Research On The Effect Of Continuous Flooding On The Bearing Capacity Of Bridge Pile Foundations In Loess Area

Due to its abundant energy reserves,the northwestern region has gradually become an important strategic battleground in China.However,there is a large area of collapsible loess in the northwestern region,which leads to the reduction of the carrying capacity of the building foundation or the excessive settlement and damage.However,previous studies on the negative friction resistance of pile foundations have been unable to meet the needs of the ever-growing engineering construction.In particular,the distribution law of negative friction resistance and the change of neutral point still need to be explored.Engineering design and other aspects have important practical application value.The on-site in-situ submerged loading test is still the most reliable way to study the negative friction resistance and neutral point.The test is a large-scale in-situ in-situ test of the static loading of submerged water to ensure that the submersion of the soil around the pile is sufficient and the single submerged state of the pile foundation is investigated.Changes in the vertical bearing capacity of the pile,and the distribution of negative friction resistance on the pile side and changes in the neutral point position during flooding,the test results indicate:(1)The ultimate bearing capacity of pile foundation under natural condition and submerged condition is obviously different.After submerged,the bearing capacity of submerged pile is 60% of the natural bearing capacity due to the loading effect of negative friction.The submerged pile settlement Far greater than the pile foundation settlement under natural conditions.(2)During the field test,it was found that the frictional resistance alternated along the depth and depth of the pile,which could be attributed to the physical properties such as moisture content,compactness and so on.Of the additional force and seepage pressure caused by the different collapsibility characteristics of soil and water swelling characteristics of the differences caused by the relative displacement of pile-soil uneven,when a layer of soil settlement greater than the settlement of the pile will have a negative friction,and vice versa will Have positive friction.(3)The negative frictional resistance above the neutral point is regarded as the load,while the effective bearing length of the pile foundation is below the neutral point.Under the condition that the positive and negative frictional resistances are staggered,the sum of the lengths of the negative frictional resistance distributions can be taken as the continuousdistribution equivalent length,that is,the equivalent neutral point depth.(4)After the preloading of the test piles,the negative friction resistance of the piles always exists,and the peak value increases with the increase of the load.The negative friction resistance is mainly distributed in the upper 7.5 meters of the pile,and the equivalent neutral depth is 4.66 to 5.23 meters.The ratio of the friction resistance to the length of the pile is 0.23 to 0.28,and the range of change is not large.During the unloading process,there is also negative friction resistance,and the neutral point is mainly 5.74 to 9 meters,and the ratio of the length to the length of the pile is 0.28 to 0.45.When designing,considering the actual stress characteristics of the pile foundation,reference can be made to the equivalent neutral point of the loading process to achieve safety,economy,and reasonableness.(5)After calculation,the total load value of negative friction resistance of S2 pile reaches about 1700 k N and the sum of limit load 2580 k N is 4280 k N,which is basically the same as the ultimate bearing capacity of S1 pile,indicating the negative friction resistance generated in working condition 2.Loading is the main reason for the reduction of the ultimate load.