Collapse of Sidewalls
If there is an area of the site where sidewall collapse occurs then the pile holes need to be temporary cased through the collapse zone. To be able to do this and still end up with the correct size pile, drilling down to the top of the collapse zone has to be undertaken using a flight about 100 mm larger in diameter than the pile size. A steel casing which has a slightly smaller diameter is then lowered into the hole. A vibratory hammer is clamped to the top of the casing and it is driven down through the collapse zone.
Using the size flight for the pile diameter the spoil inside the casing is drilled out and the remainder of the pile hole drilled. The pile is cleaned out and concreted in the manner described above, the concrete filling the temporary casing to a level above the cut-off level. The casing is finally extracted using the vibratory hammer and the concrete flows out of the casing so as to fill the larger diameter.
As a number of additional plant items are required for handling the temporary casings their use increases the cost of the operation quite significantly and to an extent that may render the solution uneconomical when compared to other possible pile types. If only a small section of the site has the problem then the overall economics will still favour the Auger pile. This may not be the case if the whole site has a collapse problem.
Water Ingress
The occurrence of ingress of ground water into a pile excavation is fairly common particularly in the soft rock at the toe of the pile. This water seeps into the pile excavation and collects in the bottom of the hole and a decision has to be made as to whether the pile can be concreted successfully in these conditions. The answer depends on the rate of inflow. If the flow is limited then a pump lowered to the bottom of the hole during the cleaning
During the short time it takes to remove the pump and discharge the first concrete a quantity of water will enter the hole. This will not affect the performance of the pile provided that the concrete that is discharged into the pile hole is concentrated in the centre and the concrete flow rate is high. This will limit the amount of water that is mixed with the concrete at the base of the pile. Some water will of course get mixed with the concrete during the concreting operation but this can be compensated for by using additional cement in the mix.
If the rate of inflow is too fast then it is better to cast the pile under water. For this to be achieved it will be necessary to temporary case the hole for the full depth prior to concreting to prevent material that may collapse off the sidewalls due to the presence of the water from collecting on the bottom of the hole. The concreting operation is then carried out using a tremie pipe which is standard procedure in piling and the casing is extracted.
Another alternative in this situation would be to drill the piles under a head of bentonite as covered under SECTION 7.7 UNDERSLURRY PILES. This would eliminate the problem but would increase the cost.
Boulders
The problems presented by boulders depend on factors such as the size, hardness and concentration of the boulders, the type of matrix and whether there is any water present in the boulder layer.
If the size of the boulders is less than one third the pile diameter, the concentration is plus/minus two or three boulders per metre of depth and the matrix is soft or loose the auger rig will drill through the layer albeit with a slower penetration rate. If the concentration increases to tightly packed the penetration rate will decrease even further. If the matrix also changes to very stiff or very dense then penetration will be very slow if at all possible.
It will not be possible to drill out boulders larger than one half the pile diameter. In these cases personnel are lowered down the pile excavation and a sling is attached to the boulder by drilling a hole through it. A crane is then used to lift the boulder out of the excavation. If a large boulder is encountered then it will be necessary to split the boulder into smaller pieces before they can be removed. Piling in these conditions can be very slow and thus expensive but the problem can be solved.
Auger piling in boulders below the water table can still be feasible provided the ingress of water can be handled by pumps to allow access to the boulder by personnel. If the ingress is too fast for this to happen then there is no alternative but to change to the Oscillator Piling System which is capable of handling this type of problem. For details of the Oscillator system see SECTION 7.10.