February_1968 — Page 37

CIVIL ENGINEERING AND PUBLIC WORKS

Part IV

Plover Cove Water Scheme, Stage II

A

LL surveys connected with Stage II contracts were based on the Colony grid and co-ordinates as given by the Crown Lands and Survey Office of Public Works Department, Hong Kong for their 1963 re-triangulation. The scale standard for this survey was fixed from a series of Tellurometer measurements.

The triangulation figure used for the basic setting-out consisted of three connected triangles of the Colony main trig system, the lengths of sides being about 4 to 5 miles (see fig. 4:1). These points covered the widely

CLOUDY HILL

ELEV. 14 42 FT

( TAI PO)

CHEUNG SHIU TAN

ELEV. 1336 FT

SURVEY AND SETTING-OUT

FOR STAGE II TUNNELS

။

by B. P. Bressler, AIAS

*

This paper outlines the methods of surveying for control in alignment and level for the driving of the tunnel system. Re- ference is also made to the types of instruments used and a description is given of the 'Photo-profil' apparatus. A triangula- tion diagram and a table of tunnel closing errors is included.

scattered works areas and formed the most convenient starting points from which to break down a minor trian- gulation network to establish co-or- dinated control stations at the various construction sites. Whenever possible, existing Crown Lands and Survey trig

HILL 1610 (ROBINS NEST)

ELEV. 1610 FT.

5000

10000

SCALE IN FEET

z

(0) PATSIN END

ELEV, 1677 FT.

(PLOVER COVE}

(5)

MA ON SHAN

ELEV. 2304 FT.

cairns were used, each contractor erecting additional control stations as necessary.

Tunnel Control Surveys

Before the commencement of tun- nel driving, control points were established at portal and shaft sites, and the co-ordinates determined from the triangulation network.

To ensure against significant errors occurring in the setting-out, cross- checking was carried out by the sur- veyors of both the contractor and the chief resident engineer. Using the main trig co-ordinates as starting points, and after verification of the associated angles by observation, the minor triangulation was then worked through independently by observation and calculation and the results com- pared.

Level Control Survey

The Crown Lands and Survey Office fundamental bench mark near the Tai Po fire station was used as the level datum point.

Closed level traverses carried out to precise standards were extended along the road to Tai Mei Tuk, the main road to Fanling and from there, along the road to Sha Tau Kok, using Zeiss N 1.2 and Watts Autoset levels with plane parallel plate micrometer attach-

ments.

Subsidiary traverses were then run from the permanent bench marks established along these roads up the various access roads to the work sites. To check against gross errors in the bench mark values at these sites, levels by trig height observations were also taken.

* This paper by Mr. B. P. Bressler, en- gineering surveyor, is the last in the series 'Some aspects of Stage II of the Plover Cove Water Scheme'. The paper was presented to the Engineering Society of Hong Kong (Session 1966-67) and is published by permission of the society.

Fig. 4:1. Main triangulation

Far East Architect & Builder February, 1968

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