Surface Settlement Profile Research Articles

DIA of centrifuge model tests on geogrid reinforced soil walls with low-permeable backfills subjected to rainfall

Geogrid reinforced soil walls (GRSWs) constructed using low-permeable backfills often experience failures when subjected to rainfall. The objective of this paper is to employ centrifuge modelling to investigate the effect of geogrid types on the performance of GRSW models constructed with low-permeable backfill, when subjected to rainfall intensity of 10 mm/h. A 4.5 m radius large beam centrifuge facility was used, and rainfall was simulated using a custom-designed rainfall simulator at 40 gravities. Digital Image Analysis (DIA) was employed to understand the deformation behaviour of GRSWs with low stiffness geogrid layers with and without drainage provision subjected to rainfall. Additionally, the effect of varying stiffness of geogrid reinforcement layers across the height of GRSW was also investigated. The interpretation of DIA helped to quantify displacement vector fields, face movements, surface settlement profiles and geogrid strain distribution with depth. Irrespective of drainage provision, GRSWs reinforced with low stiffness geogrid layers experienced a catastrophic failure at the onset of rainfall. However, GRSW reinforced with geogrid layers of varying stiffness was observed to perform well. This study demonstrates the effective use of DIA of GRSWs subjected to rainfall along with centrifuge-based physical model testing.

Longitudinal settlements during tunneling in soft Clay, using transparent soil models

The purpose of this study was to measure internal longitudinal displacements taking place during tunnel excavation and compare them to the displacements predicted using available analytical and empirical formulations. A physical model made with transparent soil permitted exploring the internal soil deformations, within the supporting soils. A soil surrogate made of Magnesium Lithium Phyllosilicate (MLPS) was used to represent soft clay. Tunneling was simulated using a moveable trap door arrangement for different face losses ranging from 5 to 30% D. Internal soil movements in the longitudinal direction were captured for three cover-to-diameter (C/D) ratios ranging from 1.5 to 3.5. Shear as well as volumetric strains were computed using the captured spatial displacements. The results demonstrate that the shape of the longitudinal profile is more dependent on the overburden depth of the tunnel in comparison to face losses, in particular: (i) subsurface settlement is approximately 2–4 times the observed surface settlement for the same C/D ratio; (ii) a longitudinal soil arch is likely to form with the increase of C/D ratio which limits the observed settlement; and (iii) shear strain fields showed that a cascading failure may occur with the increase in face movements, especially for shallow tunnels. Finally, the computed and measured surface and subsurface settlement profiles were generally in agreement especially as C/D increases, thus validating methods used to predict subsurface settlements where measurements are sparse.

A FRACTAL MODEL FOR THE TUNNELING-INDUCED GROUND SURFACE SETTLEMENT

The Gaussian function was initially adopted to model the tunneling-induced ground surface settlement, but it could not describe the porosity’s effect on the settlement. To solve the problem, Yu’s fractal theory is implemented to model the porous grand’s geometric property, and the surface settlement profile is modeled by a fractal solitary wave, furthermore, the effects of the maximal surface settlement, the porosity and Yu’s fractal dimension on the settlement’s profile are discussed. The new model offers a new view to predict the morphology of the surface settlement.

Load redistribution pattern in a RC moment frame due to excavation-induced 3D ground surface settlement profiles

Following abnormal events, identification of load redistribution pattern in the load-bearing elements is an important issue in the field of structural engineering. As a possible event, one can imply to initiation of excavation-induced large differential settlements under the columns of common reinforced concrete (RC) moment frames. A large portion of existing literature have studied the effect of single-column settlement scenarios or tunnel-induced ground movements on 2D frames, however. Using two series of real ground surface settlement profiles (called medium and large) representing deep excavation adjacent to the construction site, 8 analysis cases were defined to gradually impose differential settlements under the columns in this paper. Apart from different 3D settlement profiles, the structure was assumed to be located on the corner or center of excavation edge, and two directions were also considered for the building plan. Based on the findings of the numerical models, the axial compressive force significantly decreased in a series of highly settled columns. Also the columns which were relatively far from the location of maximum settlement made key contribution to redistribution of loads. Moreover, a large number of columns experienced tensile axial forces in some of the cases with larger surface settlement imposed. The change in the direction of beam flexural moments in conjunction with some columns (not limited to the column having the largest settlement) shed light on possibility of damage in an extensive area of the building plan.

Geosynthetic-reinforced soils above voids: Observation of soil and geosynthetic deformation mechanisms

Understanding how a geosynthetic-reinforced soil deforms in response to the formation of an underlying void is crucial to provide appropriate designs of these systems. Centrifuge models employing a trapdoor to simulate the void formation below a geosynthetic-reinforced sand were conducted to investigate the behaviour in a controlled environment at realistic stress levels. The plane-strain models allowed visual observations of the deformation mechanisms using Particle Image Velocimetry (PIV). These observations were used to validate assumptions about the geosynthetic behaviour made in current design recommendations, and address limitations related to the fill behaviour. Soil expansion was observed to be confined to a parabolic zone above the void related to the soil dilatancy, rather than with a single, unique coefficient of expansion. The zone of subsidence was characterised by an initial vertical prism with a funnel to the surface, with the surface settlement profile better described by a Gaussian distribution rather than the parabolic profile used historically. Detailed interpretation of the centrifuge tests has given new insight into the soil and geosynthetic behaviour relevant to how these systems deform in practice. This paves the way for more efficient design recommendations and consequently will facilitate better predictions of geosynthetic-reinforced soil behaviour above voids.

Effect of rock bolt support mechanism on tunnel deformation in jointed rockmass: A numerical approach

A parametric study was conducted to determine the influence of different bolt parameters (bolt length and diameter) on the maximum induced boundary displacements in jointed rockmass, using a numerical method based on finite element code. Three different types of jointed systems (Type A, Type B, and Type C) were considered and analyzed. Type A displays normal joint closure, Type B demonstrates a combination of normal and shear joint closure, and in Type C, the majority of the joint is loaded in shear condition. The ground surface settlement profiles for these three joint systems, under unsupported conditions, are presented in this work. For the same rockmass properties, the cases in Type A and Type B exhibit the maximum and minimum settlement, respectively. For each joint type, the effect of variable bolt parameters on the tunnel boundary is studied in terms of the total displacement. The simulation results confirm that an increase in the bolt length does not significantly reduce the boundary displacement, whereas an increase in the bolt diameter substantially reduces the tunnel boundary displacement.

Mining-induced ground movements and displacements using transparent soil model test

Surface subsidence induced by mining subsidence affects the overlying buildings and nearby utilities. Therefore, an investigation of the internal displacement and surface settlement is very important. A small-scale transparent soil model was developed to study the surface settlement profile and displacement field induced by mining subsidence. An analysis of the continuous change of the displacement field inside the transparent soil models indicates that the subsurface settlement at different subsidence stages can be approximated by a normal probability curve. Additionally, the observed surface settlements are consistent with settlement predictions of both the probability integral method and the Gaussian distribution method. The results indicate that it is a good way to study further the underground structure–soil interaction.

Response of building to shallow tunnel excavation in different types of soil

This research investigates the tunneling induced building responses in different types of soil via finite element (FE) method. Different simulation techniques, namely contraction factor method and stress reduction method, are applied in FE-model to simulate the volume loss during the excavation process. A wide range of soils, including limestone, dense/loose sand, stiff/soft clay and silt, are taken into account in this study. The considered building responses include the surface settlement profile, position of inflection point, deflection ratio in sagging/hogging zone, horizontal compressive/tensile strain, etc. The relationship between these building responses and soil/building properties is investigated. Local sensitivity analysis is further applied to distinguish the relative importance of soil stiffness and strength in determining the deflection ratio of the building settlement profile. The results show that soil properties play a significant role in determining the building responses. Generally, soil with larger initial elastic modulus results in higher deflection ratios and horizontal strains of the building. Furthermore, in order to provide a guideline for practical tunneling design, an upper bound design curve of modification factor to greenfield deflection ratio or horizontal strain for different types of building is derived in both sagging and hogging zones.

Deformation Characteristics of Retaining Structures and Nearby Buildings for Different Propped Retaining Walls in Soft Soil

Abstract This article reports the field performance of deep excavations of two subway station cases, including the lateral wall deflection behavior and settlement trends of the surrounding soil and nearby buildings. The retaining structures employed in these cases were contiguous pile walls (CPW), soil-mixing walls, and diaphragm walls (DW), all of which were embedded in soft clay. The measured wall deflection profiles exhibited typical bulging behavior at the end of the excavation. The ratios of the measured maximum wall deflection to the excavation depth were found to be similar for all three types of retaining wall. Furthermore, the maximum and minimum corner effects on the wall deflection development were obtained for the DW and CPW, respectively. The measured ground surface settlement increased linearly with increased maximum lateral wall deflection, while the settlement magnitude became extraordinarily large because of the presence of sludgy soil. A concave pattern was proposed for the surface settlement profiles for all three types of retaining wall. The building settlement was quantified, with the value lying between those of the surface settlement and soil settlement at 10-m depth. The soil displacement field induced changes in the side and end resistance behaviors of the loaded piles, along with additional settlement of pile-foundation buildings. In addition, the pile-foundation building settlement was influenced by the corner effect. These research results will enhance our understanding of the deformation characteristics of the retaining structure and nearby buildings. Meanwhile, the findings will provide guidance for the optimal design of the retaining structure in soft soil.

Building Deformation Prediction Based on Ground Surface Settlements of Metro-Station Deep Excavation

Building deformations are not only closely related to the distance from the building to metro-station excavation but also related to the relative positions of the building and metro-station excavation. Building deformations can be predicted using ground surface settlement profiles. Based on typical geological parameters of Nanjing metro-station excavation, ground surface settlements were numerically simulated by auxiliary planes perpendicular and parallel to the excavation and by angled auxiliary planes at the excavation corner. Results show that the ground surface settlement profiles in auxiliary planes are closely related to the relative positions of the auxiliary planes and the metro-station excavation. Partitioning of ground surface settlements was proposed according to the three types of ground surface settlement profiles; furthermore, bending deformation and torsional deformation regularities of surrounding buildings were analyzed, and an estimation method for building settlements was developed. Finally, field-monitored settlement data of 21 buildings in different zones were compared with the estimated settlement data, and the application of the settlement estimation method to different types of foundations was analyzed. The results of this study can serve as reference for metro-station deep excavation construction and protection of surrounding buildings.

Effect of Closely Spaced Twin Tunnel Construction beneath an Existing Subway Station: A Case Study

Abstract Closely spaced twin tunnels were excavated vertically beneath an adjacent subway station in service in Beijing, China. Because of restrictions on the settlements of the existing subway station in the area, a special protection methodology was adopted, and an analysis was performed using real-time monitoring data to investigate the response of the overlying subway station to twin tunnelling and surface settlement characteristics. The results indicated that advanced grouting realizes regional integral uplifts within the range of a reinforced area. The lifting work with jacks is critical to restraining the trend of settlements. The final settlement trough of the columns displays a double-trough “W” shape that is induced by nonuniform lifting loads, and the total ground surface settlement profile displays an asymmetric single-trough “U” shape. The moment when temporary inverts and middle diaphragms are demolished is important, as it causes potential excessive settlements. With monitoring, this tunnel engineering was completed without interrupting the subway traffic, and the safety of the operating subway station and ground surfaces was successfully guaranteed. This is a valuable project case for establishing criteria to guide the construction of closely spaced twin tunnels in similar conditions.

Centrifuge Modeling of Pile-Sand Interaction Induced by Normal Faulting

AbstractMany forensic studies conducted after catastrophic earthquakes revealed severe damages to pile foundations caused by bedrock fault movement. The mechanism behind these pile foundation failures is not yet fully understood. In this study, two centrifuge model tests are carried out on a single pile and on a group of three piles in sand subjected to normal faulting. The surface settlement profile and additional internal forces of the piles are measured. The surface settlement profile in sand could be well fitted and predicted with a profile function (error function). The single pile, which is located in a differential settlement zone, is displaced, and the pile top rotates toward the hanging wall, inducing an additional bending moment along the pile. The pile group is then forced to rotate toward the hanging wall because the three piles are well connected to the pile cap. Driven by the movement of the two additional piles and a pile cap in place, the pile on the footwall side experiences greater displ...

Experimental and simplified model study of the development of ground settlement under hazards induced by loss of groundwater and sand

Most catastrophic failure in underground engineering is caused by soil and water loss. However, few studies have focused on the evolution of ground deformation during such a process. In this study, model tests were conducted to investigate the flow channel boundary (the interface between the flowing and stationary ground) and ground settlement profile during the flow process of sandy soil and water, and a simplified model was proposed based on the experiment results. The model was validated against measurements from laboratory model tests with fine and medium sand, and the tendency of the simplified model to predict the subsurface ground movement boundary was verified with case records of tunneling, which can also lead to similar soil loss. Finally, parameter analysis was performed according to the simplified model. The study showed that the flow channel boundary could be described by an ellipse for a planar problem (i.e., loosening ellipse). Furthermore, the flow channel boundary was related to the soil loss volume, buried depth of soil extraction location, and eccentricity of the loosening ellipse. The eccentricity of the loosening ellipse decreased with increasing average particle diameter. Peck’s curve could be used to describe the profile of the settlement trough when the mass loss ratio was not >20 %. The ground surface settlement profile changed to the shape of an “inverted triangle” with increased soil loss. The goodness of fit of Peck’s curve to subsurface settlement increased with depth before the deep ground flowed out from the opening. The effect of groundwater flow was considered in the model tests and simplified model to analyze the disaster mechanism, which can provide a reference for the further research.

Ground surface settlements due to construction of closely-spaced twin tunnels with different geometric arrangements

This paper examines the ground surface settlement profiles due to the construction of closely-spaced twin tunnels using the shallow tunnelling method. In the concerned zone, where the twin-tunnelling was performed in stacked and offset arrangements, the ground surface settlements of in total 18 cross-sections were continually recorded during construction. To cater for different conditions of the twin tunnels in the concerned zone, partial face, full face and forepoling reinforcement schemes were adopted. The recorded surface settlements and settlement troughs of three typical sections are reported and illustrated. The surface settlement troughs induced by each of the twin tunnels are fitted by the Gaussian function. The parameters that characterize the surface settlement troughs induced by each of the twin tunnels, such as the maximum settlement, percentage of ground loss, trough width and empirical trough width parameter are presented and compared.

Twin-tunnelling-induced ground movements in clay

Modern tunnelling methods aim to reduce ground movements arising from the construction process. In clay strata the usual method of construction is by tunnel boring machine, which allows close control of the tunnelling process; however, any movements have the potential to cause damage to existing structures at, and below, the ground surface. The construction of underground rail systems often comprises two tunnels running in opposite directions. Common practice for assessing construction-generated movements around these tunnels is to make predictions based upon individual tunnel construction and utilise superposition to generate a total deformation profile. This approach does not take into account the strain- or stress-dependent effects between tunnel constructions. A delay may result in unanticipated ground movements generated by the construction of the second tunnel. The effect of this delay on the ground movements arising between the first and the second tunnel construction process was investigated in a series of plane strain centrifuge tests. The ground movements at and below the surface were monitored and were assessed against superposition-based predictions for surface settlement with the outcomes highlighting some inconsistencies. A procedure for predicting both surface and subsurface vertical settlement profiles in the plane transverse to the advancing tunnels in clay is suggested.

Effects of twin tunnels construction beneath existing shield-driven twin tunnels

This paper presents a case of closely spaced twin tunnels excavated beneath other closely spaced existing twin tunnels in Beijing, China. The existing twin tunnels were previously built by the shield method while the new twin tunnels were excavated by the shallow tunnelling method. The settlements of the existing tunnels and the ground surfaces associated with the new tunnels construction were systematically monitored. A superposition method is adopted to describe the settlement profiles of both the existing tunnels and the ground surfaces under the influence of the new twin tunnels construction below. A satisfactory match between the proposed fitting curves and the measured settlement data of both the existing tunnels and the ground surfaces is obtained. As shown in a particular monitoring cross-section, the settlement profile shapes for the existing tunnel and the ground surface are different. The settlement profile of the existing structure displays a “W” shape while the ground surface settlement profile displays a “U” shape. It is also found that due to the flexibility of the segmental lining, the ground losses obtained from the existing tunnel level and the ground surface level in the same monitoring cross-section are nearly the same.

A generalized model for geosynthetic reinforced railway tracks resting on soft clays

SummaryThe present study pertains to the development of a foundation model for predicting the behavior of geosynthetic reinforcement railway track system rested on soft clay subgrade. The ballast and sub‐ballast layers have been idealized by Pasternak shear layer. The geosynthetic layer is represented by a stretched rough elastic membrane. Burger model has been used to characterize the soft clay subgrade. Numerical solutions have been obtained by adopting the finite difference scheme combined with non‐dimensioning the governing equations of the proposed model. The results confirm that the present model is quite capable of predicting the time‐dependent settlement response of geosynthetic reinforcement railway track system placed on soft clay subgrade. The surface settlement profile and mobilized tensile load of geosynthetics has been evaluated by considering variation in the wheel load, sleeper width, thickness of ballast and sub‐ballast layers and shear modulus of ballast and sub‐ballast layers. It has been observed that an increase in the sleeper width by 24% results in the reduction in central settlement and mobilized tensile load by 6.5% and 20.1%, respectively. It was found that with a 50% increase in the thickness of the ballast layer, the central settlement has decreased by 7.3% and the mobilized tension at the zone of maximum curvature has increased by 24.6%. However, with an increase in the thickness of the sub‐ballast layer, a considerable reduction in both central settlement and the mobilization of tension on geosynthetic has been noticed. The pattern of variation of settlement and mobilized tension for an increase in the shear modulus of ballast and sub‐ballast material was found to be almost similar. Copyright © 2014 John Wiley & Sons, Ltd.

The development and implementation of a tunnel characterization method

AbstractThe Garner and Coffman method was developed to design a proposed underground facility based on an allowable settlement profile; the method may also be used to characterize an unknown underground facility based on an observed surface settlement profile. The method uses both static methods and 2‐D finite element analyses to relate the characteristics of the ground surface settlement profile to the underground facility (depth, diameter, and number of tunnels). The calibration and validation of the Garner and Coffman method, as obtained by using tunnel segment data from historical tunneling projects (Bangkok, London, Taipei, Singapore, and Heinenoord), are presented. Specifically, the method was calibrated using settlement profiles and facility characteristics from 15 tunnel segments and validated using settlement profiles from 16 additional tunnel segments. A numerical relationship (developed during this research project) was then used during the validation of the model to predict facility characteristics for the “unknown” underground structures. The predicted depth and diameter for each of the “unknown” underground structures were within ten percent of the actual diameter and actual depth of the underground structure, as obtained from the literature. For all 16 validation tunnel segments, the tunnel location was predicted within one tunnel diameter of the actual facility centerline.

Subway tunnel design using a ground surface settlement profile to characterize an acceptable configuration

Tunneling induced settlement is often a controlling factor in the design of a subway or underground facility due to the impact on adjacent or overhead infrastructure. A method is proposed that uses an acceptable ground surface settlement profile to generate a tunnel system configuration that reproduces the acceptable/tolerable settlement profile. A brief discussion of existing methods of tunnel settlement analysis and two (2) case studies illustrating the proposed method are presented. The chosen case studies are urban subgrade rail tunnels (subways) constructed in Istanbul, Turkey and Bangkok, Thailand. Each case study is used to illustrate the use of the bifurcated static back calculation – iterative finite element model prediction/solution method.The initial settlement profiles used for both case histories were measured settlement profiles obtained from the literature (Ercelbi et al., 2005; Suwansawat, 2002). These settlement profiles were assumed to be the tolerable settlement profiles for the subway systems. For each case history, the iterative static back calculation – finite element model ingested the individual literature obtained design settlement profile to identify an optimum tunnel configuration (depth and size) that will reproduce the design settlement profile. Comparisons were made between the as-built tunnel configuration and the tunnel configuration predicted using the proposed method.

Applicability of the principle of superposition in estimating ground surface settlement of twin- and quadruple-tube tunnels

Prediction of tunneling-induced ground surface settlement is important to underground constructions in the densely populated city because excessive settlement could trigger potential damage to existing structures. This paper examined the interaction of multi-tube tunnels on ground surface settlement and the methods of prediction for the transverse ground surface settlement profile induced by the closure and grouting of tail void of shield machine of multi-tube tunnels in the CH221 Contract of the Taipei Rapid Transit System. The transverse ground surface settlements of a twin- and a quadruple-tube tunnels at two sections of this contract were back analyzed using the empirical equation that assumed the settlement profile over a tunnel to be a Gaussian distribution curve, and the finite element analysis. The applicability of the principle of superposition in the analysis and the characteristics of the ground surface settlement for a green-field land (previously undeveloped land) and a brown-field site (previously developed site) was also studied using the finite element analysis. The results showed that the ground surface settlement profile of multi-tube tunnel could be conveniently and reliably estimated using the empirical equation together with the principle of superposition. Extra effort required in the model set-up and result interpretation caused the numerical method to be outperformed by the empirical method.