摘要
能量桩是一种既可以与土体进行能量交换,又可以承担上部荷载的桩基形式。上部土层冻结,下部土层未冻结,由温度变化引起的桩体自身变形及土体的冻胀融沉引发的桩体位移是能量桩在季节性冻土地区推广中亟待解决的主要问题。针对季节性冻土地区土体温度分布特点,将土体分为冻结层和非冻结层分别开展模型试验,测得冻结层和非冻结层中能量桩多次温度循环后的桩—土温度分布、桩周土体孔隙水压力及桩体位移的变化规律。结果表明:在非冻结土层中,多次循环取热后桩顶会产生不可逆的沉降位移,5次取热循环后,桩顶沉降达到0.95%D(D为桩体直径),且桩体沉降未达到稳定;在冻结层,放热过程中能量桩会发生桩体融沉现象,恢复过程中会发生桩体冻胀现象,融沉导致的沉降位移随着循环次数的增加逐渐减小,在第3轮放热循环后消失。第1、2、3轮的融沉位移分别为5.9%D、0.93%D、0.11%D。每轮循环过程中,冻胀引起的上升位移虽逐轮减小,但在5轮循环之后依旧存在,且冻胀引发的总位移呈阶梯状上升,桩体最终产生上升位移,达到3.8%D。
中国国土面积辽阔,跨度大,冻土分布广泛,尤其是季节性冻土区,约占国土面积的53.5
近年来,学者们针对能量桩—土的热力学特性,已经开展了一系列研
目前对能量桩的研究很少考虑冻土的影响,尤其是冬季季节性冻土地区土体温度与外部环境温差较大,地热能源丰
试验选用土体为饱和黏土,物理力学参数如
密度/ (g·c | 颗粒比重 | 含水率/% | 饱和度/% | 塑限/% | 液限/% | 土体比热/(J·(kg·K | 导热系数/(W·(m·K | 渗透系数/ (cm· |
---|---|---|---|---|---|---|---|---|
2.02 | 2.76 | 25.7 | 98.8 | 19.3 | 33.8 | 1 370 | 1.61 |
0.5×1 |
模型试验桩包括参照桩和能量桩,模型桩由空心钢管组成,桩体外径为20 mm(D),桩体内径为18 mm,桩长为300 mm(L0),如

图1 模型桩
Fig. 1 Model pile
模型试验布置示意图如

(a) 平面布置图

(b) 纵向布置图
图2 试验布置示意图(单位:mm)
Fig. 2 Schematic diagram of the test arrangement(Unit: mm)
采用小型高低温交变试验箱,温度控制范围可达-40~100 ℃,用于控制环境温度,冻结土体。采用保温水箱和温控水循环装置,完成水速及水温的自动控制,土体外部的热交换管采用保温隔热处理。利用数据采集仪,完成对数据的自动采集和实时监控,试验系统实物图如

图3 试验系统实物图
Fig. 3 Physical diagram of the test system
在制备好的土样中埋设模型桩和测试仪器。使用直径和长度分别为18、400 mm的薄壁土壤采样器在模型桩所处位置的黏土中钻孔,然后将模型桩竖直插入钻孔至预定深度。埋设温度传感器前,预先将温度传感器用环氧树脂固定到钢钎(直径3 mm)上,构成温度传感器序列,随后将钢钎直接插入黏土中,达到设计高度。孔压计的埋设与温度传感器类似。模型桩和测试仪器埋设完成后,将模型槽整体放置于高低温交变试验箱中,桩顶安置电子式千分表。
对于冻结土层,土体温度低于零度,其相变温度区间为-1~0
首先开展未冻结层试验,即取热试验,保持高低温交变试验箱温度为15 ℃,当土体温度基本不再发生变化后,通过热交换管向桩体内通水240 min,水温控制为0~3 ℃,水速约为0.3 L/min,之后进行自然恢复240 min,随后继续开展冷循环试验,重复5次。
重新填槽,开展冻结土层试验,即放热试验,保持高低温交变试验箱温度为-5 ℃,单向冻结,当土体温度基本不再发生变化后,通过热交换管向桩体内通水300 min,水温控制为20 ℃,水速约为0.3 L/min,之后进行自然恢复600 min,随后继续开展热循环试验,重复5次。

(a) 取热

(b) 放热
图4 桩土温度的变化规律
Fig. 4 Measured temperature history of pileand surrounding soil

(a) 取热

(b) 放热
图5 桩体温度沿深度的变化规律
Fig. 5 Distribution of pile temperature along depth

图6 未冻结层中孔隙水压力随时间的变化规律
Fig.6 Time history of pore water pressure in non-frozen layer
马田田

图7 冻结层中孔隙水压力随时间的变化规律
Fig. 7 Time history of pore water pressure in frozen layer

(a) 桩顶位移随时间的变化规律

(b) 桩顶位移随温度的变化规律
图8 未冻结层中桩顶位移的变化规律
Fig.8 Variation of pile head displacementin non-frozen layer

(a) 桩顶位移随时间的变化规律

(b) 桩顶位移随温度的变化规律
图9 冻结层中桩顶位移的变化规律
Fig. 9 Variation of pile head displacement infrozen layer

图10 循环过程中能量桩位移累积的变化规律
Fig. 10 Accumulation displacement of energy piles in each cycle
基于室内模型试验,对季节性冻土地区能量桩的位移变化规律进行研究,得到以下结论:
1)季节性冻土中,取热(放热)过程中能量桩桩体及桩周土体温度随之下降(上升),每轮循环温度变化规律基本一致。在1倍桩径处,由于相变潜热的影响,恢复过程中冻结层土体会出现一段温度稳定期,即水—冰转换过程中吸收相变潜热的阶段。2倍和3倍桩径处土体温度变化较小,不存在温度稳定期。
2)未冻结层中,取热时能量桩孔压降低,自然恢复时,孔压升高。在冻结层中,放热时能量桩孔隙中的冰开始融化,体积缩小,孔压下降。温度恢复过程中则恰恰相反。
3)未冻结层中,桩顶位移随温度升高而增长,随温度降低而减小。随着循环次数的增加,桩体产生累积沉降。在冻结层中,桩顶位移首先随温度升高而增大,之后由于冻土中冰发生融化,使桩体发生较大的融沉位移,且每轮融沉位移逐渐减小,直至消失。温度恢复过程中,桩体先发生短暂沉降,随后由于土体冻胀,桩体产生上升位移。在每轮循环中,冻胀引发的位移逐轮减小,但并未消失,5轮循环后,桩体最终产生上升位移。
4)桩体融沉引起的位移明显变化点温度随循环次数的增加逐渐升高,而冻胀引起的桩体位移明显变化点温度基本一致,约在-0.5 ℃左右。冻结层中能量桩位移累积速率显著大于非冻结层,在冻土地区工作的能量桩安全问题不容忽视。但冻土中水—热耦合机理较为复杂,后续应开展大尺度模型试验和数值模拟,进一步研究季节性冻土中能量桩工作时的热力学响应。
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