Shaft Excavation Research Articles

Response of a drilled shaft foundation constructed in a redrilled shaft excavation following collapse

Two drilled shaft foundations (DSFs) of equal size (1.2 m diameter) were installed at the Turrell Arkansas Test Site (TATS). The soil stratigraphy at the TATS consisted of 6.1 m of clay underlain by 3.0 m of silt underlain by sand. After drilling the excavation for the North 1.2 m DSF, the silty soil collapsed from the sidewall of the excavation into the bottom of the excavation. Following the collapse, the excavation was redrilled and the construction of the DSF was completed. The measured capacity, unit side resistance, and end bearing resistance of the South 1.2 m diameter DSF were predicted by using software programs and mean values of soil data. The measured response of the North 1.2 m diameter DSF was backward modeled to determine the appropriate strength and stress reduction. Based on the measured data for this site, a 10 percent reduction in unit weight within the silt layer and a modification of the soil properties within the top 3.0 m of the sand layer produced predicted responses that matched t...

Blast-Induced Damage on Millisecond Blasting Model Test with Multicircle Vertical Blastholes

To investigate the blast-induced damage effect on surrounding rock in vertical shaft excavation, 4 kinds of millisecond blasting model tests with three-circle blastholes were designed and carried out with excavation blasting in vertical shaft as the background. The longitudinal wave velocity on the side of concrete model was also measured before and after blasting. Then blast damage factor was then calculated by measuring longitudinal wave velocity before and after blasting. The test results show that the blast-induced damage factor attenuated gradually with the centre of three-circle blastholes as centre. With the threshold value of 0.19 for blast-induced damage factor, blast-induced damage zones for 4 kinds of model tests are described and there is an inverted cone blast-induced damage zone in concrete model. And analyses of cutting effect and blast-induced damage zone indicate that in order to minimize the blast-induced damage effect and ensure the cutting effect the reasonable blasting scheme for three-circle blastholes is the inner two-circle blastholes initiated simultaneously and the outer third circle blastholes initiated in a 25 ms delay.

Underground Excavation Behaviour of the Queenston Formation: Tunnel Back Analysis for Application to Shaft Damage Dimension Prediction

The Niagara Tunnel Project (NTP) is a 10.1 km long water-diversion tunnel in Niagara Falls, Ontario, which was excavated by a 7.2 m radius tunnel boring machine. Approximately half the tunnel length was excavated through the Queenston Formation, which locally is a shale to mudstone. Typical overbreak depths ranged between 2 and 4 m with a maximum of 6 m observed. Three modelling approaches were used to back analyse the brittle failure process at the NTP: damage initiation and spalling limit, laminated anisotropy modelling, and ubiquitous joint approaches. Analyses were conducted for three tunnel chainages: 3 + 000, 3 + 250, and 3 + 500 m because the overbreak depth increased from 2 to 4 m. All approaches produced similar geometries to those measured. The laminated anisotropy modelling approach was able to produced chord closures closest to those measured, using a joint normal to shear stiffness ratio between 1 and 2. This understanding was applied to a shaft excavation model in the Queenston Formation at the proposed Deep Geological Repository (DGR) site for low and intermediate level nuclear waste storage in Canada. The maximum damage depth was 1.9 m; with an average of 1.0 m. Important differences are discussed between the tunnel and shaft orientation with respect to bedding. The models show that the observed normalized depth of failure at the NTP would over-predict the depth of damage expected in the Queenston Formation at the DGR.

Analysis of Surge Shaft Excavation Process Based on 3-Dimensional Finite Difference Method

As an important part of the structure of hydroelectric power station, surge shaft is embedded in the rock mass, whose body size and loading conditions are typically complex. Thus, it is necessary to evaluate safety of the design scheme in order to ensure construction safety. With upper background, three dimensional finite difference model of the surge shaft and surrounding rock of a hydroelectric project in Zambia were established based on three dimensional nonlinear finite difference analysis method. Calculation of the surge shaft stepped excavation with supporting at all levels of the construction process was done. The deformation and stress state of the surrounding rock of surge shaft and its variation law in excavation process was analyzed. Furthermore, the stability of surrounding rocks and rationality of the designed support measures were evaluated.

Development and applications of rock fracture mechanics modelling with FRACOD: a general review

Rock failure is often controlled by fracture initiation, propagation and coalescence, especially in hard rocks where explicit fracturing rather than plasticity is the dominant mechanism of failure. Prediction of the explicit fracturing process is therefore necessary when the rock mass stability is investigated for engineering purposes. However, the fracture mechanics approach is rarely used in practical rock engineering design partly due to the inadequate understanding of complex fracturing processes in jointed rock mass and partly due to the lack of tools which can realistically predict the complex fracturing phenomenon in rock mass. Since 1990s, a new approach to simulating rock mass failure problems has been developed using a numerical code namely FRACOD. FRACOD is a code that predicts the explicit fracturing process in rocks using fracture mechanics principles. Over the past three decades, significant progress has been made in developing this approach to a level that it can predict actual rock mass stability at an engineering scale. The code includes complex coupling processes between the rock mechanical response and thermal process and hydraulic flow, making it possible to handle coupled problems often encountered in geothermal, hydraulic fracturing, nuclear waste disposal and underground Liquefied Natural Gas (LNG) storage. During this period, numerous application cases have been conducted using FRACOD, which includes: borehole stability in deep geothermal reservoir, pillar spalling under mechanical and thermal loading; prediction of tunnel and shaft stability and excavation disturbed zone, etc. This paper summarises the theoretical fundamentals of the fracture mechanics approach with FRACOD and the most recent developments. Several selected application cases are also discussed briefly in this paper to demonstrate the effectiveness of this approach.

Dewatering of a deep excavation undertaken in a layered soil

In order to carry out deep excavations under the water table in urban environments, the safety of the work site and of the adjacent buildings is a major cause for concern. One of the most common and effective methods of undertaking these excavations involves combining the cut and cover method with a dewatering system. The success of a construction depends on the stability of the excavation bottom, the effects produced outside the excavation by dewatering (soil movements) and/or the state of the enclosure (defects in the diaphragm walls). This study proposes a realistic multidisciplinary procedure to address these issues. The work emphasizes the importance of soil characterisation and underlines the need to perform a Watertightness Assessment Test (WTAT) before the excavation stage. The procedure was applied to the excavation of a deep shaft of the High Speed Train (HST) tunnel in Barcelona. An earlier geological characterisation at large scale ruled out the use of deep pumping wells. However, a subsequent hydrogeological characterisation, which involved borehole logging, grain size analyses, Natural Gamma Ray and pumping tests, revealed the presence of thin transmissive layers inside the low hydraulic conductivity materials. The dewatering system was designed by considering different model scenarios and the safest design was selected for the excavation. Depths of the enclosure and of the pumping wells differed in accordance with the scenarios. The impacts (settlements due to pumping) and the stability in each scenario were computed. The state of the enclosure underwent a WTAT before the start of the excavation, but after constructing the enclosure, to verify its low permeability. The test consisted in pumping inside the enclosure and monitoring the groundwater behaviour outside the enclosure. Numerical interpretation of this test showed a defect in the diaphragm walls below the excavation bottom. Since this defect was not repaired because of its location (below the bottom of the excavation), the dewatering system had to be redesigned to ensure safety. Surface settlements, which were also a source of concern, were small. They were computed using coupled hydro-mechanical models.

Design of shaft linings to resist time dependent deformation in evaporite rocks

Potash, a fertiliser currently in demand worldwide, lies beneath beds of salt and other evaporites deep within the ground. The ground support challenge encountered when excavating through these strata is that of time dependent (creep) deformation control. The deformation is radial, causing the shaft excavation to close up over time, eventually loading the permanent lining with the full geostatic pressure and causing lining failure. In order to achieve a significant design life for an economical shaft lining in creeping rocksalt strata, a deformation zone filled with a compressible material is employed between the excavation and permanent lining. This helps resist the ground pressure, and reduce the magnitude and velocity of creep deformation. Optimum lining designs have been developed by predicting the creep deformation at a given time and by considering the behaviour of different compressible materials available to fill the deformation zone. Foamed concrete is the favoured material for an intermediate compressible lining.

Shotcrete Support of a Ventilation Shaft in Hard Rock

Ventilation shaft is important for the operation of 18 km long Qinling highway tunnel. The main geological problems encountered in the shaft excavation are high geostress and rockburst. Shotcrete can provide effective support in the rockburst conditions during the excavation of shaft in hard rock. According to the classification of rock mass Q system and engineering experience, the parameters of shotcrete support with bolt and mesh are determined, and the numerical simulation is carried out for stability analysis. Finally the dry mix shotcrete technology is applied for the support.

Study on the Stratum Displacement Changes Caused by the Vertical Shaft Construction in Saturated Ultra Soft Soil

In view of Wenzhou saturated super soft soil, This article studied the influence of different soil parameters for the shaft excavation construction and the stratum displacement change law by using Plaxis finite element software, according to the data obtained in field tests. The considered factors included the lateral brace stiffness, the stiffness of the underground diaphragm wall, and the surrounding soil disturbance. The calculation results provided much important engineering information, such as the horizontal displacement nephogram, the vertical displacement nephogram and the total displacement incremental vector diagram of each construction steps, which can be referred for the construction of the similar underground projects in soft soil areas.

Numerical Modeling and Solution of Ground Water Inrush at the Baixiangshan Iron Mine

The Baixiangshan iron mine is a large scale underground mine under construction, and will be put into production in 2011. As hydro-geological conditions of the mine are extremely complicated, ground water inrush happened twice during excavation of the ventilation shaft in 2006 and 2009 respectively. The purpose of this paper is to control the ground water inrush and guide the safety production of the mine. Hence, the ground water state was firstly analyzed through the data of borehole and hydrologic geology. After that, hydro-geological models of the ground water system were built up. According to the hydro-geological models, mathematical models of the ground water were established. With the help of the software of Processing Modflow for Windows (PMWIN), the numerical model of the ground water was set up. Then, by using the SOR (Successive Over-Relaxation) iterative method, the numerical equations were solved. Finally, the district ground water inflow rate on the main mining levels and the dangerous districts of the ground water inrush was figured out. The results of the numerical solution are significant to the control of ground water inrush in the Baixiangshan iron mine.

An In-situ Experiment of Rock Mass Property Changes by Shaft Excavation

An In-situ Experiment of Rock Mass Property Changes by Shaft Excavation

Influence of a fault system on rock mass response to shaft excavation in soft sedimentary rock, Horonobe area, northern Japan

This paper focuses on the stress field that produced a fault system in the Horonobe area of Japan, and considers the relationship between the fault system and the rock mass response to shaft excavation in soft sedimentary rocks. In this area, hydraulic fracturing investigations in boreholes and tectonic plate movements indicate that the major horizontal principal stress is oriented E–W; however, during shaft excavation, the greatest convergence in the shaft was oriented NNE–SSW. Therefore, it is necessary to understand the influence of the fault system on the rock mass response to shaft excavation. We performed a fault-slip analysis of faults in the shaft wall and reconstructed the paleostress field that produced the fault system. The maximum horizontal principal stress was oriented mainly NNE–SSW, and the minimum horizontal principal stress was oriented mainly WNW–ESE. These directions are similar to the directions of maximum and minimum convergence in the shaft, respectively. The results show that a fault system can affect the rock mass response to shaft excavation in soft rock, and it is considered that the rock mass deformation is controlled by the fault system. In addition, it may be possible to assess and predict the rock mass behavior by focusing on paleostress field that produced the fault system.

Interaction between clay-based sealing components and crystalline host rock

The results of hydraulic–mechanical (H–M) numerical simulation of a shaft seal installed at a fracture zone (FZ) in a crystalline host rock using the finite element method are presented. The primary function of a shaft seal is to limit short-circuiting of the groundwater flow regime via the shaft in a deep geological repository. Two different stages of system evolution were considered in this numerical modelling. Stage 1 simulates the groundwater flow into an open shaft, prior to seal installation. Stage 2 simulates the groundwater flow into the shaft seal after seal installation. Four different cases were completed to: (i) evaluate H–M response due to the interaction between clay-based sealing material and crystalline host rock in the shaft seal structure; (ii) quantify the effect of the different times between the completion of the shaft excavation and the completion of shaft seal installation on the H–M response; and (iii) define the potential effects of different sealing material configurations. Shaft sealing materials include the bentonite–sand mixture (BSM), dense backfill (DBF), and concrete plug (CP). The BSM has greater swelling capacity and lower hydraulic conductivity ( K) than the DBF. The results of these analyses show that the decrease of the pore water pressure is concentrated along the fracture zone (FZ), which has the greatest K. As the time increases, the greatest decrease in pore water pressure is found around the FZ. Following FZ isolation and the subsequent filling of the shaft with water as it floods, the pore water pressure profile tends to recover back to the initial conditions prior to shaft excavation. The majority of the fluids that ultimately saturate the centre of the shaft seal flow radially inwards from the FZ. The time between the completion of the shaft excavation and the completion of shaft seal installation has a significant effect on the saturation time. A shorter time can reduce the saturation time. Since most of the inflow comes from the FZ, application of the BSM for extended distances above and below the FZ does not significantly affect the saturation time of the volume adjacent to the FZ. The application of BSM near the FZ rather than a low swelling capacity, more permeable filling material is very significant. This study assumed a perfect contact between seal materials and host rock. Limited to the assumptions used in this study, use of BSM near the FZ was found to increase the time before the centre of the shaft seal became fully saturated from between 4 and 30 years (when the DBF is used) to between 90 and 100 years (when the BSM is used).

On the Sharply Curved Shield Tunneling for 345kV Power Cables in Taiwan

Due to the rapid development of the Taiwanese economy, quality of life is continually improving. Installing extra-high voltage cables in underground tunnels to meet the increased electricity needs of urbanization and modernization is the current trend. In recent years, shield tunneling has become the main underground tunneling method for metropolitan areas because it is safe, avoids the need for ground excavation, does not affect ground transportation, does not hinder the flow of groundwater, and has a low environmental impact. This paper used the Taiwan Power Company’s “Fenglin 2nd Road Shield Tunnel Turn-Key Project of the 345kV Underground Cable Route Connecting Gaogang, Wujia, and Kaohsiung” as a case study site. It has geological properties as high water table, sandy soil, and gravel formations. This paper also introduced the construction experiences of this project, including considering the work safety of launching vertical shaft excavation, designing a divergent shaft to place directly above the shield tunnel, constructing a large-diameter, sharp-curved shield tunnel, revising the project’s management items and procedures, and pipejacking a connection adit.

A comparative micromechanical analysis of the effective properties of a geomaterial: Effect of mineralogical compositions

In order to provide a physical interpretation of the variation of the mechanical properties of Callovo-Oxfordian argillite with mineral composition, we implement three linear homogenization schemes. The argillite is modeled as a three phase material composed of a clay matrix and inclusions of quartz and calcite. It is shown that, unlike the dilute scheme and the self-consistent scheme, the Mori-Tanaka model describes the in situ experimental data well. The determined properties are finally used in a finite element computation. The aim is to evaluate the effect of mineral composition on the elastic response of the excavation of a vertical shaft in the context of the underground laboratory of Meuse/Haute Marne.

A new method of cut-blasting for vertical shaft excavation and its experimental study

Based on cutting principle and technology development of vertical blasthole cut by stage and deck in vertical shaft excavation, combined with the merits of middle space charging and toe space charging, the reinforced cutting effect of central large-diameter blasthole and the method of cutting blast by stage and deck toe space charging for the vertical large-diameter blastholes was put forward and analyzed theoretically. This new cut blasting method is provided with the advantages of high blasthole using ratio, big cavity bulk, low rate of chunk, even lumpiness, and relatively high energy using ratio. The parameter choices and practical effects of this cutting method were discussed after in situ experiment. It shows that the decked delay time of 75∼100 ms is applicable.

Application of a micromechanical model to cavity excavation analysis in argillite

This paper presents an application of a micromechanical model to the analysis of cavity excavation in heterogeneous rocks by taking into account influences of mineralogical compositions. The Callovo-Oxfordian argillite is considered here in the context of feasibility studies for geological storage of nuclear waste storage. Based on the microstructural analysis, a micromechanical model is first proposed using nonlinear homogenization techniques for heterogeneous materials. The argillite is seen as a three phase composite with elastoplastic clay matrix, elastic damaged calcite grains and elastic quartz grains. Using the proposed model, the variations of elastic and plastic properties of material with mineralogical compositions at different depths are systematically taken into account. Comparisons between experimental data and model's predictions are presented. The proposed model, implemented into a finite element computation code, is finally applied to the analysis of excavation of a vertical shaft.

Ultrasonic velocity survey in Callovo-Oxfordian argillaceous rock during shaft excavation

The excavation of a deep underground structure induces a stress field redistribution that could create an excavation-damaged zone (EDZ). The study of the EDZ is important in the framework of nuclear waste storage, where such EDZ could constitute a preferential pathway of stored material towards the biosphere. Analysis of ultrasonic wave propagation across the rock media around the excavation is one technique that can be used to characterize such zones. This method has been applied in the Meuse/Haute–Marne Underground Research Laboratory (URL) in France to characterize the initialization and evolution of damage around the URL access shaft. To achieve this, a device comprising 21 ultrasonic sensors was spread out at different distances from the shaft wall, at depths between −465 and −475 m. A transversely isotropic model enabled the measured experimental velocities on each ray path during shaft excavation to be integrated. The results highlighted the initialization and extension of the damage as the shaft front proceeded. They also showed a polarization of the velocity field and an oscillation of the transversal isotropy with a preferential orientation of the stress release and the microcracking. Monitoring of the velocity field also highlighted some significantly different behaviour linked to the complex mechanical behaviour of the argillaceous rock.

Handling the blockades while excavating the surge shaft at Tala Hydroelectric Project

Every component in a hydel project is an important element. Delay in any one component will affect the project time and cost equally. Excavation of surge shaft is one such component which happens to be the most critical as it involves multiple stages for its completion. This operation as such is slow, difficult and if it encounters adverse geology the operation puts the entire project on the critical path. This paper discusses in detail the problems encountered during the entire excavation of a surge shaft for a 1020 MW hydel project in Himalayan geology with special reference to the blockades. It deals with the methods adopted to clear the chokes at various stages of excavation and to achieve the accelerated progress by full face blasting.

大規模地下施設の建設に伴う周辺地下水環境の変化

The Japan Atomic Energy Agency (JAEA) is constructing the Mizunami Underground Research Laboratory at Mizunami, Japan to establish general techniques for the assessment of the deep geological environment. The facility including two 1,000 m shafts and sub-stages at 100 m depths between two shafts is currently under construction in the sedimentary rocks. This study aims to evaluate the environmental changes around a large underground facility. To this end, hydrochemical changes in response to shaft excavation are assessed based on the observation of hydraulic head and groundwater chemistry around the facility. The observations indicated that rock formations with low hydraulic conductivity act as barriers to hydraulic disturbances, while higher conductivity zones provide a preferential flow path. Groundwater flow to the drifts creates chemical changes by mixing among chemically different groundwaters in higher conductivity zones. It is therefore meaningful to monitor the water pressure and chemistry at highly conductive rock formations during construction and operation of underground facilities. These investigations will provide the basic information on hydrochemical buffer capacity of the natural environment. Furthermore, observations suggest that grouting of conductive rock formations is important for maintaining the groundwater at near preconstruction levels so as to retain the buffer capacity of the rock formations used for safety assessment.