COFFER DAMS
Rajesh H & Unnikrishnan J R
Roll No. 7 and 14
S2 M Tech ( Part Time)
College of Engineering
Thiruvananthapuram
introduction
• Temporary structure to enclose an area for excavation of foundation
• It generally consists of a relatively impervious wall built along the periphery
of the proposed excavation to prevent the flow of water to the excavation so
that the foundation may be laid in dry condition.
• They are used
a) Very large Excavation
b) Foundations of Open water structures (Eg. Bridge piers, docks, dams etc.)
c) Foundations on open land where with high ground water table
2
tYPES
Earth coffer dams
Rockfill coffer dams
Single – sheet pile coffer dams
Double – wall sheet piling coffer dams
Braced coffer dams
Cellular coffer dams
Earthen cofferdam
4
Earthen cofferdam
♦ constructed around the area to be dewatered.
♦ built of local soils, preferably fine sand, Clay
♦ usually have a clay core or a vertically driven sheet piling in the
middle and upstream slope is covered with riprap
♦ Due to economical reasons it is not possible to make it watertight
and hence some seepage of water into the excavation is tolerated.
The collected water is later pumped out.
♦ freeboard of 1m to prevent overtopping by waves
♦ Suitable for depth of water upto 3m
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ROCKFILL COFFER DAMS
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ROCKFILL COFFER DAMS
Made of rockfill and an impervious membrane of soil to reduce
seepage
The crest and upper part of the impervious membrane is provided with
riprap to provide protection against wave action
The slopes of rockfill dams are made steep (1.5V:1H)
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SINGLE – SHEET PILE COFFER DAMS
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SINGLE – SHEET PILE COFFER DAMS
to enclose small foundation sites in water for bridges at relatively shallow
depth
Consists of a row of cantilever sheet piles (timber / steel) whose joints are
properly sealed. The piles are sometimes heavily braced
they are provided with earth fills on one or both sides to increase the lateral
stability
suitable for moderate flow velocities of water and for depth upto 4m.
Susceptible to large leakage and flood damage
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DOUBLE – WALL SHEET PILE COFFER DAMS
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DOUBLE – WALL SHEET PILE COFFER DAMS
• two straight, parallel vertical walls of sheet piling, tied to each other and walls
filled with soil.
• The width between the parallel piles is empirically set as where h is
the height of water
• Double – wall sheet piling coffer dams higher than 2.5m should be strutted
• Sometimes, an inside berm is provided to keep the phreatic line within the berm
• The fill material should have a high coefficient of friction and unit weight so that it
performs as a massive body to give the coffer dams stability against overturning
and sliding
• Suitable measures to be adopted to reduce the uplift on the coffer dam – by driving
the sheet piling on the upstream as deep as possible
• They are suitable upto a height of 10m
• Have less leakage than single sheet pile coffer dams
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BRACED COFFER DAMS
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BRACED COFFER DAMS
• Formed by driving two rows of vertical sheeting into ground .
The vertical sheeting is held by horizontal beams called wales and
they are in turn held by suitable struts
• They are susceptible to flood damage
• Land coffer dams – braced coffer dams to prevent ground water
from entering the foundation pit on land and to support the soil so
as to prevent cave in. After dewatering the pit, concreting of the
structure is done. When the concreting has been completed above
water level, the coffer dam is removed
13
CELLULAR COFFER DAMS
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CELLULAR COFFER DAMS
• Constructed by driving sheet piles of special shapes to form a series of
cells.
• These cells are interconnected to form a watertight wall.
• These cells are filled with soil to provide lateral stability
• Two types of cellular dams are commonly used –
• Diaphragm type
• Cellular type
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DIAPHRAGM TYPE CIRCULAR TYPE
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Diaphragm type cellular coffer dams
• Consists of circular arcs in the inner and outer sides which are
connected by straight diaphragm walls.
• The connection between curved parts and diaphragms are made by
means of specially fabricated Y – element
• The cells are filled with coarse – grained soils which increases the
weight of the coffer dams and inturn its stability. The leakage through
the coffer dam is also reduced.
• Its effective length can be increased easily by lengthening the
diaphragm
17
Circular type cellular coffer dams
• It consists of a set of large diameter main circular cells interconnected by
arcs of smaller cells.
• The walls of the connecting cells are perpendicular to the walls of the
main circular cells of larger diameter
• The segmental arcs are joined by special T – piles to the main cells.
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Design aspects
19
20
PARANETERS FOR ANAL.YS1s
Assume B-0.85H for firat trial.
1. Equivalent width of cofferdam .
2. EEfective veight of cell fill. W- [B(-H,) +B(H) Yaubl
3. Average distance betveen crona
wal ls.
Horizontal active force on
outboard_ side - compute using
KA tan(45 - 0/2).
Coefficient of horizontal earth K (varies - ee horizontal
pressure. pressure - diagran)
6. Water force on outboard side.
7. Horizantal passive force due to "p pyt (Include
be ro plus vater force. tveen
sheet pile and so11
8. Net overturniag BORent due to
total horizontal force. (poiat of applicat ion of P, 1s
approximated as H4/3, see References
Ln text lor further gui dance)
9. Rasisting monent dug to cel1 Mp CB/2)
fll.
uo. Radius of cell vall.
|11. Interlock tension. T P, L
vhere P% total horizontal stress
at poínt b
Zone at naximun inter tension
located at H/4 abo
stress diagran, Inboard Sheeting
and references cited in text
12. Ult image interlock strength. T, 16 kip/in for ordiaary U.S.
Bteel sbeet piles and 28sheet
for hígh interlock U.S.
kíps/in
piles
13. 2f fective unit ve 1ght. Y veighted average of call
SUB (above and
below water in che cel1)
PIGURE 37 (eontinued)
Des Lgn Criteria for Cel lular Cofferdams
7.2-120
14. Friction angle of sofl and 8- 2/3
steel.
15. Coefficient of friction be tueen A - use 0.5 for RmOOth rock,
cell f11 and rock. for all other use tan
]6, Drained angle of shearing
resistance of so1l,.
17. Coef ficient of interlock f 0.3
Eriction.
8 Horizontal effect stress on p' - (see presSUre diagran for
a vertfal plane.
subscript)
9. Hor izontal effect force on a (see pressure diagrss for
vertical plane. subseipt)
PICURE 37 (conti nued)
Des Lgn Gritería for Cellular Cofferdaes
1.2-121l
DESIGN METHODs
COFFERDAM ON ROCK WITH BERM
l. Fac tor of safety against sliding on Base
wA
Fs 2 125(TEMPORARYITO I5(PERMANENTI
2. Fac tor of safe ty against overt urning, Fo
f o 2 y TO 3.5
3. Factor of safety agninst excessive interlock tension, F
P2I5 TO 20
Factor of safety against vertical shear on centerline, F,. (Terzaghi)
Fvs /s TAN +Ie-P,2 IL25 (TEMPORARr WALL)
50(PERNANENT WALL)
Where P' 1s calculated
errective stress diagran for the
Center Plane of c e l e 0.6; and
P 1s calculated using the effective stress d1agran of Inbosrd
Sheeting, and equa ls area ab'e'd iEh K = 0.4.
5. Factor of safety against tilting, r
EMPORARY
FOR KA TAN (45-/2) L50 (PERMANENT
6. Factor of nafety against shear at cell fill, sheet pL le interface, F.f
PERMANE
Where P,' 1s calculated using the effectíve stress díagran for
Outboard Sheeting, and is equivalent to area jkl with k 0.7 to l.0.
7. Select value of B vhich satisfies all requi reaents.
COFFERIAM ON ROCK - WITHOUT BERM
Follow des ign Steps I through 7 as above for cofferdam with be rm.
8. Put Pp - 0 in all equations to compute Mo and fac tor of sa fety.
9. In coeputíng Pue, P, 1s caleulated t estreas diagran for
Inboard Sbeetin and eguals are ith 0.4.
PIGURE 37 (Contínyed)
Design Criteria for Cellular Cofferdans
7.2-122
cOFFERDAM ON DEEP SAND rOUNDAT LON- WI THOUT BERM
10. Penet rat ion of nheet pi lir
y
Mhlch are evalt ed with f1 ow net. pend on nder Nee page fequlre
In general, thin avo 14
at Inboard toe.
D, D -
or DË s vz
ground urface. 1f water level is lowered at least below inboard
Il. Check factors of nafety for Steps 2, 3, 4, 5, and 6 above
cofferdamu on rock. tor
12. Factor of nafety for stabili ty againgt bearing capacity fallure, Pbe
2 2 (NOTE: P 40)
QultULTMATE BEARING CAPACITY POR CONTINUOUS FOOTING Oor wiOTH B(SEE OAPTER 4)
13. Penetration to avold pull-out of outboard sheeting.
15,WHERE Qull ULTIMATE PLLOUT CAPACITY PER LINEAR FOoT OF
Mo
WALL =V2 K, YD TAN8xPERIMETER (NOTE Ppr0), AND Op
COFFERDAM ON DEEP SAND POUNDATION - WITH BERM
14. Design as per steps for cofferdam on deep sand foundation vithout bera,
except that passive resul taat Pp 1s included in resiatíng overturníng
moment.
15. Stability agaiaet besring capaci ty failure is not as crieical th
presence of bero,
16. Penetrat on of sheeting requlred to avoid plping 1s evalunted with flow
aet.
17, Pene trat ion of Outboard Sheeting to avo 1d pul l-out is the sane as for
cofferdas on deep sand berm except include P, in calculation
of M
COPFERDAM ON STIPF TO HARD CLAY
18. De siga procedures saue as for cofferdams on sand. Stabil1ty againet
bearing capaclty fa1lure of Inboa tion of
sheetíng to avoid plping 1a ua D8tu oe iaportant.
ually not be
19. Penetration to avoid pull-out of Outbaard Sheeting
L51 p SAME AS STEP I3
dp o Ce DXPERIMETER( C, FROM TABLE II
PIGURE 37 (Continued)
Des Lgn Criteria for Cellular Cof ferdams
7.2-123
COFFERDAM ON SOFT TO MEDIUM STIFF CLAY
20. De afgn procedures sane as for cof ferdam s on deep sand, wIth
Bod1fications as per following steps. Penetration to avoid
piping ís uually not iaportant.
21,
Factor of Safety for atabil1ty ageinse bearíng capactty fatlure, Fhe
Fbe from Step 12 > 3
22. Because of internal instabil1ty due to settlement of comprens ible
foun da tion, factor of safety against verticsl stress on centerline
Pyg frOn Step ghould be
Fvs -xBxteo 225 (TEMPORARY)
Invest 1gate overall tabt 2ity of cof ferdam vith res pect to sliding
along a curved surface belov the bot tom of the aheeting by lope
sta bl li ty analysis fron DM-7 CHAPTER 7
23. Invest igat OUB Band ithin the clav denost r
yhfeh could d u e l e p e r v
excessive uplift pressure below the base of the
cof ferdam.
24. Eva luate penetration of outboard neating to vo1d pull-out as per Step
19.
PIGURE 37 (conti nued)
Des ign Criteria for Cellular Cofterdans
1.2-124
