4. We are the Main Contractor for a MRT Project, in constructing the Station Box Structure, we intend to use the “Open-cut Method”. The excavation size is about 28m (depth) x 50m (width) x 70m (length). We are concerned about such a huge excavation and we want to take all precautions to ensure that our excavation and strutting works do not end up like the “Nicoll Highway Collapse” case.
Our Answer to you:
From your question, you have indicated that you intend to use the “Open-cut Method” for excavation, thus I presume that you have planned to use the “Bottom-up” Method in constructing the MRT Station box.
Your excavation of 28m is indeed deep, retaining wall systems such as sheet-piled wall would not be applicable. You would need something such as Diaphragm Wall or Bored Piled Wall for your Retaining Wall System or any other suitable systems.
This would normally be the construction sequence:
A. Install of all the relevant ground monitoring devices and ensure that they are in working condition,
B. Construct the Retaining Wall System of your choice,
C. Install the 1st layer of horizontal Strutting System (with preloading) and continue with the excavation downwards,
D. Install the 2nd layer of horizontal Strutting System (with preloading) and continue with the excavation downwards,
E. Repeat step (D) concerning the lower levels of Strutting System and excavation works until you reach the base of the excavation,
F. You proceed with the construction of the Foundation System,
G. You then construct the base of the Station box,
H. After which, you proceed with the construction of the higher levels of Station box until you complete the roof level.
The aforesaid is only a very brief stated sequence for me to use as an introduction, of what I intend to present later. There are a lot of other technical considerations you need to comply with in order to make the construction a success. I have deliberately omitted them in order not to confuse the reader.
In order to performed the Excavation Works, I would now discuss only the basic considerations:
A. Earth pressure on the retaining wall,
B. Base stability,
C. Bending moments and shear forces in the wall,
D. Strut forces,
E. Displacements around and below excavations,
F. Ground water considerations, and
G. Overall Stability.
Earth Pressure on the Retaining Wall
The earth pressures exerting on the Wall depend on various soil parameters that must be obtained from proper soil testing. The construction loadings exerting on the ground around the Wall also contribute to increase the pressures on the Wall.
The stability of the base of the excavation can be achieved with sufficient Wall embedment.
In instances where you need to improve the soil quality at the base, to facilitate your construction, you may need to use suitable Ground Improvement Methods which I am not going to elaborate here.
Bending Moments and Shear Forces in the Wall
The soil pressures exerting on the wall and all possible construction loads would all contribute to the bending moments and shear forces in the wall. These are normally taken care by the Design Engineers.
All the strutting systems in the excavation designed and constructed are to resist the earth pressures exerting on the wall. They mainly serve the following purposes:
· Resist all the forces exerting on the wall and
· Limit the deflection (movement) of the wall.
One must take special precautions when installing the strutting system. You need to follow the exact procedure stated, as determined by the Designer. In this kind of strutting work design, he should have taken care of how the excavation is going to proceed with close coordination with the installation of strutting works.
For this kind of construction, the struts designed and constructed normally should be braced at short intervals. A strut that has a long un-braced length would have a low resistance to bucking, this means that it would just buckle at a low compressive force exerted on it, and this should always be avoided.
Displacements around and below excavations
I have heard many Contractors complained about this issue. They have told me, ‘The Authorities are making life difficult for us by stated such stringent requirements on the deflection criteria on our retaining wall system in excavation works. To what I see, there is no danger even the deflection limits set by the Authorities are exceeded.”
Although I have quoted the above-mentioned, please do not worry. As I did not mention any name, so you don’t need to worry about being “blacklisted” & the Authorities’ investigation on you. After all, I am always the Employer’s and Contractor’s Consultant, not the Authorities’ Consultant... We are somewhat on the same side.
Although the Authorities have stated these deflection requirements, you need to know that they are really not too concern about the “Deflection” of the Retaining Wall, but the possible damage that the excavation work would cause to the neighboring structures and possible nearby MRT structure & tunnel systems. The deflection limits set for your excavation works are helping you to ensure that you do not cause undue settlements and movements to structures around your excavation works. I am not siding the Authorities, but these deflection limits set by the Authorities are the results derived after careful analyses. After all, the Authorities obtain no benefit in antagonizing you…
Being a Professional Engineer practicing in this industry since April 1993 and have also studied law, I could only advise you that, should any excavation work cause settlement / movement on any nearby structures, evidenced by cracking on the structures, be it structural crack or non-structural crack, one would likely to be sued for “Negligence causing Property Damage” under the Law of Tort. And it would be impossible for one to escape from such liability.
So far, I have not seen any settlement problem that can be solved by any simple jacking work or without incurring a huge cost in rectification. Usually, when a structure has been affected by settlement, it would somehow be “tilted”. Anyone who is experienced in this industry would know that, even this “tilt’ problem can be resolved, the cost is definitely going to be exorbitant. Furthermore, the law requires that one who have caused “the damage”, not only to reinstate the settled structure to “structurally sound level”, but to be aesthetically acceptable (i.e. to have the “same look” as before the start of construction- the “Restitution” concept). And this is going to be real difficult, the rectification costs would definitely shoot up.
Ground water considerations
Ground water is not only relevant in determining the pressures acting on the retaining wall, they must be monitored closely during the excavation and construction stages, as any lowering of ground water table may cause settlement of neighboring structures sitting on footing, and it could also affect services nearby.
Based on the ground conditions, nearby structures configurations, ground slope profiles etc. you would need to input to your Civil & Structural Design Engineers so that they could performed the Overall Stability check on the design before you start work. Normally these analyses are performed using reliable and accredited computer software.
It is not uncommon to perform these analyses over & over again during the excavation stage, as the excavation work continues downwards and the actual soil profile are made visible, thus the analyses can become more accurate. With these thorough analyses and possible amendment of strutting system based on updated analyses results, any possible collapse could then be avoided.
With the above mentioned, you would realize that the excavation & construction work are definitely not easy tasks. It would require the cooperation of all parties in the team to make it a success. In my many years in the construction industry, I have not encountered any project that I could label as “easy”, because there are numerous happenings that could occur at a construction site in any single day, and as good Construction Professional, one would need to make quick decisions to solve these problems, thus one would require not only good engineering judgements but also good leadership qualities.
In my own opinion, anyone who wants to survive in this industry would need to be very alert and able to cooperate and work in a team. The work is hard and sometimes it would even drain off all your energy. But believe me, all is worth it, because what you are leaving behind later may not be enormous wealth, but structures that you have constructed and each of them will be a legacy for a long time.
First drafted on 10 September 2008.