Surface Settlement Profile Research Articles

Ground Response due to Buried Structure Construction Process

Deep aircraft shelter and deep fortification are major projects of buried structures in military purposes. The Greater Cairo metro and El-Azhar road tunnels have been constructed as major projects of buried structures in Cairo City. Deep fortification construction causes movements in surrounding soil. Ground movement is an important factor during buried structure construction. Buried structure construction in cohesionless soil is a sophisticated process leading to cause potential damage to exist surface and subsurface fortification structures. Ground movements depend on soil properties, buried structure depth, buried structure diameters, and method of construction. In this study, the finite element analysis (FEA) is used to predict surface displacement due to buried structure process based on case study. Surface displacement equation (SDE) proposed by Peck and Schmidt (1969) is also used to calculate surface displacement due to buriedstructure construction process. For assessing the reliability of the FEA, a case history along the Greater Cairo Metro tunnel Line 2 is considered. A comparison between the field measurements and those obtained by the FEA and the SDE is made. The surface ground movements obtained by the SDE are examined with those obtained by the FEA at different sand soil densities. However, the SDE does not consider impact of different sand soil types. The surface settlement profile computed by the SDE is in poor agreement with those obtained by the FEA in loose to medium sandy soil. The surface settlement profiles computed by the SDE agree well with those calculated by the FEA in dense and very dense sandy soil. Finally,the finite element analysis is a good tool to analyze different sandy soil densities at which different sand soil stress and strength parameters are considered in the FEA.

Analysis of Tunneling-Induced Ground Movements Using Transparent Soil Models

Ground movements induced by shallow tunnels affect the safety of nearby underground and aboveground structures. Therefore, the reliable prediction of these movements is important. A transparent soil model is used to investigate not only the surface settlement profile induced by shield tunneling, but also the distribution of soil deformation within the soil mass near the tunnel. The observed surface settlements are consistent with the normal probability curve commonly used for predicting settlement, with only the inflection points or trough width parameters somewhat different. The measured data are consistent with field measurements in that the trough width parameter is independent of the volume loss and linearly proportional to the tunnel depth. An analysis of the displacement field inside the transparent soil models indicates that the subsurface settlement trough at different depths can be approximated by a normal probability curve; and the horizontal displacement can be expressed by the trough width parameter and the volume loss, at the point at which maximum horizontal displacement occurs at the point of inflection. Additionally, the measurements indicate that subsurface ground movements can be in excess of the observed surface settlement, which can adversely affect underground utilities.

Effects of Advancing Open Face Tunneling on an Existing Loaded Pile

A three-dimensional, elasto-plastic, coupled-consolidation numerical analysis was conducted to investigate the effects of an advancing open face tunnel excavation on an existing, loaded pile. Based on the ground conditions, geometry, and tunnel construction method simulated, a significant zone of influence can be identified one tunnel diameter ahead and one behind the tunnel excavation face. Within this zone of influence, the settlement of the pile is greater than the ground settlement due to the plastic yielding at the pile toe. A computed ground surface settlement profile simulated by a normal Gaussian distribution does not accurately reflect pile head settlement and a significant nonlinear distribution of subsurface settlement with depth is induced by tunnel excavation. Under plane strain conditions, other empirical and analytical methods predicted similar trends in the subsurface settlements, but gave larger magnitudes of subsurface settlements. The open face tunnel excavation induces complex distributions of relative subsurface settlements and both positive and negative side shear stresses along the pile. Within the zone of influence of the tunnel excavation, excess positive and negative pore-water pressures are generated at the pile head and pile toe, respectively. At the pile toe, the pore-water pressure was reduced to 40% of its initial hydrostatic value. At 10 days after the tunnel face passed the pile, the excess pore-water pressures at the pile recovered to within 20% of initial hydrostatic values. Due to the additional settlement of the pile that occurred during tunneling, the soil resistance factor of safety for the pile can be regarded as decreasing from 3.0 to 1.5, according to a displacement-based failure load criterion. The tunnel excavation did not significantly affect the existing bending moment and the axial structural load distribution within the pile.

Centrifuge Modelling of Tunnelling Near Driven Piles

A centrifuge model study was carried out to investigate the effects of tunnelling on nearby single piles in dense dry sand. This paper presents a description of the centrifuge model and the test procedures followed. The surface settlement profiles recorded during tunnelling-induced volume loss are discussed, as well as the settlement of piles installed at various distances from the tunnel. A zone was identified around the tunnel in which a potential for large settlement exists. Changes in the load distribution on individual piles at various locations during increasing volume loss are presented.

Modeling of tunneling-induced ground surface movements using stochastic medium theory

Abstract Tunneling-induced ground surface movements were modeled using the stochastic medium theory. The formulated ground movement equations were coded in a computer program, which was then used to perform back analyses of 18 ground surface settlement profiles available in the literature. Results of back analyses provided the values of two key parameters—convergence and influence angle. These two parameters are primary factors for modeling the settlement profiles. The results of the study show that the developed methodology can be used to model ground settlement profiles reasonably well for both circular and non-circular tunnels. The required input parameters can be estimated using the available information as well as empirical or semi-empirical relations. For accurate determinations of the input parameters, it is recommended that correlations between input parameters and soil/tunnel conditions be established through extensive back analyses of available field data. Also, field studies are recommended to validate the developed methodology and correlations.

Systematic update of a deep excavation model using field performance data

Numerical simulation of construction staging of deep urban excavations is commonly used to estimate induced ground deformations. A novel method to extract the constitutive model of the soil behavior directly from field measurements of excavation response is introduced. This methodology develops a neural network based constitutive model of the soil, given the lateral wall deformation and surface settlement profile measurements. The resulting soil model, used in a numerical analysis, provides correct ground deformations and can be used in the forward prediction of future excavations or later excavation stages. The soil model can continuously evolve using additional field information.

Shape of ground surface settlement profiles caused by excavation

There are two general types of settlement profiles induced by excavation, the spandrel type and the concave type. Based on a regression analysis of the field observations of settlement curves with predominantly cohesive subsurface conditions, an empirical method for predicting the spandrel and concave settlement profiles is proposed. The proposed method is verified through several case histories. For comparison, the results obtained from other empirical methods are also presented. The proposed method provides a fairly good prediction of ground surface settlement and yields better prediction of angular distortion than other empirical methods for the concave- and spandrel-type settlement profiles. Furthermore, the relationship of cantilever area and deep inward area of wall deflection is established which can be used as the first approximation to predict the type of settlement profile.Key words: prediction, excavation, settlement profile, spandrel, concave.

Shape of ground surface settlement profiles caused by excavation

Shape of ground surface settlement profiles caused by excavation

Characteristics of ground surface settlement during excavation

The objective of this paper is to study the characteristics of ground surface settlement during excavation. Ten excavation cases in Taipei with good-quality construction and field observation data are selected to study the characteristics of excavation behavior. The location of maximum lateral wall deflection, magnitude of maximum lateral wall deflection, relationship of maximum lateral wall deflection and maximum ground surface settlement, location of maximum ground surface settlement, and apparent influence range are thus established based on the actual excavation cases. Finally, an empirical formula is proposed to predict the ground surface settlement profile at the center section of an excavation, where the behavior may be characterized by plane-strain conditions. Key words : excavation, surfac settlement, wall deflection, plane strain, empirical formula.

Subsurface settlement profiles above tunnels in clays

Designers assessing the possible effect of tunnelling on structures relatively close to the tunnel crown need to know how subsurface settlement profiles develop, and how these relate to surface settlement profiles. The effect on piled foundations or existing tunnels depends on the width of the subsurface settlement profile and on the magnitude of the settlement. This note uses available field measurements and centrifuge model test data to determine how the widths of the settlement profiles and the magnitudes of settlement vary with depth above tunnels constructed in clays. The results reveal a fairly consistent pattern of behaviour: the width of the subsurface settlement troughs at depth is significantly greater than would be predicted by assuming a constant trough width parameter K of 0.5. Smaller more realistic subsurface settlements and strains will be predicted if account is taken of the proportionally wider trough widths at depth.

UNDRAINED STABILITY OF TWO-DIMENSIONAL UNLINED TUNNELS IN SOFT SOIL

The paper presents the results of centrifuge model tests and upper bound calculations on two-dimensional unlined tunnels in two different normally consolidated cohesive soils with the plasticity index of 10 and 25. The upper bound calculations were carried out by taking the variation of soil strength with depth and the strength anisotropy into account. It is found that the undrained stability of the tunnels increases with increasing tunnel cover-to-diameter ratio and that observed surface settlement profiles are different for the two soils. The critical tunnel pressures as well as the collapse mechanisms deduced from the bound calculation compare well with the observations. It is confirmed that the surface settlement profiles can be represented by the error functions fairly accurately.

Effects of undrained strength anisotropy on surface subsidences induced by the construction of shallow tunnels

The implementation of an anisotropic soil model that allows consideration of the variation of undrained strength due to anisotropy is described. This analysis is then used to identify the significance of strength anisotropy on the prediction of deformation behaviour of a shallow tunnel. It is found that, for the case of an unlined tunnel, attention should be given to the effect of strength anisotropy particularly for a soil possessing type "K" anisotropy (i.e., where the smallest undrained strength occurs at an angle θ other than the vertical or the horizontal, and the smallest value usually occurs at θ = 45°). On the other hand, for a lined tunnel, the effect of strength anisotropy upon the surface settlement profile will depend upon the size of a so-called "gap" parameter. The gap represents the net effect of loss of ground and some "workmanship" factors in a plane strain finite element analysis. Increasing the gap eases the restrictions imposed by the tunnel lining upon possible soil deformations, thereby increasing the effect of strength anisotropy. However, it is shown that for a lined tunnel with a moderate value of gap, detailed consideration of strength anisotropy may not be necessary. Key words: strength anisotropy, tunnelling, predictions, surface subsidences, finite element method, soil model, soft clays, vane strength.

Ground disturbance caused by shield tunnelling in a stiff, overconsolidated clay

Attewell, P.B. and Farmer, I.W., 1974. Ground disturbance caused by shield tunnelling in a stiff, overconsolidated clay. Eng. Geol., 8: 361–381. Some of the factors affecting ground deformation around shield tunnelling excavations in stiff clays are considered. There is particular reference throughout the paper to an analysis and interpretation of measured ground deformation around a 4.146-m diameter, hand-excavated, shield-driven tunnel at a nominal axis depth of 29.3 m in the overconsolidated London Clay. The maximum surface settlement was found, by precise levelling, to be 6.1 mm but the shape of the transverse surface settlement profile conformed to a normal probability curve only up to the time of shield passage. Of the contributory ground losses at the tunnel, yield of the clay at the tunnel face appears to dominate to the extent of generating up to 50% of the eventual surface settlement. Measurement evidence suggests a rate of yield at the face that is 2 to 3 times the radial yield over the shield and implies that up to about one-fifth of the surface settlement could be attributed to radial yield into the grouted sections of the erected tunnel lining.

Ground Deformations Resulting from Shield Tunnelling in London Clay

The paper describes the results of a field measurement program aimed at determining the ground movements created by the hand excavation of a 4.146 m diameter shield-driven tunnel at a depth of 29.3 m in stiff, fissured, heavily overconsolidated London clay. Approximately half of the recorded maximum surface settlement of 6.1 mm is both measured and calculated to have taken place during passage of the shield, the remainder being attributed to post-shield deformations. Calculations are based on a field and laboratory-determined average radial yield rate for the clay at the tunnel of 0.0055 mm/min. It is concluded that a normal probability surface settlement profile is developed over the shield and tail.Post-shield inward decompressions (K0 ≈ 1.65) and possible recompression as a clay–grout interaction effect could then be responsible for a slight reconsolidation of the clay at soffit and for distortion of the final settlement profile out of concordance with an error curve.