Inspection of Construction – The inspection is done for:
- Accuracy of alignment and dimension
- Bearing capacity of the soil at the bottom
- Removal of loose soil from bottom
- Risk of loss of ground
- Process of Concrete placement
The caisson should not be allowed to shift by about 50 to 75 mm from its exact location. The tilt shall not exceed 1 % of the height or depth of caisson. These tolerances should be accounted for in the design.
The strength of the soil at the bottom shall be inspected by spot-checking. In uniform soil one out of 5 to 10 caissons should be checked. In erratic soil every caissons should be checked. The thumb test should be carried out to ascertain the strength of the clay soil. Alternatively a pocket penetrometer may be used.
The cleanness of the caisson bottom may be inspected by the aid of mirror to reflect the sunlight. But the most reliable procedure is to examine the condition by descending down to the bottom of the hole.
In caisson drilling there will be risk of ground loss due to excessive dewatering, which could threat in the stability of the adjacent structures. When danger of ground loss is confronted, the hole is to be excavated under water. Alternatively, drilling fluid is used to prevent cave-in and bottom blow out.