PLEASE DO NOT REMVE FPfil FILE - Andrews Forestandrewsforest.oregonstate.edu/pubs/pdf/pub410.pdf · Creep, the fourth class of mass movement, is the slow, continuous downslope movement

PLEASE DO NOT REMVE

FPfil FILE

REPRINT

from

Proceedings of

A SYMPOSIUM

FOREST LAND USES

AND STREAM ENVIRONMENT

October 19-21, 1970

Principal Soil Movement Processes Influenced by Roadbuilding, Logging and Fire,Douglas N. Swanston

A cooperative university extensionprogram — School of Forestry

and

Department of Fisheries and Wildlife

Directors:

James T. KrygierCoordinator, Forestry Extension

James D. HallProf. Fisheries and Wildlife

August, 1971

Oregon State University

Copies of the complete proceedings of the symposium can be purchasedat $6.25 each from Forestry Extension, Oregon State University,Corvallis 97331 or by writing Continuing Education Publications,Waldo 100, Corvallis 97331.

PRINCIPAL MASS MOVEMENT PROCESSESINFLUENCED BY LOGGING, ROAD BUILDING, AND FIRE

D. N. Swanston

Abstract

Dominant natural soil mass movement processes active on watersheds of the western United States include I)debris avalanches, debris flows and debris torrents; 2) slumps and earth flows; 3) deep-seated soil creep; and 4)dry creep and sliding. A dominant characteristic of each is steep slope occurrence, frequently in excess of theangle of stability of the soil. All but dry creep and sliding occur under high soil moisture conditions and usuallydevelop or are accelerated during periods of abnormally high rainfall. Further, all are encouraged or acceleratedby destruction of natural mechanical support on the slopes. Logging, road buildin g , and fire play an importantpart in initiation and acceleration of these soil mass movements. Road building stands out at the present time asthe most. damaging activity, with soil failures resulting largely from slope loading, back-slope cutting, andinadequate slope drainage. Logging and fire affect stability primarily through destruction of natural mechanicalsupport for the soils, removal of surface cover, and obstruction of main drainage channels by debris.

IntroductionSoil mass movements, that is, downslope movement of a portion of the land surface under the direct

application of gravitational forces constitute one of the most common but least investigated processes oferosion and slope reduction in mountainous areas of western North America. Such movements may take theform of spectacular landslides and mud flows or the more subtle, slow downward creep of an entire hillside.

In areas with exceptionally steep slopes, high rainfall, or low-strength soil, soil mass movements mayconstitute the dominant process of geologic erosion and consequently are easily accelerated by man'sactivities. Under such unstable conditions, slope soils remain in place as the-result of a delicate balancebetween forces tending to cause downslope movement and the various factors tending to resist it. Anydisturbance may upset this tenuous equilibrium resulting in initiation or accelerating of mass movements.Thus, the ability to recognize and delineate potentially unstable slopes and the evaluation of effects of man'sactivities in such areas is prerequisite to effective management of the land.

Processes •A review of current mass erosion research in the western United States reveals four dominant classes of soil

movement. These classes can be roughly differentiated on the basis of relative rate and depth of movement,and soil moisture content at time of failure. The classification units are basically those presented in Sharpe(1938) and Eckel (1958).

The first class consists of debris slides, debris avalanches, and debris flows. These often involve initial failureof a relatively shallow, cohesionless soil mass on steep slopes as a consequence of surface loading, increased soil

29

water levels, or removal of mechanical support (Fig. 1). Debris slides are the rapid downward movement ofunsaturated, relatively unconsolidated soils and forest debris by sliding or rolling and are differentiated fromdebris avalanches largely by soil water content. Debris flows involve the rapid downslope movement ofwater-saturated soil and debris by true flow processes. These types of mass soil movement serve as a dominantprocess in such diverse areas as the maritime coast of Alaska (Bishop and Stevens 1964; Swanston 1969, 1970)and the dryer intermountain areas of Utah, Idaho, and Montana (Croft and Adams 1950, Megahan 19671).Debris avalanches are also of frequent occurrence in southern California (Corbett and Rice 1966; Rice,Corbett, and Bailey 1969) during the rainy season, and Dyrness (1967) has reported them from the westernflank of the Cascades following the Christmas storm of 1964.

Figure 1.—Typical debris avalanche occurrences in steep mountain watershedsof the western United States.

Debris avalanche, debris flow combination from Prince of Wales Island, Alaska. Failure occurred during aperiod of high-intensity rainfall in a zone of concentrated subsurface drainage. The soil was less than 2feet thick overlying compacted till. The slope angle is about 34 ' (75 percent grade).

Debris avalanches along the South Fork of the Salmon River, Idaho. These failures occurred in a shallowgranitic soil following saturation during a high-intensity rainstorm. The slope is in excess of 27' (60percent).

Walter F. Megahan. Summary of research on mass stability by the Intermountain Forest & Range Experiment Station, SoilStabilization Project, Boise, Idaho. Unpublished proceedings, U.S.D.A. Forest Serv., Berkeley Mass Erosion Conf., Oct. 17-20,1967.

30

Debris slides, debris avalanches, and debris flows are genetically related, differentiated mainly by increasingwater content. All three are characterized by initial failure of a mass of soil or mixed soil and organic debrisabove a relatively impermeable boundary. Failure is either by rotational or by translational sliding followed byrapid downslope movement. Rotational sliding defines initial failure of a soil mass by outward rotation along acurved surface. Translational sliding defines initial failure of a soil mass along a relatively flat, inclined surface.The main slide scarp is bare in both cases, and may be spoon shaped or wedge shaped. The soil massdisaggregates almost immediately after initial failure producing a debris slide or avalanche, depending on initialwater content. The slide or avalanche frequently assumes the characteristics of a flow as the soil mass movesdownslope and increases in water content. Debris torrents are a special type of debris flow occurring in maindrainage channels. These may be caused when soil material, mainly from short debris avalanches insteep-walled tributary gulleys, accumulates behind temporary obstructions such as logs and forest debris. Thisaccumulated mass is then released as a large-volume debris flow by failure of the debris dam during high stormflow. They may also occur as the result of failure at the heads of narrow drainages (Fig. 2). Debris torrentshave been reported as an important mass erosion process in southeast Alaska (Bishop and Stevens 1964,Swanston 1969), in the Salmon River Mountains of Idaho (Megahan 1967), / and on the western flank of theCascades in Oregon where Dyrness (1967) classified them as channel scour events.

Figure 2.-Three debris torrents in the Maybeso Creek valley, Prince of Wales Island, Alaska. All three developedduring a high-intensity storm in the fall of 1961. Initial failure occurred at debris dams near the upperlimit of logging in all three ravines.

Channel scour event in undisturbed timber on the H. J. Andrews Experimental Forest. This eventoccurred as a result of failure near the head of the drainage.

31

Slumps and earthflows constitute the second class and appear to be closely related in terms of theiroccurrence and genetic process (Fig. 3). Both types characteristically develop on deep soils and are frequentlyassociated with deep-seated creep. They begin initially as rotational failures usually triggered by soil saturationand rapid increases in pore water pressure in the immediate area of failure. Slumping involves the downwardand backward rotation of a soil block or group of blocks with small, lateral displacement. The main scarp has asteep headwall and is generally bare and concave toward the toe. The toe is hummocky or broken byindividual slump blocks and, if an earthflow is involved, may be lobate in shape. Earthflows frequentlyincorporate much larger masses of soil which move downslope through a combination of flowage andslumping. The main scarp is usually circular or spoon shaped with a steep headwall and narrow lower orificethrough which the soil flow issued. The toe is characteristically hummocky and lobate in form. Slumping andearthflows are common to most unstable areas of western North America but are especially important as anerosion process in the Northern Coast Ranges of California (Kojan 1967), where large volumes of sediment arebeing added annually to some streams by slumping and earthflow activity.

Figure 3.—Examples of natural slumps and earthflows.Massive slump and earthflow in a weathered till bank near Hollis, Prince of Wales Island, Alaska. Depthof weathering approximately four feet. Slope approximately 15' (34 percent).

Massive earth flow in a recently logged area on the west flank of the Cascades. Depth of soil notavailable, but failure occurred in relatively deeply weathered pyroclastic materials.

32

The third class includes dry creep and sliding of coarse, cohesionless materials on steep, sparsely vegetatedslopes. This is a dominant process of soil mass movement during the dry summer season in the San GabrielMountains of southern California (Anderson, Coleman, Zinke 1959; Krammes 1960). This type of movementinvolves the mechanical sliding or rolling of individual particles or aggregates under the direct influence ofgravity (Fig. 4).

Creep, the fourth class of mass movement, is the slow, continuous downslope movement of mantle materialas the result of long-term application of gravitational stress. It occurs in varying degrees in association withmost other types of soil mass movement, but dominates as a major process in itself on slopes covered withdeep, cohesive soils. The movement is quasiviscous, occurring under shear stresses sufficient to producepermanent deformation but too small to produce discrete shear failure. Kojan (1967) has described soil creepas a major erosion process in the northern California Coast Ranges, not only as a direct contributor ofsediment to streams but also as a critical factor in the progressive failure of overconsolidated materialsultimately resulting in the slump and eartlitlow type movements of class two.

Figure 4.—Dry creep and sliding on slopes of San Gabriel Mountains.Grass-covered dry creep cones.

Dry creep cones in stream channel, Monroe Canyon.•

33

General MechanismsDirect application of soil mechanics theory to analysis of mass movement processes is difficult because of

the heterogenous nature of soil materials, the extreme variability of soil water conditions, and the relatedvariations in stress-strain relationships with time. It does, however, provide a convenient means of expressingthe general mechanism and complex interrelationships of the various factors active in development of soil massmovements.

In simplest terms, failure in a material results if the shear stress acting on the material (force producingnon-elastic deformation) equals the shear strength or internal resistance to shear stress of the material.

Soil mass movements result from changes in the soil shear strength-shear stress relationship in the vicinity offailure. This may involve a mechanical readjustment among individual particles or a more complex interactionamong both internal and external factors acting on the slope.

Shear stress (7) or the tangential component of gravitational stress along a basal zone of sliding can beexpressed by the formula

T = W sinawhere W= effective weight of the soil

a= inclination of the sliding surface

Shear strength (S) or resistance to downslope gravitational stress is expressed by the formulaS = C+W cosa tan0

where C = cohesionW effective weight of the soila= inclination of the sliding surface0= angle of internal friction.

Figure 5 shows the geometrical relationship of these various factors. Any increases in the tangentialcomponent of gravitational stress will increase the tendency for the soil to move downslope. Increases in shearstress result from increasing inclination of the sliding surface (a) or increases in the effective weight of the soilmass (W). Increases in slope angle may result from epeirogenetic and orogenic rejuvenation or local glacial andstream modification. increases in weight of the soil may result from increased water content or surfaceloading. Shear stress can also be augmented by application of wind stresses transferred to the soil through theroot systems of trees. the local build-up of internal stresses in the soil by progressive creep, frictional "drag"produced by seepage pressures, and removal of downslope support by undercutting. Shear strength is governedby a more complex interrelationship in the soil and slope characteristics. These factors include: a) cohesion,the capacity of soil particles to stick or adhere together. This is a distinct soil property independent ofgravitational stresses, produced by cementation, capillary tension, or weak electrical bonding of organicdolloids and clay particles: b) the angle of internal friction (0), which is an expression of the degree ofinterlocking of individual grains; and c) the effective weight or normal component of gravitational stress (W

cosa), which includes both weight of the soil mass and any additional surface loading plus the effect of slopegradient (a). The tangent of the angle of internal friction (0) times the effective weight (W cosa) constitute amathematical expression of frictional resistance (s = W cosatan0). Moisture content and active pore waterpressure (pressure produced by the head of water in a saturated soil and transferred to the base of the soilthrough the pore water), act to modify the component of frictional resistance by reducing the value of thenormal component of gravitational stress. This is frequently expressed by the modified equation for effectiveweight (W cosa-p) where (p) is pore water pressure.

34

4

/ at 7 7/

V •

0. = Wcos a

F,

E,E+1 = Equal and opposite normal forces acting on the soil massF,F, = Equal and opposite shear forces acting on the soil mass

W = Weight of the soil massa = Inclination of the sliding surface

= Shear stress = W sinac = Cohesion, a soil propertya = Normal stress on the sliding surface = W cow

= Angle of internal friction, a soil propertys = Frictional resistance = W cosatan0

Figure 5.—Diagram of forces acting on a mass of soil on a slope.For simplicity, the lateral and shear forces acting on the mass are assumed equal and opposite and thereforecancel. The driving forces tending to cause downslope movement then consist of the weight of the soil mass(W) and its tangential component (r) or shear stress. Resisting forces consist of cohesion (c) which isindependent of the frictional forces and frictional resistance (s) which is proportionally related to thenormal component of the soil weight (a) through the angle of internal friction (0).

35

Decreases in the cohesive properties of soils may be produced by certain processes of soil formation such asleaching and eluviation, or by destruction of capillary tension through increasing water content. As slopeangles approach or exceed the soil's angle of internal friction shear strength may also be reduced due to areduction in the component of frictional resistance. Increased weight of the soil mass caused by surfaceloading increases the normal component of gravitational stress.

Rising pore water pressures reduce shear strength as a result of decreased effective weight of the soil mass.At the same time, seepage pressures resulting from frictional "drag" of water flowing downslope through thesoil may add significantly to the tangential component of shear stress. In highly cohesive soils increasing watercontent may also help to mobilize the clay particles through saturation and further add to increased creeprates and even ultimate failure.

Root systems of trees and other vegetation can serve as cohesive binders or, if they penetrate entirelythrough the soil zone, can anchor the soil mantle to the substrate, and thus provide an effective stabilizinginfluence. In some extremely steep areas this may be a dominant factor in shear strength of the slope soil.Under natural conditions the destruction of such effective mechanical support by logging, windthrow, or firecan produce substantial decreases in mechanical soil strength. Shear strength tests on roots taken from clearcutunits of a variety of ages in southeast Alaska (Swanston and Walkotten, unpublished report) 2 have indicated amarked decrease in strength three to five years after cutting. a time period that roughly corresponds to the lagbetween time of logging and massive debris avalanching in Maybeso Creek valley, Prince of Wales Island,Alaska.

Effects of Logging, Road Building, and FiresSoil mass movements are usually the direct result of the interaction of soil and slope properties

characteiistic of much of the western cordillera and circum-Pacific mountainous belts. Generally, thetopography has a high relief with steep slopes and narrow inter-valley ridges. Locally, glacial erosion, tectonicuplift, and subaerial processes have further steepened the slopes, creating extremely unstable naturalconditions. in such areas soil mass movement is the dominant process of erosion and the general effect oflogging, road building. and fire is to disrupt the delicate balance of forces acting on the slopes, often resultingin further initiation and acceleration of mass wastage.

LoggingLogging is the major economic activity in these mountainous portions of western North America and

probably affects the greatest areas of unstable terrain. Soil mass movements occur regularly in such areas butlittle detailed work has been done to establish their relationship directly to the logging process.

Bishop and Stevens (1964) have linked increased' occurrence of debris avalanches and debris flows tologging in southeast Alaska following high-intensity storms in the fall of 19617 These were triggered by soilsaturation and high pore water pressures. Swanston 11969. 1970). in studies of the causes and mechanics ofthese mass movements, has also demonstrated the importance of the role played by deterioration of stabilizingroot systems in this increased activity. Accumulation of debris in steep ravines, both logged and unlogged. hasalso been cited by the authors as a major contribution to mass soil movements through the formation of debristorrents. Croft and Adams (1950) attributed recent increases in soil mass movement following high-intensitystorms in the Wasatch Mountains to loss of mechanical support by root systems of trees and plants, chiefly bylo gging and burning.

Dyrness (1967) reports an apparent increase in mass movement frequency related to logging disturbance onthe H. J. Andrews Experimental Forest, and Kojan (personal communication) believes that soil mass

2 D. N. Swanston and W. J. Walkotten. Tree rooting and sod stability in coastal forests of southeast Alaska. Study No. FS-NOR1604:26 on file at the Pacific Northwest Forest and Range Experiment Station. institute of Northern Forestry, Juneau,Alaska.

36

movements in the northern California Coast Ranges are "profoundly and cx tensively affected by activitiessuch as logging.—

From the evidence available, the principal effect of logging seems to be largely related to removal of surfacevegetation, creating conditions conducive to increased soil water content and progressive deterioration ofstabilizing root systems.

Road buildingRoad building. a closely related activity to logging, has long been recognized as having high risk potential

for initiating soil mass movements in areas of unstable topography. In western Oregon and Idaho road buildingactivities have been identified directly as the greatest single cause of recent soil mass movements in the areasstudied suggesting that road building may be the most damaging of man's activities in steep mountainousregions. Dyrness (1967), in an investigation of causes of mass soil movements on the west flank of theCascades in Oregon (a total of 47 events), found that slumps and earthflows caused by road fill failures, roadbackslope failures. and failures due to road drainage waters were the most frequently occurring events during aperiod of high landslide activity in the winter of 1964-1965. Road fill failures constituted the greatest singleevent. but together all three t y pes of failure made up greater than 65 percent of all the IllaSS movements thatoccurred In Idaho. Megahan 2 reports about 90 percent of the soil mass movements ,A loch occurred along ;heSouth Fork of the Salmon River durin g a storm in April 1965 (a total of No events resulted front soil !alluresalong the ri ght-of-wa y . These included road fill failure, road baekslope obstrueted road diainagesAgain, the greatest number of these resulted from toad till failures tolioNv.ed ho [allures resulting lromobstructed road drainage. (Fig. 6).

These failures die ia:-eels the result of side casting Lid addition of road rill, inadequate poorl designedroad draina ges. and oversteepened back slopes. Side castin g and the ad.lition of road fills ma:, overload theSillface below the road cut and obstruct upslope soil drainage creatin g saturated conditions in and above theroad fill during periods of high rainfall. Poorb, designed culvert s\ stems ails deep cross drainages. coupled sii l.plu gging of drainages by slope debris ma\-- a l so create saturated conditions in and above the Fivad

ersteepened back slope cuts remove support fot the soils upsiope These conditions frequently resuliserious slumps and short but destructive soil flows.

FireFire is also recognized as an effective agent for increasing soil mass movements on already unstable slopes.

In drier areas it frequently devastates large sections of watersheds in unstable terrain resultin g in almost totaldestruction of plant cover.,

Initially, fire damage removes protecting vegetation from the surface:- It also sharply reducesevapotranspiration and increases water available for surface erosion and soil saturation. Studies on the SanDimas Experimental Forest in southern California (Debano, Osborne. Krammes. and Letey 1967) indicatethat high fire temperatures at the soil surface actually reduce infiltration rates on many soils by production ofa non-wettable layer inducing increased erosion by overland flow.

Destruction of vegetation by fire can also lead to progressive deterioration of the mechanically stabilizingroot systems, a factor which Croft and Adams (1950) consider important. along with logging. to reduction ofsoil strength and resultant increase in soil mass movements in the Wasatch Mountains. In the California CoastRanges land conversion by fire for forage production and increased water yields is a common practice and hasproduced significant increases in soil mass movement occurrence. Stearns 3 estimates 10 to 20 percent of thehigh sediment yield from soil mass movements in the northern California Coast Ranges is directly attributableto land conversion, road building, and logging activities. Corbett and Rice (1966) in observations in the Sall

3Charles L. Stearns. Sediment production due to landslides and streambank erosion in the California north coastal river basinsurvey. Unpublished proceedings of the U.S.D.A. Forest Service, Berkeley Mass Erosion Conference. Oct. 17-20. 1967.

37

Figure 6.—Examples of mass soil movements resulting from failure alonga road right-of-way.

road fill failure

back slope failure

c) failure due to inadequate road drainage

38

Gabriel Mountains of southern California have found five times as many debris avalanches occurring on slopeswhere chaparral has been converted to grass as on uncovered slopes—presumably a reflection of the destructionof stabilizing chaparral root systems. Krammes (1965) also reports an increase in annual sediment productionfrom soil mass movements of 10 to 16 times following a wildfire on monitored watershed slopes in the samea rea.

SummaryFour classes of natural soil-mass movement dominate as major processes of erosion on watersheds of the

Intermountain and Pacific Coast states. These include, in order of decreasing importance and regionalfrequency of occurrence: (I) debris (soil) avalanches, debris (soil) flows, and debris torrents; (2) slumps andearth flows; (3) deep -seated soil creep; and (4) dry creep and sliding.

The dominant characteristic in each of these classes is the presence of steep slopes, frequently in excess ofthe angle of stability of the soils on them. All mass movements but those of class four occur under high soilmoisture conditions, and usually develop or are accelerated during periods of abnormally high rainfall.Further. all are encouraged or accelerated by destruction of the natural mechanical support on the slopes.

The disrupting activities of man play an important part in initiation and acceleration of each of thesesoil-mass movements. Road building stands out at the present time as the most damaging activit y . Soil failuresrelating to this activity are the result primarily of slope loading from road fill and sidecasting. inadequateprovision for slope drainage, and of bank cutting. Fire, natural and man-caused. is a second major contributorto accelerated soil-mass movement in some areas. This relates largely to the destruction of the naturalmechanical support of soils, often abetted by surface denudation of the soil mantle. openin g it to the effectsof surface erosion. Logging, the third activity, affects slope stability mainly through destruction of protectivesurface vegetation, obstruction of main drainage channels by logging debris, and the progressive loss ofmechanical support on the slopes as anchoring root systems decay.

Literature Cited

Bishop, D. M., and Stevens, M. E. 1964. Landslides on logged areas in southeast Alaska. Northern Forest Exp.Sta., U.S.D.A. Forest Serv., Res. Pap. NOR-1, 18 pp.

Corbett, E. S., and Rice, R. M. 1966. Soil slippage by brush conversion. U.S.D.A. Forest Serv.. PacificSouthwest Forest and Range Exp. Sta., Res. Note PSW-128. 8 pp.

Croft, A. R., and Adams, J. A. 1950. Landslides and sedimentation in the North Fork of Ogden River, May1949. Intermountain Forest & Range Exp. Sta., U.S.D.A. Forest Serv., Res. Pap. IN•T-21, 4 pp.

Debano, L. F., Osborne, J. F., Krammes, J. S., and Letey, John, Jr. 1967. Soil wettability and wettingagents—our current knowledge of the problem. U.S.D.A. Forest Serv., Pacific Southwest Forest & Range Exp.Sta.. Res. Pap. PSW-43, 13 pp.

Dyrness, S. T. 1967. Mass soil movements in the H. J. Andrews Experimental Forest: U.S.D.A. Forest Serv.,Pacific Northwest Forest & Range Exp. Sta., Res. Pap. PNW-42, 12 pp

Eckel, E. G. et al. 1958. Landslides and engineering practice. Highway Res. Board Special Rep. 29, NAS-NRCPublication 544, 232 pp., illus.. Washington D.C.

Kojan, Eugene 1967. Mechanics and rates of natural soil creep: Fifth Annual Eng. Geol. and Soils Eng. Symp.,Idaho Dep. Highways, Univ. Idaho, Idaho State Univ. Pocatello, Proc. 1967, pp. 233-253.

39

Krammes, J. S. 1960. Erosion trom mountainside ;lopes after fire in southern California. U.S.D.A. Forest Serv.,Pacific Southwest Forest and Range Exp. Sta. Res. Note PSW-171, 8 pp.

1965. Seasonal debris movement from steep mountainside slopes in southern California. Proc. FederalInteragency Sedimentation Con ference 1963, paper 12, U.S.D.A. Misc. Pub. 970, pp. 88-89.

Rice, R. M., Corbett, E. S., and Bailey, R. G. 1969. Soil slips related to vegetation, topography, and soil insouthern California. Water Resources Res., Vol. 5, No. 3, pp. 647-659

Sharpe, C. F. S. 1938. Landslides and related phenomena. Columbia Univ. Press, New York, 137 pp.

Swanston, D. N. 1969. Mass wasting in coastal Alaska. Pacific Northwest Forest and Range Exp. Sta., U.S.D.A.Forest Serv. Res. Pap. PNW-83, 15 pp.

-1970. Mechanics of debris avalanching in shallow till soils of southeast Alaska. Pacific Northwest Forestand Range Exp. Sta., U.S.D.A. Forest Service, Res. Pap. PNW-103, 17 pp

Purchased by the Forest ServiceU.S.Department of Agriculture,for official use.

40