No_10_October_1968 — Page 44

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Referring to table given in Fig. 2 (b), O, the angle of slope of the embank- ment will be 30o. Then, Tan

6 in. layer of sand followed by 1 ft. 6 in. of gravel on the three inner sides. The rest of the volume of the drain = 0.577, the slope of the and the outer sides will be filled with rubble. From the toe drain, a blanket of sand 3 ft. thick will extend to a distance of 5 ft. at least inside the im- pervious core of the dam.

embankment will be 1: 1.73

A slope of 12 is sufficient. How- ever, to make up for any other unpre- dictable defects, a slope of 1: 2.5 on the upstream and 1: 2 on the down- stream will be used.

The upstream slope will be protect- ed with a layer of rip rap from elevation 32.00 at the top to lowest point on the dam in the reservoir bed. The rock used for this purpose will be hard and dur- able and not subject to slaking, solu- bility or rapid weathering. The depth of the layer of rip rap will be 18 in. minimum and the stones as far as possi- ble will be square. Rounded or irregular shaped rock is less satisfactory and should therefore have greater depth for

The stability of the earth dam was also checked by the Swedish method of slip circle. For this purpose the values of the properties of soils of the three different materials A, B, and C (table 2) were used. It was found that the minimum factor of safety on the upstream was 1.3 and that on the downstream was 1.2. These values were considered satisfactory.

The downstream of the dam need the same conditions.

be protected against possible seepage through the dam. A suitable filter with a rock toe as indicated in Fig. 1 (a) must be provided. The details of the filter and the rock toe are given in Fig. 3. The depth of the filter is 5 ft. and the bottom width is 10 ft. The side slopes of the trench for the drain will be 1 1. Side slopes of the portion above the trench will be 1: 1.5. The top width of the drain will also be 10 ft.

The toe drain will be filled with a

Emergency precautions

The design flood of 13,500 cusecs is 150 per cent of the 100 year flood of 8,220 cusecs in the catchment. This normal flood of 13,500 cusecs is con- sidered adequate under normal circum- stances. A structure designed for a 100 year flood is not safe for 100 years, as this discharge or a greater flood may this discharge or a greater flood may occur earlier and perhaps just after or within a few years of the completion

of the construction of the structure.

In order to overcome this difficulty, especially in earth fill dams, which are subject to risk of failure on overtop ping, an emergency spillway to accom- modate an emergency flood need be provided. The discharge of an extra- ordinary flood for the catchment above Ulu Sembrong Reservoir was estimated at 54,000 cusecs.

A suitable location for an emer- gency spillway is available on another saddle adjoining the main spillway of 1,000 ft. Instead of clearing this saddle and reducing it to form an emergency spillway, it may be built as a weak section of the earth dam so that it may be breached in an emergency.

The flood lift over the 1,000 ft. spillway for a peak discharge of 54,000 cusecs is 6.3 ft. The elevation of the top of the dam is 35 ft. and the elevat- ion of the flood lift is 32.3 ft. There is still a free board of 2.7 ft. and before the flood elevation reaches 35 ft., suf- ficient water would have escaped to make the dam safe from overtopping.

There may not be any need to breach the weak emergency section of the dam. It is, however, desirable to allow this breaching section and take necessary precautions to breach it in case such a necessity arises.

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