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1.
Structures are commonly founded on layered soil deposits. In this study, settlement due to a circular load on a two-layer soil system overlying a rock stratum or a stiff soil deposit is estimated and compared with the values obtained using Steinbrenner’s and Finite element approaches. Design tables are proposed for settlements due to uniform circular loading (of radius equal to a) applied on two-layer soil system for top-layer thickness H 1 = 0.2a–6.0a, bottom-layer thickness H 2 = 1.0a–6.0a, moduli ratio of top and bottom layers E 1 /E 2 = 0.01–100 and Poisson’s ratio v = 0.2–0.5. The settlement factors from the study were also compared with the Ueshita and Meyerhof’s values for a finite top layer overlying a semi-infinite bottom layer. Results were compared with the observed settlement of a structure resting on a layered soil profile overlying a rock stratum. Proposed settlement of the two-layer system shows good agreement with these studies.  相似文献   

2.
This study investigates the ground movements due to mechanized tunnel excavation by applying two-dimensional finite element analyses. To assess the contribution of the compressibility and plasticity of the soil on the ground movements, different constitutive models are employed to describe the soil behavior. The influence of volume loss around the tunnel on the surface volume loss is investigated, and a quadratic correlation between them is proposed. Consequently, the empirical Gaussian distribution curve, which is generally used to determine the tunneling induced settlement trough, is improved by applying the proposed quadratic correlation between surface volume loss and tunnel volume loss. Furthermore, the settlement trough width parameter has been derived by a linear function of tunnel volume loss as well. The proposed equations are validated via a case study of centrifuge tests from the literature. The results show that the proposed modification enhances the empirical solution by having better knowledge on the model parameters. Additionally, tunnel overburden and coefficient of lateral earth pressure at rest (K0) are taken into account to study their influence on the tunneling induced surface settlements. Finally, global sensitivity analysis is applied to evaluate the relative importance of corresponding model parameters in terms of their influence on the ground movements.  相似文献   

3.
姜晓婷  路平  郑刚  崔玉娇  崔涛 《岩土力学》2014,35(Z2):535-542
以天津地铁2号线下穿多层建筑物的盾构隧道为例,建立了盾构下穿空旷场地以及下穿建筑物的有限元模型,计算结果与现场实测数据进行了对比验证。在此基础上分析了在天津软土地区盾构隧道施工对地表沉降及多层砌体结构建筑物差异沉降的影响,并对采用小应变土体本构模型与硬化土本构模型的计算结果进行比较。结果表明,采用小应变本构模型的地表最大沉降和横向沉降槽宽度与实测数据吻合良好。盾构斜下穿砌体结构房屋时,建筑物有偏向隧道轴线方向的倾斜,采用小应变土体本构模型的计算结果可以更好地反映建筑物的倾斜斜率的变化。因此,研究软土地区盾构掘进对上方建筑物沉降影响的精细化分析时应考虑土体小应变的影响。  相似文献   

4.
Debris flow density determined by grain composition   总被引：1，自引：1，他引：0  
Density is one of the most important parameters of debris flows. Because observing an active debris flow is very difficult, finding a method to estimate debris flow density is urgently needed for disaster mitigation engineering. This paper proposes an effective empirical equation in terms of grain size distribution (GSD) parameters based on observations in Jiangjia Gully, Yunnan Province, China. We found that the GSD follows P(D) = KD -μ exp(? D/Dc), with μ and Dc representing the fine and coarse grains, respectively. In particular, μ is associated with some characteristic porosity of soil in the natural state and increases with increased porosity. Dc characterizes the grain size range of the flow and increases with the grain concentration. Studies show that flow density is related to both parameters in power law. Here, we propose an empirical equation for estimating flow density: ρ = 1.26μ -0.132 + 0.049Dc0.443, which provides not only an estimation of the density for a flow, but also describes the variation in density with the GSD of material composition; this provides important information related to the design of debris flow engineering structures.  相似文献   

5.
We have collected high-resolution neutron powder diffraction patterns from MgSO4·11D2O over the temperature range 4.2–250 K. The crystal is triclinic, space-group \( \text{P} \bar{1} \) (Z = 2) with a = 6.72746(6) Å, b = 6.78141(6) Å, c = 17.31803(13) Å, α = 88.2062(6)°, β = 89.4473(8)°, γ = 62.6075(5)°, and V = 701.140(6) Å3 at 4.2 K, and a = 6.75081(3) Å, b = 6.81463(3) Å, c = 17.29241(6) Å, α = 88.1183(3)°, β = 89.4808(3)°, γ = 62.6891(3)°, and V = 706.450(3) Å3 at 250 K. Structures were refined to wRp = 3.99 and 2.84% at 4.2 and 250 K, respectively. The temperature dependence of the lattice parameters over the intervening range have been fitted with a modified Einstein oscillator model which was used to obtain the coefficients of the thermal expansion tensor. The volume thermal expansion, αV, is considerably smaller than ice Ih at all temperatures, and smaller even than MgSO4·7D2O (although ?αV/?T is very similar for both sulfates); MgSO4·11D2O exhibits negative αV below 55 K (compared to 70 K in D2O ice Ih and 20 K in MgSO4·7D2O) The relationship between the magnitude and orientation of the principal axes of the expansion tensor and the main structural elements are discussed.  相似文献   

6.
Back analysis of a large landslide in a flysch rock mass   总被引：1，自引：1，他引：0  
Flysch is a sedimentary rock consisting of a rhythmic alternation of hard (limestone, sandstone, siltstone) and weak (marl, mudstone, claystone) layers. Because of the presence of layers with different physical properties, the mechanical characterization of heterogeneous rock masses such as flysch is a real challenge. Different methods have been proposed in the literature to characterize flysch, combining empirical classification indexes with laboratory tests. Most of these methods, however, were specifically designed for tunneling and underground excavations, and their applicability to slope stability problems is not yet fully investigated. In this study, we analyze a large landslide in a cretaceous flysch rock in order to compare the mobilized strength at failure with those predicted by the modified GSI method (Marinos and Hoek, 2001). The landslide occurred in the Savena River basin (Northern Apennines of Italy) on April 6, 2013, with a volume of about 3 million m3. Soon after the failure, geological, geotechnical, and geophysical investigations were carried out to detect the failure mechanism and define the landslide geometry. Back analyses of the failed slope were performed using both limit equilibrium and finite difference methods to estimate the in situ strength of the flysch. The results show that the mobilized rock mass cohesion is very low (c '?≈?20?÷?40 kPa) and that the modified GSI method can predict the in situ strength only assuming a disturbance factor D = 1. Moreover, the analysis shows that the linearization criteria proposed in literature to compute the equivalent Mohr-Coulomb parameters remarkably overestimate the rock mass strength.  相似文献   

7.
Rainfall-induced landslides (RILs) have been a source of social and economic disruption in the mountainous Baguio area in northern Philippines. Prolonged heavy rainfall usually happens during tropical cyclone and southwest monsoon activity. A pragmatic approach to RIL mitigation is to develop rainfall-based early warning. We implemented a modified regression method to derive the empirical minimum intensity (I)–duration (D) threshold I = 6.46 D ?0.28 and a normalized ID threshold NI = 0.002 D ?0.28 for rainfall duration ranging between 24 and 264 h. Using a separate data set to evaluate the applicability of the threshold, 93% of the landslide-triggering rainfall events fell above the derived threshold. RILs also occurred when 24-h rainfall was 0.02–28% of the mean annual precipitation or after accumulating at least 500 mm of rainfall from the onset of the rainy season. The thresholds may be further refined as more landslide data become available in the future.  相似文献   

8.
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.  相似文献   

9.
Geotechnical Engineering has developed many methods for soil improvement so far. One of these methods is the stone column method. The structure of a stone column generally refers to partial change of suitable subsurface ground through a vertical column, poor stone layers which are completely pressed. In general terms, to improve bearing capacity of problematic soft and loose soil is implemented for the resolution of many problems such as consolidation and grounding problems, to ensure filling and splitting slope stability and liquefaction that results from a dynamic load such as earthquake. In this study, stone columns method is preferred as an improvement method, and especially load transfer mechanisms and bearing capacity of floating stone column are focused. The soil model, 32 m in width and 8 m in depth, used in this study is made through Plaxis 2D finite element program. The clay having 5° internal friction angle with different cohesion coefficients (c 10, c 15, c 20 kN/m2) are used in models. In addition, stone columns used for soil improvement are modeled at different internal friction angles (? 35°, ? 40°, ? 45°) and in different s/D ranges (s/D 2, s/D 3), stone column depths (B, 2B, 3B) and diameters (D 600 mm, D 800 mm, D 1000 mm). In the study, maximum acceleration (a max = 1.785 m/s2) was used in order to determine the seismic coefficient used. In these soil models, as maximum acceleration, maximum east–west directional acceleration value of Van Muradiye earthquake that took place in October 23, 2011 was used. As a result, it was determined that the stone column increased the bearing capacity of the soil. In addition, it is observed that the bearing capacity of soft clay soil which has been improved through stone column with both static and earthquake load effect increases as a result of increase in the diameter and depth of the stone column and decreases as a result of the increase in the ranges of stone column. In the conducted study, the bearing capacity of the soil models, which were improved with stone column without earthquake force effect, was calculated as 1.01–3.5 times more on the average, compared to the bearing capacity of the soil models without stone column. On the other hand, the bearing capacity of the soil models with stone columns, which are under the effect of earthquake force, was calculated as 1.02–3.7 times more compared to the bearing capacity of the soil models without stone column.  相似文献   

10.
A natural sample of clinochlore from the Longitudinal Valley area of northeastern Taiwan has been characterized by using the powder X-ray diffraction (XRD), differential thermal analysis and electron paramagnetic resonance (EPR) spectroscopic techniques. The lattice parameters of the monoclinic (IIb) clinochlore with the composition (Mg2.988 Al1.196 Fe1.6845 Mn0.026)5.8945 (Si2.559 Al1.441)4 O10 (OH)8 have been calculated from the powder XRD data and are found to be a = 5.347 Å, b = 9.223 Å, c = 14.250 Å, β = 97.2° and Z = 2. The thermal behaviour of the sample showed the typical behaviour of clinochlore with a hydroxyl content of 12.5 wt%. The EPR spectrum at room temperature exhibits two resonance signals centred at g ≈ 2.0 and g ≈ 8.0. The signal at g ≈ 2.0 shows a six-line hyperfine structure which is a characteristic of Mn2+ ions in octahedral symmetry. The resonance signal at g ≈ 8.0 is a characteristic of Fe3+ ions. The EPR spectra have also been recorded at different temperatures (123–295 K). The population of spin levels (N) has been calculated for g ≈ 2.0 and g ≈ 8.0 resonance signals. It is observed that N increases with decreasing temperature. From EPR spectra, the spin-Hamiltonian parameters have been evaluated. The zero-field splitting parameter (D) is found to be temperature dependent. The peak-to-peak width of the g ≈ 8.0 resonance signal is found to increase with decrease in temperature.  相似文献   

11.
王丽  郑刚 《岩土力学》2011,32(Z1):704-0712
利用有限元软件ABAQUS,采用对隧道洞室周边及开挖面的土体施加由盾构机引起的各种荷载的方法模拟天津市地铁1号线盾构施工。计算结果表明,有限元模型能够很好地模拟盾构施工过程。利用此模型研究隧道开挖对桩基础的影响。隧道开挖引起的桩顶沉降、桩身侧移主要发生在盾构机推进面逐渐接近桩的过程中,当盾构机推进面通过桩所在的位置后桩顶沉降、桩身侧移增加不明显;与隧道水平距离相同时,由于长桩能够充分发挥桩身下部的侧摩阻力,隧道开挖引起的长桩的桩顶沉降小于短桩的桩顶沉降;隧道开挖过程中12 m长桩的桩身发生了整体倾斜,16、19 m长桩的桩身出现了弯曲变形,16、19 m长桩的桩身最大弯矩发生在地面下12～13 m之间,即在隧道轴线附近;开挖过程中桩顶出现沿隧道推进方向的往复位移;桩顶作用的竖向荷载越大,由隧道开挖引起的桩顶沉降越大  相似文献   

12.
In this paper, numerical and analytical methods are used to evaluate the ultimate pullout capacity of a group of square anchor plates in row or square configurations, installed horizontally in dense sand. The elasto-plastic numerical study of square anchor plates is carried out using three-dimensional finite element analysis. The soil is modeled by an elasto-plastic model with a Mohr–Coulomb yield criterion. An analytical method based on a simplified three-dimensional failure mechanism is developed in this study. The interference effect is evaluated by group efficiency η, defined as the ratio of the ultimate pullout capacity of group of N anchor plates to that of a single isolated plate multiplied by number of plates. The variation of the group efficiency η was computed with respect to change in the spacing between plates. Results of the analyses show that the spacing between the plates, the internal friction angle of soil and the installation depth are the most important parameters influencing the group efficiency. New equations are developed in this study to evaluate the group efficiency of square anchor plates embedded horizontally in sand at shallow depth (H = 4B). The results obtained by numerical and analytical solutions are in excellent agreement.  相似文献   

13.
This study investigates the behavior of shield tunneling based on a complete case record of an excavation in sandy cobble ground. Both numerical and analytical analyses were conducted to estimate the surface displacement along the tunnel axis and the surface-zone affected by the shield tunneling. This paper presents the performance of the Peck method in forecasting the transversal surface settlement trough caused by the shield tunneling in a sandy cobble layer. In particular, this study has made a comparison with the vertical surface displacement monitored during the advancing excavation to the settlements estimated by using the Peck method and by conducting a three-dimensional finite element analysis. The effective measures to provide the necessary face support pressure and to reduce the formation of sinkholes and ground settlements in sandy cobble ground were also investigated and discussed.  相似文献   

14.
We used an in situ measurement method to investigate the phase transition of Fe2SiO4 polymorphs under high pressures and temperatures. A multi-anvil high-pressure apparatus combined with synchrotron X-ray radiation was used. The stability of each polymorph was identified by observing the X-ray diffraction data from the sample. In most experiments, the diffraction patterns were collected 10–30 min after reaching the desired pressure and temperature conditions. The transition boundary between the olivine and spinel phase at T = 1,000–1,500 K and P = 2–8 GPa was determined to occur at P (GPa) = 0.5 + 0.0034 × T (K). The transition pressure determined in this study was in general agreement with that observed in previous high-pressure experiments. However, the slope of the transition, dP/dT, determined in our study was significantly higher than that estimated by the previous study combined with the in situ X-ray method.  相似文献   

15.
Increasing demands on infrastructures increases the attention on shallow soft ground tunneling methods in urbanized areas. Especially, in metro tunnel excavations, it is important to control the surface settlements observed before and after excavation, which may cause damage to surface structures. To solve this problem, earth pressure balance machines (EPBMs) have widely been used throughout the world. This study focuses on surface settlement measurements, the interaction of twin tunnel surface settlement, and the relationship between shield parameters and surface settlement for parallel tunnels using EPBM shields in clay and sand soils. In this study, the tunnels were excavated using two EPBMs. The tunnels were 6.5 m in diameter, as twin tubes with a 14 m distance from center to center. The EPBM in the first tube followed about 100 m behind the other tube. Segmental lining with 1.4 m of length was employed as a final support. The results from this study showed that (1) the most important parameters for the maximum surface settlements are the face pressure and backfill; (2) in twin tunnel excavation with EPBM for longitudinal profile, the settlement rate reached its peak value when the shield came to the monitoring section and this peak value continued until the shield passed the monitoring section; (3) every shield affected the other tunnel’s longitudinal surface settlement profile by approximately 35–36.8 %; (4) S A, S B and S C values were found to be 38.0, 35.8 and 26.2 %, respectively for an EPBM, and (5) ensuring good construction quality is a very effective way to control face stability and minimize surface settlement.  相似文献   

16.
In this study, the seismicity rate changes that can represent an earthquake precursor were investigated along the Sagaing Fault Zone (SFZ), Central Myanmar, using the Z value technique. After statistical improvement of the existing seismicity data (the instrumental earthquake records) by removal of the foreshocks and aftershocks and man-made seismicity changes and standardization of the reported magnitude scales, 3574 earthquake events with a M w ≥ 4.2 reported during 1977–2015 were found to directly represent the seismotectonic activities of the SFZ. To find the characteristic parameters specifically suitable for the SFZ, seven known events of M w ≥ 6.0 earthquakes were recognized and used for retrospective tests. As a result, utilizing the conditions of 25 fixed earthquake events considered (N) and a 2-year time window (T w), a significantly high Z value was found to precede most of the M w ≥ 6.0 earthquakes. Therefore, to evaluate the prospective areas of upcoming earthquakes, these conditions (N = 25 and T w = 2) were applied with the most up-to-date seismicity data of 2010–2015. The results illustrate that the vicinity of Myitkyina and Naypyidaw (Z = 4.2–5.1) cities might be subject to strong or major earthquakes in the future.  相似文献   

17.
Thermal diffusivity (D) was measured up to ~1,800 K of refractory materials using laser-flash analysis, which lacks radiative transfer gains and contact losses. The focus is on single-crystal MgO and Al2O3. These data are needed to benchmark theoretical models and thereby improve understanding of deep mantle processes. Measurements of AlN, Mg(OH)2, and isostructural BeO show that the power law (D = AT ?B ) where T is temperature holds for simple structures. Results for more structurally complicated corundum Al2O3 with and without impurity atoms are best fit by CT d  + ET f where d ~ ?1 and f ~ ?4, whereas for isostructural Fe2O3, f is near +3 and multiphase ilmenite Fe1.12Ti0.88O3 is fit by the above power law. The positive temperature response for hematite is attributed to diffusive radiative transfer arising from electronic–vibronic coupling. We find good agreement of k and D data on single-crystal and non-porous ceramic Al2O3. For the corundum structure, D is nearly independent of T at high T. Although D at 298 K depends strongly on chemical composition, at high temperature, these differences are reduced. Thermal conductivity provided for MgO and Al2O3, using LFA data and literature values of density and heat capacity, differs from contact measurements which include systematic errors. The effect of pressure is discussed, along with implications for the deepest mantle.  相似文献   

18.
The crystal structure of Pb6Bi2S9 is investigated at pressures between 0 and 5.6 GPa with X-ray diffraction on single-crystals. The pressure is applied using diamond anvil cells. Heyrovskyite (Bbmm, a = 13.719(4) Å, b = 31.393(9) Å, c = 4.1319(10) Å, Z = 4) is the stable phase of Pb6Bi2S9 at ambient conditions and is built from distorted moduli of PbS-archetype structure with a low stereochemical activity of the Pb2+ and Bi3+ lone electron pairs. Heyrovskyite is stable until at least 3.9 GPa and a first-order phase transition occurs between 3.9 and 4.8 GPa. A single-crystal is retained after the reversible phase transition despite an anisotropic contraction of the unit cell and a volume decrease of 4.2%. The crystal structure of the high pressure phase, β-Pb6Bi2S9, is solved in Pna2 1 (a = 25.302(7) Å, b = 30.819(9) Å, c = 4.0640(13) Å, Z = 8) from synchrotron data at 5.06 GPa. This structure consists of two types of moduli with SnS/TlI-archetype structure in which the Pb and Bi lone pairs are strongly expressed. The mechanism of the phase transition is described in detail and the results are compared to the closely related phase transition in Pb3Bi2S6 (lillianite).  相似文献   

19.
Elastic behavior and pressure-induced structural evolution of synthetic boron-mullite “Al5BO9” (a = 5.678(2) Å, b = 15.015(4) Å and c = 7.700(3) Å, space group Cmc21, Z = 4) were investigated up to 7.4 GPa by in situ single-crystal X-ray diffraction with a diamond anvil cell under hydrostatic conditions. No phase transition or anomalous compressional behavior occurred within the investigated P range. Fitting the P–V data with a truncated second-order (in energy) Birch-Murnaghan Equation-of-State (BM-EoS), using the data weighted by the uncertainties in P and V, we obtained: V 0 = 656.4(3) Å3 and K T0 = 165(7) GPa (β V0 = 0.0061(3) GPa?1). The evolution of the Eulerian finite strain versus normalized stress (f E–F E plot) leads to an almost horizontal trend, showing that a truncated second-order BM-EoS is appropriate to describe the elastic behavior of “Al5BO9” within the investigated P range. The weighted linear regression through the data points gives: F E(0) = 159(11) GPa. Axial compressibility coefficients yielded: β a  = 1.4(2) × 10?3 GPa?1, β b  = 3.4(4) × 10?3 GPa?1, and β c  = 1.7(3) × 10?3 GPa?1 (β a :β b :β c  = 1:2.43:1.21). The highest compressibilities observed in this study within (100) can be ascribed to the presence of voids represented by five-membered rings of polyhedra: Al1–Al3–Al4–Al1–Al3, which allow accommodating the effect of pressure by polyhedral tilting. Polyhedral tilting around the voids also explains the higher compressibility along [010] than along [001]. The stiffer crystallographic direction observed here might be controlled by the infinite chains of edge-sharing octahedra running along [100], which act as “pillars”, making the structure less compressible along the a-axis than along the b- and c-axis. Along [100], compression can only be accommodated by deformation of the edge-sharing octahedra (and/or by compression of the Al–O bond lengths), as no polyhedral tilting can occur. In addition, a comparative elastic analysis among the mullite-type materials is carried out.  相似文献   

20.
K-substituted gonnardite, K2.18Na0.04Ca0.02[Al2.26Si2.74O10]·2.2H2O, was studied by X-ray powder diffraction method. The structure was refined with the Rietveld technique in the tetragonal space group $I\overline{4} 2d$ with a = 13.65409(16), c = 6.56928(11) Å, V = 1224.74(2) Å3, Z = 4. Most of K+ cations (1.94 apfu) statistically occupy three nearest positions to be considered as the split one. “Excess” cations are located in the position non-typical for K+. Statistics in the cation distribution is defined by the occupation of the additional position. Based on a crystal chemical positional model (C2R2A2) [T5O10], the separation of the local water–cation assemblages from an average statistical pattern has been suggested.  相似文献   

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