PLEASE DO NOT REMOVE FROM

FILES

NEW REQUIREMENTS IN FOREST ROAD CONSTRUCTION

FP 2406

Airport Hyatt House

A

December 9-11, 1974

Sponsored by: Association of B.C. Professional Foresters

and

University of British Columbia

Faculty of Forestry and Centre for Continuing Education

147.

INTERPRETING STABILITY PROBLEMS

FOR THE LAND MANAGER

by

D. N. Swanston

Introduction

Forest operations in mountainous regions have a major impact

on soil erosion processes. Under natural conditions, forest vegeta-

tion protects the soil surface, and internal soil strength is adequate

to resist the downward pull of gravity on the soil mass. In the undis-

turbed state, the forest floor of steep, mountain watersheds repre-

sents a minimum erosion site. It exists in a state of semiequilib-

riurn between erosional processes and soil forming processes.

Any disrupting influence, whether it be a natural catastrophe, such

as fire, earthquake, or large storms, or the activities of man, is

a potential initiator of more active erosion cycle.

Downslope movement of soil may take the form of surface or

single-particle erosion involving transportation of soil particles by•

running water or the form of mass soil movement involving the

transport of a finite mass of soil and forest debris primarily by

gravity. In surface erosion, the degree of movement is directly

related to angle of slope, amount of water available for surface

runoff, and ground disturbance produced by a disrupting event.

148.

Masi erosion is a much more subtle process involving the inter-.action of slope angle and soil water content with a number of

different factors which determine the physical and biological char-

acter of the soil.

We frequently think of surface erosion as the principal ero-.•sion process in forested areas, and, in fact, it does constitute the

•most visible and active process in disturbed areas and along lower

sfopes and valley bottoms. In the steep mountainous regions, how-

ever, soilmass movements are frequently a dominant erosion process

-• and present a major problem during timber harvesting and reforesta-

tion operations in these areas.

With increasing demand for lumber and pulpwood, more of

these steep mountain watersheds are being directly influenced by

, forest operations. The resulting disruption of natural slope stability

characteristics has accelerated slope failure in many logged areas,

producing excess sediment loads in streams, causing extensive•

damage to structures and roads, and effectively removing portions•

of the watershed from immediate reforestation.

It thus becomes essential, for effective forest land manage-

ment, to be able to recognize and define these unstable areas, to

determine primary mass erosion processes operating on the slope,

and to identify and understand the interaction of principal and con-

tributing factors controlling slope failure.

149.

This requires, first of all, a basic knowledge of the

geology and geologic history ofthe area being managed and at-

least a rudimentary understanding of the six landscape components

that control or contribute to unstable conditions (see Lavkulich,

"Soils of Vancouver Island";-ii and Given, Lewis and Lavkulich,

"Proposed Landform Classification System"-Y). Analytical

interpretation and technical evaluation should then be performed,

•whenever possible, in close cooperation with the engineering

geologist, civil engineer and soil scientist.

Lavkulich, L. M., Soils of Vancouver Island: paperprepared for presentation to MacMillan Bloedel Ltd. ForestryDivision Engineers and Foresters, Dec. 11-12, 1972.

Given, W. J. B., T. Lewis and L. M. Lavkulich,Proposed Landform Classification System: being prepared forpublication by the Dept. of Soil Science, The Univershy ofBritish Columbia, Vancouver.

150.

The Erosion Cycle

The coastal area of British Columbia is characterized by•

naturally oversteepened slopes, shallow, permeable soils and•exceptionally high rainfall, all vital components of unstable terrain.

Because of high soil pérmeabilities, slope drainage is primarily by

subsurface flow • with little or no overland flow outside established

channels. When overland flow does . occur, the thick mat of forest•

vegetatioh is adequate to protect the mineral soil from surface

erosion. During major storm periods, high soil moisture levels

and local areas of saturation are produced on the slope, greatly

increasing the unstable character of the slope soils. At the same

time, rapid runoff produces high stream flow, maximum bedload

movement and channel scour in the valley bottoms.

Under these conditions, surface erosion is minimal and is

mostly restricted to stream bank cutting and erosion of bare mineral

soil areas. Soil mass movements, involving the downslope motion

of soil primarily under the force of gravity dominate as the principal

natural process of erosion and slope reduction.

Classification and Nomenclature

The dominance of soil mass movements as a principal geo-

logic erosion process on steep forested slopes is not unique to this

area. Such processes are characteristic of much of the western

151.

Cordillera and circum-Pacific mountain belts where slopes are

steep, topographic relief is high and glacial erosion, vulcanism,

tectonic uplift and powerful weathering processes have created

extremely unstable natural conditions.

Within these sensitive areas, soil mass movements can

range widely in surface configuration, speed of movement and

volume of material involved. , They can take the form of spec-

tacular debris avalanches and mudflows or the more subtle slow

downward creeping movement of an entire hillside. In terms of

dominant processes, however, soil mass movements on forest

lands throughout the west can be classified into three groups

roughly differentiated on the basis of rate and depth of movement,

basic soil properties, and soil moisture content at time of failure.

(Swanston, 1974a). These groups are:

debris slides, debris avalanches and debris flows involving

initial failure of a relatively shallow, cohesionless soil

mass on steep slopes as a consequence of surface loading,

increased soil water levels or removal of mechanical

support;

soil creep, slumps and earthflows resulting from quasi-

viscous flow and progressive failure of weathered pyro-

clastics, sandstones and shales; and

152.

(3) dry.creep and sliding or dry ravel of coarse, cohesionless

material on steep, sparsely vegetated or denuded slopes as

a result of diurnal freeze and thaw and wetting and drying.

All of these occur in 'varying degrees along the B. C. coast.

but the great majority develop as debris avalanches and debris

flows involving the rapid downslope movement of a mixture of soil,

rock and forest litter of varying water content.

Befbre going into this main group in any detail, let's consider

groups two and three. -

Soil creep, sin-raping and earth flows result from quasi-

viscous flow and progressive failure of deeply weathered, clay-

rich materials such as law-silica sandstones, shales and pyro-

clastics. In heavily glaciated terrain most movements develop in

lake clays. Movement can range from imperceptible creep motion

of 1 to . 5 inches per year to high velocity flows moving at speeds

of tens of feet per second. Sizes range from small slumps and

flows of several cubic yards in volume to an entire hillside. In

areas of extremely deep, cohesive soils, a combination of creep,

progressive slumping, and earth flows frequently involve an entire

watershed. In such areas, slumps and earth flows occur in zones

of concentrated subsurface drainage. Creep is the slow, continuous

downslope movement of mantle material as the result of long-term

application of gravitational stress. It occurs in varying degrees

153.

in association with most other types of soil mass movement, but

dominates as a major process in itself on slopes covered with

deep, cohesive soils. The movement is quasi-viscous, occur-

ring under shear stresses sufficient to produce permanent defor-

mation but too small to produce discrete shear failure. For

areas where soil creep is a major erosion process, it serves

as a critical factor in the progressive failure of overconsolidated•

materials ultimately resulting in the slump and earth flow.

Slumps and earthflows are closely related in terms of their

occurrence and genetic process. Both types characteristically

develop on deep soils and are frequently associated with deep-

seated creep. They begin initially as rotational failures usually

triggered by soil saturation and rapid increases in pore water

pressure in the immediate area of failure. Slumping involves

the downward and backward rotation of a soil block or group of•

blocks with small, lateral displacement. The main scarp has a

steep headwall and is generally bare an concave toward the toe.

The toe is hummocky or broken by individual slump .blocks and,

if an earthflow is involved, may be lobate in shape. Earthflows

frequently incorporate much larger masses of soil which move

downslope through a combination of flowage and slumping.

Slumping and earthflows are common to most unitable areas

154.•

of western North America but are especially important as an

erosion process in the northern Coast Ranges of California, the

western Cascades, and portions of the Oregon Coast Range where

large volumes of sediment are being aoded annually to some streams

by slumping and earthflow activity.

. The direct effect of timber harvesting operations on this

group has not yet been clearly defined.

Road building is probably the most damaging activity. Road

construction in active or dormant creep and slumping areas is

highly likely to accelerate or reactivate the soil mass, largely

_ through alteration of the balance of forces acting on the slope.

Timber removal probably exerts its greatest impact through altera-

tion of the natural slope hydrology, producing unstable conditions

during critical storm periods.

Dry ravel or dry creep and sliding defines single particle

movement of coarse, cohesionless materials on steep, sparsely•

vegetated or recently denuded slopes. This is a common erosion

process on unvegetated oversteepened slopes throughout the moun-

tainous region of the west, caused by loss of frictional resistance

between individual soil particles due primarily to freeze and thaw

and wetting and drying cycles. In areas characterized by steep

slopes, coarse textured soils and extended summer droughts it

155.

may be a particularly important process. It constitutes the

dominant process of soil mass movement during the dry summer

season in the San Gabriel Mountains of southern California. This

type of movement involves the mechanical sliding or rolling of

individual particles or aggregates under the direct influence of

gravity.

Principal effects of timber harvesting activities on this group

are removal of surface vegetation and construction of artificial

embankments exposing bare mineral soil to rapid weathering and

cycles of freezing and thawing and wetting and drying.

Debris avalanches and debris flows, the first and most wide-

spread group, are mass movements produced by instantaneous

failure in shallow soils overlying an impermeable surface. The

soils are usually cohesionless from an engineering point of view

and range in depth from several inches to four or five feet. Move-

ment velocities are high, frequently in excess of 5 ft. /sec. , and

volume of material moved depends on width and depth of slide and

length of slope on which it develops. Sizes range from small soil

slips between trees and stumps to long downslope avalanche tracks

and associated debris deposits covering several acres.

Debris avalanches and debris flows begin as more or less

intact masses of soil that suddenly break away and move downslope.

These masses quickly become debris avalanches as stresses within

A

156.

the mass cause breakdown of the soil structure. Debris avalanches

frequently revert to debris flows as water content and volume of

debris increases down slope. Debris avalanching within V-notch

drainages frequently produces an especially spectacular, high

volume debris flow called a debris torrent. These are usually

confined within the V-notch until the valley floor is reached when

the debris then spreads out, inundating vegetation and forming a

broad surface deposit. Debris torrents usually result when debris

avalanche material either dams the channel temporarily or accumu-

lates behind temporary obstructions such as logs and forest debris.

When these temporary dams fill during periods of high stream flow,

a debris torrent results.

Investigations have shown (Swanston 1969, 1974b, O'Loughlinli)

that the most common debris avalanches and debris flow situations

develop on slopes greater than 30° either in shallow soils derived

from colluvium or bedrock with bedrock serving as the sliding surface;

or in shallow soils derived from glacial till, with impermeable, un-

weathered till serving as the sliding surface. Both types of soil are

coarse and permeable, with less than 20 percent of particles finer

than silt.

This group is strongly affected by timber harvesting activi-

ties. Road construction is the most damaging activity, largely

through disruption of the natural balance of forces on the slope by

cut and fill activities. Obstruction of slope drainage and local

..11 -Colin L. O'Louglin. An investigation of the stability ofsteepland forest soils in the Coast Mountains, Southeast BritishColumbia. Ph. D. Dissertation Faculty of Forestry, U. ofBritish Columbia, 1972.

157.

saturation of roadfills are also important initiators. Destruction

of surface vegetation and deterioration of anchoring roots by land

conversion and clearcut logging have also been linked with accel-

erated debris avalanche and debris flow occurrence.

Mechanism of Movement

Periodically high soil moisture content and oversteepened

slopes are common factors of all areas of recent accelerated soil

mass movements on forested lands. Local bedrock type, climate,

and basic soil characteristics determine the individual failure

mechanisms. External factors, primarily rooting structures of

trees and understory vegetation, have been shown to contribute

to the inherent stability of some sites.

Basic Parameters

The stability of a soil can be expressed most simply as a

ratio between shear strength or resistance of a soil to sliding and

the downslope pull of gravity, or gravitational stress. As long as

shear strength exceeds the pull of gravity, the soil will remain•

in a stable state (Terzaghi, 1950; Zaruba and Mencl, 1969).

Soil mass movements result from changes in the soil shear

strength-gravitational stress relationship in the vicinity of failure.

This may involve a mechanical readjustment among individual

particles or a more complex interaction among both internal and

external factors acting on the slope.

158.

The following figure (figure 1) shows the geometrical relation-

ship of these various factors acting on a small portion of the soil

mass. Any increases in gravitational stress will increase the

tendency for the soil to move downslope. Increases in gravitational

stress result from increasing inclination of the sliding surface (a)

or increases in the unit weight of the soil mass (W). Stress can

also be augmented by application of wind stresses transferred to

the soil through, the root systems of trees, the local build-up of

internal stresses in the soil by progressive creep, frictional

"drag" produced by seepage pressure, horizontal accelrations

due to earthquakes, and removal of downslope support by under-

cutting.

Shear strength is governed by a more complex interrelation-

ship between the soil and slope characteristics. Two secondary

forces are active in resisting downslope movement. These are:

1) cohesion (c) or the capacity of the soil particles to stick or adhere

together. This is a distinct soil property, produced by cementation,

capillary tension, or weak electrical bonding of organic colloids and

clay particles; and 2) the frictional resistance (W cos a tan 4)) between

individual particles and between the soil mass and the sliding surface.

Frictional resistance is controlled by the angle of internal friction (4)

of the soil, which describes the degree of interlocking of individual

159.

grains; and the effective weight ((W-p.) cos a) of the soil which

includes both the weight of the soil mass and any surface loading•

plus the effect of slope gradient and excess soil water.

Pore water pressure (pressure produced by the hew:1'0f

water in a saturated soil and transferred to the base of the soil

through the pore water), acts to reduce the frictional resistance

of the soil by reducing its effective weight. In effect, its action•

causes the soil to "float" above the sliding surface.

Controlling and Contributing Site Characteristics

Particle size distribution (which governs cohesion), angle of

internal friction, soil moisture content and angle of slope are the

controlling factors in stability of a steepland soil. For example,

shallow coarse-grained soils low in clay-size particles have little

or no cohesion and frictional resistance determines the strength of

the soil mass. Frictional resistance is in turn strongly dependent•

on the inherent angle of internal friction of the soil and the degree

of pore-water pressure development. A low.angle of internal

friction relative to slope angle or high pore pressures can reduce

soil shear strength to negligible values. Slope angle is a major

indicator of the stability of these soils. Slopes at or above the

angle of internal friction of the soil indicate a highly unstable

natural state.

160.

Soils of moderate to high clay content take on a much more

complex character with resistance to sliding determined by both

cohesion and frictional resistance. These factors are controlled

to a large extent by clay mineralogy and soil moisture content. In•

a dry state, clayey soils have a high shear strength with the internal

friction .angle quite high ( > 30° ). Increasing water content mobilizes

the clay through adsorption of water into the clay structure. The

angle of internal friction is reduced by the addition of water to the

clay lattices (in effect reducing "intragranular" friction) and may

approach zero in the saturated state. Some clays are more suscep-

tible to deformation than others, making clay mineralogy an impor-

tant consideration in areas characterized by quasi-viscous flow

deformation or "creep." Swelling clays of the montmorillonite

type are particularly unstable because of their tendency to adsorb

large quantities of water and the loosening effect of alternate expan-

sion and contraction during periods of wetting and drying. Thus,

clay-rich soils have a much higher potential for failure given excess

soil moisture content. Under these conditions failures are not

directly dependent on sliding surface gradient as in cohesionless

soils but may develop on slopes with gradients as low as two or

three degrees.

161.

Contributing Factors

Parent material type has a major effect on the particle size

distribution, depth of weathering, and relative cohesiveness of a

steepland soil and as such can frequently be used as an indicator

of relative stability or potential stability problems if local climatic

conditions and relative age of the geomorphic surface on which the

soil is developed are known. In humid regions where chemical

weathering predominates, transformation of easily weathered

primary minerals to clays and clay-size particles may be exten-

sive. Siltstones, clay stones, shales, nonsiliceous sandstones,•

pyroclastics, and serpentine-rich rocks are the most easily altered

and are prime candidates for soil mass movements of the creep,

slump, and earthflow type. Conversely, in arid or semi-arid

regions, slopes underlain by these rocks may remain stable for

many years due to slow chemical weathering processes and lack

of enough soil moisture to mobilize existing clay minerals. On •

steeplands, underlain by resistant rocks, especially those at high

altitude or latitude where mechanical weathering prevails, soils

are usually coarse and low in clay-size particles. Such areas

are more likely to develop soil mass movements :1 the debris

avalanche or debris flow type.

Parent material structure is a critical factor in stability of

162.

many shallow soil slopes. Highly jointed bedrock slopes with

principal joint planes parallel to the slope provide little mechan-

ical support to the slope and create avenues for concentrated sub-

surface flow and active pore water pressure development as well

as ready-made zones of weakness and potential failure surface for

the overlying material. Sedimentary rocks with bedding planes

parallel to the slope function in essentially the same way with the•

uppermost bedding plane functioning as an impermeable boundary

to subsurface water movement, a layer restricting the penetration

and development of tree roots and an active failure surface.

Vegetation cover in general helps control the amount of water

reaching the soil and the amount held as stored water against

gravity, largely through a combination of interception and evapo-

transpiration. The direct effect of interception on the soil water

budget is.probably not large, especially in areas of high total rain-

fall or during large storms when most soilmass movements occur.

The small storms where interception is effective probably have little

influence on total soil water available for activating mass movements.

The effect of evapotranspiration is much more pronounced but is

particularly region and time dependent. In areas characterized by

warm, dry summers, evapotranspiration withdrawals of soil mois-

ture have a significant effect in reducing the degree of saturation

163.

. resulting from the first storms of the fall recharge period. This

effect is reduced as soil water deficit is satisfied. Once the soil

is recharged, the effect of previous evapotranspirational losses

becomes negligible. Conversely, in areas of continuous high

rainfall or with an arid or semiarid climate, evapotranspirational

effects are probably negligible. Also of importance is the depth of

evapotranspirational withdrawals. Deep withdrawals may require

substanti`al recharge to satisfy the soil water deficit, delaying or

reducing the possibility of attainment of saturated soil conditions

necessary for major slide-producing events. Shallow soils, on the

other hand, will recharge rapidly, possibly attaining saturated condi-

tions and maximum instability during the first major storm.

Root systems of trees and other vegetation may act to increase

shear strength in unstable soils. Such an external shear strength

factor can result from roots:

Anchoring through the soil mass into bedrock fractures in

the, rock. •

Providing continuous long fiber cohesive binders to the soil

mass.

3. Tying slope together, across zones of weakness or instability,

or stable soil masses.

164.

Providing downslope support to an unstable soil mass.

Interlacing with other vegetation, providing a network of

stability through their own strength.

In shallow soils, all five items may be important. In deep

soils, the anchoring effect of roots becomes negligible, but the

other parameters will remain important. In some extremely steep

areas in the western United States, root anchoring may be the domi-

nant factbr in maintaining slope equilibrium of an otherwise unstable

area.

Snow load increases soil unit weight through surface loading

and effects delivery of water to the soil through retention of rainfall

and delayed release of large water quantities during spring melt.

Delayed release of melt water, coupled with unusually heavy storms

during a spring warming trend have been identified as the principal

initiating factor in recent major landslide activity on forest lands

in central Washington.

Damages from Mass Erosion

The impact of soil mass movements on forest lands can be

extensive. Destructive debris avalanches and debris and earth

flows frequently destroy the entire productive soil zone within

their paths. Natural vegetation may occur in as little as 10 years,

but if the landslide remains active by progressive slumping near

165.

its head, •considerably greater time may elapse before substantial

vegetation cover develops. Conversely, the zone of deposition at

the base of the slope may actually serve as an area of more rapid

regeneration because of the mixture of soil and organic debris.

Soil mass movements are also prodigious producers of sediment

directly effecting quality of water leaving the watershed. This may

be in the form of the direct addition of a slug of sediment into a•

stream &tiring sliding or through surface erosion from the zone of

deposition. Active creep adds a continual supply of sediment to the

stream largely through progressive slumping along the banks.

Finally, soil mass movements are a major problem in terms of

personnel safety and construction and maintenance costs in active

timber harvesting areas and elsewhere in the case of roads. This•

is primarily the result of reactivation of old, partially stabilized

slumps and soil flows, timber harvesting operations on over-

steepened slopes delicately stabilized by natural vegetation and

failures due to oversteepened back slopes and excessive fills on

forest roads.

Effects of Forest Management Practices on Mass Erosion

Slope disturbance produced by forest operations in moun-

tainous regions has been clearly identified as a major contributor

to accelerated soil mass movements. Roadbuilding is the most

166.

damaging operation, but timber cutting, and slash burning have

also been shown to be effective initiators of some types of mass

erosion activity.

Roadbuilding has.j.ong been recognized as having high risk

potential for initiating soil mass movements in areas of unstable

topography (Swanston and Dyrness, 1973). In western Oregon and

Idaho, roadbuilding activities have been identified directly as the

greatest single .cause of recent soil mass movements suggesting

that roadbuilding may be the most damaging of man's activities in

these steep mountainous regions.

Recent investigations of causes of mass soil movements on

forested lands have found that slumps and earthflows caused by

road fill failures, road backslope failures, and failures due to

road drainage waters, were the most frequently occurring events

during a period of high landslide activity. As an example, a

recent investigation in Idaho reported that about 90 percent of the

soil mass movements, mostly debris avalanches and debris flows,

which occurred along the South Fork of the Salmon River during a

storm of April 1965 resulting from soil failures along the right-

of-way. As expected, these included road fill failure, road

backslope failures, or obstructed road drainages. The greatest

number of these resulted from road fill failures, followed by

failures resulting from obstructed road drainages.

••••••••

A

167.

Road construction activities can disrupt the basic equilibrium

balance of steep slope forest soils in three ways: (1) alteration of

slope drainage, (2) slope loading, (3) slope undercutting. Altera-

tion of slope drainage includes interception and concentration of•

surface and subsurface flow by ditching, bench cutting, and massive

road fills. Interception and concentration of water encourages satu-

ration, active pore water pressure development, and increased unit

weight in triad prisms, side cast materials, and soils.

Slope undercutting by benching along an oversteepened slope

removes support for the soil upslope from the road. Old slumps and

landslides are particularly susceptible to disturbance by these activi-

ties. These areas have stabilized themselves naturally according to

the slope conditions existing at the time of initial failure. They are

in a state of equilibrium with the slope of either side, and construc-

tion activity which involves appreciable excavation or filling is highly

likely to reactivate the soil movement zone through changes in dis.

tribution of stress within the dormant mass. .

Cutting of trees alone does not greatly increase surface soil

erosion; however, on steep slopes there is some evidence that it

may adversely affect soil stability through changes in soil hydrology

and mechanical support provided by vegetation. Studies in Alaska

have directly linked increased occurrence of debris flows to logging

168.

following high intensity storms in the fall of 1961 (Bishop and

Stevens, 1964). These were triggered by soil saturation and high

pore-water pressures. Destruction of stabilizing root systems is

believed to be the cause of this increased activity. Accumulation

of debris in steep ravines, both logged and unlogged, has also been

cited as a major contributor to mass soil movements through the

formation of debris torrents. As early as 19 50 studies in Utah (Croft•

and Adams, 1950) attributed recent increases in soil mass movement

following high intensity storms in the Wasatch Mountains to loss of

mechanical suppoit by root systems of trees and other vegetation,

chiefly by logging and burning; and apparent increases in soil mass

movement frequency related to logging disturbance has been reported•

in the western Cascades of Oregon.

Fire is an effective management tool in conjunction with logging

to dispose of logging debris and to prepare a seed bed. It is also

an effective agent for accelerating dry creep and sliding and may

indirectly influence soil mass movement acceleration on already

unstable slopes. Unfortunately, most work done•so far which can

be directly related to effects of fire on soil mass movements has

come from the shaparral covered slopes of southern California

where past fire erosion is a major problem. Some interesting and

applicable general analogies, however, can be made on the effects

of log and slash burning.

169.

At its worst, fire removes all protecting vegetation from the

surface. This can lead to progressive deterioration of the mechan-

ically stabilizing root systems. In the California Coast Ranges,

land conversion by fire for forage production and increased water

yields is a common practice and has produced significant increases

in soil mass movement occurrence. Current estimates indicate that

10 to 20 percent of the high sediment yield from soil mass movements

in the northern California Coast Ranges is directly attributable to

land conversion, roadbuilding and logging activities. Recent

observations in the San Gabriel Mountains of southern California

have found seven times as many debris avalanches occurring on

slopes where chapparral has been converted to grass, as on

uncovered slopes--presumably a reflection of the destruction of

stabilizing chapparral root systems. Increases in annual sediment

production from dry creep and sliding of 10 to 16 times following

wildfire has been reported in the same area (Krammes, 1965; Rice,

Corbett and Bailey, 1969).

Means of Minimizing Soil Mass Movement

A basic understanding of mass wasting processes and control-

ling and contributing factors is essential to effective identification,

prediction, and control of soil mass movements on fore/alands.

Once the land manager has accomplished this he has two options

open to him. He can (1) identify problem areas and avoid operations

on unstable terrain or (2) identify and attempt to control operational

-4/Stearns, Charles E. Sediment production due to landslideand streambank erosion in the California north coastal river basinsurvey. Unpublished proceedings of the USDA Forest. Service,Berkeley, Mass Erosion Conference, Oct. 17-20, 1967.

170.

effects. In highly unstable areas or areas of questionable eco-

nomic value, avoidance of all operations is probably the best and

least expensive solution. Controlling operational effects is a much

more difficult approach which at best will probably be only partially•successiiil. It is applicable in high value areas of questionable soil

stability or where other considerations override a desire for stability

maintenance.

Identification

Identification of unstable areas is an essential part of both

options. This involves, first of all, the accurate determination

and :napping of unstable and potentially unstable slopes in the area

of proposed timber operations. This should be followed by a care-

ful analysis of the factors contributing to unstable conditions and

a classification of unstable areas according to the level of operations

that can be safely performed within them. Thus areas within the

highly unstable class should be withdrawn from timber harvesting

activities entirely or, at the very least, have - operations limited to

light selective cutting and yarding by helicopter. Road '-.uilding

activities in these areas are almost sure to cause or accelerate

landslide occurrence with little chance of effective control. Areas

in the potentially unstable class should be carefully examined for

unstable conditions on a local basis and operation criteria designed

171.

to fit each stability situation encountered.

This type of stability analysis has been done recently for

timber sales in southeast Alaska using air photos and topographic

maps (Swanston, 1973). Estimated maximum and minimum angles

of internal friction of local soils were used to define the limits of

highly unstable and questionable terrain.

A similar landslide hazard analysis has been recently per-

formed pn the 'Teton National Forest in northwest Wyoming using

air photos and field checking to determine degree of active land-

sliding (Bailey, 1971).

Air photo interpretation is an effective tool in soil stability

hazard identification and prediction. Air photo indentification of

unstable terrain in southern California has proven 80 percent

effective in predicting areas effected by landslides during a large

storm (Kojan, Foggin and Rice, 1972), and in the northern Cali-

fornia Coast Ranges aerial photos are being used extensively to

identify, delineate, measure, and interpret topographic features

related to deep-seated creep and sliding.

The slope stability information provided by slope angle

analysis and air photo interpretation should be supplemented and

refined by careful interpretation of geology and soil maps of the

area coupled with field sampling for basic soil and structural

properties which contribute to unstable conditions.

172.

Control Measures

A number of effective engineering control measures are

available for slope stabilization, but as a rule, they are expensive

and generally applicable to speCific occurrences. Current investi-

gations of stability of forested slopes have been directed toward:

avoidance of disturbances damaging to slope stability and

reduction of landslide incidence after disturbance.

The former can best be accomplished by reduction in forest

road construction in unstable areas and substantially reducing slope

disturbance by logging processes. A number of promising new

timber harvesting methods are currently being investigated or are

undergoing practical trials with this in mind. These include balloon

logging and helicopter transport, both of which have a tremendous

potential for reduciqg the environmental impact of logging in

unstable areas.

The latter is best approached through improved road design

and construction and stabilization of disturbed areas by vegetation

planting. Some effective road design and construction techniques

are already available to the engineer and land manager. It is

largely the successful and consistent application of these techniques

and the use of good common sense which determines the impact of

roading on slope stability. Stability of road cuts and side cast

173.

slopes has been substantially increased by planting of grass and

legtimes in the western states, and several debris avalanche tracks

have been partially stabilized in southeast Alaska using a mixture•

of reed canary grass and alder wildlings. Full bench cuts and

reduction or elimination of side casts and fills will greatly reduce

slope loading, blocked drainageways and potential fill failures due

to road prism saturation.

Adequate road drainage and maintenance of through-slope

drainage across roads is essential if problems with road associ-

ated failures is to be avoided. This includes adequate planning

for culvert location and size and consistent maintenance of ditches

and culverts to prevent blocking or plugging. Once water gets into

a road prism on a highly unstable slope problems multiply rapidly.

174.

REFERENCES CITED

Bailey, R. G., 1971, Land hazards related to land-use planningin Teton National Forest, Wyoming. .USDA Forest ServiceReport, Intermountain Region, 1 31 pp., illus., maps.

Bishop, D. M. and Stevens, M. E., 1964, Landslides on loggedareas in southeast Alaska. USDA Forest Service, NorthernForest Experiment S ation Res. Paper NOR-1, 18 pp. , illus.

Croft, A. R. and Adams, J. A., 1950, Landslides and sedimenta-tion on the north fork of Ogden River, May 1949. USDAForest Service, Intermountain Forest and Range ExperimentSta., Res. paper INT-21, 4 pp., illus.

Kojan, Eugene, Foggin, G. T. and Rice, R. M. , 19 72, Predictionand analysis of debris slide incidence by photogrammetry,Santa Ynez-San Rafael Mountains, California. Proceedings,24th Int. Geological CongreSs, section 13, pp. 124-131.

Krammes, J. S., 1965, Seasonal debris movement from steepmountain side slopes in southern California. Proceedingsof Federal Interagency Sediment Conference, 1963, paper12. In USDA Misc. Publ. 970, pp. 85-88.

Rice, R. M., Corbett, E. S. and Bailey, R. G. , 1969, Soil slopesrelated to vegetation, topography and soil in southern Cali-fornia. Water Resources Res., Vol. 5, no. 3, pp. 647-659.

Swanston, D. N., 1969, Mass wasting in coastal Alaska. PacificNorthwest Forest and Range Experiment Station, USDAForest Service Res t Paper PNW-83, 1 5 pp. , illus.

Swanston, D. N., 19 73, Judging landslide potential in glaciatedvalleys of southeastern Alaska. Explorers . Journal, Vol. LI,no.. 4, pp. 214-21 7, illus.

Swanston, D. N. , 19 74a, Slope stability problems associated withtimber harvesting in mountainous regions of the westernUnited States. USDA Forest Service General TechnicalReport PNW-21, 14 pp., illus.

Swanston, D. N., 1974b, The forest ecosystem of southeast Alaska,part 5: Soil mass mo,yement. USDA Forest Service GeneralTechnical Report PNW-1 7, 22 pp. , illus.

175.

Swanston, D. N. and Dyrness, C. T., 1973, Stability of steep-land. Journ. of Forestry, Vol. 71, no. 5, pp. 264-269,illus.

Terzaghi, Karl, 1950, Mechanism of landslides. In "Applicationof geology for engineering practice." Geol. Soc. Am.Berkey Volume, pp. 83-123, illus.

Zaruba, Q. and Mencl, V. , 1969, Landslides'and their control.Elsevier Pub. Co., Inc., New York, 205 pp., illus.

176.

I 1SLOPE ANGLE -ToC

Figure 1. -- Simplified diagram of forces acting on amass of soil on a slope.