11.4.3 Earth pressure on the retaining wall
Before excavation，the earth pressure on the retaining wall is the at⁃rest earth pressure.After excavation，with the retaining wall moving toward the excavation zone，the earth pressure acting on the back of the wall（outside the excavation zone）decreases.The limiting value of the earth pressure on the back of the wall is called the active earth pressure.On the other hand，the earth pressure on the front of the wall（inside the excavation zone）increases and the limiting value is called the passive earth pressure.When the earth pressure at every depth in front of the wall has reached the passive earth pressure，the retaining wall is called in the ultimate state.The safety factor of the retaining wall in the ultimate state has to be larger than or equal to the values required by codes.Measuring the total earth pressure and effective earth pressure on the retaining wall is helpful to understand the bearing behavior of the wall and the surrounding soils.What's more，it can contribute to the exploration of the deformation and stability characteristics of the excavation.Generally speaking，with the shallow depths of soldier piles and sheetpiles，the earth pressures on them will not be large.Also，as it is difficult to install measuring devices on them，the earth pressures on soldier piles and sheetpiles are not measured in engineering practice.This section will only introduce the earth pressure cell used on diaphragm walls.
The commonly used earth pressure cells are the direct earth pressure cell and the indirect earth pressure cell.The direct earth pressure cell uses strain gauges（of the resistance or vibrating type）in a cell to measure the displacement of the bearing plate，which is then converted into the pressure.Thus is derived the pressure on the bearing plate.The indirect earth pressure cell is a cell filled with oil or mercury，which will transmit the earth pressure on the bearing plate.The earth pressure is then measured by a sensor on the exterior of the instrument.Figure 11.16 diagrams the basic configurations of the two types of earth pressure cells.
Figure 11.16 Configurations of the earth pressure cell
Figure 11.17 diagrams the installation of the earth pressure cell on the retaining wall.The installation is elucidated as follows：
①Assemble a steel cage and set it on a platform.
②Weld reaction plates onto the cage and bolt a hydraulic jack to the reaction plates.
③Add a bearing plate containing an earth pressure cell to one end of the hydraulic jack or two plates on each end.Extend the cable and hydraulic pipe along with the main reinforcement to the top of the cage and fix them.
④Examine whether there exists the phenomenon of soil collapsing from the trench wall at the positions where the earth pressure cells are to be installed.If not，place the cage with the earth pressure cells into the trench and measure the value of the earth pressure in bentonite fluid.
⑤Connect the hydraulic pipe of the jack with a manpowered pump and add pressure slowly to make the earth pressure cells move toward the faces of the trench walls.When the readings on the cells change，it follows that the earth pressure cells and the trench walls are contacting each other slightly.Keep pumping with the hydraulic pump and repeat preloading and unloading to improve the contacting condition.Lastly，add a light pre⁃load（of about 105% of the bentonite fluid pressure or the equivalent at⁃rest lateral earth pressure）and fix it［Figure 11.17（b）］.
⑥When casting the concrete of the diaphragm wall using Tremie pipes，because the unit weight of concrete is heavier than that of bentonite，the trench walls may be pushed outward.In this situation，the readings of the earth pressure cells should be constantly taken.If the readings reveal signs of decreasing，more pumping is required to bring the earth pressure cells and the trench walls to full contact.
Figure 11.17 Installation of the earth pressure cell／piezometer
⑦Dismantle the hydraulic pump.The installation is then finished.
The contact between the earth pressure cells and the trench walls is a crucial point to the success of the installation.If preloading is too much，stress concentration，which will influence the accuracy of readings，will occur.If preloading is insufficient，the earth pressure cells may be enveloped by concrete during the Tremie casting，as shown in Figure 11.18.To ensure success，the installation of the earth pressure cells has to be carried out under the direction of experienced engineers.
Figure 11.18 Failures of the installation of an earth pressure cell
The earth pressure measured by an earth pressure cell is the total earth pressure.To obtain the effective earth pressure，an electronic piezometer has to be added on the bearing plate next to the earth pressure cell，as shown in Figure 11.17（a）.
According to the reason for its use，the earth pressure cell can be set on the active side，the passive side，or both.The same as with the inclinometer，rebar stress meter，strain gauge，and settlement points on the ground surface，the central section of the excavation site is the one where the earth pressure cell should be installed，considering the largest earth pressure on the section.
The maximum value of the earth pressure on the front of the retaining wall is the passive earth pressure，whereas the minimum on the back of the wall is the active earth pressure.Earth pressures on the wall usually fall between the two limiting values.An excavation design usually has taken these values into consideration.Thus，though measuring the earth pressures on the retaining wall is helpful to understand the bearing behavior of the wall and the surrounding soils，the results are not easily applied to the judgment of the excavation safety.