Research On Calculation Method Of Concrete Strut Axial Force Monitoring Value On Excavation

The depth of excavations becomes more and more deep, struts in the retaining andprotection structure is used more and more widely. Strut axial forces monitoring is animportant means to ensure that the strut internal forces of no more than its bearing capacityand to ensure the safety of excavation. The strut axial forces monitoring is usually measured by thefrequency change of the embedded vibrating string extensometer and calculated by theformula. The phenomenon of strut axial forces too large often appear in the excavationsengineering, sometimes even more than the load carrying capacity of the strut, but the othermonitoring data and site patrol are no abnormal. The phenomenon of the strut axialforce monitoring value conflicts with the actual situation often beset on all the relevant parties.Through the analysis and investigation of various factors which affecting the axialforce monitoring data, including the design theory of excavations, the sensor error, thesensor installation and concrete pouring, the external factors, the concrete shrinkage,creep, elastic modulus and so on, and finally found that the existing axial force calculationformula are not considering the nonlinear stress strain relation and concrete creep effect, andsimply multiply the strain by the modulus of elasticity to stress. According to the standard andtheory formula at present, put forward the new calculation formula which consideringthe concrete nonlinear stress strain relation and creep for the first time, and discusses thatthe average value of concrete compressive strength should be taken as the representative valueof concrete strength to calculate the axial force. Through the comparison of the existing andthe new formula, we found that the results of the new formulae are more corresponding withthe theoretical calculation values and the actual situation of engineering.The strain of the strut can be measured by the electrometric method, it can alsobe measured by the engineering surveying, including the capping beamhorizontal displacement method and the reflecting plate method. Both methods can be help toreduce the monitoring cost and the labor intensity of measuring staff, although the accuracyare less then electrometric method, but each method has its advantages and disadvantagesand applicable scope, and the results of the axial force of the strut are ableto check and supplemented each other by using various methods to measure it.