Excavation Sequence Research Articles

Investigation of response of metro tunnels due to adjacent large excavation and protective measures in soft soils

The inevitable influence of large excavation in soft soils on nearby tunnels is of great concern in practice. In this paper, the influence of a nearby large excavation on existing metro tunnels of the Ningbo Metro Line 1 in sensitive soft soils is investigated and presented. Considerable displacement in the left tunnel closer to the excavation induced by the nearby excavation was revealed by field monitoring. Visible cracks and leakages were observed in left tunnel linings. Three dimensional numerical simulations are conducted to investigate the responses of the ground and left tunnel due to the adjacent excavation. The development of bending moment and displacement of the left tunnel during different construction stages of the nearby excavation is obtained. Then the interaction mechanism between the nearby excavation, surrounding soils and existing twin tunnels is investigated, which is of significance to the interpretation of the influence of the nearby excavation on the existing twin tunnels. Several protective measures for alleviating the influence of adjacent excavation on left tunnel are studied, including divided excavation, soil improvement and a cut-off wall. It is found that the left tunnel is influenced to varying degrees during different construction stages and the time effect is distinct for this large excavation in soft soils, which would be suggestive to engineers to pay more attention to the protection of adjacent tunnel during the crucial construction stages. The bending moment and displacement of the left tunnel is strongly related to the unloading effects and displacement of surrounding soils, which can be alleviated by means of proper improvement of excavation sequence. Comparatively, longitudinally divided excavation is more effective in protecting the left tunnel than soil improvement or a cut-off wall. This study is of certain reference value for protecting metro tunnels adjacent excavation in soft soils.

Predictive models of ultimate and serviceability performances for underground twin caverns

The construction of a new cavern modifies the state of stresses and displacements in a zone around the existing cavern. For multiple caverns, the size of this influence zone depends on the ground type, the in situ stress, the cavern span and shape, the width of the pillar separating the caverns, and the excavation sequence. Performances of underground twin caverns can be unsatisfactory as a result of either instability (collapse) or excessive displacements. These two distinct failures should be prevented in design. This study simulated the ultimate and serviceability performances of underground twin rock caverns of various sizes and shapes. The global factor of safety is used as the criterion for determining the ultimate limit state and the calculated maximum displacement around the cavern opening is adopted as the serviceability limit state criterion. Based on the results of a series of numerical simulations, simple regression models were developed for estimating the global factor of safety and the maximum displacement, respectively. It was proposed that a proper pillar width can be determined based on the threshold influence factor value. In addition, design charts with regard to the selection of the pillar width for underground twin rock caverns under similar ground conditions were also developed.

Addressing the Consequences of Dynamic Rock Failure in Underground Excavations

Rockbursts are violent events that result in the ejection of volumes of rock from the walls of underground excavations. They can be extremely hazardous and have been responsible for many accidents in underground excavations. They also are responsible for significant direct and indirect costs in mining and civil engineering projects. The occurrence of rockbursting can be reduced by optimising the design with regard to excavation layout, excavation geometry and size, excavation sequence, and by the application of destressing/preconditioning measures. Thereafter, containment of damage relies on rock support. Conventional design of rock support for the containment of rockburst damage is not possible since neither the demand that is generated, nor the capacity of support systems, are known, and thus there is a state of design indeterminacy. The approach recommended in this paper is a risk-consequence one: evaluation of the risk (the product of the probability of occurrence of a rockburst and its consequence, in financial terms), and use of this quantified risk as a decision making tool regarding the justification for significant dynamically capable rock support. A typically suitable rock support system for rockbursting conditions is suggested in the paper.

On the selection of an appropriate excavation pattern for urban tunnels with big cross-section: A case study

Among various practical measures used for restriction of the ground surface settlement in such tunnels driven in soft ground, selection of an appropriate excavation method plays a significant role. In this paper, employing suggested diagram by Yu & Chern, corresponding values of Niayesh tunnel has been inserted into the diagram. Later, two excavation methods namely: central diaphragm and side drift methods have been suggested and numerically modeled using Finite Difference Method. Side drift excavation pattern has finally been selected since it causes less settlement. To reach an optimized selection of excavation sequence through side drift method, seven excavation patterns have thus been recommended and numerically modeled. Results have revealed that the first pattern causes the least amount of settlement. Consequently, the aforementioned excavation pattern has finally been considered as an appropriate excavation pattern encompassing optimum excavation sequence for Niayesh tunnel.

발파 굴착에 의한 주방식 지하구조물의 굴착공기 분석 연구

A room-and-pillar underground structure is characterized by its grid-type array of room and pillar. As a result, its construction and economical efficiency can be governed by excavation sequence. In this study, the construction period by the drill and blast method which can be treated as a main sequence for excavation was examined by considering the regulation for blasting and construction standard of estimation in Korea. To evaluate the construction period for the room-and-pillar underground structure constructed in 4 kinds of square-type area (30×30~57×57 m), the concurrent excavation pattern which was suggested in the previous researches was used. From the suggested condition, the total construction period by drill-and-blast method can be estimated with the consideration of the construction area, number of jumbo drill and faces in operation.

Differential uplift and settlement between inner column and diaphragm wall in top-down excavation

Top structure and basement will confront the risk of being damaged on account of large stress and strain fields incurred by differential uplift and settlement between inner column and diaphragm wall in top-down method. Top-down excavation of the Metro Line 10 in Shanghai was modeled with finite element analysis software ABAQUS and parameters of subsoil were obtained by inverse analysis. Based on the finite element model and parameters, changes in the following factors were made to find more effective methods to restrain differential uplift and settlement: length of diaphragm wall, thickness of jet-grouting reinforcement layer, ways of subsoil reinforcement, sequence of pit excavation, connection between slabs and diaphragm wall or column and width of pit. Several significant results are acquired. The longer the diaphragm wall is, the greater the differential uplift between column and diaphragm wall is. Rigidity of roof slab is in general not strong enough to keep diaphragm wall and column undergoing the same uplift during excavation; Uplift at head of column and differential uplift between column and diaphragm wall decrease when subsoil from −16.6 to −43 m in pit is reinforced through jet-grouting. But, as excavation proceeds to a lower level, benefit from soil reinforcement diminishes. During the process applying vertical load, the larger the depth of diaphragm wall is, the smaller the settlement is at head of column and diaphragm wall, and the greater the differential settlement is between column and diaphragm wall. When friction connection is implemented between column, diaphragm wall and floor slabs, uplifts at head of column and diaphragm wall are larger than those of the case when tie connection is implemented, and so does differential uplift between column and diaphragm wall. The maximum deflection of diaphragm wall decreases by 58% on account of soil reinforcement in pit. The maximum deflection of diaphragm wall decreases by 61.2% when friction connection is implemented instead of tie connection.

Machine scheduling in underground mining: an application in the potash industry

In this paper, a scheduling problem that occurs in potash mining is introduced, where a block excavation sequence has to be found taking into account a limited number of underground machines as well as safety-related restrictions. The aim is to minimize the maximum completion time of excavations, i.e., the makespan. The resulting problem can be transformed into a hybrid flow shop scheduling problem with reentry, unrelated machines, and job-precedences. A mixed-integer linear model is presented and small-scale instances are solved with CPLEX. In order to tackle medium- and large-scale instances heuristically, a basic and an advanced multi-start algorithm are developed, based on a specific priority rule-based construction procedure. In addition, a modified version of the Giffler and Thompson procedure is applied. Computational experiments are conducted on problem instances derived from real-world data in order to evaluate the performances of the proposed solution procedures.

Dynamic Deformation Control of Retaining Structures of a Deep Excavation

AbstractMany uncertain factors in the excavation process may lead to excessive deformations and internal forces of retaining structures; therefore, the design of deep excavations should be adjusted according to real-time monitoring data. This paper presents a case study of dynamic control of excavation deformations. The preliminary design adopted bored piles as retaining structures braced by three levels of steel-reinforced concrete struts, while the final design used only two levels of struts to reduce the construction duration. The final design reduced the safety degree of the project to a certain extent and brought the excavation to a state of emergency. Fortunately, the designers adjusted the excavation sequence based on feedback information from the field monitoring data. In addition, a variety of deformation control measures, aimed at reducing the horizontal displacement of the retaining walls, were used, such as strengthening the chamfer of the retaining structures, adjusting the excavation sequenc...

주방식 지하구조물의 최적 굴착공정에 대한 예비 분석 연구

주방식 지하구조물의 최적 굴착공정에 대한 예비 분석 연구

Deposition and Diagenesis in the Earlier Stone Age of Wonderwerk Cave, Excavation 1, South Africa

The Earlier Stone Age (ESA) sequence of Excavation 1 at Wonderwerk Cave is the longest stratified sedimentary sequence associated with hominin occupation in Southern Africa. This sequence has been constrained chronologically on the basis of cosmogenic burial age and paleomagnetic dating. Geoarchaeological analysis of two exposed profiles covering strata 12–9 that combine micromorphology and FTIR on the ESA deposits shows shifts in depositional sources, transport, and diagenesis. This analysis provides insight into the paleoenvironmental context of hominin occupation and suggests that during part of the stratigraphic sequence, there was an ephemeral body of water in proximity to the cave. These results provide the basis for ongoing geoarchaeological research on the landscape outside the cave and other components of the cave deposits.

Twin tunnel excavation in coarse grained soils: Observations and numerical back-predictions under free field conditions and in presence of a surface structure

In this paper the effects induced by twin-tunnelling on ground and structure settlements is analysed and discussed. The study refers to the case of the Milan metro-line 5, recently excavated in coarse-grained materials using earth pressure balance machines. Firstly, settlement measurements under free-field conditions are back-interpreted using the classical Gaussian empirical predictions, providing a detailed description of the modification of the subsidence profile due to the second excavation with respect to the single tunnel configuration. The in situ measurement analysis is subsequently extended to explore the evolution of the structural response of a nine-storey reinforced concrete surface building, undercrossed by the metro-line, as observed during the tunnelling activities. In the second part, the paper illustrates a three-dimensional finite element back-analysis of the above case history. The soil mechanical behaviour is described by an advanced constitutive model (named Hardening Soil model with small strain stiffness), the twin-tunnelling is properly schematised taking into account the main aspects of the excavation sequence and the building is modelled in a realistic fashion in terms of geometry, stiffness and weight properties. The computed results, in good agreement with the settlement monitoring data, are capable to capture the interaction phenomena occurring between the twin tunnels during the construction works and to describe the evolution of the deformative mechanisms affecting the building façades.

Numerical Analyses of the Stress State in Two Neighboring Stopes Excavated and Backfilled in Sequence

AbstractBackfilling of underground stopes is commonly used in the mining industry. The stress state in these stopes has to be evaluated to ensure safe application of the backfill. In recent years, much work has been conducted to assess the stresses in single backfilled stopes. The stress distribution in stopes may also be affected by the excavation of multiple openings. This paper presents key results obtained from numerical simulations of two adjacent vertical stopes created in sequence. The results illustrate the influence of stope geometry (size and spacing), natural stress state, backfill properties, and excavation (and filling) sequence on the stress distribution in both stopes. The simulations indicate that the stress distribution in the first backfilled opening, following the creation of the second one, largely depends on the fill properties. These results also show the ways in which these factors may affect the stress magnitude and distribution pattern in the case of two adjacent stopes, when the ...

Slope Stability Monitoring by Quantification and Behavior of Microseismic Events in an Opencast Coal Mine

Abstract The application of a routine high resolution microseismic monitoring system installed in an opencast coal mine for monitoring highwall slope failures is discussed. A PC based microseismic network consisting of geophones, data loggers, GPS synchronization and Ethernet antennas for wireless communication is employed to study the impact of induced seismicity on the slope failures in real time. The study aimed to understand the rock mass response to mining induced seismicity from the behavior of seismic events within rock mass. The level of induced seismicity due to underground excavation is determined from the seismic source parameters such as locations, magnitude, and seismic energy. The status of the slope is assessed by the quantification of the microseismic events. The understanding of spatial and temporal distribution of the seismic events within the mining district correlated well with the existing geological structures and the excavation sequence. The application of microseismic system in the opencast mine has not only yielded confident results, but also marked as an effective tool for continuous monitoring of seismicity on the deep opencast slopes for mitigating the seismic risks and hazard management.

Numerical Simulation Study on Technology of the Top-Down Bored Excavation with Cast-<i>In Situ</i> Arch

The top-down bored excavation with cast-in-situ arch method has many advantages, such as forming a complete and stable mechanical structure, reducing construction procedures and the frequency of structure transformation, improving the stability of structure during the construction and so on. According to the specific project of single span top-down bored excavation duct in Changchun Liberation Road station, numerical simulation method is used and finite difference software FLAC 3D is adopted in this paper to analyze and research the applicability of the top-down bored excavation with cast-in-situ arch method to this project. Then optimization analysis for excavation sequence of pilot tunnel is done and the optimum construction scheme is raised to guide the practical project. This research has certain practical significance and also provides references for similar projects.

Assessment of Hydro-Mechanical Behavior of a Granite Rock Mass for a Pilot Underground Crude Oil Storage Facility in China

The hydro-mechanical behavior of a pilot underground crude oil storage facility in a granite host rock in China was analyzed using the finite element method (FEM). Characterization of hydro-mechanical behavior of the rock mass was performed using laboratory test, field monitoring, back analysis of field measurements and permeability tests. FEM numerical analyses were used to assess the hydro-mechanical behavior of the granite to study several design and construction issues. The containment properties of the storage facility were investigated without and with the water curtain system. Results showed that the stored oil would leak into rock mass if a water curtain system is not provided, whereas the containment property of the facility will be maintained when a water curtain system is in place. On the influence of cavern excavation sequence, it was indicated that the excavation of the caverns from left to right is a better choice than right to left for the containment property of the facility. On the influence of permeable condition, it was found that the extent of plastic zones, horizontal convergence and crown settlement under permeable condition are lower than those under impermeable condition due to the different stress paths in the rock mass experienced during excavation.

Construction Technique of Long-Span Shallow-Buried Tunnel Considering the Optimal Sequence of Pilot-Tunnel Excavation

Construction technique considering the optimal sequence of pilot-tunnel excavation was investigated in order to ensure the safety of construction process of long-span shallow-buried tunnel. Firstly, optimal comparisons of the effect of different sequence of pilot-tunnel excavation on the ground settlement were implemented by the numerical analysis. Secondly, an optimal construction method was determined and applied to the construction of a practical tunnel. Some key issues and procedure of the selected construction method were described in detail. Finally, the numerical modeling and calculation of the tunnel construction process were conducted, and the effectiveness of these simulations was demonstrated by using the measured data of the practical tunnel.

曲面切羽によるトンネル切羽安定化工法

Full-face tunnel excavation has been increasingly employed for SEM (sequential excavated method) tunneling by the virtue of enhanced capacity of roadheaders as well as the ground control effect of early ring closure. As the stability of the tunnel face is essential to full-face tunnel excavation, the tunnel stabilizing method utilizing curved tunnel faces, which would naturally form as part of the tunnel excavation sequences, can be considered effective. This paper addresses an investigation into the characteristics of the shape of curved tunnel face employing numerical models, and the stabilizing effect of tunnel face reinforcing methods, such as shotcrete on tunnel face, using physical models. Further, the constructability and stability of curved tunnel face were verified through trial excavations at an actual tunneling site. As a result it is confirmed that curved tunnel faces are advantageous and the stability of the tunnel face can be improved by a simple reinforcing method such as shotcrete on tunnel face. The trial excavations undertaken have proved the favorable constructability and stability of curved tunnel face. In addition, a conceptual approach to selection of the shape of curve tunnel face and its stabilization behavior is suggested together with a concept of tunnel face stabilizing method proposed. 掘削機械の大型化および支保構造の早期閉合による変位抑制効果から全断面掘削の施工が増大している．全断面掘削では切羽の安定が重要であり，自然形成的な曲面切羽を用いる安定化工法は有効と考えられる．本論文では，曲面切羽の形状の違いによる安定効果を数値解析により，鏡吹付けコンクリート等の鏡面補強工の支保効果を模型実験により検証した．また，実トンネルの試験施工で曲面切羽の安定性，施工性の検証を行った．その結果，曲面切羽形状の優位性および鏡吹付けコンクリート等による簡易な補強で切羽安定性が大きく向上することが確認された．試験施工からは曲面切羽の安定性，施工性の良さが確認された．また，掘削目標曲面形状および曲面切羽の安定化挙動を示し，切羽安定化工法の考え方も提案する．

Influence of Concrete Time-Dependent Effects on the Performance of Top-Down Construction

The influence of time-dependent concrete behavior on the performance of top-down excavations is evaluated in light of the observed responses during construction of One Museum Park West (OMPW), a 53-story reinforced concrete tower. Performance during construction was recorded at five instrumentation stations, which included strain gauges in the concrete floor slabs used as lateral bracing during the top-down excavation sequence and inclinometers adjacent to the walls. Top-down construction was simulated with a 3D numerical model of the basement system in which concrete material time-dependent effects were considered, but not the stress relief caused by excavation or other construction activities. Time-dependent concrete response in the floor slabs and the passive resistance developed by the soil as the wall deforms due to concrete time-dependent effects are included in the model. The concrete time-dependent properties related to shrinkage, creep, and aging of the material used in the basement floor slabs are determined by means of standard concrete practice methods. The development of concrete strength and stiffness with time is compared with compressive strength tests on concrete cylinders from construction concrete reports for the concrete used in the floor slabs. Computed lateral deflections of the perimeter pile walls and strains in the floor slabs arising only from concrete material time dependence are compared with the results of performance data collected from inclinometers and strain gauges installed at five instrumentation stations. In this way, the relative contribution of concrete time-dependent effects and excavation-induced movements to the observed lateral wall movements during the top-down stages are determined and conclusions are drawn concerning the design and performance of the top-down support system at OMPW.

Fracture development around deep underground excavations: Insights from FDEM modelling

Over the past twenty years, there has been a growing interest in the development of numerical models that can realistically capture the progressive failure of rock masses. In particular, the investigation of damage development around underground excavations represents a key issue in several rock engineering applications, including tunnelling, mining, drilling, hydroelectric power generation, and the deep geological disposal of nuclear waste. The goal of this paper is to show the effectiveness of a hybrid finite-discrete element method (FDEM) code to simulate the fracturing mechanisms associated with the excavation of underground openings in brittle rock formations. A brief review of the current state-of-the-art modelling approaches is initially provided, including the description of selecting continuum- and discontinuum-based techniques. Then, the influence of a number of factors, including mechanical and in situ stress anisotropy, as well as excavation geometry, on the simulated damage is analysed for three different geomechanical scenarios. Firstly, the fracture nucleation and growth process under isotropic rock mass conditions is simulated for a circular shaft. Secondly, the influence of mechanical anisotropy on the development of an excavation damaged zone (EDZ) around a tunnel excavated in a layered rock formation is considered. Finally, the interaction mechanisms between two large caverns of an underground hydroelectric power station are investigated, with particular emphasis on the rock mass response sensitivity to the pillar width and excavation sequence. Overall, the numerical results indicate that FDEM simulations can provide unique geomechanical insights in cases where an explicit consideration of fracture and fragmentation processes is of paramount importance.

The geochemistry of biogenic carbonates as a paleoenvironmental tool for the Lower Pleistocene Barranco León sequence (BL-5D, Baza Basin, Spain)

The Barranco (creek) León section is located in the NE marginal area of the lacustrine Plio-Pleistocene Baza Basin (Southern Spain). The Barranco León BL-5D mammal and archeological site has delivered abundant lithic tools and a hominin tooth; the age ascribed to this site is comprised between the Olduvai and Jaramillo subchrons, close to 1.4 Ma, based on magnetostratigraphy, biostratigraphy and ERS dating. The studied BL-5 section consists from base to top, of dark sandy mudstones and lutites with interbedded sandy marls. The lower part of the dark sandy mudstones contains a brackish fauna of foraminifera, ostracods, and mollusks. Sandy carbonate mudstones and sandy lutites with freshwater-oligosaline ostracods and mollusks form the upper part of the archeological excavation sequence. Stable isotope (δ13C, δ18O) and trace element analyses on ostracod and mollusk shells allow depicting the paleoenvironmental conditions recorded in the lacustrine sequence. Moreover, indications on provenance for waters have been obtained from 87Sr/86Sr ratios from shells and opercula from mollusks. The isotopic composition of the biogenic carbonates shows a strong correlation between δ13C and δ18O, which is noticeable for some gastropod shells. This correlation is a characteristic feature of closed lake settings. In our case, the more negative isotopic values reflect the input of meteoric waters and the more positive ones the skeletal carbonates formed in more isotopically concentrated waters. Some of the more negative isotopic values correspond to biogenic carbonates from the BL-5D level. They correspond to isotopically diluted waters, although the trace element data and the invertebrate fauna indicate oligosaline to mesosaline waters. Moreover, the presence in BL-5D of shells from Melanopsis tuberculata, a thermophilous gastropod, together with their isotopic signatures suggest the input of oligosaline waters with a thermal influence. The available geochemical data indicate a first stage of varying influence of several water types under changing water level in a closed lake recorded in the base of the studied sequence (brackish fauna level). The overlying beds record a through flowing open lacustrine environment mainly fed by meteoric surface and ground waters with significant inputs of stream waters.