of the reservoir below this elevation was 3,000 acre feet, and allowing this as dead storage, the gross capacity of the reservoir would be 12,500 acre feet.
At elevation 26.00 the capacity of the reservoir would be 13,000 acre feet.
The best type of dam suitable for the selected site was a rolled fill earth structure. The design for an earth dam was therefore pursued.
A hydrological study of the catch- ment above the location of the dam for the reservoir was made. The area of the catchment was 54 square miles. The normal maximum flood in the catch- ment was estimated at 13,500 cusecs and the abnormal maximum flood was estimated at 54,000 cusecs.
Height of Dam
The spillway to discharge the flood is to be located on a saddle south of the main dam. It is possible to get a length of 1,000 ft. at an elevation of
Circle
NOTE :
Fig. 2
a) Force diagram, = circle method
-26
-24
-12
.20
•14
#2
•10
-04
.02
54
10
70
b) Chart Taylor number, T(after Taylor)
about 26.00 on the saddle. It was therefore decided to utilise this length and construct a chute spillway.
Routing the two maximum floods, the flood lift for a discharge of 13,500 cusecs over the spillway was found to be 2.4 ft. and that for a discharge of 54,000 cusecs was 6.3 ft.
Other factors which determine the height of the dam are: (a) wave height and (b) free board. The selection of these two factors as the margin of safety is especially important in the case of earth dams, on account of the disastrous results of overtopping. An earth fill structure when overtopped is bound to fail.
The value of the wave height h in feet is usually determined from the formula:
h = (1.5) D + 2.5 − D1⁄4
where D is the exposure or length of the reservoir in miles. For the Ulu Sem- brong reservoir D was found to be 0.935 miles. Then:
AB designates the arc.
ab
J1
» chord.
Q
For 4x 6" and "'è < 53'
40
10
A
H
h = (1.5) (0.935) + 2.5 — (0.935)1⁄4
= 2 ft.
-
To allow for the set up of the wave, the total allowance for the wave height and wave set up may be taken as 1.5 h, which is 3 ft. The elevation of the top of the dam will be:
Spill crest level + flood lift + wave height + free board. Allowing a free board of 3.6, then the elevation of the top of the dam is:
26.00 2.4 + 3 + 3.6 = 35.00 The maximum top width of an earth dam may determined by the empirical formula:
W = 2 (H)2+3
In the case of the Ulu Sembrong Dam, the top level is at elevation 35.00 and the lowest bed of the stream is at elevation 9.00. Then the height of the dam is 26 ft. and the top width will be:
W =
2 (26) + 3 = 10.2 + 3 = 13.2
The top width of the dam will be taken as 15 ft.
Soil investigations
Investigations were carried out on the foundation of the earth dam. The soil profile of the foundation is given in Fig. 1 (b) and (c). The foundation is not satisfactory but it was considered that with proper precautions an earth dam could be constructed.
Investigations were also carried out in the area where the material for the earth fill will be excavated. Samples of soil about 1 cwt. each were collected from trial pits every 5 ft. of depth or for each change of strata, whichever was less. In these investigations, it was con- sidered sufficient if the depth of the trial pits were about 6 ft. As there was no change of strata within the depth of 6 ft., only one sample from each trial pit was obtained. In all five samples from five trial pits were collected and analysed.
The tests carried out on the samples of soils were as follows: (a) specific gravity, (b) mechanical analysis, (c) compaction and penetration tests, (d) percolation tests and (e) shear tests. The results of the tests are summarised in Table 1. The classification of the soils based on the mechanical analysis is also given and the results of the permeability tests on samples of the undisturbed soils are tabulated.
The permeability of the remoulded samples are so low that the soils can be considered relatively impervious. The shear tests are on remoulded sam-
40
Far East BUILDER, October 1968.
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