November_1967 — Page 31

CIVIL ENGINEERING AND PUBLIC WORKS

MA LIU SHUI

BORROW AREA

EN MAN

-

TAI SHUI HANG

SAND DREDGING AREA

BORROW AREA

WHITE HEAD BORROW

AREA COMPLEX

TURRET HIL

DUMPING

NG KOK

SAND DREDGING AREŽ

TAI PO

Fig 1.1: View taken during the early stages of construction, showing principal working areas

At each abutment, cut-offs were formed parallel to the shore-line up- stream and downstream from the dam (fig.1.3). These involved dredg- ing trenches through the mud to re- move underlying pervious soil strata which connected with permeable beach deposits and backfilling the trenches with mud and decomposed rock. Altogether 7,150,000 cu. yd. of dredging was required.

The depth of water below mean sea level in the dredging area aver- aged 40 ft. with a maximum of 45 ft. The average thickness of the sea- bed mud was 40 ft. and the maximum approached 55 ft. The extreme dredg- ing depth was slightly more than 95 ft. below mean sea level. All mud dredged was deposited in a bay en- closed by three islands 14 miles to the west of the dam-site (fig. 1.2).

Before dredging commenced, prob- ings were done at 50 ft. centres over the whole area of the dam-site using a Dutch Cone Penetrometer to de- termine the depth of mud. From these results the dredging depths were derived; in very few instances has dredging below these depths been re- quired..

Starting from the Tai Mei Tuk peninsula and working progressively towards Harbour Island, the bulk of the dredging was carried out by the bucket dredger "Haiphong" and the grab dredger "Biarritz" (fig. 1.5) as- sisted at times by the grab dredger Lima 2400 which was mounted on a pontoon.

Mud was discharged from the dredgers into 1,200 cu. yd. capacity self-propelled barges for dumping in the mud disposal area. The ladder of the "Haiphong" was lengthened dur- ing the contract to dredge to 75 ft. below water level, beyond that depth the "Biarritz” was employed ex- clusively.

The use of cutter suction dredgers for removing mud was not permitted as, before the contract was awarded, concern had been expressed about the effect of mud pollution on fish and, more particularly, on cultured pearl

Far East Architect & Builder November, 1967

oysters in the Tolo Harbour area. A close watch was kept on the spread of turbidity from the dredging and dumping areas but no adverse effects were observed.

It had been anticipated that, unless care was taken to limit the depth of each cut taken by the dredger, large accumulations of mud debris would be present on the foundation and dif- ficult to remove. A secondary effect of deep cuts would be the difficulty of satisfactorily forming the 1 in 11⁄2 side slopes of the trench. A procedure was therefore adopted of limiting the depth of cut to 6 ft. for each lateral sweep of the dredger with a limit of 3 ft. on the final cut at foundation level.

Although great care was taken to ensure that all material within the prescribed limits was removed, echo and chain sounding surveys of the first section completed showed that the bed of the dredged trench was too high. On investigation it was found that a residue of semi-fluid mud had accumulated in the bottom to a thick- ness of 5 ft. in places. At first, efforts to remove the accumulation by re- dredging with bucket or grab dredgers were unsuccessful and a trial with an improvised air lift pump was equally ineffective.

a

It was found that if the fluid mud was left undisturbed for about month it gained enough strength to be redredged successfully. However, inevitably after redredging, some re- sidual mud was left behind. This layer, if not exceeding 9 in. in thick- ness, would not affect the stability of the dam and was therefore allowed to remain.

The mud at the side of the trench was found capable of standing at the designed 1 in 11⁄2 slopes but at first considerable overdredging took place. However, with more care and im- proved techniques to locate the dredgers, this overdredging greatly reduced as the work progressed.

Some difficulty was experienced in dredging dredging successfully granodiorite boulders, some as large as 12 cu. yd.,

from the beach deposits near Har- bour Island and the buckets of the dredgers sustained some damage.

Decomposed Rock Filling

Decomposed rock was the principal filling material used and a total of more than 7,500,000 cu. yd. was taken from the various borrow areas (fig. 1.2) to complete the dam. The borrow areas were located principal- ly in granite or granodiorite areas and the typical excavated material was decomposed by tropical weather- ing to become a silty sand with a variable percentage of silt and clay- size particles.

The three zones of decomposed rock filling in the dam are shown in fig. 1.4. These consist of the imper- vious core, a blanket resting on the foundation to prevent seepage of water through the fill and under the core, and general fill which support- ed the core. Arising from the results of permeability tests carried out the first material to be deposited in the dam, a minimum fines content of 25% passing the no. 200 B.S. sieve was adopted as the main criterion for distinguishing core and blanket ma- terial from general fill.

on

In addition, core material was re- quired to have a minimum content of stony material and to be placed in position by grabs to reduce segrega- tion of the material during deposition. Material for bottom blanket and general filling had a higher stone con- tent and was permitted to be dump- ed from bottom opening barges.

It was anticipated when work be- gan that sufficient material could be obtained from three borrow areas ori- ginally reserved for excavation at Ma Liu Shui, White Head and Shuen Wan. However, arising from intensive trial pit investigations in these areas it was evident that the average depth of completely decomposed rock in borrow areas was less than envisaged and, as a result, there would be a shortfall of about 2,300,000 cu. yd. A number of other areas were in- vestigated and it was decided to open

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