What is prestressed spun concrete pile?

Answer 1

Prestressed Spun Concrete PileFoundations of any building or structure shall be designed and constructed to withstand safely all the dead, imposed and wind loads without impairing the stability or inducing excessive movement to the building or of any other building, street, land, slope or services. The allowable capacity of the soil/rock under working loads where any foundation is founded shall be the lesser of :

• the ultimate capacity for bearing, bond or friction with an adequate factor of safety against failure; or
• the value in relation to bearing, bond or friction such that the maximum deformation or movement induced to the foundation under working loads can be tolerated by the building, any other building, structure, land, street and services.

The allowable capacity may be increased by 25% when such increase is solely due to wind effects. In determining the said factor of safety against failure, due consideration shall be given to the form and depth of the foundation, loading characteristics, the general geological conditions of the ground and its surrounding including the presence of dissolution features, jointing conditions and any other relevant characteristics for rock.

Prestressed Concrete Spun Piles should be driven to relatively stiff stratum with sufficient embedded length in residual soil or decomposed rock in order to develop the high bearing capacity and minimize the amount of long-term settlement. The effect of soil movement and percussion during driving on the stability of any adjacent building, structure, land, street and services should be carefully assessed. Stringent requirements on performance test and quality control are usually required for this type of pile. Where it is necessary to drive the pile into thick layer of stiff soil, steel conical pile shoes with cross stiffener should be used.

Segmented Prestressed Concrete Spun Piles

Large diameter spun concrete piles are often manufactured with centrifugal casting in segments 8 to 16 ft (2.4 to 4.9 m) in length. Longitudinal holes are formed during casting to re ceive post-tensioning strands or wires. Stressing follows assembly of the segments and proper application of the joint sealant material. Such sealing material (generally polyester resin) should be of sufficient thickness to fill all voids be tween surfaces. The pile sections should be brought into contact and held together under compression while the sealing material sets. After completing the prestressing, all tendons should be fully grouted and stress on tendons maintained until the grout develops the required strength. Grouting should follow the procedures outlined in the PCI Recommended Practice

Driven Prestressed Spun Concrete Piles

The high strength prestressed spun concrete piles, commonly driven with hydraulic impact hammers or preferably installed with jacked-in rigs when considering the stricter regulations with respect to noise and vibrations in more urban areas, often offer a competitive choice of foundation system for projects with medium and high loadings. They are widely used in foundations for schools, high-rise buildings, factories, ports, bridges and power plants in this region. In early years, the main construction control for driven piles was mostly based on the measurement of set of each pile coupled with a selected small number of non instrumented static load tests to verify the specified load-settlement requirements. In recent years, with critical evolution in the understanding of the load transfer and bearing behavior of piles mainly through analysis of instrumented full-scale load tests (particularly for bored cast-in-place piles), many engineers can now appreciate that the pile performance is not simply a matter of ultimate load value alone. According to Fleming (1996) some of the basic parameters required for forecasting pile deformation under loads include (a) Ultimate shaft load and its characteristics of transformation to the ground; (b) Ultimate base load; (c) Stiffness of the soil below the pile base; (d) Pile dimensions; and (e) Stiffness of the pile material. This recent development in the understanding of the load transfer and bearing behaviour of piles in fact exerted a significant and positive influence on the evolution of codes of practice and design methods for foundations in some countries. For example, the revised Singapore Standard on Code of Practice for Foundations CP4: 2003, recommends that the static load test on preliminary test pile be instrumented to measure the transfer of load from the pile shaft and pile toe to the soil. The Code also recommends that for driven piles (similar to bored cast-in-place piles), the axial load capacity can be evaluated empirically from correlation with standard penetration tests SPT N-values (which are widely used in this region) using modified Meyerhof Equation, where the ultimate bearing capacity of a pile in compression is given by:

Qu = Ks*Ns*As + Kb*(40Nb)*Ab

Where:

Qu is the ultimate bearing capacity of the pile, kN;

Ks is the empirical design factor relating ultimate shaft load to SPT values, kN/ m2 per SPT blow;

Ns is the SPT value for the pile shaft, blows/300mm;

As is the perimeter area of the shaft, m2;

Kb is the empirical design factor relating ultimate end bearing load to SPT values, kN/ m2

per SPT blow;

Nb is the SPT value for the pile base, blows/300mm;

Ab is the cross-sectional area of the pile base, m2.

For bored piles, instrumentation using sacrificial cast-in vibrating wire strain gauges and mechanical tell-tales which permit for monitoring of axial loads and movements at various levels down the pile shaft including the pile toe level had been practiced successfully within limits of accuracy posed by constraints inherent of the installation method, in this region for many decades, allowing insight evaluation of Ks and Kb factors, (Chan, S.F.& Lee, P.C.S.,1990,; S.F. Chan, 2004, Abdul Aziz, H.M. & S.K. Lee, S.K., 2005; H.M. Abdul Aziz , H.M. & Lee, S.K. 2006).

For precast driven piles, the application of instrumented full-scale static load tests is far more challenging than their bored pile counterparts due to significant difference in method of pile installation. Due to practical shortcoming of conventional instrumentation method and the lack of innovation in this area, instrumented full-scale static load tests are in fact rarely used in driven pile application in this region. Therefore, the far lacking driven pile industry is long due for a better technology to revolutionize the methodology in the acquisition of design data in a more accurate and reliable way, to catch up with the evolution in the design methods.