November_1967 — Page 35

measure horizontal movements of the fill.

Piezometers are of the closed, double-tube hydraulic type consisting of porous ceramic hollow cylinders connected by means of polythene- coated nylon leads to mercury mano- meters in the gauge houses. The leads are filled with de-aired water. More than 400 piezometers are used on the main dam, requiring more than 90 miles of double lead. Piezo- meters are installed either in bore- holes, up to distances of 70 ft. below the foundation of the dam, or placed by diver on the embankment as the deposition of fill has proceeded.

The plastic leads, which are of in. external diameter and which are lighter than sea water, are prepared in bundles together with lengths of galvanised iron wire to act as sinkers. This work is done on a specially pre- pared pontoon, arranged so that leads can be reeled off up to 14 separate drums to form a single bundle. On the pontoon a numbered plastic tag is attached to every lead at 15 ft. in- tervals, and the leads and wire bound together with adhesive plastic tape. The bundles of leads are then laid by a diver along the lines shown in fig. 1.3 out to the gauge houses.

Prior to the resumption of filling operations over the leads a protective mound of specially selected material, free from stones, was placed along the line.

Remote-reading settlement gauges were developed for use in the main dam. They were based on the over- flow type used by the Building Re- search Station in England but are capable of recording settlements at elevations considerably lower than the level at which readings are taken in the gauge house. They consist of cylindrical cells embedded in the dam with air and water leads running to a gauge house. The air pressure necessary to maintain an air water interface in the cell is measured and, from it, the elevation of the cell is derived. Successive readings provide a settlement record.

The gauges have proved satisfac- tory in service and give settlement readings accurate to ± 0.5 in. or better. The total number of settle- ment gauges in the dam is 66, with an associated 16 miles of double air and 16 miles of double water lead. The leads, wholly of nylon, are laid in a similar manner to the piezometer leads.

A total of 30 Wilson "Slope In- dicator" installations have been pro- vided for the measurement of hori- zontal movements in the dam. The "slope indicator" instrument will be lowered down grooved plastic casing set in bore-holes through the fill and into the dam foundation, and used to measure the inclination to the vertical of the casing along its entire length. Hence the profile of the casing can be plotted, and successive readings will provide a record of horizontal

movements.

Far East Architect & Builder November, 1967

Fig. 1.5: Dredger 'Biarritz'

The majority of the materials for instrumentation of the dam have been

in supplied by manufacturers the United Kingdom, but the settlement gauges and all de-airing and reading panels used within the gauge houses have been fabricated in Hong Kong.

A recurring problem in the instru- mentation of the dam has been the protection of leads from damage by marine plant. There have been a number of instances of damage by dragging anchors or anchor cables, by the buckets of dredgers during the removal of nearby unsuitable fill material and by the operations of local fishermen. Whenever damage occurs, attempts are made to locate and identify the ends of leads at each side of the break, an operation which usually requires carefully controlled dredging of the overlying fill. If this is successful, leads are repaired using bronze couplings but, if unsuccessful, the instruments are abandoned and in important cases replaced. Extensive measures are taken to minimise such damage but nevertheless about 25% of piezometers and 34% of settle- ment gauges so far installed have had to be abandoned.

In general, construction pore pres- sures recorded by piezometers in the main dam fill and foundation have been significantly less than those as- sumed during the design of the em- bankment. Excess pore pressures re- corded in the embankment and its foundation rarely exceed 10 ft. of water and, almost invariably dissipate within a few days. The highest pore pressures developed anywhere in the dam, at two points within and near the base of the core, are shown plot- ted in fig. 1.7.

It became evident, when about 40% of the bottom sand drains had been constructed, that the develop- ment of construction pore pressures in the lower layers of the fill was unlikely to endanger the stability of the dam. The bottom sand drains were consequently eliminated for the

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remaining length of the embankment, thereby simplifying the cross-section

easing construction

and

(see fig. 1.4).

problems

Supervision of the Works

Methods and Techniques

Due to the unusual nature of the project the supervision of the under- water construction work posed special problems for the engineer's resident staff. The methods selected were the recording at frequent intervals of movements of all marine plant em- ployed on depositing material in the dam. Frequent sounding surveys of levels of dredging and filling were carried out and use was also made of underwater inspection by a diver; in addition, information was obtained from percussion borings carried out into the foundation or partially com- pleted embankment.

All the main operations of dredg- ing. excavation of filling material, transporting and depositing in the dam and trimming of completed lay- ers were carried out on a day and night shift basis. As the working sites are in the majority of cases only accessible by sea, a considerably larger supervising staff was required than is usual on a normal civil en- gineering works of similar size.

Dredging of the foundation requir- ed close supervision only when the edges or bottom of the dredged trench were being excavated. The limits to which the dredger worked were re- corded by sextant and the depths to which dredging was done were mea- sured by taking frequent soundings. Before a section of the dredged trench was passed as being suitable for de- position of filling, the accuracy of dredging was assessed by conducting surveys and checks were made to en- sure that no accumulations of un- suitable material remained on the dredged foundation.

During the final stages of any dredging operation, when the founda-

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