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.

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

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.
The red circles show where the four sonic logging tubes are installed in the reinforcement cage.
The red circles show where the four sonic logging tubes are installed in the reinforcement cage.
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:

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


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