Reinforcement mechanism and parameter analysis of horizontal high-pressure rotary jet grouting piles for tunnels in tertiary semi-diagenetic water-rich sandstone
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摘要: 第三系半成岩具备弱胶结、遇水软化等特征,隧道建设时易诱发涌水突泥和坍塌等地下地质灾害。依托云南临清高速公路王家寨隧道,结合现场地质条件与现有工程案例,采用水平高压旋喷桩对软弱围岩进行超前预加固。结合地基梁理论解析、数值模拟与现场监测方法,研究第三系半成岩水平高压旋喷桩加固机理及不同桩体参数对围岩加固效果的影响,并给出桩体参数建议范围,为类似工程提供参考。研究表明:旋喷桩加固后解析解与数值解的剪力与弯矩分布规律较为一致,在开挖未支护段桩体所受弯矩、剪力最大,最易发生断裂破坏;当围岩水压力在300 kPa时旋喷桩体最大拉应力达598.21 kPa,接近桩体极限抗拉强度,水压力小于300 kPa时水平旋喷桩能有效发挥梁、拱协同作用,拱棚效应与阻水效果显著,能将围岩压力传递给桩体后端及拱肩、边墙处,围岩沉降与地下水压得到有效控制,这与现场监测结果基本吻合;桩径、桩长、咬合厚度及搭接长度变化对桩体应力影响较大,影响程度为咬合厚度 > 桩长 > 搭接长度 > 桩径,建议在第三系半成岩富水砂岩隧道围岩水压力小于300 kPa进行超前预加固时,水平旋喷桩桩体参数范围为桩径65~70 cm、桩长10~13 m、咬合厚度25 cm、搭接长度3~4 m。Abstract: The tertiary semi-diagenetic rocks are characterized by weak cementation and susceptibility to water softening, which can easily trigger underground geological disasters such as water inrushes, mud outbursts and collapses during tunnel construction. Focusing on the Wangjiazhai tunnel of the Lincan-Qingshuihe Expressway in Yunnan Province, considering the geological conditions and the relevant engineering case studies, the horizontal high-pressure rotary jet grouting piles are employed to pre-emptively strengthen the weak surrounding rock. Through the combination of theoretical analysis of foundation beams, numerical simulation and field monitoring methods, the reinforcement mechanism of horizontal high-pressure jet grouting piles in the tertiary semi-diagenetic formations is investigated, and the influences of varying pile parameters on the reinforcement effectiveness of the surrounding rock are examined, offering a suggested range for these parameters to serve as a reference for similar projects. The results indicate that the distribution of shear force and bending moment in both analytical and numerical solutions agrees with that in the unsupported section, where the pile experiences the highest bending moments and shear forces, making it most susceptible to fracture failure. Under the water pressure of the surrounding rock of 300 kPa, the maximum tensile force within the rotary jet grounting pile reaches 598.21 kPa, close to the ultimate tensile strength of the pile. When the water pressure is below 300 kPa, the horizontal rotary jet pile can effectively give full play of the combined effects of beam and arch, significantly enhancing both the arch action and the water-blocking effects. This allows the pressures on the surrounding rock to be redistributed to the pile rear, the spandrel and the sidewall. The settlement of the surrounding rock and the underground water pressure are effectively controlled, which are consistent with the field monitoring results. The pile diameter, length, occlusion thickness and overlap length significantly affect the pile stress, in the descending order of impact: occlusion thickness, pile length, overlap length and pile diameter. It is recommended that for the pre-reinforcement of tunnels in the tertiary semi-diagenetic water-rich sandstone with the water pressure of the surrounding rock below 300 kPa, the parameters of the horizontal rotary jet grouting pile should be as follows: pile diameter of 65~70 cm, pile length of 10~13m, occlusal thickness of 25 cm, and overlap length of 3~4 m.
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0. 引言
污泥往往具有极高的含水率,需要对污泥进行脱水,以方便后续处理[1]。然而污泥胞外聚合物(EPS)的成分极其复杂,使污泥中的水分很难被去除[2]。为了降低污泥的含水率,国内外已进行了很多新技术的研究,包括微波辐射、热水解、超声波以及新型絮凝剂的开发等[3-6]。超声波处理是改善污泥脱水性能的有效处理方法之一[7-8]。污泥液相在超声波的作用下产生大量的空化气泡及产生强大的剪切力和瞬时高温,能够有效地裂解胞外聚合物(EPS),使大量结合水释放[9-10]。超声波作用使絮体颗粒尺寸变小,破坏菌胶团的结构,使其间所含结合水转化为自由水[11]。超声波还可以杀灭污泥中的病毒、细菌和其他有害物质,提高重金属的浸出率和回收率等[12-13]。超声波处理污泥改善脱水性能受到超声波频率、超声时间、声能密度、pH值、作用方式、耦合方法等因素的影响[14]。目前,超声波技术仍有一些局限性,超声波处理污泥脱水的一些机理尚未能够充分认识[15]。本文拟在超声波作用时间对污泥脱水性能影响方面进行研究,以达到最佳处理效果。从而为在实际工程应用中更好地利用超声波的优势提供理论参考。
1. 材料和方法
1.1 污泥试样
试验污泥选用当地城镇污水处理厂二沉池活性污泥,为保证各组试验初始条件的一致性,采用同一批污泥,测试初始含水率为97.13%,密度为1.026 g/cm3。原污泥基本性质如表 1所示,粒径分布采用激光粒度分析仪测定,如图 1所示。
表 1 原污泥基本性质Table 1. Characteristics of raw sewage sludge含水率/% pH SV30/% 密度/cm3 污泥温度/℃ 97.13 8.87 54 1.026 24.1 ± 2 SRF/(1013·m-1·kg-1) d10/μm d50/μm d90/μm Mean/μm 4.76 13.680 49.129 172.116 73.109 注:SV30表示污泥沉降比;SRF表示污泥比阻;d10表示颗粒累积分布为10%的污泥粒径;d50为中值粒径,表示颗粒累积分布为50%的污泥粒径;d90表示“颗粒累积分布为90%的污泥粒径;Mean表示粒径加权平均值,即平均污泥粒径。 1.2 仪器设备
本试验仪器有SM-900A超声波细胞破碎仪,LT2200E激光粒度分析仪,PXBJ-287L型便携式离子计,JJ-4B六联异步电动搅拌器,CS-101-2电热干燥箱,Coxem EM-30 PLUS台式扫描电子显微镜,CR21N高速冷冻离心机等。
1.3 试验方法
取原污泥5盒各300 mL,设定超声波频率20 kHz,声能密度9.8 W,调节超声波作用时间为5,15,30,45,60 s。待每组污泥试样超声处理完成后,检测污泥的pH值、粒径分布。试样放入离心机脱水,检测离心脱水后的污泥泥饼含水率,确定超声波调理污泥的最优作用时间。取适量污泥,进行扫描电镜试验,分析污泥的微观结构。
1.4 分析方法
(1)污泥含水率
污泥加入离心机试样瓶,离心机必须同时放置4个离心试样瓶且4个试样瓶各自的质量差值小于4 g,底部用滤布进行过滤,污泥经6000 r/min,5 min离心作用后检测滤饼含水率。污泥含水率的测定采用热干燥法,离心机脱水后的泥饼放入恒温烘箱中烘至恒重,取平均值。
(2)污泥pH值
采用便携式离子计的pH电极进行测量。
(3)污泥粒径
用去离子水将污泥样品稀释至浓度为15 mg/L的混合液,采用激光粒度分析仪测定污泥粒径分布,每个样品测定3次后取平均值。
(4)污泥微观结构
污泥样本在烘箱里烘干后,采用扫描电子显微镜(SEM)对污泥样本进行分析,放大倍数分别为200,500,1000,2000,5000倍。
2. 结果与讨论
2.1 污泥含水率的变化
污泥含水率为污泥中水的质量与污泥总质量之比。超声波作用后的污泥经离心机脱水后的泥饼含水率随超声波作用时间的变化曲线,如图 2所示。
从图 2中曲线可知,当超声波作用时间小于30 s,污泥泥饼含水率随超声波作用时间的增加逐渐降低,表明污泥脱水性能得到改善。经30 s的处理时间后降至最低值。当超声波作用时间大于30 s,污泥泥饼含水率反而升高,显示污泥脱水性能逐渐恶化。这意味着额外延长超声时间,处理效果反而变差。长时间超声波作用使污泥絮体过分破碎,过度裂解了污泥絮体和微生物细胞结构,释放出的核酸、蛋白质、脂肪微粒和无机物微粒等微小聚合物,增加了污泥的黏度,致使污泥又重新吸附水分,结合水增加,脱水性能恶化。上述污泥泥饼含水率变化规律说明超声波作用时间存在一个最优值,作用时间过短和过长均不利于污泥脱水性能的提高。
2.2 污泥pH值的变化
污泥pH值随超声波作用时间的变化曲线如图 3所示。
由图 3中曲线可知,随着超声波作用时间的延长,超声处理后的污泥pH值稍有下降,但下降趋势不明显。由于污泥絮体和细胞结构被破坏,在释放细胞内部水分的同时,也释放内部的有机物质,包含有机酸或碳酸类物质,该过程改变了污泥的化学特性,使pH降低。
2.3 污泥粒径的变化
超声波作用时间对污泥颗粒粒度分布的影响,如图 4所示。
从图 4曲线可以看出,污泥颗粒粒径主要分布在10~200 μm范围内,随着超声波作用时间延长,污泥颗粒粒径逐渐变小。究其原因,由于污泥颗粒稳固的细胞结构,短时间的超声波处理可能达不到理想的能量输入,这部分能量不足以破坏大部分的细胞结构,只能破坏结合力较小的污泥絮体结构。逐渐延长超声波作用时间,能量输入不断增强,越来越多的细胞结构无法承受空化气泡崩溃时产生的巨大压力而被破坏,絮体断裂,颗粒粒径变小。继续延长超声波作用时间,能量持续输入,完全破坏了污泥絮体及细胞结构。长时间超声波作用使污泥絮体过分破碎,表现为污泥平均颗粒粒径进一步减小。
2.4 污泥微观结构的变化
图 5为原污泥及超声波作用时间5,15,30,45,60 s处理后污泥的SEM图。
图 5(a)为原污泥微观结构,从图 5(a)中可以看出,原污泥絮体结构较为完整,较多圆球状颗粒堆积胶黏在一起,微生物细胞极少裸露,被污泥絮体紧紧包裹,污泥絮体之间紧密结合,表面相对光滑完整,结构致密。图 5(b)为超声波作用时间5 s处理后的污泥微观结构,可见少量完整细胞裸露在外,污泥絮体变得松散;图 5(c)为超声波作用时间15 s处理后的污泥微观结构,可见较多完整细胞裸露在外,污泥絮体松散,可见大块状絮体聚集体;图 5(d)为超声波作用时间30 s处理后的污泥微观结构,可以看出污泥絮体更加松散,污泥颗粒粒径变小,污泥絮体结构遭到明显破坏,暴露的细胞数显著增加,污泥絮体解体,细胞壁破裂;图 5(e)为作用时间45 s处理后的污泥微观结构,可见,细胞壁凹陷破碎明显,污泥絮体重新聚集组合。图 5(e)为作用时间60 s处理后的污泥微观结构,细胞壁破碎严重,外层胞外聚合物EPS和微生物细胞破裂失活,污泥无机颗粒与微生物细胞碎片堆积胶结在一起,污泥重新变得很致密。
3. 结论
本文从污泥离心脱水含水率、pH值、颗粒粒径分布、微观结构等方面研究了超声波作用时间对污泥脱水性能的影响,得到以下3点结论。
(1)原污泥具有稳定的胶体系统,大量结合水被污泥絮体紧紧包裹无法释放,导致污泥脱水性能很差。超声波通过声空化作用和水力剪切作用裂解污泥絮体,将难以去除的结合水释放出来,改善污泥脱水性能。
(2)由于超声波作用,污泥在释放胞内结合水的同时,也将大量的有机酸或碳酸类物质的有机物质释放到污泥浆液中,使污泥pH值降低。
(3)无限延长超声时间,污泥脱水性能会变差。采用超声改善污泥脱水性能,应选择最优的超声波作用时间,超声波作用时间过短和过长均不利于污泥脱水,在实际应用时应引起重视。
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表 1 计算参数
Table 1 Parameters for calculation
围岩类型 弹性模量/MPa 泊松比 重度/
(kN·m-3)黏聚力/
kPa内摩擦角/
(°)厚度/
cm基床系数/
(MPa·m-1)半成岩砂岩 16.7 0.30 20.6 27.59 35.2 — 140 全风化花岗岩 65.0 0.32 19.1 60.00 25.0 — 1000 初支 钢拱架 210000 0.30 78.0 — — — — 喷射混凝土 23000 0.20 25.0 — — 29 — 旋喷桩体 1000 0.25 24.0 — — — — 表 2 水平高压旋喷桩参数设计表
Table 2 Parameter design of horizontal high-pressure rotary jet grouting piles
桩径/cm 桩长/m 搭接长度/m 咬合厚度/cm 60 10 1 10 65 13 2 15 70 15 3 20 75 17 4 25 80 — 5 30 -
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