## 3 Experimental Results and Analyses  
### Earth Pressure behind Abutment  
#### 3.1.1 Relationship between Earth Pressure and Displacement Load  
As discussed earlier, the T1-T16 earth pressure cells were arranged on the back of the abutment in four rows and four columns. For the limit of space, results from four typical earth pressure cells (the third row of T3, T7, T11 and T15) are discussed in this section. Figure 9 shows the relationship between earth pressure and displacement load.  
It can be seen from Figure 9 that the active earth pressures are occurred and nearly constant (about 5 kPa) when the abutment moves to the negative direction (pulled by MTS). In addition, the active earth pressure is also close to the earth pressure at rest. The passive earth pressures are occurred and increase rapidly with the increase of displacement load when the abutment moves to the positive direction (pushed by MTS), especially for the pressures near the abutment (T3 and T7) as shown in  
Figure 8: Loading scheme (unit: mm).  
Figure 7: Loading arrangement.  
Figure 9. The maximum pressure for T3 reaches 214 kPa under the displacement of +16 mm. By comparison, the passive earth pressure of T3 is over 30 times of active earth pressure.  
Figure 10 shows the relationships between the relative displacement \(\Delta\)/\(H\) (\(\Delta\) is the displacement at the top of the abutment) and the earth pressure coefficient \(K\). The passive earth pressure coefficient \(K_{p}\) from Burke-Chen [12; 28], England [13], Barker and NCHRP [29], Dicleli [30], Massachusetts [31], Rankine theory and Coulomb theory are compared with the test results.  
It can be observed from Figure 10 that the test result of earth pressure coefficient is approximately equal to all \(K_{p}\) of Burke-Chen, England, Barker, NCHRP, Dicleli, Massachusetts when \(\Delta\)/\(H\) is not larger than 0.004 (\(\Delta\)/\(H\)\(\leq\) 0.004). Moreover, it is approximately equal to two thirds of earth pressure coefficient of Rankine theory, but it is far less than Coulomb theory. When \(\Delta\)/\(H\) is larger than 0.004 and less than or equal to 0.006 (0.004 \(<\Delta\)/\(H\)\(\leq\) 0.006), the \(K_{p}\) from Burke-Chen, England, Barker, NCHRP and Dicleli are less than that of test. The \(K_{p}\) from Rankine theory is nearly close to the test result, but still significant lower that Coulomb theory when \(\Delta\)/\(H\) reaches 0.006 (\(\Delta\)/\(H\) = 0.006). The test result is larger than all of those except Coulomb theory when \(\Delta\)/\(H\) is between 0.006 and 0.012 (0.006 \(<\Delta\)/\(H\)\(\leq\) 0.012). However, it exceeds the Coulomb theory when \(\Delta\)/\(H\) is larger than 0.012 (\(\Delta\)/\(H\)\(>\) 0.012). Therefore, the coefficient \(K_{p}\) obtained from this test is larger than all of them, which is demonstrated the ratcheting effect significantly. Moreover, the coefficient \(K_{p}\) from this test is 3.33 times as much as that of Rankine theory and 1.27 as much as that of Coulomb theory when \(\Delta\)/\(H\) is equal to 0.016 (\(\Delta\)/\(H\) = 0.016). Therefore, the existing calculation method earth pressure of backfill behind abutment is not accurate for that of IAJB.  
Figure 9: Relationship between earth pressure and displacement load.  
By the way, only the calculated methods of active earth pressure coefficient \(K_{a}\) and earth pressure coefficient at rest \(K_{0}\) were obtained from the code of _General Specifications for Design of Highway Bridges and Culverts_ (JTG D60-2015) [32] in China, while how to calculate \(K_{p}\) was not given. The calculation of \(K_{a}\) and \(K_{0}\) by JTG D60-2015 was also shown in Figure 10. The coefficients of \(K_{a}\) and \(K_{0}\) are less than the \(K_{p}\) of test from Figure 10. The result further indicated that the calculation of the \(K_{a}\) and \(K_{0}\) in the Chinese code is not suitable for IAJBs, which need to be improved to consider the coefficient \(K_{\rm P}\).