Sessional_Paper_1904 — Page 110

29

P

[No. 1290.]

SIR,

PUBLIC WORKS OFFICE, HONGKONG, 23rd October, 1903.

HONGKONG WATER-WORKS.

Tytam Tuk Scheme.

I have the honour to acknowledge receipt of your letter of the 27th July last, with enclosures, re the arrangement of the Pumping Station and capacity of the plant to be laid down, and have to thank you for the full information given.

2. I have delayed answering your letter until I was in a position to submit some definite proposals as to the works to be undertaken at once. As I indicated in my letter of the 4th June last, I hoped ere now to have laid before you complete information regarding the low-level dam to be constructed.

The

3. Unfortunately, however, the operations conducted since the date of my letter have resulted so unfavourably as to render it advisable to conduct further operations and to investigate other sites besides the one originally selected. various sites are indicated on the general plan which I now forward you and are marked Nos. 1, 2 and 3 respectively.

4. The following is an account of the trial works undertaken and the results ascertained. All depths mentioned are below Ordnance Datum, which is practi- cally the level of the bed of the bay at Site No. 1.

5. Operations were at first confined to Site No. 1 which, upon inspection, appeared in every way favourable for a dam. On both sides of the bay, rock was abundantly evident, the depth of water was very shallow, the distance across the bay was a minimum, and a dam at this point would have a very large impounding capacity.

6. After several borings had been taken, which were reported to show rock at a depth of about 44 feet, the sinking of Well No. 1 was begun. This well got so seriously out of plumb that it had to be abandoned after being sunk to a depth of 35 feet.

7. Well No. 2 was next undertaken and, after great difficulty, principally on account of skin friction, was successfully sunk to a depth of 46 feet, at which depth rock was reported. The trouble arising from skin friction was overcome by in- creasing the load on the well until it ultimately reached about 200 tons. Pumping operations within such a confined space and from such depths were also a source of great trouble. The material passed through consisted of clay and silt intermingled with shells and with occasional layers of sand and shells. Just before reaching the rock, a layer of beach boulders about 2 feet in depth, was passed through. The boulders were of varying size, down to shingle, consisted of very hard stone and were densely packed together. The rate at which water entered the well from the layer of boulders precluded the possibility of pumping it dry in order to admit of an actual inspection of the bottom.

8. Work was then begun upon Well No. 3 and, after sinking it to a depth of 42 feet through clay silt and sand, the stratum of beach boulders above described was again encountered. Sinking operations were continued, and, after passing through about 10 feet of boulders, sand was again met with and it was not until the well had reached a depth of 58 feet that rock was finally found, Here also it was found impossible to dry the bottom by pumping and the extent and nature of the rock were ascertained by feeling it carefully all over. Pieces were also broken off and compared with the rock above water level at the sides of the bay, and, as the result of this comparison, there seems no room to doubt that rock in situ has been reached. Over a small portion of the area (6 feet diameter) covered by the well, the rock dipped away to greater depth. The sinking of this well has just been completed.

9. Whilst the operations above-described were in progress, a large number of prickings with Norton tubes (171 in all) were made all over the site of the dam, the result of which was to confirm the information obtained from No. 2 Well and the borings formerly taken. They showed the presence of a hard layer presumed to be rock, at an average depth of about 40 feet over the entire area.