2. Foundation
2.5 Sequence of Work II
The caissons should be done in proper sequence of steps so as to ensure quality and asfety of workers and adjacent structures.
1. After site formation and the underground drainage has been laid, casson excavation work can be started. Excavate a pilot hole with depth not exceeding 900mm and diameter around 300mm larger than the caisson diameter. CA2 and CA 17 are started first.
2. Inside the pilot holea circular steel mould, which has an inner diameter equals to caisson diameter, is set up in the excat position of the caisson to be constructed.
3. Pour concrete of grade 30D around the gap between the mould and the vertical soil surface.
4. After the concrete has gained the required strength, the circular steel mould is removed. A concrete caisson lining is thus form to prevent collapse of soil.
5. Before further excavation is continued, a supporting frame about 2.0m high is erected on the ground surface above and across the caisson opening. This temporary frame supportsan electrical chain block which is used to transport excavated material from the caisson bottom to the ground surface.
6. A safety barrier is then set up on top of the caisson opening.-
7. Continue to excavate not more than 900mm depth of soil each time and subsequent concrete lining is again formed with the steel mould. The joints in the caisson lining shall be sealed against the ingress of water. All voids between the lining and the soil shall be filled with grade 30D concrete.
The contractor shall be responsible for increasing the thickness of the lining to suit ground conditions encountered, but the shaft diameter shall not be reduced.
It is important to note that the contractor had to stabilize any unstable layers of subsoil encountered by grouting or similar. No further excavation will be permitted until the stabilization work is completed.
Also, the contractor had to carry out the following dewatering / ground treatment measures for the excavation / construction of caissons and regular monitoring of underground water conditions and settlement of surrounding features as recommended by the geotechnical consultant:
8. Continue excavation and concreting of lining until after reaching a layer of soil at about the designed bottom level of caisson, a standard penetration test ( SPT ) shall be carried out to obtain the N-value of the subsoil at the designed bottom level of caisson. If the N-value obtained is less than the designed value, the contractor shall continue the excavation work and the SPT test in the same manner as mentioned above in order to locate a soil layer of the designed N-value. The SPT test shall be witnessed by the Registered Structural Engineer ( RSE ) or his representative and the actual bottomlevel of caisson shall be decided by the RSE. The SPT test shall be carried out to all caissons prior to bell out excavation, to a depth not less than three times the bell-out diameter, at one metre interval.
When a required soil bearing stratum is reache dand after removal of any surplus spoil, a surface protection in the form of a layer of grade 30D concrete with a minimum thickness of 300mm shall be placed immediately.
9. If underground water is encountered an time during the excavation, dewatering has to be done by means of a water pump installed at the bottom of the caisson.
10. After the caisson has reached the depth required, bell-out excavation begins. The bell-outs nbormally have diameters of more than twice of the caisson shaft diameter. The contractor shall be responsible to submit the method of forming the bell-out and provide adequate support to the bell to the Registered Structural Engineer for approval.
11. Where annular sleeve is specified around the upper part of caissons, a suitable compressible material, polystyrene, with the specified thickness and capable to sustain the pressure of wet concrete during construction shall be used.
12. Reinforcement steel bars are cut and bent after adequate test. All reinforcement shall not be rolled deformed bars with guaranteed yield stress of not less than 425N/mm2 except those mild steel with a guaranteed yield stress of not less than 205 N/mm2. All reinforcements, both mild steel and high yield steel shall comply with BS4449.Steel bars Y32 and Y40 are used.
13. The caisson shafts are then concreted with the erinforcement in place. The caisson shaft shall be 'dry' prior to concreting and concrete shall be poured in one continous operation through a hopper placed centrally over each shaft and with trunking extending to within 1.5m of the concrete surface. The size of the trunking shall be designed to suit the concrete slump in order to permit the concrete to flow and fill the caisson properly compacted by vibration pokers. The concrete shall have a minimum cement content of 300kg/m3 and a slump of not exceeding 150mm.
If the concrete of caisson shaft is placed in 'wet', tremie pipe method shall be used. The concrete shall be grade 40D with a minimum cement content shall be grade 40D with a minimum cement content 400kg/m3 and minimum slump of 150mm.
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