Experimental Study On Characteristics Of Load And Deformation When Existing Buildings Reload

The first problem is foundation problem before retrofitting and reconstruction for existing buildings, it is a key problem that determining the foundation bearing capacity and sedimentation deformation of existing buildings to make the right evaluation on foundation. The institute of foundation engineering in china academy of building research made experimental study on the mechanism of the foundation bearing capacity increasing through the sustained load test on natural foundation and pile foundation with the support of national11th five-year plan. The two experiments were made in the specific time of sustained load, it is needed to make systematic study on variation regularities of foundation bearing capacity in different time of sustained load and working properties of foundation soil. The characteristics of load and foundation deformation when existing buildings reload are systematically studied through large scale indoor model tests in this topic. Different time (three days to forty-nine days) and different load (0.5fato1.5fa) of sustained load tests are made on the models in this experiment, through experiment data analysis, some useful conclusions are drawn:(1) The properties of foundation soil change under long-term load, the bearing value of the soil increases due to compaction effect. The deformation of corresponding stress period decreases when the models continue to load after sustained load, the bearing capacity and ultimate bearing capacity can improve accordingly, also the bearing capacity improves differently along with the increase of sustained time.(2) Three percent、five percent and ten percent which is the additional settlement after sustained load accounts for percentage of the total settlement before loading are chosen for allowed deformation indexs for different time of sustained load tests. The improvement range of foundation bearing capacity can be obtained combined with P-s curves. The hyperbolic-fit method is used to get variation regularities between the increasing percentage of foundation bearing capacity and time increasing. the improved limit value of foundation bearing capacity is put forward under different allowed deformation indexs. The improved limit values are28.62%、34.89%and51.1%in different allowed deformation indexs under the sustained load of110kPa, so the foundation bearing capacity of existing buildings can be increased by50%at most. (3) Three percent、five percent and ten percent are similarly chosen for the allowed deformation indexs for different load of sustained load tests to determine the foundation bearing capacity. Although the load of sustained load is different, the proportion of foundation bearing capacity increasing is similar in the same time of sustained load and same allowed deformation index. So sustained load time has a significant impact on foundation bearing capacity increasing.(4) In practical engineering application, practical foundation settlement can be changed into load test settlement through certain proportion relationship combined with evaluation of allowable deformation before retrofitting and reconstruction on the existing buildings and allowable subsoil deformation ruled by the norm, then the appropriate allow deformation index can be chosen to determine the foundation bearing capacity of existing buildings through the range of load test allowable deformation.(5) The compaction effect of foundation soil mainly occurs at the range of1.25B (B is the width of foundation slab) deep and1B wide under the foundation slab center through analysis and comparison of settlement of soil and physical and mechanical indexes of soil in different depth.(6) Foundation bearing capacity can be determined more reasonably through sustained load test in scene.Seven days sustained load test can be done on the same properties of the soil near the buildings considering the safety of engineering and the needs of time limit for a project when existing buildings increase load, then the foundation bearing capacity can be determined through P-s curves.