Excavation Walls Research Articles

Recommendation for concrete mix design to prevent bleed channels on diaphragm walls

Diaphragm walls are used either to support excavation walls on site or prevent water move through the ground. The observations recorded following excavation and planning of diaphragm walls demonstrate that concrete outside the reinforcement cage is of a poorer quality than the one inside the cage. These disorders are mainly chimneys of water upwelling. To prevent their appearance on project sites, the concrete used for the realisation of diaphragm walls must meet the requirement of EN-1538 and EN-206/CN. Despite of this requirement, some diaphragm walls present disorders. This study stands on conducted on site and completed in the laboratory. Test results showed that the bleed water yield value of 24.4 ml, conducted using a press filter (Bauer) prescribed in the recommendations is too high. A limit of 15 ml of the relative volume of water is porposed with the press filter conducted during 5 min at 5-bar.

Seepage failure by heave in sheeted excavation pits constructed in stratified cohesionless soils

In this study, experimental and numerical investigations are performed to clarify the seepage failure by heave in sheeted excavation pits in stratified cohesionless soils in which a relatively permeable soil layer (kupper) lies above a less permeable soil layer (klower) between excavation base and wall tip. It is shown that the evaluation of base stabilities of excavation pits against seepage failure by using Terzaghi and Peck’s approach leads to considerably lower critical potential differences than those obtained from the model tests. On the other hand, a relatively good agreement is achieved between the results of the model tests and the finite element (FE) analyses. Further investigations are performed by using axisymmetric excavation models with various dimensions and ground conditions, and a comparison between the results obtained from Terzaghi and Peck’s approach and finite element analyses is given.

Application of natural weathered red-bed soil for effective wall protection filter-cake formation

Abstract In the context of slurry use for maintaining pressure and stabilizing wall protection during shield opening, diaphragm wall excavation and deep drilling to further prevent disaster accidents in civil engineering projects, a method to slurry using the natural weathered red-bed soil as the primary material is proposed, which is directly stirred with water without the addition of any other materials. Orthogonal tests are designed with the rate of change of the air filtration volume rate, effective airtightness time, and water content as the evaluation indexes. Fully transparent and high-pressure filter-cake airtightness tests are carried out. Results indicate the time of filter-cake formation is determined by the change rate of air filtration volume rate in the range between 0.020 and 0.025 L/(m·min)2. As regards the degree of influence on filter-cake formation, density exerts the greatest influence, followed by viscosity and less-than-1-mm-sized particle content. Further, the optimum density/viscosity/less-than-1-mm-size particle content combination of the slurry is determined to lie in the ranges of 1.83–1.95 g/cm3/13–21 Pa·s/45–60%. Moreover, it is found filter-cake damage is closely related to water content. With similar properties to the artificial, wide distribution of natural sources, low cost, and sample process, this materials is expected to be widely applicated.

Influence of stress path on stress memory and stress fracturing in brittle rocks

A two-dimensional numerical model based on the distinct element method, previously calibrated to the laboratory properties of undamaged and damaged Lac du Bonnet (LdB) granite, was used to investigate the influence of stress path on the development of stress memory and stress fracturing in brittle rocks. Various cycles of loading and unloading, similar to those imposed during Kaiser effect tests, were first applied to undamaged numerical specimens of LdB granite. The results of Kaiser effect test simulations were found to be consistent with those of published laboratory and numerical investigations. Further simulations were conducted to investigate the influence of stress path resulting from the excavation of a tunnel on the depth of stress fracturing around the excavation boundary. For this purpose, the stress paths at points on and near the tunnel wall, obtained from a continuum finite element model, were applied to the calibrated numerical specimen. It was found that the amount of damage in the numerical specimens decreases rapidly with increasing distance from the excavation wall. The findings of this research shed some light on the influence of stress path and grain-scale heterogeneity on stress memory in brittle rocks and stress fracturing around underground openings.

The Impact of Extreme Groundwater Recharge and Dewatering Events on the Efficiency of a Deep Anchored Wall

The main objective of the present study is to evaluate possible changes in the efficiency of a deep excavation anchored wall after the excavation had been recharged for about two years. Extensive instrumentation monitoring was carried out to assess ground movements, groundwater levels, and the displacement of adjacent structures during the process of construction. Because of uncontrolled water seepage in one of the critical excavation wall, shallow pressure relief drains were installed in the wall base in the approximately deepest parts of the excavation. This was done to limit the uplift pressures and control the displacement and stability of the wall. Careful monitoring of the adjacent building and ground movement, especially in soft ground, gives confidence to contractors and designers to predict the effects of groundwater drawdown required for constructing large and/or deep excavations and controlling uplift pressures during construction. The findings of this study may offer some recommendations for the selection of methods of groundwater dewatering control when there is seepage through the excavation wall.

A Numerical Investigation on the Performance of the Brick Stair Wall as a Supporting Structure by Considering Adjacent Building

Considering that one of the most important issues in urban excavations is the interaction between excavation and adjacent buildings, it is always crucial to study the behavior of the excavation system and its adjacent structures. Today, many methods are used to protect the stability of the excavations and to provide desired safety. One of these various methods is brick stair wall, which is used in some projects due to its characteristics such as its cost-effectiveness and proper function. Therefore, in this paper, the effect of using this method as the excavation supporting structure on the stability of the excavation wall and its role in reducing the deformation created in the wall and the surrounding ground, using Fast Lagrangian Analysis of Continua (FLAC), is discussed. The results show that the amount of deformation parameters of excavation and adjacent building constantly increase as the excavation depth and building weight increase or soil density decreases. In addition, the numerical simulations indicates that applied surcharge on the adjacent area of the stair wall (caused by the adjacent building weight) has the main effect on the damage criteria.

Two dimensional finite element modeling of Tabriz metro underground station L2-S17 in the marly layers

Deep excavations for development of subway systems in metropolitan regions surrounded by adjacent buildings is an important geotechnical problem, especialy in Tabriz city, where is mostly composed of young alluvial soils and weak marly layers. This study analyzes the wall displacement and ground surface settlement due to deep excavation in the Tabriz marls using two dimensional finite element method. The excavation of the station L2-S17 was selected as a case study for the modelling. The excavation is supported by the concrete diaphragm wall and one row of steel struts. The analyses investigate the effects of wall stiffness and excavation width on the excavation-induced deformations. The geotechnical parameters were selected based on the results of field and laboratory tests. The results indicate that the wall deflection and ground surface settlement increase with increasing excavation depth and width. The change in maximum wall deflection and ground settlement with considerable increase in wall stiffness is marginal, however the lower wall stiffness produces the larger wall and ground displacements. The maximum wall deflections induced by the excavation with a width of 8.2 m are 102.3, 69.4 and 44.3 mm, respectively for flexible, medium and stiff walls. The ratio of maximum ground settlement to maximum lateral wall deflection approaches to 1 with increasing wall stiffness. It was found that the wall stiffness affects the settlement influence zone. An increase in the wall stiffness results in a decrease in the settlements, an extension in the settlement influence zones and occurrence of the maximum settlements at a larger distance from the wall. The maximum of settlement for the excavation with a width of 14.7 m occurred at 6.1, 9.1 and 24.2 m away from the wall, respectively, for flexible, medium and stiff walls.

Review of unloading tests of dynamic rock failure in compression

Dynamic rock failure at the walls of an opening, so-called rock burst or strain burst is one of the major types of rock failure capable of causing human and financial loss. Initially the rock at the place of the future excavation is in a polyaxial stress state. When excavating an opening, the rock elements at the excavation boundary are overloaded in the tangential (with respect to excavation wall) direction and unloaded in the radial direction. This situation can be reproduced by the modified true triaxial test in which only one surface of prismatic rock sample is unloaded. This paper reviews the tests under true triaxial unloading condition and the corresponding failure modes and types (static or dynamic). In these tests the violent (dynamic) ejection of rock fragments and arc-shape fractures near the free face are often observed. However these tests are affected by friction between the sample and the loading platens. We analyse the end friction effect and show that unlike the conventional end friction effect produced in uniaxial and biaxial loading tests, the true triaxial unloading test induces additional shear stress needed to prevent sliding of the rock sample from the loading platens. This additional shear stress is shown to considerably affect the geometry of fractures.

Numerical Investigation of Squeezing in Underground Hard Rock Mines

Squeezing ground is one of the major challenges associated with deep, high-stress mining. In underground hard rock mines, these conditions are often a result of the presence of prominent structural features such as foliation inside the rock mass, which imposes mechanical anisotropy and leads to a nonlinear and anisotropic response of the rock mass to mining. Understanding this response of foliated rocks to various mining conditions is essential for managing the squeezing ground conditions. The complex behavior of a foliated rock mass requires the use of numerical methods to investigate the response of the rock mass to mining. Discontinuum numerical methods are frequently used to simulate anisotropic behavior, as they can explicitly represent the fractures. However, these methods present time and computational restrictions for large-scale models and complex mining geometries. These limitations can be overcome by employing anisotropic constitutive models in continuum numerical methods. This paper presents a methodology to investigate the failure mechanism of an intact foliated rock sample and foliated rock mass around an underground excavation utilizing the ubiquitous-joint model in continuum numerical methods. The paper also presents an approach that can be used to investigate the influence of varying interception angles between the foliation and excavation walls on the severity of squeezing ground conditions. The ubiquitous-joint model employed in FLAC3D accounts for the presence of weakness planes such as foliations in continuum material and can successfully reproduce the observed displacements and failure mechanism. The numerical modeling results were in good agreement with the closure strains recorded underground.

The New Swedish Cyprus Expedition 2017: Excavations at Hala Sultan Tekke (The Söderberg Expedition). Preliminary results

During the eighth field season at the Bronze Age city of Hala Sultan Tekke, excavations in City Quarter 1 (CQ1) exposed massive industrial and domestic structures belonging to three phases of occupation (Strata 3–1) dating to the 13th and 12th centuries BC (LC IIC–IIIA). Georadar survey, penetrating to a maximum depth of approximately 1 m, guided the excavation of walls of Strata 1–2, both of which were destroyed by conflagration. Excavations 1.5–2 m below the surface and also below the maximum penetration depth of the radar revealed a heretofore buried phase of occupation with substantial architectural units. For the first time, massive Stratum 3 structures with a markedly different building technique were exposed. Copper smelting installations, much ash and slag, and storage facilities also belong to this phase of occupation. Additional excavations guided by results from a magnetometer survey were carried out in Area A, roughly 600 m to the south-east of CQ1. Numerous circular anomalies were excavated. These were identified as Late Cypriot wells, rich offering pits, and a tomb from the same period. In addition to numerous intact locally produced vessels and other finds, the tomb contained a complete Late Minoan II/IIIA piriform jar with bird motifs which have exact parallels from Knossos. Other finds from this tomb include a diadem of leaf gold, amethyst jewellery, and nine sphendonoid shaped balance weights of haematite together with a hornblende whetstone. The features from Area A cover a period from the 16th to the 13th centuries BC (LC IB–IIC).

Numerical investigation of small- and medium-diameter drain wells to stabilise deep landslides

The stabilisation of deep landslides is still a challenge for geotechnical engineers, mainly due to the high cost of structural control works that generally lead engineers to opt for drain systems. Unfortunately there are operative difficulties affecting excavation of deep drain trenches in soils dislocated by landslides (especially as regards the provisional support of the excavation walls). Small- and medium-diameter wells are a valid alternative in deep landslides, being less expensive than large-diameter wells equipped with horizontal pipes and simpler to construct than deep trenches. However, no technical procedures for their proper design are reported in the literature: design criteria commonly used in practice are derived from pumping problems and are largely inadequate, being based on Dupuit’s hypothesis that presumes stream lines to be horizontal. This paper presents numerical solutions obtained from three-dimensional seepage and consolidation analyses of a slope drained by small- and medium-diameter wells, arranged according to various positions, showing that the efficiency of the well system decreases as the depth of the landslide slip surface increases. Comparison is also made between well systems and deep trenches occupying the same draining volume in the landslide body, showing that the former attains much greater efficiency than the latter. Non-dimensional abaci for the square grid of wells (which is the most effective arrangement) are reported herein to be used for practical purposes.

Strutting systems for deep excavations – technical challenges

Constant development carried out in town centres causes that many problems related to earthworks and tunnelling must be resolved. Investors are required to ensure respective number of parking bays, which in turn demands 2-3 storeys of underground car parks. This paper presents interesting case studies of deep excavations in Wrocław, Poland. As the geotechnical works in the old city centres may bring some risks to the surrounding area, proper strutting or anchoring systems must be provided. Apart from solving engineering problems, one needs to meet very stringent requirements of heritage conservator supervision. In order to ensure the stability of the excavation walls, it is necessary to examine the foundations of neighbouring structures, and if necessary, to strengthen them for the process of installing the excavation protection walls, progressing the excavations and constructing basement storeys. Another problem refers to constructing underground storeys below the level of groundwater. This requires efficient cutting-off or long-term lowering of water table inside the excavation with a possibly limited intervention in hydrological regime beyond the project in progress. In the typical case of old cities in river valleys such “hoarding off’ the excavation and cutting off groundwater leads to temporary or permanent disturbances of groundwater flow and possible local swellings. Contemporary technologies make it possible to protect vertical fault and simultaneously cut-off groundwater inflow by means of steel sheet pilings, diaphragm walls or secant pile walls.

Behavior of braced excavation in sand under a seismic condition: experimental and numerical studies

The behavior of braced excavation in dry sand under a seismic condition is investigated in this paper. A series of shake table tests on a reduced scale model of a retaining wall with one level of bracing were conducted to study the effect of different design parameters such as excavation depth, acceleration amplitude and wall stiffness. Numerical analyses using FLAC 2D were also performed considering one level of bracing. The strut forces, lateral displacements and bending moments in the wall at the end of earthquake motion were compared with experimental results. The study showed that in a post-seismic condition, when other factors were constant, lateral displacement, bending moment, strut forces and maximum ground surface displacement increased with excavation depth and the amplitude of base acceleration. The study also showed that as wall stiffness decreased, the lateral displacement of the wall and ground surface displacement increased, but the bending moment of the wall and strut forces decreased. The net earth pressure behind the walls was influenced by excavation depth and the peak acceleration amplitude, but did not change significantly with wall stiffness. Strut force was the least affected parameter when compared with others under a seismic condition.

Mechanism of Strain Burst by Laboratory and Numerical Analysis

Strain burst is often considered to be a type of failure related to brittle rock material; therefore, many studies on strain burst focus on the brittleness of rock. However, the laboratory experiments show that strain burst can not only occur in hard brittle rock‐like granite but also in the relatively ductile rock‐like argillaceous sandstone. This result proves that behavior of rock material is not the only factor influencing the occurrence of strain burst. What must also be considered is the relative stiffness between the excavation wall/ore body and the surrounding rock mass. In order to further studying the mechanism of strain burst considering the whole system, the engineering geomechanial model and numerical model of strain burst due to excavation are built, respectively. In a series of numerical tests, the rock mass involving the excavation wall as well as roof and floor is biaxially loaded to the in situ stress state before one side of the excavation wall is unloaded abruptly to simulate the excavation in the field. With various system stiffness determined by the microproperties including the contact moduli of particles and parallel bond moduli in the models of roof and floor, the different failure characteristics are obtained. Based on the failure phenomenon, deformation, and released energy from the roof and floor, this study proves that the system stiffness is a key factor determining the violence of the failure, and strain burst is prone to happen when the system is soft. Two critical Young’s moduli ratios and stiffness ratios are identified to assess the violence of failure.

STABILITY CHARTS FOR UNSUPPORTED PLANE STRAIN TUNNEL HEADINGS IN HOMOGENEOUS UNDRAINED CLAY

ABSTRACT: This paper investigates the stability of plane strain tunnel headings in undrained soils. Using the factor of safety approach via the strength reduction technique, the idealised tunnel heading models are studied using the finite difference program FLAC. This problem is also similar to underground long wall excavations in plane strain condition. The finite difference results are presented alongside upper and lower bound limit solutions for validation. A thorough comparison between these two methods finds a very good agreement. Design charts are presented for a wide range of practical scenarios using dimensionless ratios, similar to Taylor’s design charts used for slope stability. Typical examples are presented to illustrate the usefulness for practising engineers.

MARS inverse analysis of soil and wall properties for braced excavations in clays

A major concern in deep excavation project in soft clay deposits is the potential for adjacent buildings to be damaged as a result of the associated excessive ground movements. In order to accurately determine the wall deflections using a numerical procedure such as the finite element method, it is critical to use the correct soil parameters such as the stiffness/strength properties. This can be carried out by performing an inverse analysis using the measured wall deflections. This paper firstly presents the results of extensive plane strain finite element analyses of braced diaphragm walls to examine the influence of various parameters such as the excavation geometry, soil properties and wall stiffness on the wall deflections. Based on these results, a multivariate adaptive regression splines (MARS) model was developed for inverse parameter identification of the soil relative stiffness ratio. A second MARS model was also developed for inverse parameter estimation of the wall system stiffness, to enable designers to determine the appropriate wall size during the preliminary design phase. Soil relative stiffness ratios and system stiffness values derived via these two different MARS models were found to compare favourably with a number of field and published records.

FAULT-RELATED INSTABILITY PROBLEMS OF TUNNELS - THE HOST ROCK SLIP CRITERION AND CHARACTERISTICS OF THE TUNNELING-INDUCED SHEAR DISPLACEMENTS

As one of the fault-related instability problems of tunnels, rock slip along fault plane is closely related to the shear strength of a fault, and usually causes irrecoverable and sometimes catastrophic engineering problems. In this paper, based on continuum assumption and Coulomb-slip failure, a criterion to evaluate rock slip along the fault plane was proposed for a circular tunnel in rock masses containing a fault. A mathematical equation that describes the relationship between required shear strength of a fault and horizontal stress ratio, fault spatial extension and location was established. From the equation, the influences of the important parameters on the required shear strength of a fault was analysed after a numerical validation was performed. Besides, the effects of fault spatial extension and location on the tunnelling-induced shear displacements were characterized through numerical models. Characteristics of the tunnelling-induced shear displacements at the excavation wall indicated that fault location with respect to the tunnel dominates the nonuniform rock deformations at excavation wall, and larger fault dip angles could lead to larger shear displacements in some specific pair cases. The presented investigation provides both a deeper insight into the instability problems of tunnels related to a fault and a guideline for tunnel support design.

3D Finite Element Modelling of Sheet Pile Wall Excavation: A Case study in Bangkok

Sheet pile wall has been used extensively as a soil retaining structure during the excavation process in soft ground. Meanwhile, finite element method (FEM) has been widely used as a numerical tool to predict wall movements due to soil excavation. In FEM, many factors including soil parameters, structures’ parameters and construction stages simulation influence the analysis results. This paper presents a modelling of sheet pile wall at deep excavation using 3D FEM. The study focuses on the structures’ stiffness modelling and the stages of construction simulation. The hardening soil model and its parameters adopted from previous study was employed in the analysis. To validate the model, an excavation site located in the center of Bangkok was selected to model. PLAXIS 3D – a commercial software for solving finite element problem was employed in this study. In overall, the results of wall movements from 3D FEM agree well with the instrumented data confirming that the modelling could reflect the real behavior of sheet pile walls at deep excavation in soft soils in Bangkok

The excavation of walls and roads at the Erlitou Site in Yanshi City, Henan in 2012–2013

AbstractFrom November 2012 to May 2013, the Erlitou Archaeological Team of the Institute of Archaeology, Chinese Academy of the Social Sciences found a roughly north-south orientated rammed-earth wall (Q7) and hard-trodden road surface parallel to the wall in the vicinity of the workshop zone of the Erlitou Site. The wall was most likely erected during Erlitou Phase II and continued to be in use until the early stage of Phase IV. The trodden roads took form during Phase II and ended in Phase III or early stage of Phase IV. Together with the previous findings, the excavators postulated two competing propositions on the rammed-earth wall in question. It was the west wall of the walled workshop zone or it was the east wall of a walled enclosure has yet to be discovered to its west. The roads were auxiliary facilities of the walled zone when it was in use.

The archaeological survey and excavation of walls and passes of the Hailong Tun site in Zunyi City, Guizhou

AbstractIn 2013 and 2014, Guizhou Provincial Institute of Cultural Relics and Archaeology and other institutions conducted comprehensive survey, mapping and trial excavation to the walls and passes of the Hailong Tun Site. These archaeological activities identified the walls and passes of two phases belonging to the Song and Ming Dynasties respectively and generally made clear the full layout and evolution of the relevant remains through these periods. The extant Bronze Pillar Pass, Iron Pillar Pass, Flying Dragon Pass, Flying Tiger Pass, Chaoqian Pass, Flying Phoenix Pass, Wan’an Pass, West Pass and Rear Pass and the walls built of marlstone are the remains of the Wanli Era of the Ming Dynasty and the north and south walls and the “Earthen wall” on the top of the tun (castle) and the gates associated with them are the remains of the Southern Song Dynasty. These discoveries provided important references for the chronology and periodization of the relevant remains, and also enriched the understandings to the city sites of the Song through the Ming Dynasties in the nearby areas.