Surface Settlement Trough Research Articles

Prediction of settlement trough induced by tunneling in cohesive ground

Surface settlements of soil due to tunneling are caused by stress relief and subsidence due to movement of support by excavation. There are significant discrepancies between empirical solutions to predict surface settlement trough because of different interpretations and database collection by different authors. In this paper, the shape of settlement trough caused by tunneling in cohesive ground is investigated by different approaches, namely analytical solutions, empirical solutions, and numerical solutions by the finite element method. The width of settlement trough was obtained by the finite element method through establishing the change in the slope of the computed settlement profile. The finite element elastic-plastic analysis gives better predictions than the linear elastic model with satisfactory estimate for the displacement magnitude and slightly overestimated width of the surface settlement trough. The finite element method overpredicted the settlement trough width i compared with the results of Peck for soft and stiff clay, but there is an excellent agreement with Rankin’s estimation. The results show that there is a good agreement between the complex variable analysis for Z/D = 1.5, while using Z/D = 2 and 3, the curve diverges in the region faraway from the center of the tunnel.

Double-O-Tube shield tunneling for Taoyuan International Airport Access MRT

From 1989 to 2010, 20 tunneling projects have been carried out with the Double-O-Tube (DOT) shield tunneling method in the world. In this paper, the DOT shield tunneling for the construction of Taoyuan International Airport Access (TIAA) Mass Rapid Transit (MRT) system is introduced. A 6.42m-diameter, 11.62m-wide DOT shield machine was used to build the first DOT tunnel in Taiwan. Field data indicated that, throughout the tunneling operation, the rolling angle of the DOT shield varied between +0.23° and −0.39°, which was within the limiting design values of +0.6° and −0.6° proposed by both TIAA MRT and Shanghai Metro. For the six surface settlement troughs collected from Tokyo, Shanghai and Taipei, the ground loss due to DOT shield tunneling ranged from 0.23% to 1.30%, and the average ground loss was 0.78%. As compared with the ground loss due to single-circular Earth-Pressure-Balance (EPB) shield tunneling in cohesive soils, the range of ground loss due to DOT shield tunneling was relatively narrow, and the peak ground loss value was significantly less. Underground excavation with the DOT tunneling method would increase the tunneling duration for about 32%. The cost per meter of tunnel constructed with a DOT shield was about 1.5 times that constructed with single-circular shields. The cost of shield machine and segment lining were 23% and 53% of the total tunneling costs respectively. The expensive DOT shield machine and the complicated manufacturing and assembly of DOT lining segments are the main reasons for higher cost of tunneling. However, it would cost a lot more budget and it would be much more risky to excavate three cross-passages between the single-circular tunnels under the river.

Ground movements in EPB shield tunneling of Bangkok subway project and impacts on adjacent buildings

Monitoring data of the first Bangkok subway project excavated by 8 EPB shields were analyzed to evaluate the ground movement characteristics and responses of adjacent buildings. Mostly the maximum ground surface settlements were in the range of 20–40mm, corresponding to 0.5–2.0% of tunnel volume loss, although few sections experienced larger settlements reaching 100mm. Larger ground losses tended to occur while excavating in sand or mixed soil layers and at locations of long duration stoppage. There was only a slight trend of relationship between the settlement and shield pressure. The predominant factor controlling the movement was the human related factor in shield operating which was not easy to predict. Obstructions from piled foundations and other known and unknown buried utilities and objects, resulted in non-conformance of ground surface settlement troughs to the Gaussian function, thus they could not be predicted by any empirical and analytical methods. Shield face pressures often caused outward ground displacements that complicated the final ground movements, particularly the lateral displacement which could not be predicted by available analytical methods. Long term consolidation settlements mainly occurred in the soft clay layer, but the amount and rate did not seem significant. On impacts to buildings, buildings on deep piles showed the least induced settlements. But for buildings on short piles, the settlements may be larger or smaller than ground surface settlements, depending on the distance from tunnel center line and pile tip depth.

Behavior of Ground and Response of Existing Foundation Due to Tunneling

ABSTRACT An apparatus has been developed to model the excavation of a tunnel in the laboratory. With this apparatus, 2D model tests are carried out to investigate the surface settlement and the earth pressure brought about by the tunneling. Finite element analyses using an elastoplastic subloading tij model are also conducted. The influence of volume loss on the surface settlement and the earth pressure, due to the shallow tunnelling, is illustrated based on the model tests and the corresponding numerical analyses. It is revealed that the surface settlement troughs and the earth pressure distributions around shallow tunnels depend on both the volume loss and the crown drift of the tunnel. The effect of the interaction between the tunneling and existing nearby foundations is also demonstrated in this paper. For existing foundations, the building loads control the surface settlements and the zone of deformation during the tunnel excavation. The behavior of the foundations depends on the deformation mechanism of the ground during the tunnel excavation. The induced axial force and bending moments in the piles of a piled raft are investigated numerically, and it is shown that the axial force changes due to the stress relaxation of the ground. Bending moments are induced in the piles at a lower value of soil cover due to the differential settlement of the piled raft.

Case Study of Long-Term Ground Settlements Induced by Slurry Shield Tunnelling

The Qing-chun Road Cross-river Tunnel is the first road tunnel under the Qiantang River in Hangzhou, and it is also the first experiences of slurry shield tunnelling in Hangzhou soft ground. In order to ensure the safety of construction and reduction of environmental impacts while shield tunnelling, a comprehensive monitoring system was carried out during construction, which included ground settlements, displacements and deformations of installed linings and so on. In this paper, the long-term ground settlements induced by slurry shield tunnelling were described in detail, and careful analysis of monitoring ground settlements was made. This case study shows that: 1. The surface transverse consolidation settlements do not follow the Gaussian curve; usually the largest settlements exist above the centerline of the tunnel or nearby, from where settlements descend outwards. Due to consolidation, the surface settlement troughs widen with time.2. Shield tunneling in soft ground, the turning point in settlement-time curves or settlement rate-time curves can be chosen as the time divides the immediate and long-term consolidation settlements.

Surface subsidence induced by twin subway tunnelling in soft ground conditions in Istanbul

Unlike the symmetrical surface settlement trough of a single tunnel which can be described using the Gaussian function, surface settlement over twin tunnels can be symmetric with respect to the mid-point between two tunnels or asymmetric. The paper reports the settlement troughs which developed when earth pressure balance (EPB) machines were used to excavate twin tunnels at shallow depth in the soft ground conditions beneath a developed part of Istanbul. An attempt is made to evaluate the effects of different factors on the surface subsidence. Detailed monitoring was undertaken when one tunnel was advanced ahead of the other and when only one tunnel was being driven. It was found that the shapes of the subsidence troughs over the two tunnels were different and varied with the excavation of the second/subsequent tunnel. It is concluded that changes in the subsidence trough are related to disturbance in the geo-material when an excavation is advanced ahead, as well as the nature and thickness of the overburden.

3D Modeling of an NATM Tunnel in High K0 Clay Using Two Different Constitutive Models

This paper studies the accuracy of the three-dimensional finite-element predictions of a displacement field induced by tunneling using new Austrian tunneling method (NATM) in stiff clays with high K0 conditions. The studies are applied to the Heathrow express trial tunnel. Two different constitutive models are used to represent London Clay, namely a hypoplastic model for clays and the modified Cam-clay (MCC) model. Good quality laboratory data are used for parameter calibration and accurate field measurements are used to initialize K0 and void ratio. The hypoplastic model gives better predictions than the MCC model with satisfactory estimate for the displacement magnitude and slightly overestimated width of the surface settlement trough. Parametric studies demonstrate the influence of variation of the predicted soil behavior in the very-small-strain to large-strain range and the influence of the time dependency of the shotcrete lining behavior.

The influence of previous stress history and stress path direction on the surface settlement trough induced by tunnelling

The pre-failure behaviour of overconsolidated soils is well known to be non-linear and inelastic. Over the last two decades experimental work has shown that pre-failure behaviour is also dependent on the previous stress history of the soil. However, some research has also suggested that, if enough time is allowed for creep to take place, then there is no effect of recent stress history; moreover, what affects soil stiffness is the direction of the stress path relative to the gross yield surface. This paper investigates the effect of two scenarios on the surface settlement trough associated with tunnelling. The first assumes that the previous stress history of the soil has an effect on soil stiffness, whereas the second assumes that creep/ageing periods have erased any memory of the previous stress history, and it is only the stress path direction that influences soil stiffness. A three-surface kinematic hardening model, which can simulate both scenarios, is employed in finite element analyses of tunnel construction. The predictions of the kinematic hardening model for the simulation of each scenario are compared with predictions using a non-linear elastic model and with field measurements.

Impact of constitutive models on the numerical analysis of underground constructions

The constitutive model frequently used in numerical calculations of tunnel excavation is linear-elastic perfectly plastic with a Mohr–Coulomb (MC) failure criterion. Generally, this leads to shallower and wider surface settlement troughs than those observed experimentally. It is therefore necessary to use adapted constitutive models for the design of underground works. In this paper, three constitutive models are implemented in a two-dimensional simulation of an underground excavation in plane strain: a linear-elastic perfectly plastic model (the MC model), an elastoplastic model with isotropic hardening [the hardening soil (HS) model, Schanz et al., Beyond 2000 in computational geotechnics, Balkema, Rotterdam, pp. 281–290, 1999] and an extension of this model which implies an evolution of the stiffness modulus in the small-strain range according to the strain level (the HS model with small-strain stiffness “HS-Small”, Benz, Small-strain stiffness of soils and its numerical consequences. Ph.D. thesis, Universitat Stuttgart, 189 pp., 2007). The study is based on the results of drained triaxial compression tests representing an overconsolidated clay (Gasparre, Advanced laboratory characterisation of London clay. Ph.D. thesis, Imperial College London, 598 pp., 2005); and is then applied to a shallow tunnel. The impact of the constitutive model is highlighted as well as the limits of the simplest constitutive model.

Influences of 3D Effects, Wall Deflection Process and Wall Deflection Mode in Retaining Wall Problems

Two-dimensional (2D) and three-dimensional (3D) model tests and the corresponding elastoplastic finite element analyses were carried out to investigate the influences of 3D effects, wall deflection process and wall deflection mode on the earth pressures and the ground movements in retaining wall problems. Aluminum rod mass were used in 2D model tests, and alumina balls were used in 3D model tests. The stress-strain behavior of these materials is similar to that of dense sands with positive dilatancy. In the finite element analyses, a recently developed elastoplastic constitutive model, named subloading t ij model, were used. This model can describe typical stress deformation and strength characteristics of soils such as the influence of intermediate principal stress, the stress path dependency of plastic flow and the influence of density and/or confining pressure properly. The test results show that the earth pressures on the retaining wall in 3D condition are much smaller than those in 2D condition, and these distributions of earth pressure are more influenced by the wall deflection process than by the wall deflection mode. Observed surface settlements at the backfill just behind the wall in 3D condition are larger than those in 2D condition, but 3D surface settlements occur more locally. These 2D and 3D surface settlements are influenced by the wall deflection process more than the wall deflection mode. However, at the same wall deflection process and the same wall deflection mode, there is not much difference in the shapes of the earth pressure distributions and the surface settlement troughs between 2D and 3D conditions. These differences and similarities of the earth pressures and the surface settlements are simulated not only qualitatively but also quantitatively in the analyses using the above constitutive model.

Describing Settlement Troughs over Twin Tunnels Using a Superposition Technique

It has been observed in many tunneling projects that surface settlement troughs caused by twin tunnels have a variety of shapes unlike single tunnels, where symmetric surface settlement troughs are usually observed. The surface settlement troughs observed over twin tunnels can be symmetric with respect to the midpoint between the two tunnels or symmetric but shifted toward either side or they can also be asymmetric. Settlement troughs both over single and twin tunnels (when symmetric) can often be described by a Gaussian curve. Most of the cases reported in the literature do consider the effect of ground conditions, tunnel size, and depth on the surface settlement. However, these cases do not consider the effect of construction operation. A study of settlements above tunnels driven with earth pressure balance shields in Bangkok made it possible to include operational parameters such as face pressure, penetration rate, and grouting pressure. This is possible by comparing the effect of twin tunnels which are geometrically and geologically identical but differ in the operational characteristics. These operational differences make it, however, more difficult to describe the resulting settlement troughs over twin tunnels using existing methods. Therefore, this paper introduces a superposition technique to describe surface settlement troughs over twin tunnels. It appears that one can construct settlement curves induced by the first shield and the second shield using the Gaussian function and combine the curves to obtain a total settlement trough as a result of the twin tunnels. Using extensive data from the Bangkok Subway Tunnel project, this approach was found to be suitable both for twin tunnels excavated side-by-side and also for stacked twin tunnels. Right now this superposition technique is basically descriptive. Eventually it will be possible to use this approach to predict settlement troughs over twin tunnels.

Ground Behavior Due to Tunnel Excavation with Existing Foundation

Two-dimensional (2D) and three-dimensional (3D) model tests of tunnel excavation with nearby existing foundation are carried out to investigate the influence of the existing foundation due to the interaction between ground and the existing structures. Three types of foundations: flat foundation, group-pile foundation and piled raft are considered. 2D and 3D finite element analyses using subloading tij model are also conducted. The deformation mechanism and distribution of earth pressure during tunnel excavation in the ground with nearby foundation are found to be different from those of green field condition. Surface settlement trough due to tunnel excavation in the ground with existing foundation does not follow the usual pattern of a Gaussian distributive curve, which can be observed in the case of green field. Especially, in the case of pile foundation, Dp, the distance between pile tip and tunnel is an important factor for the ground deformation and surface settlement. For a short distance Dp, although the length of pile is long, the ground deformation is concentrated at a place near the front pile and the rotation of foundation becomes larger. The maximum surface settlement in the case of existing foundation is also larger than those in the case of green field. Due to the existing foundation, unsymmetrical distributions of earth pressure occurred at the bottom of the ground due to tunnel excavation, both in model tests and numerical analyses. The earth pressure at the crown of tunnel in the case of existing foundation is almost the same as those in the case of green field. The arching at the shoulder of tunnel in the case of existing foundation, however, is much larger than those in the case of green field due to the dead load exerted on the foundation. The numerical results agree well with the results of the model tests.

Tunnel stability and arching effects during tunneling in soft clayey soil

A series of centrifuge model tests and numerical simulations of these tests were carried out to investigate the surface settlement troughs, excess pore water pressure generation, tunnel stability and arching effects that develop during tunneling in soft clayey soil. The two methods were found to provide consistent results of the surface settlement troughs, excess pore water generation, and the overload factors at collapse for both single and parallel tunneling. The arching ratio describes the evolution of the arching effects on the soil mass surrounding tunnels and can be derived from the numerical analysis. The boundaries of the arching zones for both single tunneling and parallel tunneling were determined. In addition, the boundaries of the positive and negative arching zones were also proposed.

Three-dimensional ground settlements and stress-transfer mechanisms due to open-face tunnelling

A series of three-dimensional, numerical, elastoplastic, coupled-consolidation analyses was conducted to investigate the effects of the stiffness ratio (n = E ′h/E ′v) and the initial coefficient of earth pressure at rest (K0) on ground deformations and stress-transfer mechanisms due to idealized open-face tunnelling (i.e., New Austrian Tunnelling Method or NATM) at a given unsupported length. As n increases, smaller plastic extension strain zones are mobilized at the crown and invert for a given K0, leading to a deeper and narrower transverse surface settlement trough. On the other hand, for a given n, the lower the initial K0 is, the deeper and narrower the computed settlement trough will be. Three-dimensional stress-transfer mechanisms at a tunnel heading are demonstrated through the changes in normal stresses and induced shear stresses around the tunnel. As the tunnel excavation advances, the maximum total normal stress reduction in the lateral direction (Δσxx) and in the vertical direction (Δσzz) occurs at the tunnel heading, whereas the largest total normal stress reduction in the longitudinal direction (Δσyy) takes place at about 0.5D (D = diameter) ahead of the tunnel face. A total stress reduction zone can be identified within 2.0D ahead of the tunnel face, and the stress influence zone diminishes at a distance of about 1.0D beyond the tunnel face. A significant reduction in pore-water pressure, of up to about 75% of its initial value, can be identified around the tunnel heading. Around a tunnel open face, the effective stress coefficient Kxz (= σ′xx/σ′zz) and Kyz (= σ′yy/σ′zz) rise to approximately 1.8 and more than 2.0 at the crown and invert in the initial K0 = 1.0 ground conditions, respectively. On the other hand, Kxz falls to about 50% of the initial K0 value at the springline, and the coefficient at the shoulder and heel drops to between 10% and 20%.Key words: open face, tunnelling, NATM, anisotropy, three-dimensional, stress transfer, ground settlements.

Ground movement and tunnel stability when tunneling in sandy ground

Tunneling may cause ground movements and damage to adjacent buildings and overlying facilities. In this study, the failure mechanism and ground movement behavior around tunnels embedded in sandy soil below the water table were investigated in a series of model tunnel tests in a centrifuge. The magnitudes and extents of the surface settlement troughs for the cases of various ground loss for tunnels buried at various depths are provided. A new failure mechanism is proposed and validated by comparison with the test results. The proposed mechanism enables accurate prediction of two of the key quantities in the design of linings for tunnels embedded in sandy soils, namely the minimum supporting pressure needed to retain tunnel stability and the vertical soil pressure acting on the tunnel crown.

Numerical prediction of long-term settlements over tunnels in clay

The long-term settlements over tunnels can be rather significant, particularly when tunnels are embedded in soft and compressible soils. The major factors, which have very significant influence on the development of long-term (post-construction) settlements, can be mainly attributed to the magnitude and distribution of excess pore pressure around the tunnel due to shield tunneling, the drainage condition of the tunnel linings during the evolution of long-term settlement, the vibration damage of the vehicle to the surrounding soils, and the stress-strain behavior of the soils where the tunnels are embedded. It is widely accepted that the effects of time on strength and deformation characteristics of soils, especially of saturated cohesive soft soils, play an important role in a variety of geotechnical engineering activities, including tunneling where either rapid or long-term response is concerned. Consequently, the long-term settlement can be predicted based on the stress-strain-time relationship of soft clay using numerical method. An elastic-viscoplastic constitutive model was undertaken with the framework of Perzyna and modified Cam-clay model based on extensive experimental data of undisturbed samples with k0 pre-consolidation conditions. The constitutive model coupled with Biot's consolidation theory was correspondingly implemented into the FEM code to carry out the numerical simulation. The adopted elastic-viscoplastic constitutive model with few parameters was an advantage in studying the time-dependent stress-strain behavior of soft clay. The evolution of long-term settlement with time was simulated with the elastic-viscoplastic constitutive model. A multiple-step numerical procedure was designed to simulate the different behavior and mechanical characteristics of the tunnel and the surrounding soils in the long term. The influences of drainage condition of tunnel linings on the evolution of long-term surface settlement as well as ground loss are simulated with the multiple-step numerical procedure. Consequently, the development of surface settlements, ground loss, and transverse surface settlement troughs with time can be detected, and conclusions can be drawn from the numerical simulations. Finally, a case study was also performed and a satisfaction agreement between the predicted long-term settlement and the observed one was obtained. (A) Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.

Ground movements and collapse mechanisms induced by tunneling in clayey soil

A series of centrifuge model tests of unlined single and parallel tunnels was carried out in the plane strain condition to investigate the ground movement and the collapse mechanisms induced by tunneling in clayey soil. The measured surface settlement trough due to single tunneling was first fitted by a normal probability curve. Afterwards, a superimposition method with modification factors was suggested for estimating the surface settlement troughs induced by parallel tunneling with various center-to-center distances. The relationships of surface and subsurface settlement troughs versus the ground losses were then proposed as a function of the burial depth of the tunnel. The prediction methodology of settlements based on the ground loss was verified by comparison with 12 sets of monitored field data. The collapse mechanisms of the single and parallel tunnels derived from the upper-bound theory of plasticity were correlated with the observed ground movements in the model tests.

Prediction of Settlement in Existing Tunnel Caused by the Second of Twin Tunnels

In 1994 to 1996, three 9-m-diameter parallel tunnels were constructed crossing below twin existing tunnels at London Heathrow Central Terminal Area station for the Heathrow Express project at Heathrow Airport in the United Kingdom. Extensive monitoring was undertaken during construction. Settlement curves at various construction stages in the existing tunnels were measured. Marked asymmetry was noted in the settlement curves caused by construction of the two outer tunnels. It has been noted by several authors that the surface settlement trough caused by the second of two closely spaced tunnels is markedly asymmetrical. This has been accounted for by the fact that when twin tunnels are closely spaced, the ground in the region where the second tunnel is to be constructed has already been subjected to appreciable shear strains associated with construction of the first tunnel. This reduces ground stiffness, hence a higher volume loss for the second tunnel, producing an asymmetrical settlement trough with settlements greater on the side nearest to the first tunnel. The data from London Heathrow have been added to a database of case histories of twin tunnels prepared from reported information worldwide. A new empirical method was developed for estimating the settlement trough caused by the second of twin tunnels, along with an exercise demonstrating the use of the method, which is proposed for use as a preliminary predictive tool.

Surface settlement during shield tunnelling at CH218 in Taipei

This paper presents the surface settlement performance induced by the foamed type of earth pressure balance shield in contract CH218 of the Hsintien Line of the Taipei Rapid Transit System. The surface settlement characteristics caused by the single tunnel and by twin tunnels with reference to two sections spaced at 87 m are studied. Field observations indicate that the surface settlement trough due to the single tunnel can be represented by the normal distribution. The distance of the inflection point to the tunnel center and maximum surface settlement value are consistent with those found in the literature. The characteristics of the surface settlement trough are related to the type of the soil, particularly where the crown of the tunnel is located in a layered soil deposit. The ground surface settlement induced by twin tunnels was found to be larger than estimated using the principle of superposition.Key words: shield tunnelling, surface settlement, field observation.