Abstract
After a field test campaign conducted in Dunkirk (north of France) on open-ended steel piles aiming to study the ageing phenomenon, laboratory scale shear tests were designed to study the behaviour of the sand-steel interface. In order to carry out this laboratory investigation, the direct interface shear apparatus was used for characterizing Dunkirk sand (in dry or unsaturated conditions with about 6% water content as in the field) consolidated on initially smooth mild steel plates at different consolidation time intervals (0, 1 and 7 days) and different consolidation stresses (50, 100, 200 and 300 kPa). The test program also included two normal boundary conditions (Constant normal load CNL and constant volume CV) so that they can be compared to the field results and determine the most approaching configuration. More, the unsaturated condition induced a corrosion of the mild steel plates, causing a layer of sand remaining glued to the plate after removing the shear box. Traces of corrosion were also observed on the lower part of the sand samples (in contact with the plate). These observations lead to the interpretation of an increase of the mechanical properties (local cohesion and increase of the friction angle). In order to follow the evolution of the corrosion for each plate, thickness measurements of the sand layer stuck on the plates were carried out.
Highlights
In order to investigate the effect of time on the tension capacity of micropiles driven in Dunkirk sand, a series of pull-out tests were conducted, and the conclusions reported in [1]
In order to explain this increase of capacity, a series of direct interface shear tests were designed with mild steel plates and Dunkirk sand, with two different normal boundary conditions: Constant Normal Load (CNL) or Constant Volume (CV)
The CNL and CV tests were conducted at different time intervals (0, 1 and 7 days), different constant normal stresses (50, 100, 200 and 300 kPa) predicted from field CPT results (Figure 2) and reproducing the radial effective stress around the micropiles at different depths
Summary
In order to investigate the effect of time on the tension capacity of micropiles driven in Dunkirk sand (a fine and uniform sand), a series of pull-out tests were conducted, and the conclusions reported in [1]. Some of the micropiles (2m long, about 50 mm in diameter) tested in Dunkirk are made of mild steel In order to explain this increase of capacity, a series of direct interface shear tests were designed with mild steel plates and Dunkirk sand, with two different normal boundary conditions: Constant Normal Load (CNL) or Constant Volume (CV). The CNL and CV tests were conducted at different time intervals (0, 1 and 7 days), different constant normal stresses (50, 100, 200 and 300 kPa) predicted from field CPT results (Figure 2) and reproducing the radial effective stress around the micropiles at different depths. The tests were conducted on dry and unsaturated sand specimen (6% water content as in the field)
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