A pile is a long slender foundation member, made either of timber, structural steel or concrete which might be cast-in-situ or driven and acts as a structural member to transfer the load of the structure to a required depth in deep foundations carrying a load which may be vertical or lateral or lateral plus vertical. The existence of piles goes a long way back beginning with timber piles used to create settlements by the river beds. The art of pile foundation has improved over time. Notable advances include the invention of pile driving equipment by Christoffoer Polhem in 1740, usage of steel piles since 1800 and concrete piles since about 1900 (A. Ascalew and I. G. N. Smith, 2007).
According to A. Ascalew and I. G. N. Smith (2007), “Piles may be classified into three categories with respect to their effect on the soil during installation; large displacement, small displacement and replacement piles”. Other classifications of piles are by:
– Material: timber, steel, concrete and composite piles
– Load transfer mechanism: end bearing and friction piles.
– Method of installation: driven and bored
The interaction between the pile face and the soil is of importance to the engineer. As piles are usually long members, they come in contact with a large amount of the soil. As with any two solid bodies in contact which are in motion, friction is developed within this pile-soil interface. Movement of either the pile or the soil leads to an increase in friction and can bring about considerable amount of shear stress on the pile surface. This induced shear stress can be termed positive or negative depending on the relative movement between the soil and the pile. Positive shear stress is developed when the soil is moving upward relative to the pile while negative shear stress arises when the soil is moving downward relative to the pile.
Skin friction and shaft resistance are used interchangeably to refer to this interaction. The difference is that positive skin friction is observed when the soil heaves or swells while positive shaft resistance occurs when there is a push load on the pile. In both instances, positive shear stresses are developed on the pile face. Negative skin friction is the reverse, in which the soil is moving downward (consolidating) or termed negative shaft resistance if a pull load is exerted on the pile.
The fact that negative skin friction induces shear stresses to the pile is of concern because if the pile is not designed to accommodate the additional shear stresses it can result in serviceability issues or even outright failure. It is therefore necessary to have a fair knowledge of the possible shear stresses that will arise and make appropriate designs to cater for it.
Smith and Pole identified possible situations where the problem of negative skin friction can arise in practice (smith and Pole, 1980).
• Bearing piles driven through a recently placed fill material
• Bearing piles driven through a soft compressible soil over which a layer of fill material has been placed prior to construction
In addition to the instances above, activities which result in lowering the ground water table can also bring about consolidation and thereby negative skin friction. A case in point is the 1942 failure of a theatre wall in North London, where it was discovered that the growth of a line of poplar trees nearby reduced the water table and caused downdrag to the foundation (J. Feld and K. L. Carper, 1997). Another scenario occurred in Utah where the wetting of an unsaturated soil with a high compression index induced negative skin friction and imposed a downdrag force of over 182 tons (R. C. Hepworth, 1993).
1.2 Aims and objectives of this research project
The overarching aim of this study is to employ relevant techniques to predict the downdrag force to be experienced in piles. The calculated values would then be assessed against the actual values recorded on a test site located at the south side of the Forth estuary, Scotland. The 11 hectare site consists of normally consolidated estuarine clay which was deposited over 9000 years ago.
The clearly defined objective of this research is to predict some key requirements, the requirements to be predicted by this research and compared with the results from the test site are:
1. The initial pore pressure response of the clay from original ground level to the top of the gravel layer due to the additional 2500mm blaes surcharge.
2. The initial settlement of the ground surface due to the additional 2500mm blaes surcharge.
3. The excess pore pressure distribution in the clay in July 1992.
4. The consolidation settlement of the clay surface due to the additional 2500mm blaes surcharge as at July 1992.
5. The profile of force with depth in the centre pile of the friction group in July 1992