黄土场地震陷时桩基负摩阻力的现场试验研究

Field tests on negative skin friction along piles caused by seismic settlement of loess

有关强震诱发黄土震陷时桩基负摩阻力的研究资料极少,其影响尚未能引入桩基设计之中。通过开展爆破模拟地震动诱发黄土震陷条件下桩基负摩阻力的现场观测试验,分析研究了强震引发黄土震陷时桩基负摩阻力的基本特征。本次现场试验在选定的黄土场地内布置试桩两根,每根试桩内按2 m间距对称植入钢筋应力计20个;地震动由15组炸药微差延时爆破模拟生成,试验场地布设地震计17个。现场试验期间,整个场地所有沉降观测点都产生了不同程度的震陷,最大震陷量达33 mm;桩基负摩阻力量值显著,桩身负摩阻力平均值达54 kPa,桩身总的负摩阻力约1654 kN。试验的观测结果表明,地震诱发黄土震陷时的桩基负摩阻力不容忽视,应当在桩基设计时加以考虑。

Pile foundation has been widely used in the earthquake-prone loess area of China.However,the negative skin friction（NSF） along piles caused by a sudden settlement of loess during strong earthquakes have not been taken account into the design of pile foundation due to the lack of NSF data from field tests.In order to investigate NSF along piles in loess during the seismic settlement and to develop a method to estimate it,field tests were performed by means of a series of explosions（short delay blasting） at a loess site where two piles were grouted.The ground motion created by the explosions was strong enough to induce an obvious settlement in the site.There were 40 stress gages amounted on the two piles with an interval of 2 m to collect the data of NSF along depth during the tests.At the same time,the ground motion and settlement of loess site created by the explosions were respectively observed by accelerographes and a level gauge.During field tests,the maximum seismic settlement of loess field reached 33 mm while the average NSF along piles attained 54 kPa and the corresponding total NSF was about 1654 kN.It was shown by these test data that the NSF along piles induced by seismic settlement of loess was significant and should not be ignored in the design of pile foundation.