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This map is preliminary and has not been edited for conformity

Geological Survey standards or nomenclature.C

Landslide susceptibility map of the SJnsworth 7i-rainute quadrangle,Allegheny County, Pennsylvania

3y J. S. Fameray ~ </v;^;

The purpose of this map is to identify areas with potential slope-

stability problems significant to development. Essentially, it is a

guide to areas of past landslide and present landslide susceptibility.

The map is not designed to replace detailed studies of specific sites

by competent technical personnel. Rather, it delineates areas where

such detailed studies are most vital to the safety and welfare of the

general public. In these areas, site examinations are necessary in order

to seek firm evidence of the degree of difficulty that slope instability

may pose to a contemplated land use, and so to define whether costs of

hazard prevention are commensurate with the value of the contemplated use.

Preparation of the map was sponsored by the Appalachian Regional Commission

(ARC contract no. 7U-31).

The map is based on an interpretation of large-scale (1:12,000) aerial

photographs (series GS-VTGY) taken on April lU, 1973. About one week of

field work during fall 1973 supplemented the aerial photograph interpreta­

tions.

'^ U. S. Geolo£'cal ?<:rvcyCPZN r 1LH *.'.-,? /'-/"'/

This :...-!p is prel i^ii.j i y --,- d <- - not teen edited for : -. . :~ - - <.i i / r ith Cv-lcpical S.rvc-y . ../.:.

>

JUN 2 G iS74 J?

J3\l^

Information, from soil surveys by the Soil Conservation Service

;/.S. Dept. of Agriculture, 1973) v:as integrated with data from an early

ceologic map (I'ur.n, 1911). and other reports listed in the references.

Large recent landslides are readily s-zen on aerial photographs.

The aerial photographs also are an excellent means of locating ancient

slump benches and the hummocky areas at the bases of slopes so indica­

tive of landslide-prone areas. In addition, arcuate scars at the heads

of s_ide areas are v;ell displayed on aerial photographs. In contrast,

on topographic maps the contour interval and the configuration of the*

contours alone are not sufficiently detailed to allow for the delinea-

.icn of r'anv landslide ^"""one areas.

'.-"any landslides in the Emsvorth quadrangle are too snail to be

if.e:.'cified with certainty by aerial photo study alone. The smaller

landslides shown on the map were identified and plotted by direct field

observations. This map does not purport to show all recent landslides/

because most slide_s are so small that they could not be shown at the

scale of the map. Such a thorough landslide inventory would require

many weeks or months of effort in the quadrangle.

The rocks exposed in the Emsworth quadrangle are -ore or less flat-

lying shales, r.udstcnes, sandstones, siltstones, and minor coal beds and

limestones of the Conem=ugh Group of Pennsylvanian ace. Of these,

nonbcdded red rudstone and related residual and colluvial"soils are

particularly susceptible to landsliding. Most areas with r.oderate to

severe slope stability problems are underlain by the principal red

r.udszone horizon, the "Pittsburgh recbeds," which ranges from 20 feet

(6.1 m) to 65 feet (19.8 n) thick north of the Ohio River (Winters,

1969) . A lesser known rcdbed sequence ("Clarksburg") of red mudstona*

and related soils higher in the section has also been involved in minor

landsliding in the quadrangle.

It can be inferred that most slopes in the quadrangle are relatively

stable under natural conditions, but, as is shown on the map, many

slopes are sensitive and their natural equilibrium can be readily

upset. By far, the greatest number of landslides in the region occur

when a slope is oversteeoened, overloaded, or otherwise modified by man

in the course of development of housing, roads, pipelines, and other

features. The largely prehistoric slump benches probably were formed

under extremes of climate no longer characteristic of the area. Rela­

tively recent landslides on natural, undisturbed slopes largely are

caused by unusual conditions, such as extremely heavy and prolonged

rainfall.

The delineation of probable older landslide areas should prove

useful in light of the experience expensively acquired by highway con­

struction personnel in the routing of Interstate 279 at the extreme

southwest edge of the quadrangle. In studying this area, Kamel and

Flint (1969) clearly demonstrated that recognition of ancient slump

r.asses in advance of highway construction is of utrr.ost importance. In

fact, any type of nan-induced earth disturbance can reactivate older

slide areas if care is not exercised. Probable ancient landslides

(slump benches) also have been delineated in the deep valleys east of*

the Interstate 279 area and elsewhere in the quadrangle.

Numerous small landslides (mostly slumps) have occurred in rock

and earth fill throughout the quadrangle. Potentially dangerous rock-

falls and minor slides are particularly numerous along I'cKnight

Boulevard in the southeastern quadrant where man has extensively

modified existing slopes.

Selected references

.-cl-.3r.heil, A. C. , 1054, A soil mechanics and engineering geolocy analysis

of landslides in the area of Pittsburgh, Pennsylvania: Univ.

Pittsburgh ur.pub. Ph.D. thesis, 120 p.

Fisher, S. P., Far.aff, A. S., and Picking, L. V7., 1968, Landslic.es of

southeastern Ohio: Ohio Jour. Sci., v. 68, no. 2, p. 65-80.

Ha-el, J. V., and Flint, N. K., 1969, Analysis and design of highway

cuts in rock a slope stability study on Interstate routes 279 and

79 near Pittsburgh, Pennsylvania: Pennsylvania Dept. Highways

Bur. Materials, Testing and Research Rept., 130 p.

::unn, M. J., 1911, Sewickley folio, Pennsylvania: U.S. Geol. Survey

Geol. Atlas, no. 176, 16 p.

U. S. Department of Agriculture, Soil Conservation Service, 1973, Soil

survey r.aps for Allegheny County, Pennsylvania.

Winters, D. M. , itf/72^, Pittsburgh redbeds stratigraphy and slope

stability in Allegheny County, Pennsylvania: Univ. Pittsburgh

unpub. M.S. thesis, 49 p.

r---' 1 2. S B74 FACTORS AFFECTING LANDSLIDE SUSCEPTIBILITY / "> ..

IN ALLEGHENY COUNTY, PENNSYLVANIA ^""<"

(t-b accompany U. S. Geological Survey open-file ii

* '. landslide-susceptibility maps of Allegheny County) I

-- Significant factors bearing on landslide susceptibility include:

6 (1) rock types; (2) nature of rock layering: (3) rock fracturing: ;

7 ; (4) attitude of rock layers: (5) composition and thickness of soil i

3 cover: (6) permeability of rocks and soils: and (7) steepness of

9 slopes.

:c ~ 1. i^ock types. --Outcropping rocks are largely sandstone, silt-

: * stone, shale (or clayscone), and limestone. Coal, though only a

12 relatively small part of the total rock volume, is widespread and

i0 significant. Sandstone and limestone commonly are harder, more

resistant to weathering, than are siltstone and shale. This differ-

=~ ential weathering explains why sandstone and limestone crop out on

10 i many slopes as ledges and cliffs, whereas siltstone and shale are

* 7 , rarely well exposed except in cut banks of streams, in other very

* s ' steep natural slopes, and in manmade exposures such as highway cuts.

19 [ 2. Rock layering.--The rocks form layers comnonly 1 to 10 ft

20~ j thick, but in places layers exceed 30 ft. For example, a 2-ft layer

21 ; of limestone may rest on 7 ft'of shale which in turn rests on a sand-

I stone layer 10 ft thick. It is also common to find that a layer of

shale as thin as 1 inch lies between two layers of sandstone each many

24 i r \\ feet thick. If a shale layer is decomposed to some depth by weathering, I i

'L__j^^-^~\_cj> 7 '/ri_r_LYjL"' r-! _lie2Z_d.__r_o ^k_is_12 c s i~"Lrrnly sii'V1 2r_nzd_a"i_tends_fn ".QV^___1

slope in response to gravity. r. s. c>

borne rocx layers are continuous over a number or miles, but most

: sandstone layers, for example, probably grade laterally into another

3 rock type, perhaps silts tone, in shorter distances, and some conspicu-j

.: . ous lateral changes are seen within a single outcrop. I

5-' 3. Rock fractures.--Two nypes of r^ck fracture occur: faults, j

5 fractures along which rocks on one side are offset from rocks on the ;

other side' and ^oi^ts fractures seme ti r7 ht some ooen alor.^ which :

6 ^ little or no evidence of movement can be seen. Faults are relatively :

9 rare in Allegheny County. The harder rock layers, sandstone and lime-'

10- scone, are well jointed in outcrop, with joints commonly open and one ;i

11 to several feet apart. Joints also occur in siltstone and shale layers:

12 but the joints are chiefly tight rather than open. Most joints are 1i

ij more or less perpendicular to the plane of layering.

: ~ Joints contribute to landslide susceptibility, for if rock layers j

15- were not jointed, their tendency to fail when underlying rocks are

1° removed would be less. Joints are also an important factor in rock

17 , permeability.

18 I 4. Attitude of rock layering.--In Allegheny County, cost rock

19 , layers dip at such small angles that their attitudes can best be meas- j

2°- i ured in feet per mile rather than in degrees or in percent of grade. i

21 '; In some areas, layers dip more than 200 ft per mile (about 2° or 4 per-

22 |cent grade), but most layers have gentler dips, and locally they are

i23 |horizontal. In Allegheny County, rock attitude is most critical to

i24 '' landsliding on cverdip slopes, where rock layers dip in the same -.enerai

-I ' i~ = ~ i direction as the slooes but at lesse_r_j,n'?l^,s_Jiliail-tLhe_sloDtiS-^________1

_o _ V. s.

5. Soil cover. --Soils are composed chiefly of f ir.e-greined mineral

constituen.ts derived from rock decomposition during weathering. :;ow-

3 ever, soil means different things to different people. For example,

i co a soil scientist, soil supports plant life and has undergone near-

5_ i surface zor.ation resulting from the interaction of climace and living

6 , matter, conditioned by slope and relief. An agricultural soil rarely :i

7 is :r.ore than 6 ft deep and rr.ay rest on and be developed from a parent

s . material that is itself decomposed rock. In contrast, to an engineer,

s soil includes all unconsolicated r.aterial above hard bedrock, and so \

12- includes the parent r.aterial of many agricultural soils. Only where

:i deoth co bedrock is relatively shallow will there be virtual agreement'.j ii

12 be~ween a soil scientist and an engineer as no thickness and corr.pcsi- i

:s tion of a soil. For present purposes, soil is used in the engineering;i

:4 sense; iz applies not only to material resulting from rock weathering i

15-' in place, but also uo masses of fragmented, and d'ecor.posed rock particles: |

ID that have been transported and redeposited elsewhere. Examples of

17 transported soils are colluviura and alluvial terrace deposits, both

is : of which can be subject to landsliding.

-9 In Allegheny County, soils of the hill tops are relatively thin,i

20- ! less than 6 ft thick in many areas. Soils of hill slopes are absenti

21 ' where bedrock crops out, are relatively thin on many upper slopes, and!

i22 are nade up of rr.ore than 20 ft of colluviutn near and at the base of

i

22 ; r.any slopes. Valley-bocton soils generally have nearly level surfaces! I

24 j and so are not a significant factor in r.vost landsliding; they r:.ay I

2 -- ! exceed 100 fu in thickness.i_________________________________

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6. Perr.j;ability of ronks and scils^ Permeability as used here is

.apacity of bedrock and soil to trar.sr.it water. Sandstone in Alle--

3 gher.y County cor.n:or.ly is moderately permeable; vat<=r may passes' around :

- grains of sand and through intargrain voids in r.-.ar.y of these rocks. In,

5 _ addle ion, sandstone layers may have closely spaced joints ihat facili-

c naze passage of water. Although lirr.2stone is fina grained and is

-i inherently ir.ors or* lass irroeTiieab"'e rr.ost l"*"^2£tO'r<.2 lavers are oerir.iabla

s because they are closely jointed, and these joints corrr.only are enlarged

& by solution and removal of -inerals by noving ground vater. In contrast

:c- siltstone and shale are fine grained, inherently less permeable than ',

most coarser grained rocks, and joints in siltstone and shale layers \

12 'corr:.only are relatively tight. Thus, sandstone and liir.estone layers i

'.2 in southwestern Pennsylvania are ir.ore likely avenues for ir.over.enc of !

^ ground x-;ater than are siltstone and shale layers. Similarly, most ii i

15- sandy and rocky soils are appreciably iriore perr.eable than are soilsj

is composed largely or entirely of clay. Saturation of rocks and soils by

17 water is most likely to be cor.plete in zones where perir.eable materials

is overlie relatively impermeable materials. This saturation, coupled

is with lateral movement of water in these zones, enhances lubrication,

20-and so potential instability.

21 j Because water is a key agent in landslide susceptibility, perme-

i22 ability of rocks and soils, or the relative lack of it, is of particular

i/.7.oor tance.

f sloces.--Allagr.eny County is a land of hills and

: riuges tacr. 01 wr.icn is more or less tne same ^aigr.t as its

3 Separating these hills are valleys through which streams and rivers

^ flow at levels coi.-~only 300 to 400 ft and locally more than 600 ft

5- below adjacent ridge crests. The valley walls are relatively sceep;

e slopes of. 25 percent (about 14°) or greater occupy more than onc-zenth

7 of the area. This large incidence of steep natural slopes is a leading

s factor in the prevalence of landslides.

9 Relative importance of factors.--All of the above factors are ;

ic- interrelated. At a given place one factor may be the chief control of .

landslide susceptibility, whereas at another place the same factor may

2 be less important than others. For example, where a major stream is :

undercutting its bank, oversteepening will occur and slope failure

14 ultimately will ensue, whether the bank material is rock or soil; where!I

is- a thick soil cover becomes saturated with water, failure may occur even!

16 on relatively gentle slopes. Some reverse-dip slopes, contrary to what!

j17 might be expected, can be consistent landslide hazards because of

18 natural or manmace steepness or excessive rock fracturing; some over-

19 dip slopes,.on the other hand nay be less susceptible to landsliding

2C ~ because only one type of rock is present.

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d references

n the area of Pittsburgh, Pennsylvania: Univ. Pittsburgh

5 _ Brig^s, R. ?. , 1S74, MLO of ovardip slopes uhc.r. can affect landslicing

in All«i"r.Lny Cour.iy, Pennsylvania: U.S. Gaol. Survey I-'.isc. Field

19

5 Zckel, Z.3. , ed. , 1555, Landslides and engineering practice: Highway ;

5 Research I: = rc Spec. Rept, 29, mS-NRC 544, Vfashington, D. C. , 232 p.;

1 <

:c- Gray, R. I. , 1S70, Landslides, _in Wagner, W. R. , and others, Geology of

uhe ?i-E£;-_rgh area: Pennsylvania Gaol. Survey, 4th ser. , Gen. Geol'.

2 _ Repc. G--'-.' |

s Xilsen, T. X. , 1972, Preliminary phocointerpreLation :r.ap of landslide

- , and ocher surficial ceposics of pares of the Los Gatos, Morgan Kill,': i

:=- ' Gilroy Hot Springs, Pacheco pass, Quien Sabs, and Hollister 15' cuadi

5 \ rangles, Santa Clara County, California: U.S. Geol. Survey Misc. ji

7 i Field Studies Map MF-416, 2 sheets.-

» 'Sharpe, C. ?. S. , 193S, Landslides and related phenomena; a study of

nass-rr.cver-ancs of soil and rock: New York, Columbia Univ. Press,

136 p. _/repr. 1960, Paterson, New Jersey, pageant Hookas/-

'Winners, D.M. , 1972, Pittsburgh red beds stratigraphy and slope

|stability in Allegheny County, Pennsylvania: Univ. Pittsburgh M.S.

dissert. , 49 p.

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-OL'.SE 'JO

v.C- 5,'C'l1 ~.?<S, SLJC?? JS'J'.C

Jertr.f'c^: celluvial rr.otcr-.al3 that -eve :c*.",s1 cpe in a rr.ar,r,er si~,r,ar to a viscous fluid.

ROCKFALL

Creep: Corr_T.or. evidences - (A) Moved joint blocks of layered rock: (3) trees with curved trur.ks ccn- cave upslope; (C) displaced posts, poles, and rr.cr.u-ents; (D) broken or displaced retaining walls and foundations; (E) roads and railroads rr.oved cut o~ pi ? c?i-.-np-*f- (~?} uu-f rolls dovnslooe from w*.«-*_ Q.-... »* -3 \*y ^

creeping boulders; (G) -stone-line at appro:-:ir.ate base of creeping soil.