Figure 7.7.1
A A short length of starter casing is drilled into the ground on the pile position. The starter casing is filled with bentonite slurry and drilling commenced.
B The drilling continues until the founding level is reached, the level of the slurry in the excavation being maintained at all times.
C An airlift pump is lowered into the excavation and is used to clean the base of the pile and to pump the contaminated slurry to storage tanks for treatment.
D The reinforcing cage is placed in position and the pile shaft concreted using a tremie pipe. Displaced bentonite slurry is pumped to storage tanks.
E The concreting of the pile shaft is carried out in one continuous operation. F The completed pile after the starter casing has been extracted.
Barettes
A Barette is constructed in a very similar manner to that described above except that the excavation is carried out using a grab and not an auger rig. The grab is normally of the cable or hydraulic type and excavates a rectangular hole. The width of the hole can be varied by changing the jaws on the grab. The depth of excavation is virtually unlimited.
To achieve penetration into material of rock consistency, chisels can be used to break up the rock for removal by the grab. This is a costly exercise, however, and normally the piles are founded on the rock and not socketed into rock.
Excavations using a grab can be carried out in multiple passes in the same manner as that for a diaphragm wall. In this way the shape of the excavation can be extended and altered. These larger units are referred to as load bearing panels and can be arranged to form part of a diaphragm wall as well as performing a load bearing function.
The grab for excavating Barettes is shown in Plates 7.7.6.
VARIATIONS IN INSTALLATION TECHNIQUE
Integrity Testing
As the whole pile installation is carried out under bentonite slurry some form of integrity testing is often considered desirable. Rotary core drilling can be used to check the contact between the concrete and the rock. The integrity of the pile shaft itself can be checked with either nuclear or sonic methods as described in SECTION 9.0 PILE LOAD AND INTEGRITY TESTING. To enable these tests to be carried out three or four small diameter steel tubes are cast into the pile. These tubes are normally fixed to the reinforcing cage and lowered with the cage.
Composite Steel Pile Shafts
In Europe a novel building technique is sometimes used to accelerate the construction of multi-storey buildings with basements as well as to reduce movements associated with basement excavation. In essence this technique calls for the construction of the basement of the building from the ground floor downwards while at the same time constructing the building itself from the ground floor upwards. The saving in time with this form of construction is considerable.
These steel columns are cast into the concrete pile shaft with suitable shear connectors. Special adjustable guiding devices have been developed for the accurate positioning of the steel column sections in the excavation. This is necessary so as to ensure that the prefabricated steelwork fits at each floor level.
The system has not been used in South Africa to date but the expertise is available within the Franki Group should there be a suitable project where the savings in construction time are meaningful to the client.
POTENTIAL PROBLEM AREAS
Collapse of the Pile Shaft Excavation
There is a risk of collapse of the sidewalls of the excavation but this can be reduced to a minimum by observing correct procedures and ensuring that the bentonite slurry itself is within the specification and that the differential head is not less than 1.5 metres at any stage. A minor localised collapse can occur occasionally but this should not be detrimental to the pile. A major collapse of a pile shaft excavation is serious and the quick fix is to backfill the excavation as quickly as possible. The cause of the collapse should be determined before any further piling work is carried out. Additional care should be exercised with long panel excavations, excavations in extremely soft clays and silts and excavations carried out in close proximity to existing loaded foundations..
Delays Before Concreting
Ideally a pile should be concreted the same day as its excavation is completed. If there are delays before the concrete is placed the bentonite cake tends to increase in thickness and this has been found to be detrimental to the pile's friction capacity. If a delay is unavoidable then the pile excavation should be reamed out again using the excavation tools prior to concreting the pile.
Tremie Concreting
This is an operation which needs a large amount of experience on the part of the contractor for it to run smoothly and produce a sound pile shaft. Unfortunately even the best trained crews can run into problems especially if the concrete itself is not one hundred percent. The most likely problem is a blockage in the tremie. This requires the tremie to be removed and cleaned out. If only a small amount of concrete has been discharged into the pile the best plan of action is to remove the steel reinforcing cage and permanent liner and drill out the wet concrete. Once cleaned out the steel cage can be installed and the pile concreted.
If a large amount of concrete has already been placed when the blockage occurs the tremie should be removed and cleaned out. A water tight end cap should be placed on the toe of the tremie and latter lowered until it penetrates the wet concrete by at least two metres.
In this operation, as with all tremie operations, it is vital that the joints in the tremie pipe are absolutely watertight. This can be checked by shining reflected sunlight down the tremie pipe using a mirror. The tremie is then filled with concrete and raised slightly at which time the end cap comes off due to the weight of the concrete. Thereafter the concreting operation can proceed as normal.
Re-starting a tremie operation is a tricky process. For this reason the end result should be checked using one of the integrity testing methods. The alternative is to abandon the pile and install a replacement pile.
Concrete bleeding
For the concrete to flow through the tremie it has to have a slump of about 200 mm. To obtain this slump without having excess water in the mix is extremely difficult if not impossible with the available aggregates and admixtures. On deep piles the excess water in the concrete bleeds out and makes its way up through the concrete to the surface. The channel or run it forms in so doing is normally found near the centre of the pile where the tremie tends to form a zone of excess mortar. The concrete in this zone often shows signs of excess water and may have had some of the cement washed out of it.
This is a problem which cannot be completely eliminated with deep piles cast using a tremie but certain measures can be taken to ensure it is reduced to a minimum. By adopting suitable techniques the percentage area of the pile shaft affected in this manner can be kept low so that the overall load bearing capacity of the pile is not affected.
Before commencing the contract the gradings of the available aggregates should be determined and trial mixes designed and tested to arrive at the optimum mix design. During the contract there should be adequate site control to ensure the mix is stringently adhered to and in particular that no additional water is added to the mixer trucks while they are on site.