Centrifugal model tests on influences of unsupported length of tunnels on stability of excavation face
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摘要: 考虑了隧道开挖过程中无支撑长度的影响,结合机械手系统开发了一整套离心试验装置。通过开展不同无支撑长度下C/D、内摩擦角的离心试验,研究了在考虑无支撑长度下不同土质开挖面支护力变化规律及失稳模式机理,提出了应考虑无支撑长度上方土体的双块失稳体。结果显示,在考虑无支撑的条件下,土体的失稳由无支撑上方土体和开挖面前方土体两部分组成,且两部分不是同时发生失稳,打破了传统对于开挖面失稳的认知;随着C/D的增大,土体的失稳由开始的两部分组成逐渐转变为一部分组成,且支护力也逐渐减小;无支撑长度与开挖面支护力呈线性增加,同时缩短了土体进入失稳阶段的时间;随着内摩擦角的增大,极限支护力逐渐减小并趋于稳定值,但并未对失稳模式产生影响。Abstract: Considering the influences of unsupported length during the process of tunnel excavation, a complete set of centrifugal test device is developed combined with the manipulator system. Based on the centrifugal tests on C/D and internal friction angle under different unsupported lengths, the variation laws of support force and the mechanism of instability mode of different soil excavation faces under the consideration of unsupported length are studied, and the double-block instability body above the unsupported length should be considered. The results show that considering the unsupported condition, the instability of soil is composed of the soil above the unsupported part and the soil in front of the excavation face, and the two parts do not lose stability at the same time, which breaks the traditional understanding of the instability of the excavation face. With the increase of C/D, the instability of soil gradually changes from two parts to one part, and the support force also decreases. The length of unsupported part increases linearly with the support force of excavation face, and shortens the time for soil to enter the instability stage. With the increase of the internal friction angle, the ultimate support force gradually decreases and tends to a stable value, but has no effects on the instability mode.
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Keywords:
- tunnel /
- centrifugal test /
- unsupported length /
- support force of excavation face /
- instability mode
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0. 引言
我国已建、在建的一批重大水利工程采用了特高心墙坝,如苗尾(131.3 m)[1]、毛尔盖(147 m)[2]、长河坝(240 m)[3]、糯扎渡(261.5 m)[4]、两河口(295 m)[5]、双江口(312 m)[6]、如美(315 m)等。心墙坝具有就地取材、适应复杂地形、施工机械健全的优点,但也存在一个直接关系到大坝安全性能的凸出技术难点:接触黏土与岸坡和心墙变形协调性问题。对于高心墙堆石坝来说,接触界面处复杂的应力和变形条件及可能存在的剪切裂缝,是导致事故发生的可能原因。
接触黏土是心墙坝重要的特殊用途的土料。设置接触黏土的目的在于提高心墙与坝基岸坡接触部位抗冲刷能力和抗裂性能,保证心墙出现不均匀沉降时不与岸坡脱裂。接触黏土必须具备良好的塑性和黏性、良好的抗渗变形能力,一般采用高塑性黏土填筑[7-8]。
如美水电站位于西藏自治区芒康县境内,是澜沧江上游河段(西藏境内河段)规划一库七级开发方案的第五个梯级。工程规模为一等大(Ⅰ)型工程,采用砾石土心墙堆石坝,最大坝高315 m。心墙与岸坡之间设置水平厚度4 m的接触黏土层。研究接触黏土层在大坝填筑期和运行条件下的变形特性,分析是否有大剪切变形及剪切裂缝的产生可能,对于大坝的安全具有重要的理论和实际工程意义。因此,本文重点关注不同坝高区域内高塑性接触黏土层在坝体填筑加载过程中的变形,以及变形在竣工后的发展。
1. 试验原理和设备
1.1 离心模型试验技术
土工离心模型试验技术[9]是一项崭新的土工物理模型技术。通过施加在模型上的离心惯性力使模型的重度变大,从而使模型的应力与原型一致,这样就可以用模型反映、表示原型。离心模型是各类物理模型中相似性最好的模型。我国岩土力学的开拓者、两院院士黄文熙先生称“离心模型是土工模型试验技术发展的里程碑”[10]。
1.2 试验设备
试验在南京水利科学研究院NHRI60gt中型土工离心机上开展。该机的有效半径2 m,最大加速度200g,最大负荷300 kg;配有40路应变信号和20路电压信号高精度数据采集系统,以及图像实时监控采集系统。试验用模型箱的内部有效尺寸为700 mm× 450 mm×350 mm(长×高×宽),其一侧面为有机玻璃窗口,便于监控试验过程。
1.3 数据图像分析系统
通过数据图像分析(PIV)系统,记录试验过程中土体照片,该系统由高清摄像机、支持POE供电的Hub、无线路由器、监视PC机组成。试验时,通过摄像机透过模型箱一侧的有机玻璃板,实时记录模型土体在试验过程中的变化情况。对模型土体变形照片进行镜头校正后,应用PIV技术[11]进行分析,得到土体的变形情况。
2. 试验方案
2.1 模型设计
如图 1中虚线框所示,沿着左侧坝基的岸坡,选择了4个不同高程的位置,开展4组接触黏土层变形特性离心模型试验(表 1)。模型几何比尺为1/20,每组试验分别模拟了特定上覆压力下(采用铅丸作为等效荷载),高度为6 m范围内的接触黏土层和部分心墙土体的变形情况。模型的布置如图 2~4。
表 1 试验条件Table 1. Details of tests编号 高程/m 上覆压力/MPa 岸坡情况 L1 2850—2856 0.59 1∶1.2 L2 2800—2806 1.23 1∶1.2 L3 2764—2770 1.56 变坡处 L4 2710—2716 2.33 1∶0.85 2.2 试验土料
试验需要模拟岸坡、接触黏土层、心墙。岸坡采用混凝土模拟。接触黏土和心墙土料均取自工程现场,级配如图 5。
2.3 试验过程
(1)在模型箱内浇筑混凝土,充分振捣、整平后进行养护(28 d以上),模拟河谷左侧的岸坡。
(2)整理接触黏土和心墙防渗料(5 mm以下),剔除杂质,密封24 h后,采用四分法取样测定风干含水率[12];再按施工含水率分别进行配制,密封24 h以上备用。
(3)采用先分层击实再切削成的方法制备接触黏土层。接触黏土施工含水率为14%,按98%压实度控制,制样干密度约为1.93 g/cm3。
(4)采用分层击实法制备防渗心墙。5 mm以下粒径心墙土料按照含水率6.3%配制,掺入5 mm以上级配料,拌和均匀后,加入模型箱中击实。击实时按98%压实度控制,制样干密度约为2.15 g/cm3。
(5)打开模型箱侧面有机玻璃,在模型土体侧面绘制变形网格和标记点。
(6)安装模型箱侧面有机玻璃面(其上布置有固定的参考点,用于在PIV分析中把像素坐标转换为实际坐标),在模型土体上部加铅丸,模拟上部坝体的应力。将制备好的模型吊入离心机吊篮平台,调整配重。
(7)开启离心机,逐级提高离心加速度至设计值,增加铅丸的自重,对模型进行分级加载,加载至设计上覆压力即认为上覆荷载施加稳定,而后稳定运行4 h(相当于原型约66.7 d),模拟长期稳定荷载作用。试验期间记录土体的图像。加载过程如表 2。
表 2 试验过程Table 2. Loading procedures of tests编号 上覆压力/MPa 逐级加载过程/kPa(至设计压力后运行4 h) L1 0.59 0→148→295→442→590 L2 1.23 0→308→615→922→1230 L3 1.56 0→390→780→1170→1560 L4 2.33 0→518→1036→1553→1942→2330 (8)试验完成后停机。
3. 试验结果
本文给出的试验结果均已换算至原型。
3.1 变形矢量图和网格图
图 6给出了4组模型填筑完成时(加载至设计上覆荷载)的土体变形矢量场(放大4倍)。可以直观地看出:上覆荷载越大变形越大;心墙土体在荷载下产生以竖向下沉为主的变形;接触黏土层与岸坡之间的相对变形较小,有轻微错动但没有明显的分离现象;接触黏土与心墙土体之间也没有错动,土体的变形是协调的。
3.2 接触黏土变形分解
如图 7所示,接触黏土在荷载作用下发生了变形,对于接触黏土层中的某一点,变形量可以分解为沿坝基切向的剪切变形和沿坝基法向的压缩变形。本文选择模型顶部接触黏土与心墙结合点(土体变形最大点)进行分解,以开展进一步的分析。
图 8和图 9给出了4组局部模型接触黏土最大压缩变形和剪切变形与上覆荷载的关系。可以发现:①接触黏土变形随着荷载逐渐增加,大体呈现出随着荷载对数线性增加的趋势;②压缩变形和剪切变形大体相当,接触黏土处于压剪状态;③接触黏土变形随荷载的发展程度,在坝基坡度较缓时较小,坝基坡度较陡时较大,在坡度变化处则介于两者之间。
3.3 接触黏土长期变形规律
图 10和图 11给出了坝体填筑完成后运行期接触黏土最大压缩变形和剪切变形随时间的发展情况。接触黏土层的压缩和剪切变形在运行期初期有所增加,而后增长速度较慢,平均约为0.8 mm/d;在试验所模拟的约66.7 d时间内,变形渐趋稳定,且始终保持压剪状态。
4. 结论
通过4组局部模型试验,得到以下结论:
(1)接触黏土层与岸坡之间的相对变形较小,有轻微错动但没有明显的分离现象;接触黏土与心墙土体之间也没有错动,土体的变形是协调的。
(2)上覆荷载引起接触黏土产生垂直坝基的压缩变形和平行坝基的剪切变形,荷载越大变形越大;压缩和剪切变形均大体呈现出随着荷载对数线性增加的规律。
(3)接触黏土变形随荷载的发展程度随岸坡的坡度而加剧,在坝基坡度较缓时较小,坝基坡度较陡时较大,在坝基坡度变化处则介于两者之间。
(4)填筑完成后的运行期,接触黏土层的压缩和剪切变形在初期有所增加,而后变形增长速度较慢,平均约为0.8 mm/d;在试验所模拟的约66.7 d时间内,变形渐趋稳定。
(5)填筑过程和运行期,接触黏土层压缩变形和剪切变形大体相当,接触黏土始终处于压剪状态。
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