Multi-point Extensometers Research Articles

In situ monitoring of tunnel deformation evolutions from auxiliary tunnel in deep mine

Deformation caused by excavation is of great effects on tunnel stability and tunnel support design. Monitoring using instruments that are installed in place within the examined tunnel, cannot record a large amount of surrounding rock deformation that usually takes place ahead and close to the tunnel face. For this important reason, the overall deformation process was observed using instruments installed prior to excavation from auxiliary tunnel in a 900m deep coal mine tunnel. Two monitoring methods, i.e. multipoint extensometers and wave velocity tomography were used at two monitoring sections with different rock lithology. The results showed that a large amount of deformation at both monitoring sections had already been occurred when the excavation face passed the monitoring sections at a very small distance. The size of the plastic zone at monitoring section 1 is identified as 1.8m (0.36D, D denotes the diameter of the examined tunnel) and the velocities within this zone decreased from 4.5km/s to 2km/s. The primary deformation was generated in a distance from −0.8D (ahead the excavation face) to 2–3D (behind the excavation face). The induced deformation was also observed after the primary deformation period, and some additional deformation was still occurred in the plastic zone although the size of plastic zone remains constant. The displacements and p-wave velocities at the two monitoring sections showed little differences.

Rock Bolting at the Room and Pillar Method at Great Depths

A part of the coal reserves in the Karvina subbasin of the Upper Silesian Coal Basin is situated in protection pillars that lie under built-up areas. The longwall mining method is not suitable in these areas because significant deformation of the surface is not allowed. The room and pillar method with stable coal pillars has been proposed to minimise strata convergence. The method has been examined within the shaft protective pillar located in CSM-North Mine coal seam No. 30, where the mining depth ranged from 700 to 900 meters, being perhaps the deepest room and pillar panel in coal mining in the world [1–5].As there is no relevant experience of using this method in the Upper Silesian Coal Basin, an extensive monitoring system has been implemented to enable the mining trial to continue safely. The monitoring is focused on the load-bearing capacity of the coal pillars and strata deformation changes induced by the room and pillar mining method. Precise monitoring was carried out in two adjacent coal pillars located within the row of pillars forming the panel. To monitor roof deformation, fourteen pairs of 5-level multipoint extensometers monitored roof displacements and eleven strain-gauged rockbolts were installed at various locations. Seven hydraulic dynamometer load cells measured the cable bolt loads were installed at the roadway intersections around the monitored pillars.The integral constituent of the extraction method (using driving machines like the Bolter Miner) is bolting as the sole support system of the roadways. The contribution deals with the behaviour of roof bolting, including the loading of the bolts, yielding of the rock mass and convergence in the roadways. The monitored parameters’ database used to measure whether the room and pillar method is successful at this depth forms a necessary condition for verification of this method and its future application in conditions experienced in Upper Silesian Coal Basin.

Safety monitoring and stability analysis of left bank high slope at Jinping-I hydropower station

At the Jinping-I hydropower station the excavation height on the slope of the left bank is about 530 m. The stability of the excavated slope is very important because of the complicated geological conditions. To analyse the process of deformation evolution of the left abutment slope during construction, a comprehensive monitoring system was employed, which combined surface deformation observations, multi-point extensometers and graphite rod extensometers. This paper describes this monitoring system and analyses the deformation development with slope excavation. We also established a 3D numerical model of the slope and simulated the whole process of excavation using the finite-difference method. Results from both monitoring and numerical simulation show that the deformation depth in the left bank slope of the Jingping-I hydropower station is over 150 m owing to large-scale excavation unloading, far greater than that of general engineering slopes. The 3D limit equilibrium method and the strength reduction method are applied to evaluate the stability of the excavated slope. The calculated factors of safety indicate that the left abutment slope is stable and safe as a whole, which is in agreement with the deformation monitoring results.

Performance of the reinforced right abutment slope of Zipingpu Dam during magnitude 8.0 earthquake, Wenchuan, China

On 12 May 2008, an earthquake of magnitude 8.0 on the Richter scale occurred at Wenchuan in China. Zipingpu dam also experienced the earthquake. The maximum ground acceleration near the top of the 156 m high rockfill dam exceeded 2 g . The right dam abutment slope, which accommodates all the discharge facilities, remained intact after this severe earthquake. However, the natural slopes on the opposite side of the reservoir were severely damaged. It has been found that the reinforcement provided by more than 1000 prestressed cables in the right abutment slope helped to stabilize it. Measurements from multipoint extensometers and load cells installed in the slope showed that there were sudden changes in displacement and stress during the earthquake. This is clear evidence that reinforcement with anchored cables can be successfully used to stabilize a slope.

Displacement measurement techniques and numerical verification in 3D geomechanical model tests of an underground cavern group

In this paper, the stability of an underground cavern group is investigated on the basis of large-scale 3D geomechanical model tests and numerical simulations. Multiple measurement techniques are developed to measure the convergence of deformation in the cavern, displacements at key points in the side walls and the damage pattern during the excavation simulation process. The digital photogrammetric technique and fiber Bragg grating (FBG) based displacement sensing bars are applied to measure displacements in the surrounding rock masses. Mini multi-point extensometers with high-precision grating scales are developed as transducers for displacement monitoring. Afterward, a nodal-based Discontinuous Deformation Analysis (NDDA) method is applied to simulate the whole process of the physical model tests and to compare with experimental results. The variation process and law curves of the displacements in the surrounding rock and at key points in the side walls are obtained at excavation stages. The study shows that the application of the displacement measurement methods used in the surrounding rock masses for large-scale model tests of cavern group under true 3-D stress state have achieved satisfactory results.

Progressive degradation of rock properties and time-dependent behavior of deep tunnels

Time-dependent response of deep tunnels is studied considering the progressive degradation of the mechanical properties of the rock mass. The constitutive model is based on a rock-aging law for the uniaxial strength of the rock and for the Young’s modulus. A semi-analytical solution is developed for the stresses and displacements around a deep circular tunnel taking into account the face advance. The evolution of the plastic and damage zones over time is determined. Numerical examples are presented for the case of Saint-Martin-La-Porte access adit in France of the Lyon–Turin Base Tunnel. The computed results which are compared with the field data in terms of the convergence of tunnel wall and of the displacements inside the rock mass monitored by multi-point extensometers show the efficiency of the approach to simulate the time-dependent deformation of a tunnel excavated in squeezing ground. Simple relationships are proposed to evaluate the parameters of the constitutive model directly from those of the empirical convergence law presented in previous work.

Comprehensive evaluation of excavation-damaged zones in the deep underground caverns of the Houziyan hydropower station, Southwest China

The disturbance of a rock mass by blasting or stress redistribution can significantly influence the overall performance of an underground excavation. The characteristics of excavation-damaged zones (EDZs) during the excavation of underground caverns at the Houziyan hydropower station in Sichuan Province, China, were investigated using various in situ tests. This study presents a comprehensive evaluation of the evolution of EDZs in the surrounding rock mass in the underground powerhouse caverns using microseismic (MS) monitoring and conventional testing methods, including multi-point extensometers, acoustic wave testing and borehole TV. First, by analyzing a series of conventional testing and MS monitoring results, the deformation and failure characteristics of the surrounding rock mass were determined. Next, the formation mechanisms of fractures in the surrounding rock mass subjected to excavation-induced unloading in the underground powerhouse caverns were determined. The relationships between the EDZs of the surrounding rock mass, crack evolution and construction status were then analyzed to investigate the crack formation, development and coalescence processes. Finally, the thicknesses of the EDZs were quantitatively determined, the relationships between the fracture evolution and construction progress were established, and the EDZ formation and evolution mechanisms were evaluated. The results not only provide direct data for geological exploration but also contribute to optimizing excavation design and support for analyzing the deformation behaviors of underground powerhouse caverns.

An integrated approach to study of strata behaviour and gas flow dynamics and its application

This paper presents an advanced and integrated research approach to longwall mining-induced strata movement, stress changes, fractures, and gas flow dynamics with actual examples of its application from recent studies for co-extraction of coal and methane development at Huainan Mining Group in China, in a deep and multi-seam mining environment. The advanced approach takes advantage of the latest techniques in Australia for mine scale geotechnical characterisation, field measurement, monitoring and numerical modelling. Key techniques described in this paper include coal mine site 3D geotechnical characterisation methods, surface deep downhole multi-point extensometers and piezometers for overburden displacement and pore pressure measurements during mining, tracer gas tests for goaf gas flow patterns, and advanced numerical modelling codes for coupled coal mine strata, water and gas simulations, and longwall goaf gas flow investigations. This integrated approach has resulted in significant insights into the complex dynamic interaction between strata, groundwater, and gas during mining at Huainan Mining Group in recent years. Based on the findings from the extensive field monitoring and numerical modelling studies, a three-dimensional annular-shaped overlying zone along the perimeter of the longwall panel was identified for optimal methane drainage during mining.

Inversion Analysis Based on the Unloading Rock Mass of Underground Powerhouse Monitoring Achievements of Jin Ping Cascade II Hydropower Station

On the basis of unloading rock mass theory, we identify the excavated rock mass parameters by means of neural network training in each step, in which the subjective factors of the parameters selected process have largely been decreased. Through inducing the parameters of inversion into two -dimensional modeling, we aim to do the numerical simulation computing in various conditions. By comparison, the calculated displacement trend of the measuring points following the multi-point extensometers is basically identical with the monitoring displacement trend.

Large-scale geomechanical model testing of an underground cavern group in a true three-dimensional (3-D) stress state

The stability of a large cavern group at great depth is discussed on the basis of large-scale three-dimensional (3-D) geomechanical model tests and numerical simulations. The model tests are described in detail. Improvements in the tests were made in terms of experimental techniques and advanced measurement methods. The model tests utilized active loading on six sides of a rock mass in a true 3-D stress state. During the model construction, precast blocks were fabricated and monitoring holes were defined prior to test initiation. Newly developed combination ball-sliding walls were installed on each of the major loading surfaces to reduce the friction induced by model deformation. A unique grouting and installation technique employing prestressed cables was adopted in the tests. A digital photogrammetric technique, displacement sensing bars using fiber Bragg grating (FBG) technology, and mini multipoint extensometers were developed for measuring deformation. Overloading tests were then conducted for different overburden depths, and 3-D numerical analyses were performed to simulate the testing procedures. Conclusions regarding the stability of the cavern group were developed based on a comparison between the experimental and numerical results.

Rockfill Embankment Settlement

The Hoover Dam Bypass project (US-93) crosses the Colorado River between Arizona and Nevada below Hoover Dam, 30 mi southeast of Las Vegas, Nevada. The Arizona approach to the river includes the seven-span, 900-ft long Sugarloaf Mountain Bridge. The east bridge abutment footing is supported on a welded-wire–reinforced, special embankment fill zone within a 130-ft high rockfill embankment. Reinforced zone backfill consists of 4-in. maximum size select granular backfill; the remainder of the embankment consists of 2-ft maximum size rock. Backfill and rockfill were obtained from project roadway cuts, and construction and acceptance of the reinforced zone and rockfill embankment were in accordance with FHWA standard specifications and project special provisions. Postconstruction settlement of the embankment was monitored for 2 years with conventional survey monuments on abutment wingwalls and the embankment and multipoint borehole extensometers (MPBX) in the roadway shoulders. The monitoring period included 3 months before placement of the closure span and deck. Roughly 1 in. of settlement beneath the reinforced fill was measured during construction, with an additional 1/2 in. of settlement within 2 years after fill placement. Postconstruction settlement of the wingwalls was approximately 1 in. The rate of settlement decreased through the 2-year period, and settlement from construction and self-weight is nearly complete. Use of the MPBX was deemed partially successful in this application.

Field measurements during construction of a compressed air tunnel in Cairo

An in-sity monitoring program was implemented during construction of the spine tunnel of the Greater Cairo Wastewater Project. The test section was located at an area where the 5-m shielded tunnel was advancing under compressed air through alluvial Nile deposits. Soil displacements around the tunnel were measured during the tunnel advance using an array of magnetic multi-point extensometers and surface settlement points. Piezometers were also utilized to monitor the groundwater response at different distances from the tunnel centreline. This paper presents field measurements compiled from the instrumentation monitoring program. The general trends of ground and groundwater responses to tunneling under the encountered conditions are identified. The results are compared with available guidelines of prediction of soil settlement above shielded tunnels.

Performance of a high precision multi-point borehole extensometer in soft rock

A simple and accurate method of measuring relative displacements along a borehole using circular magnets as markers and reed switches as sensors has been developed and is now widely used.This note describes a new precise sensing system which is easily assembled and installed, together with a modified method of placing magnets. Details are given of the performance of the new system over a 2 year period for an investigation into the deformation properties of an heavily loaded week rock.