Piles formed by the auger method fall into the category of “replacement” piles. The term “replacement” refers to the removal of the soil during excavation of the pile shaft and the replacement of this soil with concrete. A spiral auger flight of specified diameter is screwed mechanically into suitable ground using a mobile drilling rig. The loosened soil is then brought to the surface on the blades of the flight and removed by “spinning off”. The process is then repeated. Excavation proceeds until either a suitable level is reached or refusal of the drill rig occurs.
After drilling a bulbous or enlarged base is formed by compacting semi-dry concrete by the repeated blows of 75 to 130kg drop hammers falling from a motorized tripod rig.
On completion of basing a reinforcing steel cage is lowered into the pile hole. In view of the small size of the pile and relatively shallow depth the reinforcing cage is maintained vertically by hand, spacer blocks are not recommended as they could interfere with the flow of concrete and cause “necking”. The pile reinforcing cage is 1.5m long to provide nominal bending resistance in the top one third of the shaft.
The pile load capacity is entirely dependent on the end bearing provided by the enlarged base and the densification of the soil below the base. The following formula, established from many pile load tests, can be used to establish the ultimate pile capacity Qult = 400 x DPSH blow count x area of base. The base area Ab = equal shaft area x 1.1.
DRIVEN TUBE PILING
Piles formed by the driven method fall into the category of “displacement” piles. The term “displacement” refers to the displacement and densification of the soil at the toe of the pile
during the driving in of the permanent casing.
The pile hole is initially pre-drilled to locate the pile accurately and to allow the 177 or 192mm diameter casing to be stabilized within the preformed hole before commencing installation by bottom-driving the casing with the drop hammer.
The pile load capacity is entirely dependent on the end bearing provided by the pressure bulb formed during driving and the densification of the soil below the base of the pile. The bottom of the leading casing is closed-ended and plugged with dry concrete and driven into the soil with an internal drop-hammer.
Driving of the casing is carried out using a 110 or 130kg drop hammer falling from a motorized tripod rig and proceeds until either a suitable level is reached or “refusal” occurs. Practical “refusal” is considered to have been reached when a “set” of 10mm/10blows of the drop hammer is achieved, i.e. the steel tube does no penetrate more than 1.0mm/blow of the drop hammer. The casings are joined by welding successive 1.0m lengths of steel casings having a 3 to 4mm wall thickness.
The pile capacity may vary depending on the ground conditions encountered but in general the following safe allowable capacities are relevant at DPSH blow counts of 25 to 30 (N=20 to 25):
- 177mm diameter pile – 175kN.
- 192mm diameter pile – 200kN.
On completion of driving a reinforcing steel cage of specified composition and dimensions, is lowered into the pile hole. In view of the small size of the pile and relatively shallow depth the reinforcing cage is maintained vertically by hand, spacer blocks are not recommended as they could interfere with the flow of concrete and cause “necking”. The pile reinforcing cage is 1.5m long to provide nominal bending resistance in the top of the shaft.
UNDERPINNING – JACKED PINS
The jacked pin involves the installation of 127 to 177mm diameter steel tubes with a 4,0mm wall thickness. The steel tubes are installed from access pits excavated to a depth of 1.5m below the underside of the existing foundations. Excavation work is carried out in terms of the regulations stipulated in the OHS Act (1993).
The leading steel section is closed ended and the following sections are jacked in short lengths against the resistance of the superstructure. Jacking is carried out using a 55 ton hydraulic cylinder and continues until the jacking force equals at least 1.5 times the building load.
On completion of jacking the tubes are filled with 25 MPa concrete and propped against the foundations to support the loads. The props comprise 110mm P.V.C sleeves that are pre-cast with 25MPa concrete to minimize the problems associated with the near surface corrosion of the steel sections.
Once the piles have been installed the access pits are backfilled by re-compacting the excavated soil in 200mm layers using hand compactors.