2.1. Test Setup
To run tests for model piles, a rigid test box, B × L × H = 1.15 m × 1.25 m × 1.50 m, was prepared. Internal wall surfaces were lined with PVC foil to protect them against moisture and to reduce side friction and boundary effects. The completed box was filled with sand up to a total height of 120 cm.
Siliceous medium sand from a sand mine near Wroclaw, Poland, was chosen for the study. A particle size distribution curve of the sand is presented in
Figure 1, and the sand’s parameters are presented in
Table 1. The moisture content was considered as insignificant to the test results [
20].
The compaction ratio can be calculated as Is = 1.66/1.74 = 0.95. Based on some correlation formulae for sands, the following estimation of the relative density can be derived: Dr ≈ 0.57, so this is a medium-density sand.
The sand was gradually poured into the crate, and twenty-centimeter layers were formed, which were subsequently compacted. The control of sand compaction was carried out using a cylinder and a ZORN dynamic plate with accuracy of 0.1 MN/m
2 (
Figure 2). At various locations in the crate, three measurements were made at three depths, 40, 70, and 100 cm, for cylinder measurement and four depths, 40, 70, 85, and 100 cm, for the dynamic plate. The results of the measurements are summarized in
Table 2 and
Table 3. Both tests showed higher soil density at the middle of the box and lower soil density at the bottom. A constant humidity was kept in the room during measurements.
To a certain extent, the situation is sensitive to changes in the relative density, Dr, of the sand. In the authors’ opinion, a relative density Dr~60% is representative, i.e., it corresponds to geoengineering practice, because the use of displacement piles is not recommended in very dense sands. Moreover, the paper is focused on the study of relative relations between the shaft and the base bearing capacities, not on the absolute values; in this way, the results and conclusions are less sensitive to the compaction level.
2.2. Pile Models
Six steel pipes, 1.0 m long, with an outside diameter 4.2 cm and wall thickness 0.25 cm, were used as pile models. The layout of the pile models in the box is shown in
Figure 3. Pipes used in testing were identified successively as 1, 2, 3, 4, 5, 6, which corresponded to the order they were installed (
Figure 3). The pile heads were situated 20 cm above the top of the sand level, so 80 cm of the pipes were embedded in sand, with 40 cm between the end of the pipes and the bottom of the box. This ratio 40 cm/4.2 cm is close to 10; Polish codes of practice for piling works pay special attention to soil conditions within a layer thickness of 5D below the pile base, any deeper mineral layers are not considered in capacity analysis of a single pile. As all the piles were tested separately, not in a group, it was assumed that there were no bottom effects. A similar assumption was made concerning the distance from sandbox’s walls. Again, Polish codes require that anchoring piles or the points of support of reference systems for displacement control should be out of the 4D-wide zone around the tested pile (it was more than 6D in the tests in
Figure 3).
The pile base included a steel cap, with a diameter equal to an external diameter of the pipe, loosely placed under the pipe during driving (
Figure 4a). Additionally, all caps included a threaded bolt, enabling a steel pole to be screwed to them (
Figure 4b).
Pile models were driven using a light dynamic penetrometer to the depth of 0.8 m (
Figure 5). While piles were driven, the number of necessary blows, N
10, to settle the pile by successive 10 cm increments was determined. The results for the successive pile models are given in
Figure 6.
2.3. Measuring Information
During testing, measurements were taken of the pile model head settlement, s, versus the applied force, Q; thus the Q–s curve was constructed. The force was applied using a jack and was taken by means of a load cell with 0.02 kN sensitivity. Displacements were measured with two sensors: an electronic displacement sensor, Keyence type GT2, with an accuracy of 0.001 mm, and analogue sensor ensuring displacement accuracy of 0.01 mm.