Report on Investigation of Bored Piles at Blocks 1, 2, 3, Phase I, Nga Ying Chau, TYTL 129, Tsing Yi

Report on Investigation of Bored Piles at
Blocks 1, 2, 3, Phase I,
Nga Ying Chau, TYTL 129, Tsing Yi

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Purpose

The investigation of the bored piles at the captioned building project, generally known as "Villa Esplanada", was initiated as a result of the findings of Buildings Department (BD) from its investigations into similar foundation works at the northern site of the Lantau and Airport Railway Hong Kong Station development. The Villa Esplanada investigation commenced in early June 1998 when a complaint about possible defects in the foundation works was also received. The purpose of our investigation is two-fold :

to ascertain the overall structural safety of Blocks 1, 2 and 3 of Villa Esplanada; and
to find out if there were any deviations from statutory provisions and requirements.

Desk Study

2. The investigation began with a desk study by experienced engineers of the Department. It involved principally a compatibility check between site investigation reports, approved foundation plans, as-built records and previous pile proofing test records, having regard to three major factors of overall stability : founding levels of piles, rock characteristics and design loadings. From this compatibility check we saw a possibility of doubt and we decided that to clear up such doubt, further investigations of a sample of the bored piles were necessary.

3. We acknowledged that core-drilling of the piles was the most direct and reliable method of detailed investigation. Piles were then selected for core-drilling based on the likelihood and consequences of deviation from approved plans and statutory requirements. In particular, the lengths and the reserved capacities of the piles were the most important factors. On this basis we decided that five bored piles should be core-drilled for each block and the following bored piles were therefore selected for core-drilling :

Block 1 : BP1-1, BP1-4, BP1C5-2, BP1C6-2 & BP1-13
Block 2 : BP2C6-1, BP2C9-2, BP2-10, BP2C12-1 & BP2C14-3
Block 3 : BP3C4-2, BP3C8-1, BP3-12a, BP3C13-2 & BP3C14-1

4. The suggestion of core-drilling as indicated above was made to the Registered Structural Engineer (RSE) of this project on the morning of 4 July 1998. The core-drilling was required to be carried down to such a level as to achieve a minimum penetration into bedrock of not less than 1.5 times the diameter of the pile to be core-drilled.

The Core-drilling Process

5. "Bachy Soletanche Group", a contractor specialized in core-drilling, was appointed by the developers and RSE to undertake core-drilling, which commenced at about 11:00 am on 4 July 1998.

6. In the process of core-drilling at Block 2, the RSE reported accessibility difficulties and requested that the following four bored piles be not core-drilled for reasons indicated :

BP2C9-2 : Pile under wall
BP2-10 : Pile under water pump room
BP2C12-1 : Pile under sprinkler water tank
BP2C14-3 : Pile under water tank

We acceded to the RSE's request and as an alternative, suggested that drill-holes be sunk in two other acceptable places outside but adjacent to Block 2 to establish the factual rock stratum. The exact locations of the two drill-holes were agreed on site on 6 July 1998 between the RSE and BD Structural Engineers and work commenced the following day. No practical difficulties were encountered in the core-drilling at Blocks 1 and 3 where five bore piles were successfully core-drilled for each block.

7. The core and hole-drilling process was carried out round-the-clock for four days under the full-time supervision of the RSE and continuously witnessed by BD Structural Engineers. In this exercise, BD mobilised 14 Structural Engineers and 5 Technical Officers (Structural).

Assessment Report of the Registered Structural Engineer

8. On 6 July 1998, the RSE reported some findings to BD. We accepted that as verified by the core-drilling results on 5 bored piles, Block 1 complied with the standards of safety laid down in the Buildings Ordinance and the subsidiary legislation.

9. As to Block 2, no decision was taken pending the submission of results of the hole-drilling and an engineering assessment from the RSE. On Block 3, we noted the RSE's reference to a 230 mm layer of "highly decomposed moderately weak" rock at the founding level of bored pile BP3C13-2. We considered this to be a weak spot which required to be examined further. On 7 July 1998, we suggested demonstration of safety by the RSE.

10. The RSE submitted to BD a full report on 8 July 1998. The RSE concluded that his investigation results were consistent with the piling records submitted and the foundations at Blocks 1, 2 and 3 were in his opinion structurally safe.

Evaluation by Buildings Department

11. From the findings and observations of BD's Structural Engineers, we concluded that the rock characteristics at the founding level of the bored piles at Block 1 were satisfactory. We were able to arrive at this conclusion on 6 July 1998 (para 8 above).

12. As regards Block 2, we had taken note of the RSE's reference to a 430 mm layer of marginally acceptable rock at the founding level of bored pile BP2C6-1. However, the results of hole-drilling in two places made it possible for the factual rock stratum to be established and earlier geological records compared, thereby clearing up the earlier doubt. Furthermore, the RSE indicated in his engineering assessment a figure of 6 mm as the worst scenario of settlement which he regarded as negligible. We accepted the RSE's assessment, having regard to all other relevant engineering factors.

13. On Block 3, the rock characteristics of the founding levels of the bored piles as revealed in the core-drilling were satisfactory except BP3C13-2 which is founded on a seemingly weak rock. We proceeded therefore to consider two scenarios which could possibly arise from such weakness :

(a) the weakness being localized -- In this scenario, we decided that such localized weakness should fall within the normally accepted engineering variation; and

(b) the weakness being more extensive -- In this scenario, we had to consider the design of the foundation system, the loadings, the reserved capacities of the bored piles and the core-drilling results as observed by our own staff. The RSE's carried out an analysis on the assumption of the entire bored pile BP3C13-2 being defunct. Based on this analysis, the RSE took the view that the group of piles, without the defunct pile, could safely support the load. We examined the RSE's calculations and accepted his view.

Conclusion as to Safety

14. Given the results of core and hole-drilling, the detailed engineering assessment of the RSE and our own evaluation, we concluded that Block 1, 2, and 3 of Villa Esplanada were safe and confirmation to this effect was made on 8 July 1998 in a public announcement and a letter to the developers. We also took the opportunity to clarify that the investigations initiated in this case were intended to clear the doubts raised in recent allegations.

Continuing Investigations

15. We will address the question as to whether any person carrying out building works has contravened the Buildings Ordinance. Investigations for this purpose are continuing.

Buildings Department

15 July 1998