摘要
研究饱和黏性土中静压沉桩引起的桩土界面孔压增量及有效径向应力的变化规律对工程实践具有重要意义。利用自制的大比例模型试验系统,通过双壁开口和闭口管桩,实现了开口和闭口桩桩身表面嵌入式安装微型测试元件,得到了考虑孔压增量的桩土界面有效径向应力的变化规律。研究结果表明:入土深度越大,桩土界面孔压增量及有效径向应力越大,闭口桩桩土界面超孔压大于开口桩;桩身上部桩土界面超孔压和有效径向应力小于桩身下部;同一入土深度,随着桩身h/L的增加,桩土界面土压力存在侧压力“退化”的现象;特定试验条件下,闭口和开口桩桩土界面超孔压与上覆有效土体自重比值最大值分别是61.2%和52.1%,桩土界面有效径向应力是超孔压的3.76~5.46倍。桩土界面超孔压和有效径向应力与桩身h/L位置有关。
在饱和黏性土地基中,对静压桩沉桩效应及长期承载力的发挥产生影响的重要因素是桩土界面超孔隙水压力和土压
许多学者通过试验研究了桩周孔压增量和径向应力的变化规律。Hwang
为便于观察沉桩过程,试验模型箱的正面安装尺寸为500 mm×500 mm的钢化玻璃窗。加载系统主要由反力架和横梁、液压千斤顶、电控系统、静载控制系统等组成,以实现试验模型桩的静力沉桩过

图1 大比例模型试验装置
Fig. 1 Large proportion model test device
进行压桩试验前,对模型箱中的土体进行采样,根据《土工试验方法标准》(GB/T 50123—2019
室内模型试验共有2根试桩TP1(闭口)、TP2(开口),试桩TP1桩端安装与桩身等直径的底板。模型管桩采用铝制材料,弹性模量为72 GPa、泊松比为0.3。试桩TP1和TP2桩长均为1 000 mm,内、外径分别为100、140 mm。双壁模型管桩的结构示意图如

(a) 闭口管桩

(b) 开口管桩
图2 双壁模型管桩结构示意图
Fig. 2 Schematic diagram of double-wall model pipe pile structure

图3 传感器布设
Fig. 3 Sensor placement
为了充分利用模型箱空间,沉桩桩位布置见

图4 桩位布置
Fig. 4 Layout of pile position
从

图5 压桩力
Fig. 5 Pile driving pressure
沉桩过程中试桩TP1和TP2孔隙水压力传感器量测的孔隙水压力总量与入土深度的关系如

(a) 试桩TP1

(b) 试桩TP2
图6 试桩TP1和TP2沉桩过程中孔隙水压力分布图
Fig. 6 Pore water pressure distribution diagram of test pile TP1 and TP2 during sinking process
从
传感器位置 h/L | 超孔隙水压力/kPa | 上覆有效土重/kPa | 比值/% |
---|---|---|---|
1/20 | 4.21 | 6.88 | 61.2 |
1/10 | 3.2 | 5.76 | 55.6 |
1/5 | 2.35 | 4.64 | 50.6 |
2/5 | 1.66 | 3.52 | 47.2 |
3/5 | 0.79 | 2.4 | 32.9 |
传感器位置h/L | 超孔隙水压力/kPa | 上覆有效土重/kPa | 比值/ % |
---|---|---|---|
1/20 | 3.58 | 6.88 | 52.1 |
1/10 | 2.68 | 5.76 | 46.5 |
1/5 | 2.24 | 4.64 | 48.3 |
2/5 | 1.56 | 3.52 | 44.3 |
3/5 | 0.87 | 2.4 | 36.3 |

(a) 试桩TP1

(b) 试桩TP2
图7 试桩TP1和TP2沉桩过程中桩侧土压力分布
Fig. 7 Distribution of lateral earth pressure of test pile TP1 and TP2 during sinking process
为了解释桩侧土压力“退化”现象,将相邻传感器侧压力退化值用“Δ”符号在

图8 桩土界面有效土压力与超孔隙水压力的比较
Fig. 8 Comparison between effective soil pressure of pile-soil interface and excess pore water pressure
通过开展均质黏性土体中静压沉桩室内试验,研究了沉桩过程中桩土界面超孔隙水压力和有效土压力的变化规律,得到以下结论:
1)桩土界面孔隙水压力和孔压增量与入土深度成正比关系,在同一桩身位置处闭口试桩桩土界面孔隙水压力大于开口试桩,桩土界面孔隙水压力增量幅值均随着h/L增大而减小。
2)桩土界面径向土压力在不同h/L位置处随着入土深度的增加呈线性增长趋势,桩土界面径向土压力受桩端形式影响不大,闭口和开口桩桩土界面径向土压力随着h/L增大而减小。
3)闭口和开口桩桩土界面径向有效土压力均为桩土界面土压力的主要组成部分。随着h/L增大,桩土界面有效土压力递减速率大于孔压增量。
由于研究仅限于给定的均质黏性土、桩端形式不同的模型桩和h/L位置,其他不同土层、模型桩和h/L位置条件下的桩土界面土压力、超孔隙水压力及有效土压力的变化规律还有待进一步深入研究。
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