Contents preface (VII) introduction 1—37

(ii) Slurry Trench Method
In deep alluvial deposits, provision of a rolled-earth cutoff may be expensive and difficult. In such situations, a deep narrow trench is kept open by filling it with a thick slurry of clay and bentonite. This trench, known as slurry trench, is backfilled with different types of soil, cement, and bentonite mixtures or unreinforced or reinforced concrete to act as a seepage barrier.
(iii) Sheet Piling
Interlocking steel sheet piling and interlocking wood are also used to construct thin cutoffs through deep alluvial foundations of embankment dams. Such thin cutoffs make the best under seepage barrier when alternative cutoff methods are costly and time-consuming. However, small openings in cutoffs and gaps at the bottom or top can result in large amount of seepage. Studies have indicated that a cutoff with 5 per cent open area at one point reduces the seepage by 60 per cent, whereas a cutoff with the same amount of open space equally divided among four openings is only 30 per cent efficient (8). Another drawback of sheet piling is the damage to itself by boulders during the driving process. Sheet piling is more effective in a homogeneous foundation than in a stratified foundation. Sheet piling is less frequently chosen because of its relatively higher cost and inherent leakage through interlocks between individual piles.

506 IRRIGATION AND WATER RESOURCES ENGINEERING
Water surface

Zoned dam

A
Water table
B

Pervious soil

C
Rock (Impervious)
(a) Cross-Section and flownet for partial cutoff

Zoned dam

A

B Water table

Pervious soil Q ~ O

C
Rock (Impervious)

(b) Cross-Section and flownet for complete cutoff

100

%	80
quantity,	60
seepage
	40
	Relative
20
	0

0

0
20
	,%
40	inseepage
	Reduction
60
	80

20 40 60 80 100

Percent of depth penetrated

(c) Relationship between depth of cutoff and seepage quantity

Fig. 15.15 Rolled-earth cutoff
(iv) Concrete Cutoff Walls
Concrete cutoff walls of 1.5 to 2.0 m thickness can be easily constructed by backfilling the dewatered trenches with unreinforced concrete. The trenches are braced and sheeted to keep them open. Another new method of constructing a concrete cutoff wall is by installing a continuous row of overlapping concrete piers with the help of special drilling rigs or other equipment (4).

EMBANKMENT DAMS