November_1967 — Page 34

skips were unloaded by a moored crane which overturned them at water level.

Tunnel spoil, the lighter gradings of riprap and some of the filter rock were loaded by chute into self-propel- led barges suitably reinforced and then dumped directly in position. The heavier riprap was transported by cargo barges of the type common in Hong Kong Harbour, the individual pieces being loaded and unloaded by the barge's crane. The core of the rock mounds outside the closure sec- tion was deposited by the same type of vessel but using cargo nets.

In order to reduce tidal currents across the closure gap to a minimum whilst the closure mound was being raised, layers of rock deposited were limited to a thickness of 3 ft. These layers needed to be formed by work- ing from each end towards the centre of the gap above 4 ft. P.D. As the closure operations had to be done quickly, rock was stockpiled on the sill protection adjacent to the mound beforehand and then dredged and placed by the "Biarritz".

Grouting

For the reasons mentioned exten- sive foundatiton grouting was not con- sidered necessary under the main dam. Limited fissure grouting of rock in the foundation between the beach cut-offs and the abutments was envisaged. At the Tai Mei Tuk abutment, where previous rock was exposed on the shore, grouting was associated with a concrete cut-off wall so as to minimise seepage at the

contact between the foundation and the embankment fill.

At each abutment curtain grouting consisted of a single line of grout holes, generally at 10 ft. centres ex- tending to a maximum depth of about 200 ft. into

The the foundation. lines of holes extended over lengths of 300 ft. at Tai Mei Tuk abutment and 550 ft. at Harbour Island abut- ment. Limited numbers of blanket grout holes were carried out at 20 ft. centres on two lines 15 ft. on either side of the line of curtain grout holes.

At the subsidiary dam-sites, due to the closeness of the abutments and the absence of the mud blanket, more extensive foundation grouting was re- quired and provision was made for sealing by grouting any leaks on the perimeter of the reservoir when the sea water was pumped out.

Instrumentation

As this is the first known instance of a major dam being built under- water with decomposed rock filling, a high degree of instrumentation was considered desirable. The instrumen- tation provided for the dams was planned to fulfil the following re- quirements:-

(i) To give warning of incipient failure of the embankment at any stage of construction or dur- ing draw-down following com- pletion.

(ii) To indicate the most suitable cor-

rective measures should failure of the embankment appear to be imminent.

(iii) To provide information suitable

In

for checking design assumptions on the shear strength of the de- composed rock filling. (iv) To provide information suitable for the prediction of settlement allowances to be built into the dam as it neared full height. (v) To reveal the pattern of seepage through the embankment and foundation during pumping-out of the reservoir and following completion of the project. Instruments were installed at ten cross-sections of the main dam. order to avoid excessive lengths of lead between the instruments and their reading points, five gauge houses were provided, one on land near each abut- ment and three on off-shore steel towers spaced along the reservoir side of the dam. To avoid the occurrence of negative pressures in the hydraulic leads linking instruments to the off- shore gauge houses, the gauge houses were arranged for lowering within their towers during pumping-out of the reservoir and subsequent raising during refilling. The layout of the five gauge houses and ten instrument- ed cross-sections is shown on fig. 1.2.

Two of the ten cross-sections were planned to provide full instrumenta- tion of the section, while the remain- ing eight are partially instrumented. A typically fully instrumented cross- section is shown on fig. 1.4. Three types of instruments are installed in the dam. They consist of piezometers to measure pore pressure, settlement gauges to measure pore pressure, set- tlement gauges to measure vertical movements of the fill, and devices to

PLOVER COVE RESERVOIR

EXISTING SEA-BED (LEVEL VARIES)

SANO

BOTTOM OF MUD (LEVEL VARIES)

30%

ROCK SEA-BED PROTECTION

SPILLWAY CREST LEVEL +27′ PD

RUBBLE MOUND

FOUR-RAP WAVE PROTECTION

OPERATING DRAM-DOWN LEVEL -18 ́P D

KEY -

TO REZOMETER INSTALLED BEFORE CLOSURE

2 *: PEZCHETER INSTALLED AFTER CLOSURE

4

3 X SETTLEMENT GAUGE

SLOPE INDICATOR

DUMPED DECOMPOSED ROCK

WAVE WALL

TOLO

HARBOUR

ROADWAY

GRASSING

DĚCOMPOSED ROCK,

IN LAYERS

PROFLE OF DAM AT COMMENCEMENT OF PUMPING-OUT

__AP-RAP ROCK PROTECTION

+4PDMSL

ORALLE

FILTER ROCK

SAND DRARS

DUMPED DECOMPOSED ROCK

SAND DRAINS

DUMPED DECOMPOSED ROCK BOTTOM BLANKET

CLOSURE RUBBLE MOUND

RAISED IN LAYERS

\SAND DRAIN

PLACED DECOMPOSED

|DUMPED DECOMPOSED ROCK

ROCK CORE

SAND DRAINS

DUMPED DECOMPOSED ROCK

DUMPED DECOMPOSED ROCK BOTTOM BLANKET

270"

A

TYPICAL CROSS SECTION

(OUTSIDE CLOSURE)

PROFILE OF BAM AT COMMENCEMENT

OF PUMPING-OUT

SPORÁRY

JROCK PROTECTIONÉ

ROCK DRAIN

DUMMED

DECOMPOSED

SAND DRAIN

DUMPED DECOMPOSED

ROCK

O

DUMPED DECOMPOSED ROCK BOTTOM

O'MIN

110

о

❤

ROCK DRAIN

DUMPED DECOMPOSED ROCK

SANO

DRAIN

DUMPED

DECOMPOSED

DUMPED DECOMPOSED ROCK BOTTOM BLANKET

0

O

205

B TYPICAL SECTION THROUGH CLOSURE GAP

(INSTRUMENTED SECTION)

Fig. 1.4: Main dam cross-sections and closure arrangements

60

✪

O ROCK

10 MIN

EUSTING SEA-BED (LEVEL VARIES)|||

SAND

BOTTOM OF BUD

LEVEL WRES)]

20

ECTION

50

100 FEET

Far East Architect & Builder November, 1967