**2. Large scale in situ load tests of piles**

Load tests of piles, that are performed in-situ on the construction site are the best opportunity for the determination of the load-deformation behavior [3]. For the determination of the bearing capacity, the loads on test piles can have a vertical resp. horizontal direction. Vertical loads can be compression loads or tension loads depending on the construction task. The tests can be static load tests or dynamic load tests. Detailed descriptions of these different test types are given in [4, 5]. In the following, only the static pile load test for determining the vertical bearing capacity is presented.

Normally counterweights or anchors are used as an abutment for the pile load. The installation of counterweights or anchors necessitates large technical and financial input. Using hydraulic jacks like the Osterberg-cell (O-cell) is more convenient. **Figure 1** shows the variations of static pile load tests.

By using the Osterberg-method, hydraulic jacks are installed in a test pile to detect to determine the skin friction in different pile segments that correspond to different soil layers. The single pile segments serve as counterweights for the different test phases.

The result of a pile load test with vertical load is described by a resistance settlement curve Rc,k(s) which can be used as the basis for the analyses of stability and serviceability. In **Figure 2** a qualitative trend of a resistance settlement curve is shown. Two straight reference lines help to determine the pile resistance Rc,k. These two straight reference lines draw a tangent at the beginning and at the end of the resistance settlement curve. The interaction of both lines defines the stability limit state.

Based on one or several pile load tests, the measured value Rc,m is determined, which has to be reduced by the factor ξ taking straggling into account. According to [6] the pile resistance has to be calculated by Eq. (1) if the superstructure is not able to transfer loads from softer to stiffer piles.

$$R\_{c,k} = \text{MIN}\left\{ \frac{(R\_{c,m})\_{av}}{\xi\_1} ; \frac{(R\_{c,m})\_{min}}{\xi\_2} \right\} \tag{1}$$

The superstructure is able to transfer loads from softer to stiffer piles if the superstructure has sufficient rigidity. In this case, the straggling factors ξ<sup>i</sup> can be *Reducing Carbon Emissions by Combined Pile-Raft Foundations for High-Rise Structures DOI: http://dx.doi.org/10.5772/intechopen.104559*

#### **Figure 2.** *Determination of the pile resistance by a resistance settlement curve.*


#### **Table 1.**

*Straggling factors ξ<sup>i</sup> for resistance of pressure piles.*

divided by 1.1 (ξ<sup>1</sup> is always ≥ 1.0). To the measured average pile resistance belongs the straggling factor ξ1. To the measured minimum pile resistance belongs the straggling factor ξ2. The straggling factors for pressure piles are given in **Table 1**.
