December_1965 — Page 38

100

200

100.

COPD..

300

296

ROCKFILE

FILTER

QUI

RIPRAP SURFACE

SCALE

700 ET

0014

281

かじゅ

cio Eu

9011

OD12

US

99

SHOULDER

SHOULDER

OUT

eve

°C?

0010

004

OUS OU2

a‍.

CORE

007

оро

096

OCA

003

• DI

004

+ D2

GROUTHOLES

CUT-OFF

DF6

DF7

DRAINAGE BLANKET

FILTER,

DFB

IIS TWL (SUPPLY BASIN

ROCKFILL

DAM

Fig. 2. Cross section

In the original design, horizontal drainage blankets of 18 in. thick at vertical intervals of 20 ft. were re- quired in both the upstream and downstream shoulders. The purpose of these drainage blankets was to accelerate the dissipation of pore water pressures during construction in both shoulders, and during rapid draw-down in the upstream shoulder. However, as the constructional pore water pressures in the shoulders prov- ed to be negligible, only two such blankets were put in each shoulder.

The 6 ft. thick mass concrete cut- off wall which was constructed into fresh to slightly weathered granite along the centre line of the embank- ment extends well into the left and right abutments, in places about 120 ft. deep (Fig. 3).

Three lines of AX-size groutholes, one along and one 10 ft. each side of the centre line, were put down under the core to seal up any cavities or fissures in the bedrock by injecting cement grout. The depths of the groutholes varied, being governed by the permeability of the rock encoun- tered.

The criterion was that the treated rock should have a coefficient of per- meability of not more than 1 Lugeon

ALL LEVELS ARE IN FEET ABOVE PD

PIEZOMETERS ARE SHOWN THUS O

Unit* obtained by water pressure testing in the drillhole.

The maximum height of the dam is 186 ft. and the crest length is about 750 ft. Its maximum width at the base is about 1,100 ft. The width of the core varies from 100 ft. at the base to 20 ft. at the top.

The quantities of materials for the dam are shown in Table 1 below:—

Table 1

Quantities of Materials

Material

Core fill

Quantity (cu. yd.)

158,200

Shoulder fill

983,700

Rock fill

317,700

Filter

Total

158,000

1,617,600

Embanking Materials

The main embanking material for the rolled earthfill was completely decomposed granite. This occurs in

* Water loss of 1 litre per minute per metre length of drillhole, under a pres- sure of 10 kg. per sq. cm.

abundance in the neighbouring hills. The weathering profile of granite has been described by Ruxton and Berry, 1957 (1).

A zone of about 4 to 10 ft. of more clayey structureless residual soil exists below the top soil, and this material was selected as core fill. Below this zone, where the weather- ing is less intense, the residual soil is coarser, and the granitic structure is still apparent. This material was selected as shoulder fill. The upper and lower limits of particle-size curves for each type of material are shown in Fig. 4.

Although the specification stipulat- ed that the core material should con- tain not less than 25 per cent by weight particles passing a No. 200 B.S. sieve, it was found that this alone was not adequate to identify the core material, because the grading limits of core and shoulder materials over- lap, as shown in Fig. 4. The cri- terion of a plasticity index of 20 per cent was used to supplement the identification of the two types of materials, as suggested by Lamb. 1962 (2). This was found to be quite reliable, as will be seen in Fig. 5.

It was also found that with some experience, the two types of materials

400

LEVEL IN FT.

CH 800

AX-SIZE GROUT HOLES

SPILLWAY TUNNEL

CONCRETE

CUT-OFF WALL

— - CH 1200

Fig. 3. Longitudinal section along centre line (looking downstream)

74

CM 1300

CH 1400

OOST HO、

DAM CREST 306

-TW.L.296

ORIGINAL GROUND LINE,

CH 1600

CH 1700

TOP OF CONCRETE CUT-OFF WALL

AX-SIZE GROUT HOLES

SCALE

མ་་

OOK HƏ

Far East Architect & Builder

December, 1965