classes rather than the hazard within those classes. Some of the damage attributed to the relatively unstable areas actually occurred in adjacent, more stable areas. Landslides originating within the relatively unstable areas shown on plate I can affect both upslope areas (by the retrogressive failure of slumps) and downslope areas (by the generation of debris avalanches). In using slope-stability maps to make decisions concerning landslide hazard, it is necessary to consider the possible effects of landslides originating within a relatively unstable area upon adjacent, more stable areas.
Influence on landsliding during 1971-1972
While the spatial distribution of landsliding in Seattle during the winter of 1971-1972 was related to certain geologic factors, the temporal distribution appears to have been the result of climatic factors. The specific dates of movement are known for 29 of the 47 landslides included in Appendix 1. The relationship between the dates of landsliding and daily precipitation is shown in figure 22; for landslides having more than one date of known movement only the first date given in Appendix 1 is illustrated. Nearly 75 percent of the landslides occurred on two of the three days during the winter of 1971-1972 having more than 1.5 inches of precipitation and nearly 90 percent of the landslides occurred on three of the four days having over an inch of precipitation.
Tubbs (1974b) discussed possible reasons for the apparent lack of landsliding on January 20, despite nearly two inches of precipitation. Because the several months prior to that date had experienced slightly below-normal precipitation, the suggestion was made that cumulative as well as short-term precipitation might influence landsliding. It was also suggested that a period of sub-freezing weather after January 20 may have increased the subsequent susceptibility to landsliding. General conclusions were drawn concerning the climatic conditions under which future episodes of widespread landsliding could be expected in Seattle, but because the landslides considered in that paper were restricted to a single year it was not possible to adequately develop a predictive model based on climatic factors.
Influence on landsliding during 1932-1972
To facilitate analysis of the relationship of landsliding to annual and cumulative precipitation, the period from August 1, 1932 to July 31, 1972 was divided into 40 precipitation years, each beginning on August 1 and ending on July 31. These dates were chosen because the mean daily precipitation in Seattle is at its minimum in late July and early August and at its maximum in December and January (figure 23). The annual precipitation and number of landslides listed in Appendix 2 for each of the precipitation years are shown in figure 24. There is a significant linear relationship between landsliding and annual precipitation (figure 25), with more landslides occurring in years having high annual precipitation, but the low correlation coefficient and the large time intervals involved limit the usefulness of the diagram for predictive purposes.
In consideration of the apparent influences of daily and cumulative precipitation on landsliding during the winter of 1971-1972, an analysis was conducted of the relationships between cumulative precipitation, short-term precipitation, and the landslides included in Appendix 2 for which the specific dates of occurrence are known. The cumulative precipitation was calculated for each date of reported landsliding, as was the precipitation during five intervals (of length n = 1 to n = 5 days) including the day of the landslide and the (n - 1) preceding days. The number of reported landslides is plotted against cumulative and short-term precipitation in figures 26 through 30. Cumulative precipitation is plotted in 1.0-inch intervals and short-term precipitation is plotted in 0.1-inch intervals. Combinations of cumulative and short-term precipitation for which more than one date of landsliding was reported are plotted as the greatest number of landslides reported for any of the dates rather than as the sum of the number of landslides reported for all the dates.
Several aspects of these figures require explanation. The general clustering of landslide dates toward the center of the cumulative precipitation range appears to be the result of the annual distribution of periods of intense rainfall. Relatively few dates having two or more landslides are shown at cumulative precipitation values of less than about 25 inches, but this is at least partly the effect of inadequate data; several occurrences of two or more landslides at cumulative precipitation values of <20 inches are listed in Appendix 2 but are not plotted in figures 26 through 30 because the dates of occurrence are known only to within two days. Although dates having two or more landslides tend to involve more short-term precipitation than dates having no landslides or only one landslide, the two fields almost completely overlap. There is no obvious relationship between cumulative precipitation and the amount of short-term precipitation necessary to produce widespread landsliding.
These results differ somewhat from those of Nilsen and Turner. They reported a more distinct separation of the conditions under which larger and smaller numbers of landslides occurred in urban areas of Contra Costa County, California, and suggested that less storm-period precipitation was required to produce widespread landsliding as cumulative precipitation increased.
There are several differences, however, between the precipitation parameters used by Nilsen and Turner and those used in this study. Nilsen and Turner used storm-period precipitation, rather than precipitation during intervals of constant length, as the measure of short-term precipitation. Storms were defined as any number of consecutive days having measurable precipitation, and storm periods as periods including storms spaced less than four days apart. Although useful in their study area, these definitions are not useful in Seattle because much of a typical Seattle winter would fit the definition of a single storm period.
Nilsen and Turner attributed all landslides occurring during or immediately following a storm period to that period. However, because the length of a storm period is variable and the amount of storm-period precipitation is partly a function of the length of the period, storm periods receiving greater total precipitation are generally longer and can be expected to include a correspondingly greater number of landslides. Figures 26 through 30 avoid this effect by treating each day as a separate event and relating the number of landslides occurring on that day to the record of previous precipitation, both short term and cumulative. Individually, the figures depict the effect of precipitation intensity and exclude the effect of duration. The durational effects are implicit in the differences between the figures.
The cumulative precipitation expressed in figures 26 through 30 includes the immediately preceding short-term precipitation; this facilitates comparison between the figures because it allows each landslide date to be plotted in the same column in all five figures. Nilsen and Turner, in a similar figure, did not include storm-period precipitation in the cumulative precipitation, but this difference does not account for the differing results of the two studies.
Some of the difference between the results of this study and those of Nilsen and Turner may be due to differences in the nature of landsliding in the two areas. The landslides they described in the San Francisco Bay region generally involved ancient landslide deposits of considerable thickness, while landsliding in the Seattle area usually involves a relatively small thickness of regolith. To the extent that landsliding in both areas is related to the degree of saturation of the potential slide mass, this difference might cause landsliding in Seattle to be more susceptible to relatively short periods of intense precipitation.
The relationships between landsliding and short-term precipitation in Seattle are expressed in figures 31 through 35. The average number of reported landslides per day is based on the landslides included in Appendix 2 for which the specific dates of occurrence are known. Precipitation is plotted in 0.2-inch intervals except where larger intervals are necessary to include at least one landslide (thus allowing the data points to be plotted on semi-log paper); the intervals are indicated for each point.
The relationships can be represented by the regression lines shown in figures 31 through 35. The general form of the equations for these lines is
where L is the average number of reported landslides per day, P is the short-term precipitation (in inches), and a and b are constants. All the relationships are significant at the .01 level and their correlation coefficients range from .92 to .97. The correlation coefficients are highest for the relationships involving one-day and two-day precipitation (figures 31 and 32), and progressively decrease with longer periods. Landsliding in Seattle appears to be most responsive to periods of intense precipitation but short duration. The correlation coefficient for one-day precipitation (.95) is slightly lower than that for two-day precipitation (.97), but this effect is probably due to the larger fraction of one-day precipitation that may occur subsequent to landsliding.
The regression lines shown in figures 31 through 35 are plotted in figure 36, which has applications in landslide prediction and in the analysis of historic landsliding. The accuracy of figure 36 depends on the constancy of the relationships between precipitation and landsliding and between landsliding and the number of reported landslides.
The relationships expressed in figure 36 can be used to evaluate the influence on landsliding of possible freeze-thaw effects such as decreased strength or increased infiltration capacity. During the 40 precipitation years considered in this paper there were 35 intervals of one day or longer during which the maximum air temperature did not exceed 0 degrees C. The minimum air temperature during these intervals was generally considerably lower, suggesting the possibility of soil freezing; unfortunately, soil temperature data is not available for the 40-year period. Sub-freezing air temperature is necessary, although not sufficient, to produce sub-freezing soil temperature, and in the absence of soil temperature data the specific landslide dates listed in Appendix 2 were compared to the dates of sub-freezing air temperature. Figure 37 shows the cumulative sum of reported landslides during the 10-day periods following each of the 35 intervals of sub-freezing temperature, and the cumulative sum that could be expected on the basis of the relationship between two-day precipitation and landsliding shown in figure 36. (In cases where two or more intervals of sub-freezing temperature occurred within a 10-day period, reported and expected landslides were attributed to only the most recent interval.) The number of reported landslides following intervals of sub-freezing temperature does not significantly differ from the number that could be expected on the basis of short-term precipitation. Freeze-thaw effects do not appear to be an important factor affecting landsliding in Seattle.
Figure 36 predicts the number of landslides expected to be reported in The Seattle Times and for which the specific date of occurrence is reported. In order to predict the number of landslides that actually occur on a particular day as a result of an episode of short-term precipitation, it is necessary to multiply the number given in figure 36 by a scale factor. A reasonable scale factor for predicting the number of landslides that cause significant damage is 10, and a reasonable factor for predicting the total number of landslides is 20. The scale factor for damaging landslides was estimated by multiplying
specific reported slides
where the first term is the ratio of the number of damaging landslides during the winter of 1971-1972 to the number of landslides included in the Federal disaster assistance records (approximately 1.25:1), the second term is the ratio of the number of landslides included in the Federal disaster assistance records to the number of landslides during the winter of 1971-1972 reported in The Seattle Times (approximately 5:1), and the third term is the ratio of the number of landslides reported in The Seattle Times to the number of those landslides for which the specific date of occurrence was reported (approximately 1.5:1). The scale factor for total landslides was estimated in the same way, except that the ratio of the total number of landslides during the winter of 1971-1972 to the number of landslides included in the Federal disaster assistance records (approximately 2.5:1) was substituted for the first term. The uncertainties in these approximations are sufficiently large that such scale factors were not incorporated into figure 36; instead, the expected number of reported landslides is presented as an index of temporal landslide hazard.
Considerable landscape modification accompanies urbanization, and can be expected to influence both the spatial and temporal distribution of landsliding. More than 80 percent of the landslides included in Appendix 1 involved one or more human factors that may have contributed to landsliding (table 4). The large number of landslides involving human factors may be partly due to the bias of the data, as discussed under Data sources, and may also be partly due to recognizing possible human influences in landslides that would have occurred regardless of the human factors. It is usually more difficult to assess the importance of possible human influences than to determine the geologic and climatic causes of landsliding.
Diversion of water onto (and into) a slope was the most common human factor, noted in more than 40 percent of the landslides. The water was usually the result of runoff from roofs and paved areas, but other sources were occasionally involved. Steepening of slopes by excavation was also recognized in over 40 percent of the landslides. This can contribute to sliding either by the removal of lateral support, often resulting in immediate failure, or by the creation of unnaturally steep slopes upon which debris slides are likely at some future date. Only one landslide listed in Appendix 1 was of the former type, but the latter variety was quite common. The placing of artificial fill upon a slope can contribute to landsliding, especially on steep slopes underlain by an impermeable substrate; over 30 percent of the landslides involved some fill. Finally, about 10 percent of the landslides were associated with retaining wall failures, due to inadequate design, construction, or maintenance.
Although human factors can result in occasional departures from the previously described relationships of landsliding to topography, stratigraphy, and precipitation, they generally reinforce those relationships. The diversion of water onto a slope, for instance, usually occurs during periods of intense precipitation. The creation of artificially steep slopes is most conducive to landsliding in areas underlain by an impermeable substrate. Artificial fill is most susceptible to landsliding during periods of intense precipitation on steep slopes underlain by an impermeable substrate. The widespread involvement of human factors does not greatly diminish the predictive value of the relationships described in this paper.
Landsliding in Seattle is related to certain geologic, climatic, and human factors. Landslides typically occur in areas sloping >15 percent and underlain by either the Lawton Clay or pre-Vashon sediments; they are often associated with the contact between those units and the overlying Esperance Sand. On the basis of these relationships, a slope-stability map was constructed that predicts the spatial distribution of landsliding (plate I). The temporal distribution of landsliding in Seattle is primarily a function of short-term precipitation. Precipitation data can thus be used to estimate daily landslide hazard (figure 36). Human influences affect both the spatial and temporal occurrence of landsliding, and cause occasional departures from these predictions.
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LANDSLIDES DURING 1971-1972
Appendix 1 includes the 47 landslides that occurred in Seattle during the winter of 1971-1972 and are included in the Federal disaster assistance records.
Location of each landslide is given according to the street name and block number. The slides are listed in roughly counter-clockwise order beginning in the southeast comer of the city.
Dates of initial movement are given as precisely as can be determined from newspaper accounts, Seattle Engineering Department records, and discussions with property owners and neighbors. Dates of major subsequent movement are also noted where such information is available. Dates of occurrence that cannot be determined to within two days are listed as indefinite.
Classification is based on the scheme presented by Varnes (1958). Where the type of landslide listed is followed by an arrow and the name of another type of landslide, the slide is believed to have originated as the first type and subsequently developed into the second type.
Slide material refers to the material that moved (i.e. the material above the failure surface).
Substrate refers to the stratigraphic units underlying the regolith. Except for landslides in which the failure surface was totally within the regolith, substrate refers to the material immediately below the failure surface.
Causes listed include only the specific geologic and human factors discussed in the text that are believed to have had contributed to the landslides. No attempt has been made to list the general causes common to all the slides, nor are climatic factors included.
LANDSLIDES DURING 1932-1972
Appendix 2 includes all landslides occurring in Seattle between August 1, 1932 and July 31, 1972 that were reported in The Seattle Times and for which the dates of initial movement are known to within two days. Landslides known to have occurred on specific dates are listed as separate entries from those for which the dates of occurrence are only known to within two days. Where movement was reported at the same location on two or more dates during a precipitation year (from August 1 to July 31) only the earlier movement is listed. The total number of landslides in Appendix 2 is 160.
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