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About 90% of H-piles adopt Hiley’s formula for design. However, this formula is only applicable to pile lengths less than 30m and is suitable for course-grained materials (not suitable to fine-grained soils) as suggested by GEO (1996). In Hiley’s formula, by observing the penetration of piles after the hammer impact, the pile capacity could be readily obtained from the response of the impacting force. Therefore, the individual pile capacity could be obtained by this dynamic method.
When a hammer with initial motion collides with a stationary pile, the transfer of energy is most efficient when the two masses are comparable. That is the reason why some codes limit the ratio of weight of hammer to the weight of pile to be more than 0.5.
Boussinesq’s closed form solution for a rod fixed at its end and hit on its top by a mass shows that compressive stress in the rod increases with the mass of hammer. On the other hand, a larger relative mass of hammer leads to lower tension stress. When an impact is made on friction pile, the compressive stresses are highest at pile top. When an impact is made on end-bearing pile, the compressive stresses may be highest at the top or the bottom of the pile.
In pile driving operation, proper selection of piling hammers is essential to prevent the damage of piles. For instance, a light hammer with higher drop causes a higher impact stress than a heavy hammer with lower drop provided that they generate the same energy per blow.
Cap block is installed between the hammer end and the drive cap to control the hammer blow in order to protect both the hammer and the pile from damage. When the hammer hits the cap block, it compresses elastically and reduces the peak forces, thereby lengthening the time of hammer blow. Moreover, it should be capable of transmitting the hammer energy effectively to the piles.
In Hong Kong, the local practice of driven H-piles is to adopt dynamic formula like Hiley’s Formula to drive piles to set and verify its capacity by static load test for a certain proportion of these piles.
The basis of all dynamic formulae is the conservation of energy in which energy applied to the piling system by driving hammer is equal to the energy required to penetrate the soils with energy losses during the process. The Hiley’s formula is the most popular dynamic formula used in Hong Kong.
The load transfer mechanism is summarized as follows:
When a socketed foundation is loaded, the resistance is provided by both rock socket wall and the socket base and the load distribution is a function of relative stiffness of foundation concrete and rock mass, socket geometry, socket roughness and strength. At small displacements the rock-socket system behaves in an elastic manner and the load distribution between socket wall and socket end can be obtained from elastic analysis.
Micropiles are defined as bored piles with diameters not exceeding 250mm. Micropiles were first constructed in Italy in 1950s and were given the name “root piles”.
Whenever bored piles (larger diameter) and driven piles are considered feasible, they should be more economic than micropiles. Owing to the small size of piles, only small dimension of equipment is needed for construction. Moreover, it can be used to drill through any type of soils, boulders and hard materials.
For the construction of friction barrettes, some grout pipes are designed at the periphery of the barrettes. Within a short duration (e.g. 24 hours) of concreting of barrettes, the fresh concrete cover is cracked by injecting water.
