A Study Of Pile Capacity For The Heather Platform
Authors: Parry R.H.G. 
Published: December 2007

The unusually high undrained shear strengths which have been measured in the soil at this site are probably a consequence of high lateral effective stress brought about by a substantial degree of overconsolidation. By assuming high, but realistic, values of lateral stress it is shown that these high undrained shear strengths can be predicted. Skin friction values are calculated as a proportion of the undrained shear strength of the soil, the proportion being estimated on the basis of assumed effective stress conditions in the soil adjacent to the pile. Alternative values of skin friction are also calculated by a direct effective stress method.

Introduction

The site investigation by Fugro at the Heather Field showed the soil to consist of hard sandy silty clays with some very dense sand layers below about 50 m depth. The high shear strengths, together with the large pile size and pile loading, were outside the range for which field load test records exist for driven piles, and it was felt that the more traditional methods of calculating pile capacity should be supplemented by the approach described here.

Pile Details

The piles specified were 1.524 m (60 in) outside diameter steel tubes, driven open-ended with internal driving shoes. Wall thickness was 63.5 mm (2.5 in). The driving shoe had a length of 457 mm (18 in) and an internal diameter of 1.346 m (53 in).

If it proved impossible to advance the piles to their design depth it was planned to use insert piles 1.270 m (50 in) OD, but it was hoped this expensive and time consuming procedure would not be necessary.

The maximum working load for the pile was taken to be 29.5 kN.

Soil Conditions

The soil strength profile for boreholes 5, 5a, taken to be fairly typical is shown in Figure 1. A range of shear strengths are shown for penetration tests at any level, the limits of which define undrained shear strengths equal to one-fifteenth and one-twentieth times the measured cone resistance. A number of features of the soil and soil profile are of particular interest:Within depths of about 26 m to 33 m below sea bed level, a transition occurs from high shear strengths above this level to lower strengths below it. Various methods were used to assess shear strengths, including the CPT, torvane, pocket penetrometer, unconfined compression test and unconsolidated undrained tests. These have led to a scatter of results, but above 26 m tend to indicate peaks of shear strength at 5 m depth and 21 m or so, the peak at 5 m ranging up to an undrained shear strength exceeding 800 kN/m2 and at 21 m exceeding 600 kN/m2. The minimum value between these depths is between 250 and 350 kN/m2. A reasonable mean strength for design purposes between sea bed level and 26 m below it in 575 kN/m2. Below 33 m depth the undrained shear strength drops down to generally between 250 and 350 kN/m2 for depths less than about 45 m.The overconsolidation ratios, discussed more fully in a later section, have values greater than 4 above 26 m depth, the maximum values exceeding 20. The soil above 26 m depth can therefore be regarded as heavily overconsolidated. Below 26 m depth the values drop from 4 at 26 m depth to less than 2 at about 55 m depth and thus the soil below 26 m depth is comparatively lightly overconsolidated.