Mass soil movements in the H.J. Andrews Experimental Forest

Historic, Archive Document

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“MASS SOIL MOVEMENTS
“IN THE HJ. ANDREWS

EXPERIMENTAL FOREST -}
A C. 1. DYRNESS |

U.S. DEPT OF AGRICULTURE
MATIONAL RG RICUETURAL LIBR ARY

OCT 5- 6/

= tam

CURRENT SERIAL RECORDS

1 p/

PACIFIC NORTHWEST we
FOREST. AND=RAMGE EXPERIMENT STATION. + 7/5

qh. > \U.S=DEPARTMENT OF AGRICULTURE

U.S. FOREST SERVICE Research Paper PNW— 42),

5967, I

PESDOP ECBO SOLS RO BODS LE ES LEP BOKE DS EE ES BORO GODS EO DPBS BOR

CONTENTS
Page
LINE BUG) DNOKO BPI KOWN ae eh a A SNS a) le 1
STUDY AREA. : . . ° . : . . . ° ° 1
ALISO D) SyIOURIWAL Gis : : : Sumner. Go hoy) Moe a 1
METHODS . . . . . . : . . : : . . 2,
RESULTS . 4 . . . 5 . . . . . . ° 2
Mass movements by morphological class . 2
Mass movement by mode of action
Or GistuT Da NGem eas) Fe whee iene ae inter tues 4
Relationship between mass movements
and certain site characteristics. . .- 9
DISCUSSION AND CONCLUSIONS . . - - « 12

SRPSKA SL SEDS AF SEAL SF OG OF SF SHOF AKLEOQPSFLF DS SFOBARAGAR

INTRODUCTION

Each winter a number of mass
soil movements, such as earthflows
and slumps, occur in the Coast Ranges
and Cascades of western Oregon.
These movements, generally occurring
during periods of heavy and prolonged
rainfall, often disrupt roads and dam-
age other improvements. Some, how-
ever, remain undiscovered in remote,
undisturbed areas.

During the winter of 1964-65,
there was an unusually large number
of mass soil movements in western
Oregon. <A high proportion of these
movements occurred during the severe
storm that struck northern California
and Oregonthe week before Christmas.
Soon after this storm, it was found
that the H. J. Andrews Experimental
Forest was in one of the hardest hit
areas and was in an excellent location
for an intensive survey of stormdam-
age with emphasis on the origin of
large mass soil movements. Accord-
ingly, asurvey of allmass movements
in the Experimental Forest was initia-

ted April 1 and concluded July 1,1965.

The study had several aims:

(1) to completely describe and photo-
graph all major movement areas in
the Andrews so that changes in the
future may be followed and documented,
(2) to identify the types of movement
which had occurred and estimate the
amount of damage to the site and/or
improvements, (3) to attempt to de-
fine the relationship between the mass
soil movement and (a) amount of man's
disturbance and (b) certain site factors
such as geology, soil, elevation, and
aspect.

STUDY AREA

Wine ele aig GMalchaxe ws IDagoKsigi
mental Forest comprises the entire
15,000-acre drainage of Lookout
Creek, located approximately 40 miles
east of Eugene, Oreg., and part of the
Willamette National Forest. The area
lies within the Western * Cascades
physiographic province and is pre-
dominantly mature topography with
sharp ridges and steep slopes. Bed-
rock in the area is made up of pyro-
clastics (tuffs and breccias) and basic
igneous rocks (basalt and andesite).
Forest cover is comprised of old-
growth Douglas-fir (Pseudotsuga
menztesitt) , with varying amounts of
western hemlock (Tfsuga heterophylla)
and western redcedar (Thuja plicata).

THE STORM

Fredriksen has described the
characteristics of the 1964 Christmas
storm in the H. J. Andrews area in
some detail. < At lower elevations,
more than 13 inches of rain in 4 days
was supplemented by an undetermined
amount of snowmelt. This storm has
generally been classified as an event
with a 50-year return period. How-
ever, Fredriksen pointed out that in

ay
Berntsien, (‘Carl Min 7 and
Rothacher, Jiacks A guide sto: the Hi:
J. Andrews Experimental Forest.
Pacific Northwest Forest & .Range
IDe§5 Sus}. Glas 4 aUllwiso aOao.

gy Fredriksen, R. L. Christ-
mas storm damage on the H. J.
Andrews Experimental Forest. U.S.
Forest Serv. Res. Note PNW-29, 11
Dido wikis, MYOS.

only 12 years of record at the Experi-
mental Forest, this was the second
storm to total more than 13 inches of
precipitation within a 4-day period.
He concluded, "we believe 4-day
storms which deliver 13 inches are
probably not unusual.'' The fact re-
mains, however, that this storm
caused the most extensive damage re-
corded establishment of the
Experimental Forest in 1948.

since

METHODS

Information recorded at each
soil movement site included type and
specifics characteristics of @ soil) tox
debris movement, general character-
istics of the area, and assessment of
factors influencing the soil or debris
movement. Photographs were also
taken at each site and a sketch made
showing the outstanding features. An
attempt was made to find and describe
all events where 100 cubic yards or

more of soil or debris were involved.

RESULTS

Descriptions of 47 mass move-
ment events were made in the Andrews
INope@mis (blero 1h))s Estimated
ranged from 100 to 75, 000 cubic yards,
and the total quantity of material
moved was about 347, 800 cubic yards.

sizes

Mass movements

by morphological class

The movements have been
grouped into 10 classes on the basis
of their morphological characteristics
(table 1). Earthflows occur when the
soil becomes saturated, and pore-
water pressure builds up to such an
extent that the soil mass flows out in
a generally tongue- shaped form.
Slumps, generally occur in
saturated soils; however, they ane
characterized by rotational movement

and spoon - shaped

also,

failure planes.

Table 1.--Mass movements occurring on the H. J. Andrews Experimental Forest

during the winter of 1964-65, by morphological class

Morphological class

Earthflow

Slump with earthflow

Slump

Earthflow causing channel scour
Slump causing channel scour

Debris slide causing channel scour
Channel scour

Roadfill washout

Avalanche with rilling

Debris avalanche with earthflow

Total

Events Material moved

Cubic yards

104,600
14,525
4,050
82,350
106,300
7,500
26,300
1,050
100
1,000

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347,775

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SMAHYHONY "HH

Channel scouring is generally caused
by massive and sudden failure of the
entire unconsolidated mantle, most
often in areas of the oversteepened
drainage heads. Debris avalanches
involve the sudden downslope move-
ment of largely rock material in steep
terrain. The earthflow, slump, and
avalanche classes are defined by
Eckel. 2/

Earthflows were the most com-
mon class of soil movement with 18
out of the total 47 events falling within
this class (table 1). Channel scouring
events ranked second with a total of
14. Only three of these events were
confined entirelyto the drainage chan-
nel; the remainder were triggered by
a separate mass movement which fre-
quently began at a road. The third
most commonly occurring movement
class was the slump with some earth-
flow characteristics. Soil material
which has slumped down, when satu-

rated with water, will frequently flow
out at the toe and may travel a great
distance downslope. Only three slump
events were completely devoid of flow
characteristics. These were backslope
slumps where the material came to
rest on the road surface.

A consideration of amounts of
material moved emphasizes the im-
portance of earthflow and channel
scouring events (table 1). The re-
maining forms of movement contrib-
uted very little to the total amount of
material moved.

Mass movement by mode of

action or disturbance

Mass movements on the
Andrews were also classified accord-
ing to type of disturbance or mode of
action (table 2), with attention focused
on the source area of the event. For

Table 2.--Mass movements occurring on the H. J. Andrews Experimental Forest

during the winter of 1964-65, by mode of action or disturbance

Type of disturbance

Roadfill failure

Road backslope failure

Road backslope and fill failure
Events caused by road drainage water
Road removed by stream

Events in logged areas

Events in undisturbed areas

Total

3/

— Eckel, Edwin B., ed. Landslides and engineering practice. High-
way Res. Board Spec. Rep. 29, 232 pp., tlws.. Washington) Dr Grasse

Events Material moved
Number Cubic yards
1b) 40,900
5 14,400
6 38,325
8 86,450
3 4,350
8 227, le)

5 141,200
47 347,775

when a channel scouring

example,
event originated in an untouched stand
of timber, itwas classifiedas a move-
ment occurring in an undisturbed area
even though at adownstream location,

roads may have been severely damaged.

Although only five events oc-
curred in undisturbed areas, this type
contained the largest volume of mate-
rial. Events caused by road drainage
water ranked second, with the other
five types far behind. Each class is

considered separately below:

1. Roadfill failure.

The most common single type
of mass movement on the Andrews
was the roadfill failure (12 events)
(fig. 2). Thisis apparently a rather
simple type of movement caused

by the saturation of the roadfill
embankment. Most of the fill ma-
terials, especially those derived

from pyroclastics, appear to flow
readily when saturated. As nearly
Asp mCouldsuber wdetermuned,) sthese
failures were not due to improper
rather, were
large

road drainage but,
caused by’ the unusually
amounts of rainfall or perhaps by

improper fill compaction.

2. Road backslope failure.

This isa common event, which
frequently results in road blockage
Gaige'S) kn inhie ss tanlunensmost |oLten
takes the form ofa slump, but the
slump material may have
earthflow characteristics near the

some

toe. Themost failure-prone back-
slopes were those constructed in
areas of deep, well-weathered tuffs
and breccias. These
materials fail easily when exces-
If failures did occur
inareas of andesite or basalt parent
material, invariably the local bed-
rock was highly fractured. Like
the majority of other mass move-

saprolitic

sively moist.

ment events, backslope siumapis
occur only when the mantle is satu-
rated and are also more common
inareas where mass movement has

occurred repeatedly in the past.

Figure 3.--Backslope failure in an area
of soil derived from andesite

colluvium.

Figure 2.--Roadfill failure in the
H. J. Andrews Experimental Forest.

3

Road backslope slump causing fill
failure.

Six events in the Andrews fit
within this type (fig. 4). In effect,
this is a ''chain reaction''! with the
follows: A
portion of the backslope becomes
saturated and slumps down onto the
road surface, thus blocking the in-
side drainage ditch.

general sequence as

The drainage
water is then diverted across the
road surface and down onto the fill
slope from the outside portion of
the road. The fill slope becomes
saturated with water and an earth-
flow type of movement follows, en-
croaching to a varying extent onto
the road surface. In some cases,
the ''chain''is further lengthened by
the fact that the earthflow on the
fill slope may trigger channel scour-
ing in a drainage below.

Mass movement caused by concen-
tration of road drainage water.

Thereare twoclasses ofevents
under this heading: (a) those due to
the concentration of road drainage
water, even though the road drain-
age system was apparently function-
ing properly (fig. 5), and (b) those

caused by failure of some portion-

of the drainage system--for exam-
ple, a clogged, culvertion inside
ditch.

Events caused by concentration
ole road) jidrainagemmavatek. our

events in the Andrews fall. within
this category. In ~all™ cases, ‘the
event was associated with stream
channel scouring. The typical se-
quence was as follows: Extremely
large quantities of water flowing
from aculvert completely saturated
the soil mantle below the road. In

mostcases, this saturatedarea was

Figure 4.--View of typical backslope slump
which has also caused roadfill! failure.

near the head of a tributary drain-
age. When the soil became satu-
rated, the entire mantle either
slumped or flowed downslope and
temporarily blocked the drainage
channel. When a sufficient head of
water was built up, this temporary
dam breached, and a wall of water
and debris rushed down the channel
removing everything in its path.
The endresult was adrainage chan-
nel from which virtually all uncon-
solidated material has been remov-
ed exposing bare rock.

All four events of this type
occurred in areas of hard, frac-
tured, greenish breccias and tuffs.

Figure 5.--Soil movement at the head of a drainage
caused by high flows through the culvert and
inadequate energy dissipation at the outlet end.

Events caused by failures in
the road drainage system. --Of four

of these events in the Andrews,
three were 200 to 500 cubic yards
in size, while the fourth involved
approximately 75,000 cubic yards
of material. In every case, these
failures occurred in areas of deeply
weathered tuffs and breccias. All
were flow-or slump-type movements
of roadfill or sidecast material,
none of which resulted in channel
scouring. In these cases, the con-
centration of water resulting in the
necessary saturation came from the
clogging of aculvert or inside ditch
(fig. 6). These blockages had grave
consequences because of the tremen-
dous volumes of water diverted onto
the roadfill or sidecast materials.

Road removed or damaged by
stream.

In three cases, culverts carry-
WAS jo wmEiboMENL Org shiny S i iaalaliy is Oooh
streams failed either because of
inadequate size or, more probably,
because they became blocked by
debris (fig. 7). Asa result, roads
were completely washed out or
were severely damaged.

Figure 7.--Debris which blocked the culvert and

caused several feet of material to be deposited
on the road surface. (Road, in foreground, has
been repaired.)

Figure 6.--Massive roadfill failure caused by
blockage of road drainage ditch.

SM SNP ON

y

6.

Mass movement events in logged
areas.

The eight mass movement
events in logged-over areas were
principally earthflows (fig. 8), al-
though two also had some slump
characteristics. They ranged in
size from 150 to 9,500 cubic yards
of material. Four ofthe movements
occurred inareas which gave ample
evidence of considerable mass soil
movement in the past (i.e., very
uneven, hummocky relief). The
remaining four were on smooth,
steep slopes. In two cases, where
the movements occurred at the
heads of drainages, there was ex-
tensive channel scouring.

Althoughthese movements took
placein disturbed, logged areas, in
every case it is impossible to say
whether the logging per se caused
the instability. It is probably safe
to assume that at least one or two
of them would have occurred even
if the area had not been logged.

Mass movement events in undis-
turbed areas.

Of five mass movement events
in completely undisturbed areas,
all were connected with channel
scouring (fig. 9), but three alsohad
earthflow characteristics at the
source area. Size of these move-
ments ranged from 200 to 75,000
cubic yards--four were among the
largest slides that occurred on the
Andrews. The average for all five
events was well over 28,000 cubic
yards.

Figure 9.--Channel scour event in

undisturbed timber.

Figure 8.--Massive earthflow in recently logged area.

With one exception, these
events constituted failures at the
heads of drainages. These areas
are often very steep and unstable
due to the headward erosion of the
stream channel. When these areas
become completely saturated, the
weight of water adds to the driving
force and at the same time de-
creases the strength of the soil.
Sudden, massive failures frequently
Result bhwts es) uncdoubtedly,) thie
normal course of geologic erosion
in this area, and there is little that
man can do to forestall events of
this type.

Relationship between mass movements

and certain site characteristics

The influence of roads on mass
movements inthe Andrews is clearly
indicated in table 3. Although over 72
percent of the mass movements
occurred in connection with roads,
only 1.8 percent of the total area of
the forest is in road rights-of-way.
On the other hand, only about 11 per-
cent of the events occurred in undis-
turbed areas. Since 84.6 percent of
the total area is undisturbed, this is
far less than would be expected if the
events occurred in a random manner.

In the western Cascades, it has
been observed that mass soil move-
ments occur much more frequently in
areas of pyroclastic rocks (tuffs and
breccias) than in areas wherethe bed-
TOE 1S, COladyoirigsecl VOR) lyeiseule 9 ee
andesite. It has also beennoted else--
where that greenish tuffs andbreccias
are more unstable than are their red-
dishcounterparts. Theserelationships
are borne out by the data presented in
table 3. About 94 percent of the
events occurred in areas with a tuff

and/or breccia substratum, even
though only 37 percent of the total area
is made up of these rocks. Moreover,
64 percent of the mass movements
were on greenish tuffs and breccias
which make up only 8 percent of the
total area--clearly indicating the un-
stable nature of these materials. Al-
though about 63 percent of the area is
underlain by basalt and andesite mate-
rials, only 6.4 percent of the mass
movement events occurred there.

When soil series in the move-
ment area are considered, the rela-
tionship described above still holds
but to a lesser extent. Eighty-seven
percent of the events occurred in
areas of soilfromtuffs and breccias--
the breakdown being almost 49 percent
on soils from reddish tuffs and brec-
cias and 38 percent ‘on ‘soul's from
greenish tuffs and breccias. The
apparent discrepancy between these
figures and those for the substratum
reported above may be attributed to
the fact that many soils have been
formed in transported parent mate-
rials and oftenthere is little relation-
ship between the soil profile and the
substratum. It was noted in several
cases that the discontinuity between
the soil profile and the underlying
bedrock constituted the failure plane,
probably due to inherent weakness and
water flowing through the zone.

Elevational range in the An-
drews Experimental Forest is approx-
imately 1, 400-5, 300 feet. All but two
of the mass movements occurred at
elevations below 2,900 feet (fig. 10).
The great bulk of the events (over 72
percent) occurred in the elevational
range from 2,000 to 2,600 feet (table
3). There are two possible reasons
for the lack of mass movements at
high elevations: (1) Most of the de-
posits of tuffs and breccias occur at

pp eS eee

Table 3.--Relationship between occurrence of mass movement events and certain site factors

in the H. J. Andrews Experimental Forest during winter of 1964-65

Site factors

Disturbance:
Undisturbed
Logging
Road construction

Substratum in movement area:
Greenish tuffs and breccias
Reddish tuffs and breccias
Andesite colluvium
Basalt and andesite residuum

Soil series:

Soils from reddish tuffs and breccias

Frissel Series
McKenzie River Series

Soils from greenish tuffs and breccias

Limberlost Series
Budworm Series
Slipout Series
Soil from andesite colluvium
Carpenter Series
Rock land

Elevation (feet):
1,400-1,700
1,700-2 ,000
2,000-2, 300
2 ,300-2 ,600
2,600-2,900
2,900-3,200
Over 3,200

Slope (percent):

0-15

15-30

30-45

45-60

60-75

75-90

Over 90

Aspect:
North
Northeast
East
Southeast
South
Southwest
West
Northwest
Level

10

Mass movement
events

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Events per
1,000 acres

----------- ---- Number ----
84.6 0.4
13.6 3.9

1.8 N29)
8.0 25.0
29.2 342
42.7 3
20.1 3
22.8 14.2
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355) 4.9
9.1 BoP
Goel 19.5
3.0 4.4
2.0 1343
34.9 9
4.1 1.6
Pest 7255)
6.2 4.3
8.8 11.4
10.9 11.6
ibsyoal 3.0
10.9 6
47.4 1
5.9 ieee
25.2 0)
34.6 133}
22.8 5.0
8.4 11.9
Dies) 18.7
-6 0
16.8 6
5.0 9.3
5)q8) 4.5
11.4 Arey;
11.9 6)
16.8 8
9.9 Grell!
19.3 4.1
3.0 DS

lower elevations; (2) temperatures
prevailing above 3, 000 feet at the time
of the December storm may have been
low enough to maintain some snow
cover.

As would be expected, steep-
ness of slope and frequency of slides
were apparently positively correlated
(table 3). Only about a sixth of the
events occurred on slopes with a gra-
dient of less than 45 percent (fig. 10).

For some reason, mass movements
were almost nonexistent on slopes
with a south or southwest aspect--
probably because rock weathering and
soil formation proceeds much more
slowly on the drier aspects. The re-
sulting shallow soils and less deeply
weathered rocks may give rise to a
greaterdegreeof stability. It appears
that mass soil movements occur more
readily inareas of deep soilsand well-
weathered bedrock.

20 Figure 10.--Relationship between number of mass movement events and

number of events
i) w pS ui a | © ive) (=)

N NE E SE

Ss SW WNW

aspect

elevation, aspect, and slope.

ml

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DISCUSSION AND CONCLUSIONS

Mass soil movements are the
end result of acombination of causative
factors. In our area, these generally
include soil materials of low internal
strength, sufficient water for satura-
tion or near saturation, and at least
moderately steep slopes. Man's acti-
vities may contribute to movement by
changing the slope or other character-
istics of the mantle and by influencing
the distribution of water. Since a
complex of factors interact in the pro-
duction of mass
often extremely difficult to pinpoint
the specific cause of amass movement
In a very real sense,
differs in
respect from any other event, and

movements, it is

event. every

movement event some
each represents a particular combina-
tion of conditions which culminates in

massive mantle failure.

Perhaps one of the most im-
portant problems facingus isto deter-
mine the extent that man's activities
contribute to the occurrence of mass
soil movements. From our experience
in the H. J. Andrews Experimental
Forest, we are forced to conclude that
man-caused disturbance has had an
important influence in the occurrence

Although
apparently

of mass soil movements.
logging disturbance also
increased movement frequency, this
relationship is especially marked in
the case of road construction. How-
ever, it should be borne in mind that
inanarea such as the Andrews Forest,
where slopes are steep anda large
portion is underlain by soft, deeply
weathered pyroclastic rocks, the stage
is set for extensive mass movements
during high rainfall periods whether
or not disturbanceis afactor. There-
fore, it is perhaps often true that
man's activities accelerate the occur-
rence of mantle failure events in an
rather than

already unstable area,

2

contribute in any significant way to
this basic instability. In other words,
disturbance may cause some small
and, by itself, insignificant change
whichisnonetheless sufficient toupset
the tenuous equilibrium and trigger
mass soil movement.

It is perhaps of some signifi-
cance that the largest mass movement
events encountered in this study were
those which occurred in undisturbed
areas. All involved extensive stream
channel scouring, andthe source areas
were oversteepened drainage heads.
This general pattern is interpreted as
the normal course of geologic erosion.
On the other hand, man-caused move-
ments are generally smaller, since
the entire slope--from drainage head
to main stream channel--is generally
not involved.
disturbed involve only
rights-of-way, or perhaps onlya small
portion of the slope below the road,

Instead, many events in

areas road

Since roads are so oftenan im-
portant factor in causing mass move-
ments, the problem is) to ,determunie
means of minimizing their effect.
Perhaps the most obvious means is to
reduce road mileage to an

minimum. In

absolute
steep, mountainous
terrain, this may be done by the use
of skyline and, possibly, balloon log-
ging methods. In many areas, it is
possible that improvements in road
location may appreciably reduce the
frequency of mass soil movements.
Unstable soils and landforms should
be identified, and the route selected
should avoid

possible.

these areas wherever
In addition, improvements
in road design and construction may
also contribute substantially to in-
creased mantle stability. Modification
of waste handling to avoid sidecasting
on steep slopes and provision for ade-
quate road drainage are two of the
more important means to minimize
mass movement hazard,

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The FOREST SERVICE of the

U. S. DEPARTMENT OF AGRICULTURE
is dedicated to the principle of mul-
tiple use management of the Nation’s
forest resources for sustained yields
of wood, water, forage, wildlife, and
recreation, Through forestry research,
cooperation with the States and private
forest owners, and management of
the National Forests and National
Grasslands, it strives — as directed
by Congress — to provide increasingly
greater service to a growing Nation.

H.J. ANDREWS , |
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