A Study On Prestress Loss Of Group Anchor

The technology of prestressed anchor cable has been widely used to reinforce rock and soil, the prestress loss variation has been the focus of the engineering studies at home and abroad. Through proper analysis and forecasting the prestress of prestressed anchors in long-term changes, and ensure the service life of the anchoring class structure of the key projects, it is very important to the national economy and the people’s livelihood. But because of it with comples mechanism, many influencing factors, long observation period and high cost, so long-term changes of prestressed research subject to an number of conditions, especially the anchor group of the design and calculation are developing slowly.Based on the research status of prestress loss about single anchor and anchor group, analysis and comparision of existing theoretical prediction model, FLAC3D is used to simulate single anchor and anchor group, and taking some excavation engineering in KumMing as an example, through contrasting the theoretical prediction model, the results of numerical simulation and the datum of on-site monitoring, this paper gets some conclusions:(1) Comparision the prediction models of K-H, Burgers coupling with anchor, the anchor equivalent elastic modulus formula Es=ExAs/Ar is amended as Es=ExAs/Ar+E’, using the two models to compute the prestressed anchors of the51document’s indoor test, it finds that the K-H model is more suitable for comparison with the experimental data. Besides, if the prestressed anchors in the early monitoring results are known, the lstopt fitting sofeware to fit the pre-monitoring data, this method can predict the changes of the post-prestressed anchors, and the fit is very good.(2) FLAC3D is used to analog the prestressed instantaneous loss and long-term loss of a single anchor, to study the lithology, free section, anchoring section, pad pier and tensioning force of prestress loss. The result show that:the higher the rock mass strength, the smaller the loss of prestress; if free length is fixed, anchor length meets the demand of the uplift, increased the length of the anchoring section to improve the prestress loss of little significance; fixed anchor length, the shorter the free section, the greater the anchor instantaneous prestress loss and long-term loss, if the increase in the length of the free section, the anchoring section axial force peak moves towards the end; pre-stress increases, the greater the value of instantaneous prestress loss, but the loss rate has no effect of tension value, the greater the value of prestressed anchoring section axial force decay faster; the smaller the size of the pad pier, the anchor prestress loss rate is higher.(3) FLAC3D is used to calculate the3,5,9anchor group system prestress loss, considering the different anchor spacing and length of anchor cable, The result show that:the smaller the anchor spacing, the bigger the stress loss rate of the cable has been tension, the more serious of anchor group effect, the anchor group’s axial force does not mean that the simple addition of a single anchor axial force; anchor spacing is too large, although the anchor cable loss of the axial force is very small, but the compressive stress superimposed effect of the rock surface is bad, and it does not reach the mechanism of anchor group’s toughening and crack arrest; Arranging anchor equal length, the prestress loss rate is bigger than unequal length arrangement, and anchoring section tensile region superposition is easy to form a coherent cracks, it’s harmful to the stability of the anchoring section, so the best use of the arrangement of the anchor is the different length; With the increase in the number of the anchor, the anchor that at the center of the group, the prestress gradually decreases, the prestress loss rate gradually increases, but the loss rate, the decreased rate gradually decreases, the prestress tends to balance, the law is consistent with the actual monitoring prestressed anchor; the prediction model of K-H coupling with anchor’s result is close to the numerical results, it indicates that the formula, calculating a single anchor prestress loss, is the same use to estimate the prestress’ change of the anchor group system of any one of prestressed anchors.(4) To Kunming excavation engineering as an example, take the use of FLAC3D simulation, the prediction model of K-H coupling with anchor calculation, and contrast with the on-site monitoring data, it can get that:FLAC3D and theoretical prediction model formula can be a good predictor to the prestress loss of the anchor group system.