若基底压力恰好等于P1 ，塑性区最大开展深度为
4B4
B4?0.75m?2m 所以A点不会被剪破。
第4题
解：由几何关系可以求出
?0?78.7
?当t=0时刻
M点超静孔隙水压力?u?26kPa
?1?3＝p??d'?(?0?sin?0)＋?（d?z）?'250?(3.14?78.7180??sin78.7)＋10.5?3?
?218.9kpa/62.7kpa?1??1??u?218.9?26?192.9kPa ?3'??3??u?62.7?26?36.7kPa ?/1m'???tg(45?2?/32??/2)?2Ctg(45??/??/2)?36.7?tg(45?292)?2?19?tg(45?292
)?170.7kPa//∵?1m?170.7kPa??1?192.9kPa ? 该土体单元处于破坏状态。
当t=≦时刻
M点超静孔隙水压力?u?0
''?1??1?218.9kPa ?3??3?62.7kPa
?/1m???tg(45?2?/32??/2)?2Ctg(45??/??/2)?62.7?tg(45?292)?2?19?tg(45?292
)?245.8kPa//∵?1m?245.8kPa??1?218.9kPa ? 该土体单元处于稳定状态。