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Focusing on three-dimensional finite-element analysis (FEA) for the assessment and optimization of reinforcement treatment using deep soil-mixing and umbrella vault methods to stabilize a cross-passage excavation in soft alluvial soils, this paper presents parametric studies, interpretation analyses, and details of the modeling technique for the re...
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... and f c 9 5 unconfined compressive strength of the mixes of soil cement (psi). As per the testing report for the project issued by the local authority, the reinforced soils using the soil- mixing technique were classified as Type IV rock in accordance with the Chinese railroad geological investigation standard (Ministry of Railway of China 2001). Fig. 7 shows the cores of soil cement produced with the soil-mixing method. Based on the rock-quality designation (RQD) of the core samples, 20% of the calculated modulus of elasticity of the cemented samples using Eq. (2) was used for analysis (Bieniawski 1992). The values of cohesion c and friction angle f of the soil cement produced by ...
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... Soil cement behaves similar to weak concrete, greater the strength of typical soils [14]. Unconfined compressive strength (UCS) is the widely accepted parameter for evaluating soil cement mixtures, serving as both a design and quality control measure [25]. In the review of the literature on the properties of the sabkha cement mixer, only a small amount of research was done in the field. ...
Excavation in Sabkha soils, found in coastal regions of Saudi Arabia, presents complex challenges in construction due to their unique geotechnical characteristics. These soils are known for their high compressibility, collapsibility, and variable sediment deposition, rendition them unsuitable for excavation without adequate support systems. As urbanization and industrial activities continue to expand into these areas, the demand for effective excavation support systems becomes gradually critical. This study investigates the feasibility of employing Deep Soil Mixing (DSM) as an excavation support system in Sabkha soils. Utilizing finite element analysis through PLAXIS 3D software, the behaviour of DSM walls in Sabkha soil under various conditions is rigorously simulated. The findings of this research establish that DSM walls can serve as an efficient excavation support system in Sabkha soils, provided that crucial factors, particularly excavation depth, are considered during the design phase. The study demonstrates that properly designed DSM walls, in combination with appropriate anchoring methods, can substantially reduce horizontal displacement and bending moments, ensuring the stability of excavations in Sabkha soils. The research highlights the need for further investigations into the cost-effectiveness of DSM walls in comparison to other excavation support options to evaluate the economic feasibility of using DSM in Sabkha soil applications.
... However, its implementation in a tunnel junction area is seldom discussed. With the exception of [21], which assessed the effect of both deep soil-mixing and umbrella vault methods to stabilize the excavation of a cross passage, and [22] who studied the effect of the umbrella arch technique on a cross passage using a 2D homogenized numerical model, few studies have examined the effect of the umbrella arch technique solely on the stability of a cross passage excavation in a 3D simulation. To bridge this gap, this paper presents a finite element modelling (FEM) approach of an umbrella arch method modelled as a homogenized area using the theoretical tools of homogenization for periodic media applied to a cross passage excavated with the conventional method (NATM). ...
The stability of tunnel cross passages excavated in soft soil has always been a major challenge. In recent years, new techniques based on the installation of pre-reinforcements before tunnel excavation have been developed to control excavation-induced deformation and surface settlements. In this paper, a finite element numerical simulation was conducted to study the reduction effect of an umbrella vault element modelled as a homogenized area on the deformations induced after the excavation of a cross passage. The results of this study show that the ground deformations can be controlled efficiently by using this type of pre-reinforcement. However, the findings showed that there is no effect of the umbrella arch length on the reduction of the ground deformations. This paper represents a very good demonstration of 3D modelling of tunnel junctions using pre-support techniques; it is the most advanced/appropriate research tool for studying the behaviour of cross passages and is useful as a paradigm for other researchers and practitioners.
... The cross-section span of the cross passage is generally 2.0-3.0 m, the height of the wall is 2.5-4.0 m, and the section is rectangular, circular, or straight wall arched [5][6][7]. The construction of the cross passage changes the main tunnels segment from a complete cylindrical structure to an unstable force structure with notches; thus, the overall force (bending and shearing) performance of the tunnel is considerably decreased [8][9][10][11]. After the support construction of the cross passage, the tunnel and cross passage form a 3D complex intersection structure [12][13][14]. ...
The construction of the cross passage changes the main tunnels segment from a complete cylindrical structure to a notched unstable stressed structure. Thus, the overall force (bending and shearing) performance of the main tunnels is considerably decreased. The excavation of the cross passage also destroys the stability of the original soil layer and main tunnels and causes difficulty in avoiding the settlement of the surface and main tunnels. Numerical simulation analysis of the mechanical characteristics and influencing factors analysis were carried out using FLAC3D. Results show that (1) the construction of the cross passage largely disturbs the main tunnels. The settlement of the intersection part of the main tunnels and cross passage is mainly concentrated from −7 m to 7 m. (2) During the construction of the cross passage, the original main tunnels are subjected to stress redistribution, which causes stress concentration in the intersecting segments, especially in the opening side segment has an unfavorable force form in which the upper and lower sections are integrally pulled. (3) The construction of the cross passage causes a large tensile stress area in the structure as well. As a result, the overall tension of the upper and lower sections of the structure and the compression of the hance are unfavorable. (4) The influence of the excavation of the cross passage on the surface and main tunnels is related to the buried depth, intersection angle, excavation method, and grouting range. Therefore, the influence of different parameters can be analyzed by changing the model parameters.
... Engineering practise adopts many empirical methods in addition to theoretical and numerical analyses, especially for ground improvement and pre-support design, which are mostly experience-based. As for using soil-cement in reinforced excavation, unconfined compressive strength (UCS), q u (at 28 d, and hereafter), is widely adopted as the design and construction quality control standard (Wang et al., 2014a). Parameters used in numerical analysis, e.g. ...
... Statistical analysis for in situ testing results can reduce uncertainty, but it is not always suitable to be carried out to obtain sufficient information prior to or during construction period. Some data can be gained after soil-cement mixture hardening, but it is too late to update the design (Wang et al., 2014a). Consequently, how to interpret other properties from q u of soil-cement mixture for numerical analysis is an important issue in applying this state-of-the-art technique to engineering practise. ...
... (2) and (3) has a trend to be lower than that calculated by other methods, but the values calculated by Eq. (3) are slightly larger than the average range of these values. In engineering practise, the typical range of the UCS of soil-cement mixture is 0.5e2.5 MPa when applying the grouting technique to reinforcing and bracing excavation (Wang et al., 2014a). It may be appropriate to use a value between the ranges calculated by Eqs. ...
soil-cement is a mixture produced by grouting or mixing cement with soils. This paper reviews and discusses the general classifications of grouting techniques and the suitability of their applications. The mechanical properties of soil-cement mixture and the influence of sodium silicate added are discussed. Design considerations for deep soil mixed wall (DSMW) for excavation support and vault arch for tunnelling stabilisation are presented. Parameters for the numerical analysis of soil-cement mixture are evaluated and recommended. © 2018 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences
... The deformation behaviors of intensive crushing-calcite caused by earthquake and its permeability characteristics were tested [12]. Recently, some factors such as grain shape and gradation [13], grain mixing [14], flow forces [15], and water table level [16] have been considered to analyze the impact of flow induced grain rearrangements. Based on the MTS815.02 ...
A test system for water flow in granular gangue mineral was designed to study the flow characteristics by compaction treatment. With the increase of the compaction displacement, the porosity decreases and void in granular gangue becomes less. The main reason causing initial porosity decrease is that the void of larger size is filled with small particles. Permeability tends to decrease and non-Darcy flow factor increases under the compaction treatment. The change trend of flow characteristics shows twists and turns, which indicate that flow characteristics of granular gangue mineral are related to compaction level, grain size distribution, crushing, and fracture structure. During compaction, larger particles are crushed, which in turn causes the weight of smaller particles to increase, and water flow induces fine particles to migrate (weight loss); meanwhile, a sample with more weight of size (0–2.5 mm) has a higher amount of weight loss. Water seepage will cause the decrease of some chemical components, where SiO 2 decreased the highest in these components; the components decreased are more likely locked at fragments rather than the defect of the minerals. The variation of the chemical components has an opposite trend when compared with permeability.
... Historically, many researchers have investigated water flow through non-cemented media (e.g., in dam filter layers) to find out the effect of flow on the grain size ratios and consequently on the hydraulic properties of the grain mixtures [18][19][20]. More recently, a good number of studies has been carried out to analyze the correlations between the seepage-induced grain rearrangements with seepage forces [21]), grain mixing [22], water table level [23], channel network formation [24] and grain shape and gradation [25]. Furthermore, several experimental studies have been reported in the literature on the investigation of hydraulic properties due to water flux in non-cemented coal samples and other non-compacted sedimentary rocks [26][27][28][29]. ...
In order to better understand groundwater influx and protection in coal mining extraction works, an in-house water flow apparatus coupled with an industrial rock testing system, known as MTS 815.02, were used to study the effects of grain size mixtures on the compaction and flow properties of disintegrated, or non-cemented, coal samples. From the Reynolds number evaluation of the samples with different grain mixtures, and the relationship between the water flow velocity and pore pressure gradient differences, it was found that seepage through the mixtures are of non-Darcy flow type. The porosity of coal specimens was found to be highly affected by compaction, and the variations of the porosity were also influenced by the samples’ grain size distribution. It was found that the sample porosity decreases with increasing compaction and decreasing grain sizes. Grain crushing during compaction was observed to be the main cause of the appearance of fine grains, and the washing away of fine grains was consequently the main contributing factor for the weight loss due to water seepage. It was observed that during the tests and with the progression of compaction, permeability k decreases and non-Darcy factor β increases with decreasing porosity φ. The k-φ and β-φ plots show that as the sizes of disintegrated coal samples are getting smaller, there are more fluctuations between the porosity values with their corresponding values of k and β. The permeability value of the sample with smallest grains was observed to be considerably lower than that of the sample with largest grains. Non-Darcy behavior could reduce the hydraulic conductivity. It was found that the porosity, grain breakage and hydraulic properties of coal samples are related to grain sizes and compaction levels, as well as to the arrangement of the grains. At high compaction levels, the porosity of disintegrated coal samples decreased strongly, resulting in a significant decrease of the permeability at its full compression state; Non-Darcy flow behavior has the slightest effect in uniform samples, therefore, indicating that disintegrated coal in uniform grain size mixtures could be treated as an aquicluding (water-resisting) stratum.
... As for the functions of chemical grouting in tunneling, Aksoy and Onargan (2010) studied the role of umbrella arch and face bolt as deformation preventing system. They employed 3D finite element model to study the effects of umbrella arch and bolts in tunneling face, the results indicate that the arch and the face-bolt system have significantly diminished the risk of settlement damage on buildings by reducing the ground settlements in an order of 69 %. Wang et al. (2013) evaluated the effects of umbrella arch and soilmixing used to reinforce the soft soils and optimized the design based on settlement control criterion. Recently, Valizadeh Kivi et al. (2012) reported a research using Mohr-Coulomb elastic-plastic constitutive model with 3D ABAQUS to investigate the ground settlement caused by excavation of a metro station in Tehran. ...
... Unconfined compressive strength is often utilized as the design and construction quality control criterion for soil-cement utilized to reinforce and pre-support excavations in soft ground. The dilemma of applying this state-of-art technique is uncertainty existing in the properties of soil-cement made very limited specific information available for advanced analysis prior to construction, some data can be obtained after construction but it is too late to revise the design (Wang et al. 2013). Hence, interpretation method based on the unconfined compressive strength for soil-cement mixtures will be very useful. ...
... The dilatancy w was related to the friction angle / 0 and the constant critical state friction angle / cv using the assumption of Rowe's stress-dilatancy theory. When the friction angle of the native soil is less than 30°, the dilatancy of c -/ material is negligible (Wang et al. 2013). When the friction angle greater than 30°, the dilatancy can be evaluated by using Eq. ...
Uncertainty of properties of improved soft ground with chemical grouting often results in discrepancy between simulated results and observed data and makes prediction for ground subsidence difficult. It also made practice usually adopts unconfined compressive strength of soil–cement mixtures as design control criterion. This paper presents a methodology to interpret the strength and deformation parameters from unconfined compressive strength of soil–cement mixtures for advanced numerical analysis which is widely used in reinforcing the soft ground and pre-supporting excavation. Observed data are used to compare with the simulated results and validate the interpretations. Numerical experiments with two-dimensional finite element model for covered undercutting of large cross-sectional twin tunnel under an existing subway station are presented. Issues with respect to design and construction of excavation that potentially affect the ground surface subsidence are explored. It was found that the ground subsidence caused by tunneling in soil–cement medium under existing subway station is similar to that in a greenfield site but with flatter settlement trough. Unconfined compressive strength of soil–cement mixtures ranging from 1.5 to 2.0 MPa can significantly reduce the settlement to an acceptable control criterion. Settlement magnitude is influenced by pillar width but not obviously by the location of umbrella arch installed in twin tunnels.
The fully mechanized tunneling (FMT) method was recently developed for metro cross-passage construction. In the construction phase, plugging and backfilling grouting must be carried out, and lifting grouting for correcting the cantilevered cross-passage abnormal settlement is an efficient option. In this study, the grouting effects were investigated via numerical simulation and field monitoring. For the simulation, a cantilevered/constrained multiple foundation beam (CMFB) model was developed and verified using a benchmark test. Parametric sensitivity analyses with definite parametric value ranges were conducted for different construction stages. With the CMFB model, the grouting construction steps were simulated for a particular case, and the simulation results were validated by field monitoring. Regarding structural deformation, the values were within 4 mm after the first-time plugging grouting or backfill grouting and close to 21 mm in lifting grouting for dislocation displacement correction. The monitoring data further revealed the variation features of ground swelling and subsidence, including a maximum ground displacement of within 2–3.5 mm, and a more obvious displacement effect from the subsequent plugging or backfill grouting than that of previous grouting steps. Overall, this study shows the feasibility of the proposed CMFB model and demonstrates the safety of the grouting technology adopted in FMT cross-passage construction in soil strata.
In order to study the mixed permeability characteristics of broken coal medium, the parameter changes during water-gas permeation are analyzed and discussed in this study. It includes the law of two-phase flow state transformation, the law of permeability evolution and the principle of effective stress action. Firstly, the water-gas mixed permeation test system was designed by independent design. Then the gas and liquid two-phase permeation parameters were measured in real time by using multiple sets of sensors. The results are as follows: In one hand, the two-phase seepage process in broken coal medium, the seepage apparent Reynolds number is mainly distributed between 1230–2207. The difference of two orders of magnitude between the kinematic viscosities of the two fluids leads to an asynchronous transition process between the liquid and gas phase flow regimes. On the other hand, there is a competitive relationship between the relative permeability ratio of water and gas phases. When the proportion of gas phase relative permeability is distributed in the interval 71.7%–85.1%, the proportion of gas phase relative permeability is dominant, and it is most sensitive to the influence of the compaction deformation process of the initial pore structure. At the same time, the permeability k decreases with the increase of the effective stress σov in all three stages of the compaction deformation of the skeleton of the broken coal medium, in accordance with the relationship: σov=0.678/k+0.608. The effective stress will directly determine the pore deformation process of the structure of the broken coal medium.
Unconfined compressive strength (qucs) and maximum shear modulus (Gmax), which are essential properties of grouted sands for quality control and stable design, exhibit a nonlinear behavior with curing time that makes it difficult to estimate the long-term qucs and/or Gmax. This study investigates the applicability of the hyperbolic model to capture the nonlinear development of qucs and Gmax of grouted sands relative to curing time, with the ultimate goal of estimating the long-term qucs. Three sands with varying particle sizes were grouted with microfine cement at three different water-to-cement ratios (W/C=1, 1.5, and 2), after which unconfined compression tests and bender element tests were performed according to curing time. The results of this study demonstrate that the hyperbolic model can effectively capture the time-dependent variations of both qucs and Gmax of the tested grouted sands. Investigation of the hyperbolic coefficient k of the tested materials reveals that the sand particle size and W/C affect the required curing time for completion of the hydration process, and relatively constant Gmax values can be obtained at a relatively earlier curing time compared with qucs. Finally, the direct relationship between qucs and Gmax is investigated in this study.
