11.4.1 Sturt load
The loading of the struts has to be monitored constantly during excavation lest it may exceed the allowable value and endanger the safety of the excavation.Some commonly used devices to measure the strut load are the strain gauge，the load cell，and the hydraulic jack gauge.
（1）Strain gauge
The strain gauge is a widely used device to measure the strut load.The types of strain gauges are many，though the commonly used in excavations are the resistance strain gauge and the vibrating wire strain gauge.The two types of strain gauge take advantage of the change in resistance and vibrating frequency of the wire，respectively to measure the strain.The resistance type is more sensitive while the vibrating wire type is less influenced by pollution，humidity and temperature.Therefore，placed outdoors and exposed to sun，rain，and dust，the vibrating type is more suitable and favored.
Figure 11.12 Photo of strain gauges
（2）Load cell
The load cell can be divided into the mechanical type，the hydraulic type，and the electronic type.The electronic type is used most commonly.Figure 11.13 is a photo of a load cell and its configuration.As shown in the figure，the main part of the load cell is a circular box made of steel or aluminum alloy where several strain gauges are fixed inside or attached to the exterior.The accuracy of the electric load cell is the same as the strain gauge but it costs more.The load cell is usually applied when the strain gauge is not easily installed，on a wood or steel pipe of struts for example.
Figure 11.13 Load cell and its schematic configuration
（3）Hydraulic jack gauge
After struts have been installed in an excavation，hydraulic jacks are often necessary to preload the struts so that they can be tightly connected with the wales and the lateral deformation of the retaining wall can be decreased.After preloading，the pressure gauge of the hydraulic jack can also be used for the measurement of the strut load.Figure 11.14 is a photo showing how a jack is set between struts.Nevertheless，misalignment of struts，offcenter strut loading，non⁃parallel bearing plates，and transverse relative movement of bearing plates all cause friction between the valve and cell cylinder，and render the measurement of the strut load inaccurate.Temperature changes and pressure gauge inaccuracy may cause additional error.Using pressure gauges of hydraulic jacks to measure the strut load is not reliable as a result.
Figure 11.14 Photo of jacks between struts and walls