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Piling Works

Yuen Chau Kok Development Project

The Yuen Chau Kok site had been formed in the early 1980s by reclaiming an area of foreshore in Sha Tin valley in the eastern New Territories. The site development blueprint called for the construction of two 41-storey buildings, three 33-storey buildings and one 4-storey facilities complex with a car park.

The Yuen Chau Kok site

The Yuen Chau Kok development project was undertaken by the Housing Department (HD), the executive arm of the Housing Authority (HA).

On 17 October 1997, HD invited 27 contractors from their approved contractor list to submit tenders for the necessary piling works. This was a design-and-build project, meaning that the contractor winning the bid would be required to design and recommend piling methods in accordance with contract specifications as well as carrying out the piling works.

Zen Pacific Civil Contractors Ltd (“Zen Pacific”) was the successful bidder for the piling works of all five buildings. Its bid, the lowest of all that were submitted, amounted to $63M. This might have seemed low, considering that Zen Pacific had proposed using large diameter bored piles for all the buildings — known to be a relatively costly method. However, an analysis carried out by the quantity surveyor appointed by HD showed that the quantity of steel that Zen Pacific were proposing to use for reinforcement was somewhat less than might have been expected. It was also noted that more construction machinery was available in the market now that the construction of Hong Kong International Airport had reached its final phase, and that this might have contributed to the low bid price. So, low though it was, the bid did not arouse real concern.

Zen Pacific was a known experienced construction industry contractor. It had previously operated under another name and title, being renamed Zen Pacific Civil Contractors Ltd only in April 1997, and the present piling contract was the first that the company had ever signed with HA.

Subcontracting Arrangements

The other main player in what would turn out to be a major scam was Hui Hon Contractors Ltd (“Hui Hon”).

Hui Hon was not on HA’s list of approved contractors. Shortly after Zen Pacific received the tender invitation, Hui Hon approached Zen Pacific, indicating its interest in the project. In November 1997, Hui Hon entered into a pre-contract agreement with Zen Pacific to the following effect: Hui Hon would undertake the design and construction of the piling foundation and prepare the bid, while Zen Pacific would be responsible for the procurement of construction materials such as concrete, reinforcement cages, and permanent linings, charging 4 percent of the total contract price and 3 percent of the material costs as administrative fees.

In February 1998, Zen Pacific subcontracted the works to Hui Hon immediately on being awarded the contract by HD. As the entity that had signed the contract with HD, Zen Pacific should clearly have informed HD of the subcontracting arrangements which they had entered into. Yet they never once disclosed this arrangement, even when HD requested them to report the details of any sub-contractors during an initial contract meeting. At this same meeting, Zen Pacific even introduced members of the staff of Hui Hon as their own staff.

Throughout the entire construction period, Hui Hon’s role as a subcontractor was concealed. For example, when the site agent appointed by Hui Hon corresponded with HD, he always used the Zen Pacific letterhead.

Large Diameter Bored Piles

The Yuen Chau Kok site was on reclaimed land. Ground investigation works showed that the site was underlain in succession by fills, marine deposits and in situ decomposed materials. The bedrock was generally dominated by an igneous rock, quartz monzonite, with some minor intrusion of granite. To support the proposed buildings properly, all the piles would have to be founded on the quartz monzonite layer.

Major procedures for consulting large diameter bored piles

The major standard procedures for constructing large diameter bored piles are:

Pre-drilling:
Pre-drilling at the intended pile locations must reach right down to bedrock to verify the eventual depth of the piles. This ensures that it is the bedrock that will be supporting the foundations and thus the whole building.
Excavating the pile shaft and installing temporary casing:
While excavating each pile shaft, a temporary casing must be driven down to bedrock at the same time. Installing this casing stabilizes the walls of the pile shaft and prevents soil from these walls from collapsing into the shaft.
Enlarging the pile base:
After the pile founding level has been confirmed, the pile base is enlarged to form a bell-out, thus increasing the support provided by the pile.
Cleaning the pile shaft:
After constructing the bell-out, tremie pipes are placed inside the pile shaft. Tremie pipes are readily connectable pipe sections and must be placed so as to reach right to the bottom of the shaft. This positioning enables water to be delivered to clean the whole shaft effectively, in particular displacing debris or sediments that have collected at the bottom. The cleaning process continues until the water being pumped out runs clean and clear.
Installation of reinforcement cage:

After the pipe shaft is cleaned, a steel reinforcement cage is installed, which must be as long as the shaft is deep. At the same time four sonic logging tubes are fixed to the reinforcement cage. These tubes also must reach the founding level of the shaft, and are later used to verify the integrity of the whole pile. The pile shaft is then cleaned again before concreting begins.

Four sonic logging tubes


The red circles show where the four sonic logging tubes are installed in the reinforcement cage.
Concreting:
The pouring of the concrete has to be continuous and meticulously scheduled. This is to avoid interruptions in the pouring process which might allow the formation of cold joints — weak areas which can allow water to enter. During concreting, the temporary casing is progressively removed.
Sonic tests:
After concreting, all large diameter bored piles must undergo sonic tests. An emitting probe and a receiving probe are placed inside two separate sonic tubes, and are hoisted up and down the bored pile. The speed at which the sound waves travel indicates the quality of the concrete and the length of the pile.
Concrete core tests:

Next, concrete core tests are conducted on 5 percent of the piles. Testing staff drill into the full length of a pile to take out concrete core samples section by section, with each section no longer than 1.5 m. The core sections removed are assembled in order to measure the total length of the pile and then delivered to the laboratory for compression testing. The whole process is carefully documented, and the core samples ought to be securely stored according to standard procedures.

Core samples

Core samples ought to be securely stored to minimize opportunities for malpractice.
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