Experimental study on influences of shield machine reception on existing shield tunnels during construction of connecting channels
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摘要: 联络通道机械法施工中盾构机接收对已建盾构隧道受荷变形影响机制暂不明确问题,通过设计1∶10的缩尺模型试验,开展了联络通道机械法施工中盾构机接收对已建盾构隧道影响试验研究。试验结果分析表明:联络通道机械法施工时,盾构机开挖面附加水平土压力将导致接收端已建盾构隧道对侧的土压力在中部有显著的增大,而两端则减小,即导致顶推对侧发生了水平被动土拱现象;在中间附加水平土压力作用下,隧道在中间发生了显著的竖椭圆变形,而在两端则发生了一定的横椭圆变形;隧道中间发生竖椭圆变形对上下地层形成挤压,在竖向上同样形成了被动土拱现象;隧道纵向挠曲变形时出现了一定的反弯现象。盾构隧道作为地层中的管状结构物,在地层中纵向变形分析时需要考虑横断面变形的影响。Abstract: The influence mechanism of shield machine reception on the deformation of the existing shield tunnels under loads during the mechanical construction of connecting channels is not clear. By designing the scale model tests of 1∶10, the influences of shield machine reception on the built shield tunnels during the mechanical construction of connecting channels are investigated. The analysis of the test results shows that during the mechanical construction of connecting channels, the additional horizontal earth pressures on the excavation face of the shield machine cause the earth pressures on the opposite side of the built shield tunnels at the receiving end to increase significantly in the middle, while decreasing at both sides. That is to say, the horizontal passive soil arching occurs at the opposite side of the jacking. Under the action of the additional horizontal earth pressures in the middle, the tunnel has a significant vertical elliptical deformation in the middle. At both sides, a certain transverse elliptical deformation occurs. The vertical elliptical deformation in the middle of the tunnel compresses the upper and lower strata, and also forms the passive soil arching phenomenon vertically. A certain reverse bending phenomenon occurs during the longitudinal deflection of the tunnel. As a tubular structure in the strata, the shield tunnel needs to consider the influences of the cross-section deformation when analyzing the longitudinal deformation in the strata.
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0. 引言
江苏各市广泛分布的软弱土,具有高含水率、高压缩性、低渗透性、低抗剪强度、高含盐量及显著的结构性与流变性等特点,对其处理较为复杂。针对此类软黏土,电渗法有较好的处理效果,但是电渗法存在耗能过高、加固不均匀的问题[1]。为缓解城市用地紧张,根据已有研究成果,提高电渗加固软土性能主要有两种研究思路:①通过改变电极材料[2-3]、电极布置形式[4-5]和通电方式[6]等初始条件;②将电渗与其它方法联合使用,常见的有电渗-堆载[7]、电渗-真空预压[8]、化学电渗[9]等。本研究在第二种思路的基础上,将电渗法与堆载预压和化学灌浆结合,以期使电渗法更加经济可行。为探讨该法的可行性,本文开展电渗-堆载-化学灌浆联合法(Electro-Osmosis-Surcharge Preloading-Chemical Grouting,简称EO-SC-CG)和化学电渗(Electro-Omosis-Chemical Grouting,简称EO-CG)的对比试验,从排水量、通电电流、有效电势、十字板剪切强度、含水率等方面证实电渗-堆载-化学灌浆联合法的有效性。
1. 试验材料与试验方法
1.1 试验材料
室内模型试验所用土样为取自江苏盐城地区的滩涂软土,通过室内土工试验对软土的基本物理性质进行测试。试验前,将原状土烘干后击碎,然后倒入搅拌桶中充分搅拌均匀后静置24 h,再对软土进行重塑,使试验用滩涂软土的含水率达到40%,最终得到重塑土的基本物理性质指标如表1所示。
表 1 重塑土的基本指标Table 1. Basic parameters of remolded soil含水率 w/% 液限wL/% 塑限wP/% Gs 不排水抗剪强度cu/kPa 40 30.8 13.9 2.71 ≈0 1.2 试验装置
室内模型试验(EO-SC-CG与EO-CG)采用自制试验装置,主要由土样室和排水室两部分组成,其中排水室内的排水孔为直径25 mm的圆形孔洞,如图1所示。EO-CG装置模型与前者相同,区别仅在于EO-CG方法没有施加充当均布荷载的上覆砂。模型箱采用亚克力板材制成,模型箱尺寸为400 mm×300 mm×200 mm。阳极采用尺寸为350 mm×150 mm×3 mm的铁板;阴极所用电极尺寸与阳极相同,在电极板上均匀打下48个孔径为4 mm的小孔。注浆管采用内径9 mm,外径11 mm的PVC管,管壁均匀设置小孔,并将管底封闭,有利于注入的化学浆液向土体扩散,同时能够有效控制化学浆液过快的向土体底部沉积。阴极注浆材料选用Na2SiO3溶液,阳极注浆材料选用CaCl2溶液[10]。电导线采用多股铜芯电导线,导体材质为无氧铜,绝缘材料为聚氯乙烯。装置图1的上覆砂均匀铺在土样层上,既起到堆载的作用,又可以消除电渗模型几何边界引起的尺寸效应[11]。
1.3 试验方案
本文主要研究EO-SC-CG与EO-CG两种加固方法对盐城地区滩涂软土的加固效果,试验分为两组,基本参数如表2所示。试验的初始含水率为40%,电势梯度选取1 V/cm[12],电源电压均为23 V。
表 2 试验基本参数Table 2. Basic parameters of experiments组别 试验时间/h 堆载大小/kPa 注浆材料与注浆量 EO-SC-CG 48 1.5 CaCl2 (45mL)+Na2SiO3(45mL) EO-CG 48 0 CaCl2(45mL)+Na2SiO3(45mL) 试验开始前,将阳极电极放置在远离排水室一侧,阴极电极放置在靠近排水室一侧;两侧注浆管均放置在距电极3 cm处;分别放置两根测针在电极与注浆管中间。因为EO-SC-CG涉及施加堆载时间,故先开展EO-CG试验。两组试验开始通电后实时观测记录通电电流,电势与排水量。待排水量不再增加时,关闭电源,分上、中、下三层按距离阳极0,5,10,15,20 cm,取土样测量十字板剪切强度与含水率,土样测试点位置如图2所示。过程中两组试验注浆时间均定在电流大幅降低且保持稳定的时刻。依据袁国辉[13]进行的电渗-堆载联合试验,当电渗固结度达到40%时为最佳堆载时间。故EO-SC-CG可根据EO-CG得到最终沉降量S∞,利用平均固结度表达式:Uavg=St / S∞,得到固结度达到40%时的沉降量,施加堆载。
2. 试验结果与分析
2.1 排水量与排水速率
排水量与排水速率随时间的变化曲线如图3所示。因为施加堆载的作用,EO-SC-CG的排水量最终高于EO-CG。两组试验的排水量分别为1360,1170 mL,EO-SC- CG的排水量相对EO-CG增加16.2%。由图3可知,排水速率随时间呈现出逐渐减小的趋势,并且在化学注浆后排水速率均会先达到一个峰值点,之后逐步下降。EO-CG和EO-SC-CG分别在试验进行至10 h和8 h时注浆,注浆后排水速率1 h内增幅分别约为28.9%和14.3%,达到峰值时增幅分别约为34.1%和37.5%。因堆载预压的加持作用,EO-SC-CG的峰值增幅稍大。结合微观观测,随着电渗的进行,注入的浆液在直流电作用下生成CaSiO3并填充土体孔隙,导致土体的渗透性降低,进而影响土体的排水速率。试验后期,EO-SC-CG的排水速率高于EO-CG,说明EO-SC-CG因施加堆载预压,在一定程度上能够缓解土体后期排水效果较差的趋势。
2.2 有效电势
有效电势随时间的变化曲线如图4所示。由图4可知,两组试验的有效电势均呈现出先增加后减小的趋势,且其变化速率在注浆后都呈现出加快的趋势,说明注浆后,土中可移动的离子浓度增加,促进了土体内的离子移动速率,导致电阻减小,有效电势增加。EO-SC-CG在11 h施加堆载时,其有效电势较前一时刻没有明显变化,且达到第一次峰值的时间与EO-CG基本一致,说明施加堆载对有效电势的提升有限。两组试验的有效电势在第一次峰值后均呈现下降趋势,但是EO-SC-CG的下降速率较缓。因为阳极不断发生电化学反应,生成的胶结物附在土体表面,导致电极与土体接触界面上电阻增大,有效电势减小;加之阳极附近土中的水不断向阴极移动,致使阳极区土体失水产生裂缝,接触电阻增大。而EO-SC-CG的有效电势下降速率较缓是因为堆载作用能够有效抑制裂缝的产生,使得电阻增大缓慢。比较两组试验后期的曲线可知,EO-SC-CG的有效电势相对较大,进一步说明堆载作用在一定程度上能够抑制裂缝产生,减缓有效电势的减少,使有效电势总体上变化较为均匀。
2.3 十字板剪切强度与最终含水率
将所得结果在同一距离不同深度的强度以及含水率取均值,得到抗剪强度与最终含水率在电极间的分布如图5所示。由图5可知,抗剪强度随距阳极的距离增大而减小,阳极附近土体的抗剪强度最大。土中的水在电渗作用下,自阳极移动至阴极,阳极附近因为铁质电极的腐蚀,生成Fe2+、Fe3+的氧化物与氢氧化物等,一定程度上能够胶结土体。同时因为注浆作用,阳极附近发生化学反应生成Ca(OH)2、CSH和CAH等填充土体孔隙,使阳极附近的土体强度得到提升。两组试验中,EO-SC-CG的平均抗剪强度相对EO-CG提高约14%,故堆载对土体抗剪强度的提升具有一定作用。因为堆载产生的自重作用对土体进行了压密,导致土体抗剪强度的提升。由于电渗作用,孔隙水不断自阳极流向阴极,含水率的分布呈现出从阳极到阴极逐步增大的规律。相比EO-CG,EO-SC-CG处理后的土体含水率较低,减少约17.8%。将同一深度不同距离的抗剪强度与含水率取均值,得到抗剪强度与最终含水率随深度分布如图6所示。由图6可知,土体的抗剪强度沿深度逐渐降低,呈现出表层>中层>底层的规律,EO-SC-CG得到的平均强度相比EO-CG提高了14%。相比EO-CG,EO-SC-CG试验处理后同一深度的土体含水率较低,减少约17.6%。
3. 结论
通过电渗-堆载-化学灌浆与电渗-化学灌浆两组室内试验,分析试验过程中排水量、排水速率、有效电势、十字板剪切强度与含水率等,得以下结论:
(1)在EO-CG的基础上增加堆载对电渗排水有一定的促进作用,相对EO-CG,EO-SC-CG的排水速率增加25.8%,平均抗剪强度提高14%。同时,EO-SC-CG的有效排水时间更长,堆载作用在一定程度上能减缓土体后期排水速率降低的趋势。
(2)堆载一定程度上抑制裂缝产生,阻止有效电势减少,进而使有效电势总体上变化较均匀。
(3)EO-SC-CG不仅能促进土体排出水分,提高土体的密实度与强度,同时也能改善电极与土的接触性,实现电渗、化学灌浆和堆载预压的共同加固。
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