Model Tests On Stability Of Reinforced Concrete Pile Foundation Under The Condition Of Deep Excavation

With the continuous advance of China’s urbanization process,many of the existing buildings in the use of functional defects continue to show due to the lack of underground parking or other facilities.More and more people pay attention to the development and utilization of underground space.When the basement is added to the existing building,it is necessary to use the original pile foundation or to set up a new pile foundation to bear the load of the existing building,and then excavate the earthwork and add the basement.In the process of excavating the soil around the pile,the stability of the pile foundation is obviously reduced.Reinforced concrete pile foundation is one of the most common forms of pile foundation.It is of great significance to study the stability of reinforced concrete pile by means of large scale model test.In order to carry out the model test,the 2.5m × 2.5m × 3.0m geotechnical model box was designed and manufactured.The utility model is composed of a vertical steel plate which is surrounded by the periphery and a horizontal hoop which is connected with the periphery through bolts.The angle steel of the four big horns of the model box is fixed with the adjacent vertical steel plate through bolts,and the temporary skeleton of the model box can be formed,and the soil filled in the box can be prevented from leaking along the joint of the two steel plates.This paper presents the design method of the vertical steel plate,the horizontal hoop and the connecting bolt of the model box.The design of model test considering different excavation ratio,reinforcement ratio,pile diameter,eccentricity and support setting parameters such as different positions of influence on the stability of reinforced concrete pile,in the experiment the strain ofconcrete pile and ultimate load under different conditions.According to the test results,the calculation formula of the stability of reinforced concrete pile under the condition of soil excavation is proposed.Based on the analysis of the model test results,the following conclusions are drawn:(1)With the increase of excavation ratio,the stability of reinforced concrete square pile decreases gradually.Especially when the excavation is relatively large,the ultimate load of the axial compression pile decreases sharply.(2)The reinforcement can not only improve the ultimate compressive strain of reinforced concrete pile,but also improve the ultimate load of reinforced concrete pile.The reinforcement ratio is increased by 1.25 times,the ultimate load is increased by about 18%,the reinforcement ratio is increased by a factor of3,and the ultimate load is increased by more than 33%.(3)The stability of reinforced concrete pile can be improved significantly by increasing the pile diameter.(4)When the excavation is relatively small,the top of the pile support can effectively improve the stability.When the excavation is relatively large,the setting of the support is more effective.(5)The eccentricity of the bearing capacity of pile effect is particularly remarkable,when the offset is 25mm(0.3125 times of the pile diameter),the ultimate load of the pile is reduced about 40%.When the offset is 50mm(0.625 times of the pile diameter),the ultimate load of the pile is reduced by about 70%.(6)The failure location of reinforced concrete pile is more in the lower position of the pile-soil interface.(7)The calculation formula of the stability of the axial compression and eccentric compression piles is put forward.The calculated values are compared with the experimental values.The formula can provide technical support for the design of the pile underpinning of the existing building.