All caissons are to be founded on completely / highly decomposed granite
soil stratum with a N-value not less than 200 for caissons which designed
for a soil bearing capacity not exceeding 1000kPa, or with a N-value of
not less than 80 for caissons which are designed for a soil bearing capacity
not exceeding 400kPa.
Excavate each caisson shafts in stages not exceeding 900mm deep. Cast
the required concrete lining as each stage of excavation is completed and
allow the concrete therein to achieve adequate strength before the next
stage is commenced.
Stabilised any unstable layers of subsoi; encountered by grouting or
similar. No further excavation will be permitted until the stabilisation
work is completed.
Construction of caissons shall be sequenced such that at any one time,
excavation below the level of ground water table may only be carried out
in only one pairs of caissons. The level of ground water table shall be
verified on site by the Registered Structural Engineer or his representative.
Excavation of a subsequent pair of caissons shall onlyproceed after concreting
of the preceding pair of caissons.
All necessary precautions and measures shall be taken to ensure that
no undue water draw down occurs during the caisson constructions. Should
there be any undue water draw down, all caissons excavation and dewatering
should be suspended and the engineer informed immediately.
After reaching a layer of soil at about 3m above the designed bottom
level of caisson, a standard penetration test shall be carried out to obtain
the N-value of the subsoil at the designed bottom of the caisson. Standard
penetration test shall be carried out to all caissons prior to bell-out
excavation, to a depth not less than 3 times the bell-out diameter at 1m
interval. If the N-value obtained is less than designed value, the contractor
shall continue the excavation work and the standard penetration test in
the same manner as mentioned above in order to locate a soil layer of the
designed N-value. The standard penetration test shall be witnessed by the
Registered Structural Engineer or his represnetative and the actual bottom
level of caisson shall be decided by the registered structural engineer.
When the required soil bearing stratum reached and 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.
All joints in caissonlinings 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 increasingthe thickness of
the lining to suit ground conditions encountered, but th eshaft diameter
shall not be reduced.
When bell-out is required to enlarge the caisson base, the contractor
shall be responsible to submit the method of forming th ebell-out and provide
adequate support to the bell to the registered structural engineer for
approval.
The concrete lining shall be constructed with grade 30D concrete. The
concrete in caisson shafts be grade 30D.
Where annular sleeve is specified around the upper part of caissons,
a suitable compressible material with the specified thickness and capable
to sustain the pressure of wet concrete druing construction shall be used.
Samples and information on the characteristics of this material shall be
submitted to the registered structural engineer of approval.
