PANEL ON PLANNING, LANDS AND WORKS
in respect of Piling Works in Construction Sites
This paper sets out the monitoring mechanism in respect of piling works in construction sites.
2. Two types of piles are in common use in Hong Kong, namely, percussion piles and bored piles. These piles are generally constructed with concrete and steel. When used as foundation to a building, a system of piles properly installed is an essential ingredient of the structural stability of the building. It is therefore very important that control is exercised over the design and construction of piled foundations.
3. The Buildings Ordinance provides for a control mechanism to ensure that this is done. Under the mechanism, Authorized Persons (APs), Registered Structural Engineers (RSEs), Registered General Building Contractors (RGBCs) and Registered Specialist Contractors (RSCs) appointed by the developer of a building site are assigned with specific responsibilities for the design and construction of the piles of buildings in the private sector.
4. The Director of Buildings, in his capacity as the Building Authority (BA), assumes a monitoring role to see to it that APs, RSEs, RGBCs and RSCs discharge their statutory responsibilities and the piles comply with safety standards.
Design of Piling Works
5. Given the pace, type and intensity of building developments in Hong Kong, we need high standards of safety design and professionalism for the piled foundations to our buildings. Safety standards are laid down in the Buildings Ordinance and the Building (Construction) Regulations, which are supplemented by Practice Notes for Authorized Persons and Registered Structural Engineers issued by the Buildings Department (BD). RSEs design piling works and submit plans for the works to BA for approval. Both RSEs and BD structural engineers (who check the piling plans and works) are highly experienced professional engineers. We have a good track record and our standards are among the highest in the world.
Execution of Piling Works
6. In addition to the requirement for piling plans to be approved by BA, the statutory procedures require a consent from BA before piling works may commence, thereby providing one more opportunity to ensure safety standards and requirements are fulfilled. In issuing his consent, BA may impose conditions prescribing requirements and procedures that normally relate to precautionary and protective measures, sequence of works, submission of reports and certificates and related documents and supervision of works by RSEs and other technically competent persons.
7. After obtaining BA's consent, a developer appoints an RSC specialised in foundation works to carry out the works. While the piling works are in progress, the RSC is under a statutory duty to provide continuous supervision and the RSE to provide periodic supervision and to make inspections to ensure that the works are being carried out in accordance with the approved plans and the relevant provisions of the Buildings Ordinance and regulations.
8. For percussion piles, BD structural engineers are always present in a driving test of the first pile. The test is conducted so that BD structural engineers may witness the process which serves to demonstrate the required bearing capacity of the pile. BD structural engineers may also have to attend further driving tests if circumstances so require.
9. For all types of piles including percussion and bored piles which are constructed in concrete and steel, BD structural engineers check and take appropriate action on the following -
- concrete cube and steel reinforcement test reports from laboratories recognised under the Hong Kong Laboratory Accreditation Scheme;
- steel reinforcement mill certificates from manufacturers;
- pre-drill records;
- amendment plans and any further site investigation reports; and
- monitoring records and reports on adjacent buildings, structures and land.
10. During construction, particular attention is paid to site investigation material testing records and drilling or driving records. BD randomly checks these records to ascertain if they are compatible with earlier findings and assumptions. These compatibility checks are a useful tool to determine if doubts should be raised about the integrity of any pile. In parallel, BD monitors the activities of all construction sites, including those where piling works are in progress. Staff of the Site Monitoring Section of BD carry out inspections of piling sites to ensure site supervision is provided in accordance with the supervision plan prepared by the AP, RSE and RSC and lodged with BD as a requirement for the grant of consent.
Proof Tests on Completion of Piles
11. When piling works have been completed, the AP, RSE and RSC certify, as appropriate, to BA that the piling works for which they are responsible have been constructed in accordance with the approved plans and comply with the Buildings Ordinance. At the same time, the RSE submits to BD record plans, other as-built records and any additional test reports and certificates on the building materials used in the piles and at the founding levels of the piles. These documents are randomly examined by BD staff and any deficiencies or anomalies identified will be pursued.
12. Upon receipt of all plans and documents following completion of piling works, BD will consider initiating action for proof tests. Piles will be selected for such tests. The sampling rate for a proof test is about 1% of the total number of piles constructed. Two types of proof tests are normally used: loading tests for percussion piles and small diameter bored piles, and drilling tests for large diameter bored piles. In the 15 building sites under BD's recent investigation, drilling tests were conducted on the large diameter bored piles.
13. For large diameter bored piles, because of the impracticability of providing the very large loading required for performing a loading test, proof tests by core drilling are prescribed instead. The piles selected for core drilling are bored through the shaft to a certain depth below the founding stratum as a validity check on the design assumption. All concrete and rock cores so retrieved are inspected by engineers from BD to verify the founding levels of the piles and the acceptability of the founding materials which provide support to the piles and the building above.
Further Tests in case of Doubt
14. If any proof tests or any other evidence indicate doubtful or unsatisfactory results, BA may require more or other tests to be carried out in order to ascertain that the piles constructed are capable of performing their intended functions. No superstructure will be allowed to be built on the piled foundation until BA is satisfied as to the integrity of all the piles designed and constructed for the superstructure.
15. In summary, pile design, construction records, reports and documents are submitted by the AP, RSE and RSC and are vetted by BD to ensure works are carried out in a quality-assured manner. Professional staff of BD check the design and monitor the standards of building construction through an auditing system. On completion of piling works, they require and witness proof tests to ensure safety standards are secured. In the case of driven piles, BD requires additional driving tests to be carried out and witnessed by BD staff.
Planning, Environment and Lands Bureau