Ground freezing monitoring techniques

Ground-freezing is used in mining and civil engineering to make water-bearing strata temporarily impermeable and to increase their compressive and shear strength by transforming joint water and interstitial water into ice. Freezing of rock or soil layers is achieved by means of an array of cased boreholes, through which a cold liquid is circulated in order to remove heat energy, comparable to cooling coils.

Frost expansion from the boreholes into the ground is monitored, for a better and more economic control of the freezing process. For this purpose, instruments for the determination of borehole deviation, for measurement of the temperature field and for an assessment of the degree of freezing with ultrasonic waves, have proved useful. These three techniques are described based on results obtained from field measurements.     

Ground response curves for rock excavations supported by ungrouted tensioned rockbolts

Summary A numerical analysis has been developed for the design of ungrouted tensioned rockbolts as support of excavations under axisymmetric conditions. The bolts dimensions (length, crosssection, longitudinal and circumferential spacings), their stiffness, their pre-tension load and the delay of installation are taken into account. Moreover, the method effects three main improvements in the usual theory, taking into consideration: 1. the reaction force transferred to the rock mass in the bolts anchoring zone, 2. the elastic recompression of the carrying ring surrounding the excavation due to the bolts preload, and 3. the relative displacement of the bolts ends which has a repercussion on their tension.Since the usual rock-support interaction analysis is only available when the rock mass and the support behave independently, an alternative solution has been explored for the bolting system (since it cannot be considered as an internal support). It consists to include the effect of the rockbolts into the ground reaction curve.In this paper, the principles of the analysis are explained and a numerical application is taken.     

Current and future UK practice for the permanent support of shaft excavations

Summary Mining development work in the UK is primarily concerned with the coal industry. Sedimentary rocks of the sandstone, marl and limestone types and coal measures are the predominant strata through which new shafts or existing shaft deepenings penetrate. The paper discusses hydrogeological information, the loadings imposed on shaft linings in such strata and outlines design philosophy. Design formulae are quoted and design practice is reviewed with reference to the current UK Codes of Practice for concrete construction.Included in the overall shaft lining design concept should be a consideration of the installation factors. Construction aspects and the influence of strata water control measures are therefore discussed in relation to recent advances using high-strength concrete and water pressure relief. New concepts in shaft lining design are also proposed and commented upon. Finally, the significant features of shaft lining design and construction are emphasized.     

Design of tunnel support systems using ground freezing

One of the more promising techniques in soft ground tunneling through urbanized areas is the use of artificially frozen ground for temporary tunnel support. This paper describes the general design considerations involved in the ground freezing method. Various factors are discussed which influence the selection of the freezing temperature, the thickness of the frozen zones and the spacing of the freeze pipes. The time required to achieve freezing is discussed in addition to the amount and rate of frost heave caused by the freezing.

To illustrate the applicability of the freezing method, various considerations in the design of an 8-ft. diameter tunnel in upstate New York, a 75-ft. diameter tunnel in Georgia, and a 12 1/2-ft. diameter tunnel in Washington, D.C. are discussed. All three of the tunnels were to pass immediately beneath mainline railroad tracks. A laboratory testing program was implemented to determine the effects of the repetitive train loads on the zone of frozen soil around the tunnel perimeter. Stress-controlled repeated load triaxial tests were performed on both undisturbed and remolded samples frozen from temperatures of −7°C for the New York tunnel to −10°C for the Atlanta and Washington tunnels. Static testing consisted of both quick triaxial tests and creep tests on frozen samples of the various soil types.

It was found that there was little difference between the cumulative strain response from repeated load tests and static tests for the low frequencies investigated (one-quarter to one-half cycles per minute). Hyperbolic stress—strain functions were developed to simulate the stress—strain relationship for various cumulative loading times.

The stresses and strains in the frozen soil tunnel configuration were computed by the finite element method, using both linear and hyperbolic stress—strain functions. Tangent modulus values were varied to reflect the decreasing modulus with increasing loading time. The analyses indicated that zones of frozen soil of approximately 3 ft. thick were required for both the New York and Washington tunnels. However, high tensile stresses were calculated for the Atlanta tunnel, precluding the use of the freezing method.     

Drainage of supported excavations

Summary The paper provides the approximate closed-form expressions for the estimation of the amount of ground water infiltration, the seepage exit gradient and the hydraulic pressure distribution on the cutoff elements in the case of sheeted excavations below the water table. The expressions are obtained by a multiple regression-type fitting of the results of an extensive parametric finite-element analysis for the two-dimensional steady-state flow in the soil around a sheeted excavation. In addition to the problem geometry, the effects of an impermeable base and a thin low permeability layer penetrated by the sheeting are also studied. The predictions of the approximate expressions deviate from the corresponding finite-element results by less than 15%, for a wide range of geometries and soil characteristics. It is shown how shape factors might be used to adapt the semi-empirical formulae for the case of excavations with limited length (circular and rectangular).     

Factors affecting the stability of deep excavations in clay with consideration of a full elastoplastic support system

Acta Geotechnica - This study investigates the stability of internally braced excavations in thick, saturated clay using a finite element method with reduced shear strength. The support system was...     

Application of rock mass index (RMi) to the rock mass excavatability assessment in open face excavations

Earthwork and surface excavation activities play an important role in construction projects. Selecting the best technique to loosen the overburden material within the surface excavation in open mining and geotechnical projects is of great importance from economical and technical viewpoints. Surface excavation includes direct digging, ripping and blasting. To select the most effective method and plan for excavation, geotechnical investigation is very important. It is also a big help in avoiding conflict between contractors and clients when they do not reach mutual agreement regarding the price of rock and soil excavation. There are many engineering classification systems used to assess rock masses for excavation purposes. All of these systems consider several geotechnical parameters to assess the earth masses. This study reviews these systems and then offers a new categorization based on the Rock Mass index (RMi) classification system and block volume to assess excavation in rock masses. The original dataset was obtained from the literature review as well as the surface excavation in Upper Gotvand dam and Hydro Power Plant (HPP). The offered system was also validated through the data extracted from the surface excavation in Sardasht dam and HPP in Iran.     

Polyhedral modelling of underground excavations

The use of key-block theory for the analysis of blocky rock mass structures in mining and civil engineering is well established. Such methods have traditionally been used to analyse the stability of rock blocks at free surfaces only. These methods have also been limited to the analysis of relatively few discontinuities, usually assumed to be infinitely persistent.     

土钉支护基坑抗隆起稳定性计算方法研究

以基坑隆起破坏的根本原因是地基承载力不足为前提,运用Terzaghi地基极限承载力理论,推导了土钉支护条件下基坑抗隆起稳定性的计算方法。采用地基土单侧滑动失稳破坏假设,同时考虑了单侧滑动体上竖向抗剪力对抵抗基坑隆起所起的作用。通过基本算例,对影响基坑抗隆起稳定性的参数进行了讨论,并得出了各抗隆起安全系数与各因素之间的部分变化规律。与现有的基坑抗隆起稳定性计算方法对比分析表明,所建议的方法在实际工程应用中是可行的。     

Transient analysis of excavations in soil

A finite element approach is used to assess the transient stability of excavation in elasto-plastic soils. Stability is shown to be a function of the rate of excavation, the soil permeability and the drainage path lengths. Sequential excavation has been modelled rigorously in the finite element analysis, together with the transient effects through a fully coupled Biot formulation. Results are presented which demonstrate the effects of the rate of excavation and permeability on stability. Both drained and undrained behaviour of the problem are retrieved as special cases of the transient analysis, and comparisons made with classical solutions where available.     

Finite element analysis of vertical excavations

The critical height of vertical excavations in both plane strain and axisymmetry has been considered using fiement analysis incorporating plasticity theory. The numerical solutions obtained are found to compare favourably with values from centrifuge tests and limit analyses. The results are presented in a form which shows the variation of the stability number at failure with hei height/width ratio of the excavation. Graphical output is also included to emphasise the changing mechanisms as this ratio varies.     

Assessment of apparent earth pressure for braced excavations in anisotropic clay

Acta Geotechnica - The evaluation of strut forces is critical to ensure the system stability of braced excavation in urban areas. The most common empirical approach to determine the forces in the...     

基坑变形监测及地下水的处理

结合南昌某建筑基坑,介绍了基坑支护结构水平位移和沉降监测点的布设、观测方法、监测数据处理等,并根据监测结果,发现可能发生危险的先兆,判断工程的安全性,防止工程破坏事故的发生;并以施工监测的结果指导施工,进行信息化反馈优化设计。由于基坑开挖后地下水丰富,坑内局部出现涌水、涌砂,导致电梯井等加深部位不能开挖到位。在基坑外部排水能力有限等情况下,提出了在电梯井的四周用旋喷桩形成止水帷幕、井底用旋喷桩封底的方法。实践表明：在旋喷桩施工完成后1 d即可垂直开挖电梯井,并没有渗水,取得了良好地加固效果和经济效益。     

深基坑支护结构的实用计算方法及其应用

对深基坑支护结构的受力和变形计算提出了一套系统的实用计算方法,较好地解决了基坑支护结构设计、计算的关键问题,并在广州地铁和许多重大基坑工程应用中取得了较好的效果。该方法把支护结构简化为一竖放的弹性地基梁,支撑、锚杆及岩土体用弹簧系统来代替,岩土的弹簧刚度可用岩土的变形模量来计算, 针对基坑工程施工和结构的动态特点,提出了一套系统的计算方法,其中包括考虑施工过程的增量计算法、合理确定支护结构入土深度的计算法、支撑加预应力及支撑拆除的计算方法等,并应用提出的增量法,首次较完整地对国际上著名的支护土压力Terzaghi-Peck表观土压力给出了理论解释,对岩土的变形模量提出采用承载力反算的经验方法,为岩土参数的确定提供了更简便的方法。这一系列的研究成果为深基坑支护结构的设计提供了一套新颖的实用计算方法,较好地解决了基坑支护结构中的一系列设计计算难题,已在工程实践中成功应用。     

Modeling small-strain behavior of Taipei clays for finite element analysis of braced excavations

We examine the small-strain behavior of Taipei clays in braced excavation through a detailed analysis of a well-documented case history. Specifically, we analyze the case of the Taipei National Enterprise Center (TNEC) excavation using two soil models, the Modified Cam-clay model (MCC) and the three-Surface Kinematic Hardening model (3-SKH). Our finite element analysis includes a consideration of the over-consolidated stress state and the high initial shear modulus of the clay. Results show that the observed wall deflection and surface settlement can be satisfactorily predicted simultaneously using the 3-SKH model. This is an improvement on the MCC model, for which only wall deflection, not ground settlement, can be accurately predicted. This study re-confirms the importance of considering small-strain non-linear behavior for the over-consolidated stress state in finite element analyses of braced excavation responses.     

Stability of unsupported conical excavations in non-homogeneous clays

New plasticity solutions for the undrained stability of unsupported conical excavations in homogeneous and non-homogeneous clays were solved by axisymmetric finite element limit analysis. Three parametric studies were performed on excavated height ratios, slope inclinations and dimensionless strength gradients. In all cases, the exact stability factors were accurately bracketed by computed bound solutions within 0.6%. An accurate closed-form equation of the stability factor was proposed from nonlinear regression analysis of lower bound solutions. New conical stability factors for soil cohesion, strength gradient, and coupling effect of these components were deduced to conveniently and accurately predict a safe solution in practice.     

Advances in discrete element modelling of underground excavations

The paper presents advances in the discrete element modelling of underground excavation processes extending modelling possibilities as well as increasing computational efficiency. Efficient numerical models have been obtained using techniques of parallel computing and coupling the discrete element method with finite element method. The discrete element algorithm has been applied to simulation of different excavation processes, using different tools, TBMs and roadheaders. Numerical examples of tunnelling process are included in the paper, showing results in the form of rock failure, damage in the material, cutting forces and tool wear. Efficiency of the code for solving large scale geomechanical problems is also shown.