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CILtPTEIl 4

OPEN EXCAVATIONS

1. VARIOUS METHODS OF EXCAVATIONS t

Most of the civil engineering structures are built below the

surface of existing ground and it is required an excavation in the

undisturbed foundation soil. It is not the function of the soil«

engineers, in general, to select the equipment for excavating at

the proposed construction site. The soils engineer may not even

design the bracing system if any required. On the other hand» the

soils engineers are the specialists who solve difficult and unusual

excavation problems and they are consulted very often to approve the

method of excavation and they are asked to analyze the stability of

the -slopes of the excavation, to evaluate the magnitude of the earth

pressure against bracing systems and to check the stability of the

bottom of the excavation.

The problems of excavation.are different for each particular

foundation and it is almost impossible to set up some general rules

V'to be applied to all kind of excavations Each excavation that ; • (s -• • ^ dealswith a particular foundation may require special attention and

its b*n particular solution. Tlie—enginee rj?ho^ is_in_ charge of.the

excavation^desires to know the possibilities andlimitationa of various

excavation technique«, for the sakeof safety und economy. In a

difficult excavation problem he may not be the right person to make a

decision and he usually consults to a soils engineer who has enough

experience with the excavation problems. Thereforef ¿t is imperative

for a soils engineer to be f&miliar to the various excavation problems.

He must be able to show to the contractor the'most reasonable solution

( Technically feasible and safe* economically sound ) under existing

local conditions.

It should be pointed out right here that there are various

methods for excavating a site., An excavation for a particular

foundation may be made by_an. open excavation vitlPunsupported slopes,

03

cant i 1everibrac e tl or cellular )f sheetpile

vail* { cantilever or anchored ) and caissons. Cofferdam , sheetpile

vaTTi~and caleeOn« are going to be studied in Foundation £ngineering

II Course. In this chapter, braced cute and excavations with

unsupported slope will be discussed. Howeverf it is hard to make

dinstiction sometimes between braced cuts, sheet piling walls and

braced cofferdams«

Vhen we study open excavations we shall first assume that the

ground water table is below the bottom of the excavation or that it

has been temporarily lowered. The methods for lowering ground water

table before or during excavation in various soils will be studied

at the end of this chapter.

1. Open Excavations with Unsupported Slopes |

Temporary or permanent i halloy cuts or excavations can be made «

with unsupported slopes.if there^is adequate space to establish a

-proper slopeit which the soil material can stand safely with an

acceptable factor of safety. The excavation with unsupported slopes

is the most simple and preferable method, but it is n6t always possible

to excavate foundation soil with slopes because of lack of *P*c«

particularly in the cities« In most Cases, the engineers have to

consider a vertical excavation for a particular foundation because of

the existing adjacent structures. If an excavation with unsupported

slopes is possible, then the engineer has to make a decision.about the

steepness of the slopes. The steepness of an excavation s oj e depend^

on various factors such as the mechanical and hydraulic properties of

soil, weather conditionsthe..depth of excayation and the duration of

foundati<^ £Onjtj^ engineers will naturally desire to

excavate the soil with a slope that is as steep as possible, because

steep slopes will require minimum space and excavation. On th« oilier

.hand, it is known that largely depends on the

physical properties of the soil material to be excavated.

The maximum angle j>f elojie that a soil material can safely b*

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excavated may be determined by subsurfuce exploration and field —

laboratory teatej>r by experience. The permanent excavation« such

aa railway and highway cute have to be studied more carefully than

temporary excavation« such aa foundation excavation!. But, in

either case the factor of safety should be large enough to protect

the human life and property against unexpected-slope failures. The

method of analysis of slope stability has been considerably improved

by the development in theoretical and applied solljtae chanic•. On the

other hand, the empirical methods are not so reliable being many times

over conservative or sometimes inadequate. It is our understanding

that both, overdesign and underdesign are undesirable and lead either

economical failure or actual engineering failure. ^

large and deep excavations, subsurface exploration, j,cçompanied by

field tests and sampling, laboratory

of the test, results must be followed.

a) Stability of Cuts in Bedrock.'

Unfortunately, the slope design in bedrock has not yet reached

to a satisfactory level that permits to built safe and economical

slopes in bedrock. It is generally observed that the excavation of

bedrock may cause falls. Rarely, the rock excavation may lead to

slides. Falls that are,more common than slides are caused by surface

weathering process of the bedrock. Slides along curved slip surface

are not observed very often and will rarely occur in cuts less than

about 30 m. in height.

To analyze the stability of rock slopes is a difficult problem,

because, the theories developed in rock mechanics are not well enough

to give quantitative results for evaluating the problem of' rock-slope.

Also, the chemistry of rock weathering is not well developed. ïhenever

an important rock excavation is encountered in the field it is

essential to consult geologists to ask their cooperation, because

geologists can detect much more quickly than civil engineers the.

presence of dangerous faults in bedrock. One of the main difficulties

in the analysis of the stability of rock slope is the surface

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weathering proceae that causes disintegration and erosion of the

surface material. Except for very hard ms.terial,' almost all kind of

materials will disintegrate and fall down.- Blasting during the

excavation of bedrock will also produce additional weak surfaces.

During or after the excavation weakspots in the'bedrock will be

exposed Mid the material will fall down. It should be pointed out

here that the weathering process takes timef_ Therefore the stability

of rock-slope may not be important for v^efipo'rarybuts, but extremely

important for^perman^ent c its. It may be remembered that a rock-fall

in bedrock cut (rs^lrtnr^than a rock slide ) occured on Ankara —

Istanbul highway at the vicinity of Kizilcaharaatn nearly ten years

after it has been excavated and it caused a catastrophic death of

sixteen passangers of a. travelling bus. It is also very common to

read in daily newspapers that rock slides or falls occur very often

in all over the country and cause heavy damages to the property,

particularly along railway and highway cuts. It is hoped that the

slope failures in bedrock will be predicted and controlled in the

future by the rapid advancement in the flold of rock mechanics and

engineering geology. In general, for permanent cuts there are four

methods ( Wood«, Highway Engineering Handbook, 1960 ) used for rock

slope design as shown in Fig. 4.1. Uniform slope as shown in Fig.

4«la is recommended when weathering process is not very important

because of the existence of hard massivo bedrock. Uniform slope

should be normally used up to 10 m. heights and it is not recommended

to.excavate rock-slope by uniform slope method if height exceeds

15 m. The variable slope angle method may be used where layers

with different hardness underly each others. Then" the proper

slope for each layer should be used as shown in Fig. 4.1 b. In

stratified sedimentary deposits, excavation may be made by

utilization of permanent or temporary berms as shown in Fig. 4.1 c

and 4.1 d. Jleruis may keep debris off the pavement and whenever

fractures exist, berms are essential. Unfortunately,_tH"éré are no

reliable:..J.j!Uboratory or field tests yet for measuring the weathering

characteristic of rock, The best information may be obtained by

visual inspection and comparison at the site. Experience and

7

judgment is required in the prediction of the weathering process.

Only experienced geologist* and engineering geologists may reach

to a sound decision by judgment.

(?) Uniform Ck)\£triaUc. &rtn<rttenh G^)Temporary

s/opz anpfe. henr? i w m

Figure 4,1 Various Bedrock Cut Methods. ( Hoods, Highway

Engineering lUndbook )

For general use, the values in Table 4.1 m«,y be recommended for cuts

in bedrock. These values, however, must be verified by reliable

field inspection and study.

' TADLE i 4.1

Average Slope Values for Bedrock Excavations ( Vfoods )

1. Igneous Rocks. . 0.25/l to 0.50/l

(Granite,Syenite,Diorit«,Gabbro,...) (O.ii horizontal, 1 vertical)

2. Sediuentary Rocks

a) Uassive sandstone and limestone 0.25/1 to 0,50/l

b) Interbeddsd sandstones, shales and

1imestones 0.SO/l to 0»75/l

c) Massive claystone and siltstone 0.75/1 to l/l

07

3, Metamorphic Rocks

a) Gneiss, schist, and marble 0.25/1 to 0.50/l

b) Slate 0,50/l to 0.7ö/l

NOTE:

Sandstone :. They are Basses of . water borne sand, cemented

together into a hard and compact mass.

Limestone I It is a crystalline sediment primarily composed

of calcium carbonate.

Shale t It is a laminated rock composed predominantly of

clay-sized particles although a small percentage

of sand or silt sizes also may occur. Clays and

silts are changed into shales by the process of

adhesion, compaction, and cementation.

Clays tone I It is rock composed of clay-sized particles not

having any lamination.

Siltstone : They are composed primarily of silt-sized grains

( approximately 0.05 to 0.005 mm )

Schist ' t It is a finely foliated rock containing & high

percentage of mica. During excavation blocks may

tend to separate along the weak planes.

Gneiss t Group of rocks similar to the schists but coarsely

grained and with alternate bands of minerals of

different composition.

Slate i It is dark-colored,'plate shape rock with extremely

fine texture and easy cleavage. In excavations,

large slate blocks may become detached vhen undermined.

b) Stability of Slopes in Soil Cuts

Stability of slopes in soil excavations depends on various

factors such as the type of material to be excavated, the degree of

consolidation of that material, the. degree of uniformity of soil

layers, and the position of ground watir table. In nonhomogeneous

soils, the presence of an effective layer ( loose soil on a firm

effective layer or a firm soil on a loose effective layer ) control«

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the type of failure surface, On the other h»nd, the failure surface

in homogeneous materiala can be approximated by a circle or a

logarithmic spiral.

During an excavation.operation a mass of soil has a tendency

to flow downward under its own weight and the surcharge on the

ground if any. This tendency of movement should be counteracted

by the shear strength of the soil, developed along the failure

surface and the slope should be safe if the shearing resistance

of soil overcomes the driving forces.

It is a common observation that the cuts in favorable ground

conditions should be stable if the following slopes were used :

Temporary Cuts l/2 to 1 (l/2 horizontal, 1 vertical)

Highway and railway cuts 1 l/2 to 1

Flooded.Cuts 2 or 3 to 1

Cuts in sand: Sands above water table, can be considered stable and

they can be excavated using standard slope. On the other hand,

loose saturated sands are not«table andthey cause slides during

excavations. Fine sands and silts below ground water table are the

worst kind of excavation materials'and they flow easily like a liquid.

If the 1 ateral confine^^ flne^sand or silt is

disturbed it is almost impossible to preVent slide failure.

Saturated fine sands should not be attempted to be excavated without .

any lateral supporting system.

Fortunately, most of the sands encountered at the s.ite contain

some cementing material Or a small amount cohesion and they may be

excavated using standard slopes.

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Cuta in Loan Lo««« io a uniform coheaive wind - blown sediment

with a grain aize diameter «.bout 0.06 ram. Th« particle« «re aouewhat

cemented by calcium carbonate or clay. It ia uaually vary poroua

and it can atand in almo«t vertical alopea if it ia not saturated.

Cuta in loaaa are not atable if itia permanently submerged. On

saturation! cementing material i* removed and loeaa depoaits

transform into almoat cohesionleas material«.

Cut» in Soft Clay ! If » standard «lope of 1,6 to 1 ia uaed in a

thick layer of aoft clay, a slope failure will occur when the

excavation reachea a depth of about 3 meter« aa it ia shown in

Fig. 4.2 a. and a riae at the bottom of the excavation will be

observed.

Ca) Typic-af base -fai/are.

in sofj- <zfay

Figure 4.2 Cuta in Soft Clay.

Cty TZtt/uns //? a so//- c/t ty

i/n^cr/di/7 hy & A<?r<?

Coyer.

If the aoft clay atratuin ha* a atiff overlying layer, the heave may

-occur when the excavation reachea the aoft clay. If the aoft clay

ia underlain by a hard atrata at a ahort diatance below teh bottom

of the cut, failure occur» along a circle tangent to the hard layer

( Fig. 4.2 b ).

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Cut« in Stiff Clay i Stiff clays are weakend by hair cracks as it

can be observed in stiff red fissured clays of Ankara («lay in UETU

Campus)« llair cracks in stiff clays are known as joints and vhen a

stiff clay specimen is pressed between fingers it can be seen angular

pieces with shiny surfaces* Slides in stiff clays may start shortly

after excavation or may not occur until many years after the cut is

made. li is easy to observe in Ankara that the deep vertical

excavations (up to 10 meters) in stiff local clay are stable temporarily,

and there is usually no risk if the required time of excavation and

foundation construction is short and the job can be completed in dry

season. On the other hand, nobody can be sure that an excavation in

stiff clay will be stable if the time of excavation and construction

is delayed and the open excavation is exposed to changing climati-fr

conditions. It can not be also sure that a cut in stiff clay can

stand safely if some water bearing sand or silt layers are encountered.

The fissured clay at the site is very rigid and the fissures

( or joints ) are closed. ïhen an excavation starts in stiff clay,

the lateral stresses will be relieved and consequently thé clay

mass will tend to expand. Yater may enter into open fissures and it

causes softening the clay. This gradual process of softening may

cause to reduce the shearing strength of the clay and slidesoccur.

The stability of cuts in stiff clay is difficult to analyze

because of the joints in the mass. The shearing strength can not be

determined easily- as it can be in other clays ( soft, medium ). It

is necessary to run triaxial tests on large specimens that include a~

representative number of defects ( joints )' of stiff fissured clays.

Cuts in Clay Containing Layers of Water-Bearing Sand î If a cut is

made in a homogeneous clay containing layers or pockets of water -

bearing sand or silt, water seeps out of the slopes at various

points. They require special attention, particularly if the

water - bearing strata slopes down toward the excavation as shown

in Pig. 4.3

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£ Oft CfoUHD

J&CA&

€&*y ever uiaMr

¿fearing strata,.- * . i , „

epcatefterr* Sand siff-

Jfa&om of-; 4 e-*c<vix/t0/7

Stnahz

Figure 4.3 Cut in Clay Containing an Inclined Water-Bearing

Strata«

In this case, the inclined water-bearing strata works as an

effective layer and it cause* a slide toward the excavated area.

2, Braced Cuts t

It 1* desirable, ofcourse, that the sides of temporary cuts

should be made as steep as possible (preferable vertical) without

use of any bracing system* Vertical or steep cuts without bracing

will furnish a free working space for the construction of substructure

and foundation, and it will also be economical. Unfortunately most

of the soil conditions at the side does not permit deep and wide

excavations without any bracing system. It is also desirable to

complete the excavation and construction in dry, but it should not

be forgotten that in many cases it will be nec*Knmry to lover the

ground water table.

It is known that the cohesive soils can be cut vertically up to

a certain depth ( Critical Height H^ - ) without bracing.

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The approximate value« of critical height are given below (Teriaghi

and Peck) :

Note : It ha« already pointed out that,the very deep cut« in stiff

clay« can stand safely temporarily, but their apparent stability

cannot be considered for permanent cuts.

(a) Bracing of Shallow Cuts i

Up to 5 or 6 meters excavations may be assumed as shallow cuts.

The bracing of shallow cute is more or less standardized and these

standard arrangement« can be uaed under different «oil conditions.

Theoretically, cuts in cohesive soils can be made without bracing

if the depth' of excavation does not exceed the critical height* But,

it should be remeubored .that some tension cracks, will appear onJLhe

ground near to the cut after_ascertain time. Theae cracks reducea

crïtTcaï~height and the aides of the cut may collapse. To prevent

such failures the upper edge« of narrow cuts in cohesive soils are

braced as shown in Fig* 4.4 .

If the depth of narrow cut is more than l/2 llc» strut«, are

placed as excavation proceeds. Horizontal boards ( lagging ),

vertical members ( soldier beams ) and «truta protect« the side of

the cut as «hown in Fig. 4.G . It »ay bo Jieoessary to fit. tho lagging

boards tightly together in son* oases»It u j not bo necossary to place

the lagging boards tightly together and some space nay bo loft botwoon

thea if tho excavated soil is clay, clayey silt orclayey sand, in other

words, if tho soil to be oteavated can not flow b«tw«en tho spaces of

the lagging boards*

Soil

Very soft clay

Soft clay

Medium clay

Stiff and very stiff clays

Critical Height (m)

Up to 1.5

1.5 to 3.0

3.0 to e.o

as low as 3f0

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'Struts C o r ^n/ce-s) >

V O k / 5 or Z g a Z O CI77.

¿frtber, a/yrox.

every rrj.

Figure 4.4 Narrow Cut in Clay. tl >

7imt>e.r p/anfiS

Cm Hjfck. f

Z.o "C %*} csn iy*We

>

. - fI7.

So/</ier I— . r ¿»earns

¿//v/j-/ «flp/>ro*.

¿.very rn.

¿.ay^iaj Cborizortf-af

boards). Tbe,y frrcy

rrof- ft tte. /vjc-Mtcs-

Figure 4.T» Narrow Cut,.

An alternative procedure ia to uae Vertical Sheeting - Vale ayatem

aupported by horizontal atruta aa ahown in Fig. 4.0 a and 4.0 b ,

In perfootly cohesion!*«« ailt, Band or grav*!, particularly below

tabl*, only vertical sheeting can bo us*d<

Figure 4.6 Vertical Sheeting in Braced Cute.

In soft «oil» where the danger of bottom heave ii critical or in

granular soils where lose of ground is possible,.vertical eheeting

must be driven into the »oil on each aide of the cut (Fig. 4.6 b),

(b) Bracing of Deep Cut» \

The bracing of deep cut» cannot be standardized and they deserve

a serious.study, particularly in soft or loose soils. Large and deep

excavations in soft soils are difficult and expensive* Therefore,

the'engineer should investigate every possible solution before a

decision is reached* s ' . - '

The most common methods for bracing of deep cuts (more than 6

meters,in depth) aro shown in Fig. 4.7 .

The vertical sides of a cut may not be excavated evenly.

Therefore some adjustment is necessary to fit the struts in their places

• firmly. Struts may be selected somewhat long and held in their position

by end cleats ( Fig. 4.8 a ), or wedges may be used ( Fig. 4.8 b ).

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« o a )

r Craoad S^r-fuce. V

e « ;

^—* :

f -

t&fro/-

— - , ¿>&¡rrr

t

/•4.ŞŞİ*,tp

-ít/tf/e

3

\ £ 3

Sefdier Seatrj

H ftk) _

ü W e .

StCob

B Î

E

c J

/Laßp/iJ

r-St ruh

'""r A I

Sotdier J>£¿zm H

secrtófj

c H-piU

1 F

5 7 ! ; 5 W

^UJz/e

7 \5fru/-

Ps/inj

SeCTtotJ

B-G

SecT/ofJ

Figur« 4.7 Different Method« for Con«truction Open Deep Cut«

( Terfceghi & I'eck ).

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VJALV

(c) Bracing of Tide Excavation« t

In many, cases excavations are too vide to be bracod by horizontal

atruta, and bracing ayatem may be aupported by inclined supports as

shown in Fig* 4,9, Vhen inclined struts are used, it ia necessary to

check unbalanced vertical forcea if any (an illustrative numerical

example will be given in the claaa room). In aoue caaea, the central

portion of the site may be excavated first with unsupported slopes

( Fig. 4.9 b )» part of the footing may be built, and this completed

part ia used as support for inclined struts for the other part of

the excavation.

In all types internal bracing systems, the excavated area is

obstructed by horizontal or inclined struts which do'not allow a*

fast,.easy and economical construction of foundation and substructure.

Anchored shoring systems ( anchored soldier beam — lagging - wale

system, anchored sheeting - wale system, anchored cast, in.place pile-

wale ayatem or anchored alurry wall ayatem ) by the use of drilled

tie-backs are becoming quite popular becauae they permit an

unobstructed excavation area as can be aoen in Fig. 4.10 .

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Figure 4.9 Bracing of Vide Excavation«.

Figur« 4.10 Ti»-back Anchorag«.

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A considerable depth of excavation can be obtained by placing row«

of tie-back« at different elevation* while the excavation proceeds.

This ay«teo ha* *o many advantage* that it can *ucce«*fully be u«ed

almost under any *oil' and ground water conditions for almost any depth of

«xcavatiOh desired. Particularly slurry wall-tioback systems have many

advantages to make larg* and doop oxcavations in water. Slurry wall is

a concrete diaphragm wall built in soil. This n*w systau is based

upon the possibility of excavating^under bentonite slurry^a self -

supporting deep trench (or hole) with no ne*d of internal bracing, to

b* filled with concrete, reinforced if doaired. For various application

of slurry walls see I.C.O.S. publications (or others), for example, The

I.e.O.S. Company in the Underground Works, Italy. The tie-backs are

made of steel bars or cables inserted: into drilled holes and grouted

with cement for a length variable with the load.

II. EARTH PR&SSURE AGAINST KtACING SYSTEMS i

The lateral earth pressure against bracing'of open cuts should

not be calculated by the classical earth pressure theories, because

the yield of bracing does not satisfy the basic assumptions, of these

theories. It is known from soil mechanics that the normal retaining

structures can tilt or rotate about bottom an amount equal to the

minimum yield to produce active earth pressure. If a structure can

not yield or deflect, then it will be subjected to the earth pressure

at rest. If a retaining structure is prevented any latoral movement

at the top but it is permitted to yield at the lower.depth, the

lateral earth pressure may approach the earth pressure at rest near

the top and reduces to a very small or zero pressure at the bottom.

This is the case of braced open cuts^ because the movement at the

ground surface is restricted to a very small amount by the u*e of

the top tier of struts. The movement at lower levels increases a«

the depth of excavation increases. This type of yielding produces

a roughly parabolic distribution of earth pressure as shown in

Fig. 4.11 .

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Jlfr/JtSTJW*

€<xrMf pressor**

f&sjunt by earth

\ pressors /¿jeorJes . 7 W W

Figure-- 4.11 Theoretical and Observed Shape of Pressure

Qiagrams in Braced Cuts,

It can be easily seen from the theoretical and observed pressure

areas that the upper struts should be under-designed and lover ones

over-designed if the theoretical hydrostatic pressure distribution

was used. This is the reason why there were many failures reported

in braced cuts due to buckling of upper struts.

These discrepencies between the hydrostatic pressure distribution

calculated by earth pressure theories and the actual roughly parabolic

distribution measured at the site, forced the investigators intensive

full scale experiments, particularly during subway excavations in

cities. It was observed that the actual distribution of lateral

pressure might considerable change from place to place and it was then

necessary to make a simple assumption as an envelope of all possible

.curves of distribution of pressure. According to Terzaghi and I'eck

( 1948 ) lateral earth pressure against bracing of open cuts may be

evaluated by trapezodial shaped pressure areas as shown in Fig. 4.12 .

For calculating earth pressure on braced cut in sand, jf values (angle

of internal-friction) may be estimated by Standard Penetration Tests in

the field. For bracing type shown in Fig". 4.7 a, £ , angle of wall

friction may be taken equal to zero. For the types shown in Fig. 4.7 b

and 4.7 c, the value of S in dense sand may be taken up to 20°.

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i; ,* O.SoH O.ZoH

V/to

0.3o H

// 0.60 H O.80 H 0.55H

O.2oli

o.Qj^CcsS

Ca)t>£HS£ SAjVD

SHfJIff

Gai

Ck>) loose SAU D C C ) SOPT AtJD M E D I U M CLAY

Figure 4.12 Earth treasure Again»t Bracing

( Terzaghi & Peck, 1D48 )

whore p^ - active earth pressure at the bottom of the cut

calculated by Coulomb Theory.

$ - angle of wall friction ( 0° to 20° )

q » Unconfined compression strength of clay.

The pressures against the bracing of cuts in sands and soft

to medium clays have been given in slightly different forms

( Feck, 1008 ) aa shown in Fig. 4,13 .

The aiinpliried preaaure area for cuts in Bands (Fig. 4.13 a) will

be used with caution where the depth of cuts exceeds about 12 m ,

The preaaure diagram ahown in Fig. 4.13 b is baaed on a reasonable

conservative aaauwption and it may safaly be uaed for soft to medium

clays except for a deap deposit of clay with a very low initial

tangent modulus.

- Ill -

0J5H

65 TH

it&m

I*—*-•]

T H - Z f a

WPt/fJ-

(p) sat/£>s (6) So// to"

medium tZ-f&ys

O.ZVH C5 0.4 VH

Oz) 3/tff fissarec/

c/ays

Figure 4.13 Earth Treasure Against Bracing (peck, 10G8).

Earth pressure on bracing in stiff clays increases considerable^

in time. The results of the experiments made by Kirkdam ( Proc.

Brussels Conf.' on Earth Pressure Problems, 1958 ) indicated that the

final maximum earth pressure at the base ( hydrostatic distribution )

may be calculated on the basis of c *> 0 and tf ( ft determined by triaxial test ). There is also a tentative pressure diagram for.

stiff—fissured clays as shovn in Fig. 4.13 c. Lover pressure to be

used only when movement can be kept to a minimum and construction

time is short. Otherwise the higher values are applicable. No data .

as yet available for cuts in soils that exhibit values of both c and

ff ( sandy clays, clayey aands, cohesive silts, and a variety of other

soils ). Therefore, design rules for such materials cannot be worked

out until adequate field observations have been made, and the bracing

systems for soils other than sands and clays should be designed with

caution ( Ter'zaghi & Peck, 1S08, p. 412 ).

- 112 -

Earth pressure on braced cuts in stratified soils may also be

calculated by a trapezoidal distribution as shown in Fig. 4.14

( Peck, Earth Pressure Measurements in Open Cuts, Chicago Subway,

1943 ).

/ / 4

y~rr

TT7TT

0.55 H

0./5H

Figure 4.14 Earth Pressure on Braced Cuts in Stratified

Soils (Peck, 1043 ).

where

q - l/n I ^ K H 2 tg fl + H .n.q "1 «qwiTftl»nt shear «trength A I S f I . • C U l -

^ - l/H + equivalent unit weight

H K - Hydrostatic pressure ratio for sand, (may be taken as 1.0) 0 j= n - Coefficient of• progressive failure (0.5 to .1.0), This

% value varies With the creep characteristic« of the clay,

and the length of time during which the excavation remains

open.

- Unit weight of sand.

^ •» Unit weight of clay,

$ - Angle of internal friction of a and.

q u »» Unconfined compression strength of clay,

(laboratory shear strength from tub« sample*)

q" - Actual field shear strength.

- 113 -

Sheet Pile» i They »re designed «.a a continuous member ( also long

soldier beams as shown in Fig. 4.7 b ) supported by struts and at the

bottom of excavation. The struts may be arranged so that the maximum

bending moments are equal in all spans.

Hales are designed as continuous members too. They ar« subjected

lateral forces .front sheeting and axial loads from the perpendicular

wales at the corner of the excavation.

Struts are pure compression members and they must be designed with

a minimum factor of safety of 2 against buckling. According to

Terzaghi, strut loads may be calculated by means of simplified

procedure shown in Fig. 4.15, which ignores continuity effects.

B+a

E+ O

F+G

H

e

H

Figure 4.15 Simplifying Assumption for the Computation

of Strut Loads ( Terza^hi & Peck ).

There will bt no support at the bottom for the bracing type shown

in Fig. 4.7 a, where soldier beams not driven into the soil. To obtain

the strut loads, ofcourse, the total reaction at the level of each

strut is multiplied by the horizontal spacing between struts.

- 114 -

III. STABILITY OF BRACED CUTS t

A braced cut may fail aa a unit due to unbalanced external

forces, heave of the bottom or piping. Particularly, failure« due

to heave of the bottom of excavation in eoft clays, and piping in

pervious soils are not very uncommon.

1. Heaving t If an excavation is made in soft clay, the bottom may heave

because of the inability of the clay to support the weight of the

overburden on either side. If the depth of an excavation in soft

clay exceeds a certain value, then the soil on both sides of the

cut moves downward, together with the system of bracing, and the

bottom of the cut rises. The weight of the column of soil abed

(Fig. 4.16), acts as a surcharge load on soft clay and the bottom

will rise if the surcharge^at the level of bottom of excavation

exceeds the hirJrTngjft«parity of aoi}.-

w^fiT

T U B

V b ..

Figure 4.10 Heaving of Bottom of Excavation in Clay (Teng, 1002).

- 115 -

For stability analysis it may be assumed that the angle of internal'

friction of clay ia equal to zero, and the «hear e engJ. ie. ftqjMdL

to one—half of the unconfined compreeeira^aj^ength. If a hard layer

is located at a short distance below the'bottom of the cut, the

surface of rupture (it is assumed to be a circle) is tangent to the

upper surface of the hard layer. The shear resistance which will

develop^ along the vertical surface <ul is equal to,

S - 1/2 q u ( H - q u / f )

where qu/^f is deducted from the depth of cut because of the presence

of possible tension cracks near to top. The weight of this block

will also be resisted by the cohesive resistance on surface c«, and

by the passive earth pressure on surface Jbe (passive pressure may

be taken equal to q^ if the weight of the soil below the line _bc is

neglected)..

Taking moment about point b,

1/2 TH B* - 1/2 q u (11 - ) ^ - ^ { f l * qJ ^ l/2 q u B*

or

f H D]l - q u (H - qu//f ) - V/i ^ q u Bx

If the total passive earth pressure is less than the terms

in the left hand side, braced cut

is not safe in spite of strong

bracing system. To prevent any collapse of the sides of cut together

with bracing system, sheet piling ( or soldier beams ) must be

driven enough into the soil. The driven piling must resist a force

equal to the difference between the left hand and the right hand

sides of the equation. The sheet piling should be driven to a depth

at least equal to 2/3 x D^, preferably into hard stratum.

If there is no hard layer within a short depth, the width of

may be taken as large as 0.707 B.( See, Terzaghi & Peck, Soil

Mechanics in Engineering Practice, art. 32 ), In order to obtain

the permissible safe depth of braced cuts, the values of q^ in the

above analysis should be divided by the factors of safety suggested

in the following table ( Tschebotarioff ).

- 116 -

TAULE î 4.2

Suggested factor« of safety

Degree of Sensitivity

High

Medium

Slight

Not sensitive

Sensitivity ratio

• à. 4 ..

2 to 4

1 to 2

Z.X

Permanent structures

3.0

2.7

2.5

2.2

Temporary structures

2.5

2,0

1.8 1.6

Note I Sensitivity ratio of a clay is defined as the ratio of the.

unconfined compressive strengths in the undisturbed and in

the remolded conditions.

The above method is applicable to the cases where the excavation

is very long. Bjerrum and Oide offered ( Stability of Strutted

Excavation in Clay, Geotechnique, 1956 ) a method which can be applied

square, rectangular or circular excavations in plan. The investigators

ass tuned that the braced cut is a deep fo.oting and this imaginary

footing may fail in an identical manner to the bottom of braced cut '

failed by heave, with the exception that the shear stresses in these

cases are in opposite directions. The factor of safety against

bottom heave is then given,

F.S. - N 2 ( II + q )

It - the depth of excavation

q « the surcharge lo*d on the area surrounding the excavation

» the unit weight of soil above the bottom of excavation

q u - the average unconfined compression strength of the clay

beneath the bottom of the excavation

This equation does not include the resistance of sheeting if they

¡riven, T)

in Fig. 4.17 .

are driven. The values of the bearing capacity factor, N , are given c

- 117 -

9

' 2 u / 3

H / &

4- 5

B - ïidth, H - Depth

L « Length of excavation

N c (Rectangular) « (0.84 + 0.10 B/L).N '(Squaro)

Figura 4.17 Stability of Braced Cute (Bjerruiu & Eide).

2. Piping : llany braced cut« in «and have failed by the audden formation

of a discharge channel at the bottom of excavation. Sheet piling

ia used for bracing cùta where the «oil ia granular and where the

excavated area must be dewatered. To prevent failure due to piping,

the aheet piling muat be adequately driven into the aoil. It ia

known frpci aoil mechanica that when piping takea place, the granular

aoil' at the bottom of excavation will have no contact pressure

between the grain«, and consequently soil can not support sheet

piling*. Piping, thua, cauaea a complete failure of bracing ayatem

together with the adjoining aoil.

It ia difficult to study 'piping in natural aoil deposit

because of presence of pocketa of different soil material. In

stratified soila a piping analyaia may be made aaauming that the

118

•oil is homogeneous and isotropic with a coefficient of permeability

equal to y k v kj ( k^ and k^ are coefficients of permeability in

vertical and horizontal directions, respectively ). For this type

of analysis the soil profile should be transformed by using a reduced

horizontal scale equal to ky/k^ .

According to the investigations made by Marsland ( Model

Experiments to Study the Influence of Seepage on Stability of a

Sheeted Excavation in Sand} Geotechnique, 1953 ), the penetration

of sheetpiling.required to prevent piping may be taken from Table

4.3 ( See also Fig. 4.18 ). Values in this table furnish à factor

of safety of 1.5 against piping.

1

^ ; • 1

t

1 .* * \ »

b

1 t

/ / / / / / / / / / 9 / / / / 7 / / J / S / / S

tMPEfÇ Vtous J~ftY£^

C*) ct>)

Figure 4.18 Minimum Penetration of Sheet Piling (Maraland).

- 119 -

TABLE : 4.3 Minimum penetration (D^) of Sheet Piling to Prevent

Piping (F.S. - 1.6) - (liar*land)

Vidth of Excavation

Site Condition* 8D 41) 2D D 0.5D Û.25D

1. Homogeneous loose sand

with infinite depth

( Hx - O 0 ) Dx - 0.7D 0.8D 0.9D 1.0D 1.2D 1.4D

2. Homogeneous dense sand

with infinite depth Dj - 0.4D 0.5D 0.6D 0.8D 1.0D 1.3D

3. Impervious layer under

homogeneous dense sand

,(Fig. 4.18 a)

Hj/D - 1 Dx " - °«4D °- 4 D °«6D

^/D - 2 Dj - - 0.4D 0.50 0.8D 1.1D 1.3D

4. Coarse sand layer under

fine sand (Fig. 4.18 b)

a) D + H^ B as in 1

h) D + Hj B use flow nets

c) Coarse layer reaches

above excavation

level (H^ « 0) Homogeneous values (l and 2) are safe.

5. Coarse layer above

fine layer Generally safer than homogeneous values.

Sheeting and Open pumping can be used for excavations if the

sheeting is designed and braced to take into consideration seepage

forces and loss of soil strength. If the hydraulic head is large,

seepage forces can Cause the soil to become quick, loose its shearing

strength, and place excessive load on the bottom struts, with the

possibility of the bracing collapsing.

C Vf - 120 -

# V-IV, DEfalTKKlNG

Construction of civil engineering structures many times requires

excavation below water table. To ensure safe and dry working

conditions in the excavated area, the flow of water into the excavation

must be eliminated or reduced. Therefore, the inflow of water must be

controlled by lowering the ground water table below the slopes and

bottom of the excavation. Ground water lowering operation cannot be

standardized, and it is possible to use one or several types dewatering

systems depending to the size and depth of excavation, and the

hydraulic characteristics of soil. By means of a proper devatering

operations excavation and construction safety requirements can be-

satisfied, because,

a) Stability of excavated slopes will increase

b) Danger of heaving of the bottom of excavation will decrease

c) Lateral loads on sheeting and bracing may be reduced

d) Excavating characteristics can be improved.

Lowering the water-ta^e_Cjm^aJLs^b^jised to increase "the

effective weight of the soil and consolidate both the soil above

and~TjeToiTlLhe~Tow^ w<ster table.

Customary methods for controlling the ground water were to

collect the water in the_juimp in the bottom of excavation and to

pump outside of the excavated area. To facilitate pumping from

suups~|T"*fieeting were frequently used. In these days, however, other

methods listed below way also be used for controlling ground water,

a) Veil points

b) Deep, large — diameter wells

c) Vertical sand drains combined with deep wells

d) Electroosmosis and ground freezing methods.

On large jobs, evaluation of coefficient of permeability and

pumping tests are necessary for determining the quantity of water

and the time required for a dewatering process. The amount of water

- 121 -

to be removed from the excavated area depends upon the height of water

head, the permeability of the soil below the water level and the size

of the area to be dewatered. In general, extensive dewatering is

necessary for deep excavation in permeable soils and little dewatering

for shallow e^a^tions^t»r<ww«e^ations in impervious soils,

f^ 1 • Pumping from Sumps'^

permit seepage from the slopes,' and'collect the water in ditches

and suuips, and pump it out of the^e^^atajm. A suuip is a simple

hole in the ground ( square, rectangular or circular ) to collect

the water for pumping out. For small areas one sump is enough, but

several suuips may be used for dewatering large excavations. Although

pumping from sumps is a very simple operation, it has also some

disadvantages. pumping from sumps may cause softening of the lover

part of slopes due to high seepage forces. Pumping from suiups in

fine sands and silts tuay cause an underground erosion starting at

the bottom of excavation and working backward. The loss of material"

toward the bottom of the excavation may endanger the stability of

existing structures. Sometimes a row of. sheeting around the sump may

reduce the risk of undermining the exiating atructure. In some

large jobs braced cuts in sand with deep sheeting should be

accotapained by sumps surrounded by another row of sheeting.

Sheeting and pumping can be successfully used for controlling

ground water.

2. Well Points j J

M~1wint' is a metal pipe approximately 2 to 3 in. in

diameter and 1 to 4 ft. in length, that is perforated and wrapped .

with a metal filter mesh (usually stainless steel mesh). Veil

points are connected to 2 ~ 3 ill. diameter pipes ( riser pipes )

and are driven into the ground. The upper ends of riser pipes are

connected to a header pipe that is connected to a pump, A well

point system is schematically shown in Fig. 4.10 .

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