11.4.2 Stress of the retaining wall

The stress of the retaining wall sometimes has to be measured constantly during the process of excavation lest the stress may exceed the allowable value and endanger the excavation safety.

The stresses of soldier or sheetpiles can be measured by fixing commonly used strain gauges on them.As for column piles or diaphragm walls，the rebar stress meter has to be installed onto the steel inside the retaining wall to measure the load.The rebar stress meter contains a strain gauge，which is usually the resistance type.The exterior of the strain gauge bas to be enveloped in many layers of water repellent membranes，because，when the steel or the retaining wall is acted on by a force and bends，repellent membranes may break，causing the strain gauge to contact water.Since water is also an electric conductor，water will change the resistance of the strain gauge and influence the measurement.Figure 11.15 is a photo where a rebar stress meter is fixed on the steel cage.The figure shows cutting a main reinforcement at the designed depth and collecting a rebar stress meter onto the main reinforcement by welding or a coupler.

Figure 11.15 Rebar stress meter on a steel cage