## 3 Experimental Results and Analyses
### Horizontal Deformation
#### 3.3.2 Horizontal Deformation along the Depth  
Horizontal Deformation under Positive Displacements  
Figure 21 shows the distribution of horizontal deformation of specimen along the depth under the positive displacement loads. It can be observed from Figure 21 that the deformation of the abutment increases linearly from the bottom to the top of the abutment under different positive displacement steps, thus the abutment movement can be regarded as a rigid body motion with a certain rotation. In Figure 21, it can be also observed that the deformation of the pile along the depth increases firstly to maximum, then to decrease to zero and continuous to be a negative value. After the deformation reaches a maximum negative value, it begins to decrease to zero at the end of pile. The horizontal deformation of pile head is relatively small under the displacement loads, but the deformation of pile body grows rapidly at deeper depth along the pile and it reaches the maximum value at the pile depth of 0.4 m (2.58\(b\)). Then, the horizontal deformation of pile reduces to 0 mm at depth of 0.9 m (5.8\(b\)), where it begins to change into a negative value. The maximum negative deformation of the pile is observed at depth of 1.2 m (7.74\(b\)), and gradually diminishes to 0 mm as the depth increases. It can be seen from Figure 21 that the distribution of horizontal deformation of the specimen is significantly different from the those of previous test and traditional theory of soil-pile system [38; 39; 40]. The traditional distribution assuming the maximum horizontal deformation is occurred at pile head, is also shown in Figure 21 with a dashed line. To be specific, the maximum horizontal deformation of pile in this test is occurred in the pile body rather than at the pile head. Under the displacement load of +12 mm, the maximum deformation of pile body reaches +11.08 mm, which is far larger than that of the pile head (+1.82 mm); under the displacement of +14 mm and +16 mm, they reach +15.38 mm and +21.62 mm respectively, which even exceed their displacement loads. In Figure 21, the significant difference between the traditional theory and this test is caused by the unbalance of earth pressure on the front and back of the abutment and the movement of soil around the pile. When the specimen moves to the positive direction, then a gap is formed between the pile and front soil. The gap will be filled by the soil of backfill due to the difference earth pressure that is shown in Figure 22. This will induce the residual deformation greatly in the positive direction. The residual deformation grows constantly and accumulatively under the reciprocating loads. Here we define the growing phenomenon as the ratcheting effect of deformation or accumulative deformation. In practical engineering, when the integral abutment moves towards the bridge span due to reduction in temperature or concrete shrinkage, a gap develops between the abutment and backfill. Then under the action of vehicle weight and dead weights of backfill, the soil will fill the gap, which leads to the development of a void below the approach slab. This phenomenon was also observed in this experiment when the test specimen moves to the negative direction. Figure 22a shows the process of void formation, and Figure 22b shows a photo of void behind the abutment. Therefore, the reasons for bumping at bridge-head and settlement of soil are also related to the mechanical behavior of the soil-abutment-pile interaction in addition to the properties of soil. Besides the soil, the void phenomenon also occurs around the interface between abutment and pile, as shown in Figure 22a, which is caused by the reciprocating movements of pile and soil. Figure 23 shows the distribution of horizontal deformation of specimen along the depth under the negative displacement loads. Figure 23a,b show the deformation under displacements of \(-2\) mm\(\sim\)\(-10\) mm and \(-10\) mm\(\sim\)\(-16\) mm respectively.