(24 September 2018, Hong Kong) In response to the recent public concerns on ground and structures settlements, The Hong Kong Institution of Engineers (HKIE) expresses its professional views on the issue as follows: -
Causes of Ground Settlement
With ever-growing population, the need for housing and accommodation as well as infrastructure facilities continues to grow which demands more developments and construction works in Hong Kong. These construction activities will inevitably induce soil or underground disturbance to the adjacent ground and buildings in particular for sites underlain by soft marine deposits and weak fill layers.
Pile and raft foundation construction are conventional systems to support building structures. Excavation and lateral support works (ELS works) is normally called for in the construction of pile caps, basement, underground drainage and tunnel works.
In response to various ground and site conditions, professional civil/structural/geotechnical engineers will develop a most suitable foundation and works to limit the induced settlement and possible disturbance to the adjacent ground/structures so as to ensure public safety.
Statutory Control
The Buildings Department (BD) has established a stringent controlling system under the Buildings Ordinance for foundation and basement works to ensure safety of the buildings/structures and the public.
A series of Practice Note and Code of Practice stipulate the settlement limits for piling and excavation works have been promulgated to the registered professionals (i.e. Authorised Person/Registered Structural Engineer/Registered Geotechnical Engineer under the Buildings Ordinance) and the building industry for reference and to follow.
Prior to commencement of site works, a comprehensive instrumentation and monitoring plan with a set of Alert, Alarm and Action (AAA) values shall be proposed by the project registered professionals to form a mechanism for controlling site activities with regular review of the site situations.
For general case with no sensitive adjacent buildings, structures and services, the ground settlement limit of 25mm will be adopted as generic figure for the Action level.
The project registered professionals will submit an investigation report regarding the excessive settlement issue together with technical justifications for higher ground settlement limits with respect to the insitu ground conditions, existing structural forms and conditions of adjacent buildings, existing utilities and services for subsequent construction works for BD’s approval.
For developments within the Railway Protection areas, the practice note issued by BD has stipulated certain settlement and movement limitations. However, these figures are only generic values and do not imply the railway related structures will be adversely affected.
In case the settlement is to be exceeded or has been exceeded, the registered professionals shall submit an investigation report and design checking for the existing MTR structures to support their request for relaxation of settlement limits. BD and MTRCL-Railway Protection Section will review the submission for approval if structural, railway and public safety can be justified.
Engineer’s Assessment of Ground Settlement Limit
The ground settlement limit of 25mm as described above is of general case. Different structures and services will have different tolerance level in accommodating movements.
For the case where there is nearby listed heritage or pre-war buildings, the tolerable ground settlement limited shall become more stringent to much less than 25mm, or even as low as 5mm. There is no single figure on tolerable ground and buildings settlement for all types of design and construction. The acceptable ground and structure settlement limits should be assessed on case by case basis according to different project requirements, site and existing structures’ conditions.
Similar assessment shall also be conducted by the registered professionals to protect the utilities around construction site from damages.
Conclusion
In conclusion, construction works will inevitably induce certain amount of soil or underground disturbance. While there is a set of general guidelines for the consideration of Engineers on the tolerable ground settlement in their design, Government Authorities have set up stringent controlling mechanism to ensure building and public safety would not be compromised by any developments and/or construction works.
Tolerable ground and building settlement limit is not any single figure which can be universally applicable to all design and construction, but shall vary case by case and should be reviewed and agreed by the Government Authorities (and/or the MTRCL – Rail Protection Section) upon to meet specific criteria of individual projects and works.
The HKIE has jointly organised with the Faculty of Engineering of The University of Hong Kong a public seminar titled “The Mystery of Ground Settlement” at 6:00 – 7:00 pm at the Lecture Theatre A, Chow Yei Ching Building, The University of Hong Kong on 26 September 2018.
We hope this seminar can further explain the meaning and essence of ground settlement to the general public.
Attachment:
Poster of “The Mystery of Ground Settlement” / 公開講座：「沉降之謎」海報
View video:
公開講座：「沉降之謎」
Video from Department of Civil Engineering, The University of Hong Kong
All information and views expressed by speakers and in their presentation materials do not reflect the official opinion and position of the HKIE. No responsibility is accepted by the HKIE for such information and views including their accuracy, correctness and veracity.