Soft Soil Deposits Research Articles

Nonlinear response of bridge piers on inclined pile groups: the role of rocking foundation input motion

This paper presents first results of an on-going research focused on the effects of piles layout and inclination on the nonlinear seismic response of bridge piers. The analysis methodology, based on the substructure approach, is firstly presented. The soil-foundation system is studied in the frequency domain according to a numerical model developed by the authors while the inertial interaction analysis of the superstructures is carried out in the time domain to capture the nonlinear structural behaviour. A suitable lumped parameter model is used to approximate the frequency dependent behaviour of the soil-foundation impedances in the time domain analyses. The procedure is applied to some case studies constituted by single bridge piers founded on medium stiff and soft clayed soil deposits. Pile groups with piles of different inclinations are considered, as well as piers with different fundamental periods and yielding bending moments of the base cross sections, to simulate systems with different ductility capacity. Analyses results show the key role of the foundation rocking on the superstructure response and demonstrate that inclined pile foundations may have a significant impact on the superstructure response, reducing the pier head displacements and ductility demand.

Ground improvement of soft soil by geotextile-encased columns

This paper compares the field response of two test embankments constructed on similar soft soil deposits. The un-strengthened foundation had vertical drains underneath the embankment and the strengthened foundation had geotextile-encased granular columns, both provided with basal geogrid reinforcement. The effectiveness of the encased granular columns was assessed by comparing the settlement below the embankments, the horizontal displacement of the soft foundation and the excess pore pressures in the middle of the soft clay layer. The results showed that the granular encased columns significantly reduced the maximum settlement occurring below the embankment as well as the maximum horizontal displacement in the clayey foundation. At the same settlement level, the load-carrying capacity of the strengthened foundation was found to increase by as much as around 2·5 times that of the un-strengthened one. It was also observed that the geotextile-encased granular columns caused a noticeable reduction in the maximum excess pore pressure following load application. Furthermore, the degree of consolidation achieved at the end of embankment construction was quite high when encased granular columns were used.

Small Stone-Column Groups: Mechanisms of Deformation at Serviceability Limit State

AbstractStone columns are a popular form of ground improvement that can be applied to a variety of foundation solutions involving soft soil deposits. Many design methods are based on an assumption that columns with a length-to-diameter ratio greater than 4 bulge uniformly along their length. This mode of deformation is not valid for small groups of columns in which interaction between individual columns and the footing gives rise to more complex behavior. The deformational behavior of stone columns at serviceability limit state is of utmost importance, because settlement rather than bearing capacity criteria are generally more onerous in soft soils. The results of some high-quality laboratory studies have been informative in this respect, but the extrapolation of results from small-scale tests comes with the caveats associated with single-gravity laboratory testing. The FEM, in combination with advanced constitutive models, can provide a link between the deformational behavior and settlement performance o...

The comparison of modelling inherent and evolving anisotropy on the behaviour of a full-scale embankment

The behaviour of a full-scale embankment constructed on a soft soil deposit has been studied using two different anisotropic elasto-plastic constitutive models, namely S-CLAY1 and Sekiguchi–Ohta (SO) inviscid, in order to investigate the influence of modelling (initial) inherent and evolving anisotropy on boundary value level simulations. The initial inherent anisotropy which is generally modelled by a rotated yield surface can also influence the model prediction capability. A Finnish test embankment known as Murro embankment was chosen for finite-element analysis using the two advanced soil models. The predictions of each model were studied and compared with measured field data. For the purpose of model comparison, the influence of evolution of anisotropy was further investigated by using only the inherent anisotropy feature of S-CLAY1 model, by setting the values of rational hardening parameters to zero. Overall, the S-CLAY1 model predictions are in good agreement with the measured data, which is due to the incorporation of a rotational hardening law (evolution of anisotropy) into the model in addition to the consideration of inherent anisotropy. However, the SO inviscid model predictions are less comparable with the measured data due to the consideration of only the inherent anisotropy into the model. The results clearly demonstrate the importance of including soil’s evolving anisotropy in the analysis and how the inherent anisotropy is modelled (i.e. the shape of yield surface).

Comprehensive nonlinear seismic ground response analysis of sensitive clays: case study—Leda clay in Ottawa, Canada

A series of time domain nonlinear and frequency domain equivalent-linear ground response analysis was conducted for a representative site in Ottawa, Canada. The objective is to provide insight into the performance of different site response analysis approaches in predicting site amplification factors for the soft soil deposit over a wide range of shear strain. The surficial geology of the site is predominantly composed of the sensitive Leda Clay typical of this region in eastern Canada. A combination of results and observations from monotonic and cyclic laboratory tests, as well as field geophysical experiments, was used to calibrate the dynamic properties of the Leda clay, more specifically, the variation of the clay’s shear stiffness (Gmax) with the in situ stress state ( $$\sigma_{vc}^{{\prime }} ,OCR$$ ). The calibrated relation was then successfully validated through material-specific site response analysis using the strong motion recordings from the 2010 Val-des-Bois earthquake and the shear wave velocity profile from borehole data. Comparison of the results with the hazard spectra proposed by NBCC 2010 reveals that the code-based site class E hazard spectra for the Ottawa area can be sufficient for seismic design, except around the natural site period. The results also suggest that the nonlinear analysis using the shear strength-adjusted hyperbola for modulus reduction and best-fitting trend to the laboratorybased damping would provide a reliable estimation of ground motion spectra at the natural site period. The performance of the ground response analysis alternatives for the sensitive clays, over a wide strain range was compared and discussed.

Detection of Local Site Conditions in Tripura and Mizoram Using the Topographic Gradient Extracted from Remote Sensing Data and GIS Techniques

The contribution of remote sensing and geographical information system (GIS) techniques to estimate the surface level seismic hazard was investigated using topographic gradient maps in the states of Tripura and Mizoram. This paper addresses a vital issue on the estimation of the seismic hazards of Northeast (NE) India, where the hazards are high and the risk is also high. In developing site response maps or seismic hazard (surface acceleration) maps, it is difficult to obtain near-surface soil data (site-specific data) through geophysical/geotechnical field investigations at a smaller resolution scale (a state/region or country level). The topographic slope provides a reliable proxy because the rocky materials maintain a steep gradient/slope compared to soft soil deposits. In this paper, peak ground acceleration (PGA) at the bedrock for the states of Tripura and Mizoram in NE India has been estimated using probabilistic seismic hazard analysis (PSHA), which considers linear sources and events (from 1731 to 2010) with appropriate ground motion prediction equations applicable for NE India. Further, the authors have adopted the square root impedance model to estimate the spectral amplification of the study area to bring the hazard from bedrock level to surface. Authors have classified the entire study area as site classes B, C, and D categories and thereafter estimated spectral amplifications for the site periods (0.40, 0.21, 0.11, and 0.01s). The surface level PGA (g) and amplification maps have been developed using a probabilistic approach considering PGA at the bedrock level. The results indicate that this area is highly sensitive to spectral responses, such as spectral acceleration or spectral amplification rather than PGA. (C) 2016 American Society of Civil Engineers.

Observed Behavior of Reinforced Concrete and Unreinforced Masonry Buildings in April 25, 2015 Nepal Earthquake

In order to investigate the effects of devastating 7.8 M Gorkha Earthquake that struck Nepal on 25 April, 2015 and its aftershocks on reinforced concrete and unreinforced masonry buildings, a small survey team was dispatched from Institute of Engineering, Thapathali Campus. From the observation it was found that the earthquake caused extensive damages on the low-strength stone masonry buildings with no seismic detailing. In contrast, most of the reinforced concrete buildings sustained minor to some non-structural damages except in some localized places where the buildings were found to be collapsed by soft storey mechanism. The localized failure of reinforced concrete buildings in Kathmandu w valley and outside may be attributed to amplifcation of waves in thick soft soil deposits, poor quality of construction, inadequate column sizes and lack of ductile detailing.Journal of the Institute of Engineering, 2015, 11(1): 177-181

Development of synthetic soil mixture for experimental shaking table tests on building frames resting on soft soils

ABSTRACTIn this study, a synthetic soil mixture has been developed and proposed for experimental soil-structure interaction shaking table tests on building frames with shallow foundations resting on soft soil deposits. The proposed mix provides adequate undrained shear strength to mobilise the required shallow foundation bearing capacity underneath the structural model while meeting both criteria of dynamic similarity between the model and the prototype to model soft soils in shaking table tests. To find the most appropriate mixture, different mixes with different proportions of mix components were examined in the soils laboratory. Performing bender element tests, the shear wave velocity of the soil specimens was acquired at different cure ages and the results were examined and compared. Based on the test results, a synthetic clay mixture consisting of kaolinite clay, bentonite, fly ash, lime and water has been proposed for experimental shaking table tests.

Piezocone-based prediction of secondary compression settlements of coastal defence structures on natural silt mixtures

This paper discusses the evaluation of long-term settlements of a predominantly silty seabed under a rubble mound breakwater using a simplified one-dimensional approach in conjunction with an empirical method for the estimate of one-dimensional creep characteristics of soils from cone penetration test data. In particular, the procedure is applied to the assessment of long-term settlements of two existing breakwaters built along the Venetian coastline as part of the coastal defence works included in the ambitious Mo.S.E. project for the temporary closure of the lagoon inlets. The effectiveness of two different CPTU-based empirical correlations for the estimate of the secondary compression index Cαε in sand and silt mixtures, recently calibrated on field data from a Test Site located in the Venetian Lagoon area, has been examined by comparing the computed settlements with actual vertical displacements obtained from Persistent Scatterer Interferometry (PSI). The paper aims at providing some practical guidance on the assessment of long-term settlements in soft soil deposits of different coastal areas, by illustrating a general and simple approach for the evaluation of secondary compression characteristics of a large variety of sediments, based on the use of a site investigation technique routinely employed in nearshore and offshore projects.

인천 송도지역 준설토의 침강 및 압밀특성에 대한 실험 및 해석적 연구

연약지반의 특성에 부합하는 이론을 이용하여 침하량을 합리적으로 예측하는 것은 건설방재적 관점에서도 대단히 중요하다. 특히, 준설매립지반과 같은 초연약지반의 압밀거동 모사를 위해서는 비선형 유한변형 압밀이론을 적용할 필요성이 있다. 본 연구에서는 인천 송도지역 준설토의 침강 및 압밀특성을 파악하기 위하여 침강압밀시험, 자중압밀시험 및 CRS 압밀시험을 수행하였으며, 그 결과를 PSDDF를 이용한 수치해석의 입력치로 사용하여 준설매립지반의 거동을 해석하였다. 본 지역의 준설토는 통일 분류법에 의해 저압축성 실트(ML)로 분류되었으며, Yano법 및 수치해석을 통하여 얻어진 최종 체적변화비는 각각 1.56, 1.17로 나타났다. 이러한 결과의 차이는 계면고가 상대적으로 높고 투수성이 큰 해당 구간 준설매립토의 토질 특성에 기인한 것으로 사료된다. Accurate settlement estimation of dredged soft soil deposits is significantly important to prevent potential disasters during land reclamation. An application of the non-linear finite strain consolidation theory is inevitable in dealing with a very soft ground formation such as dredged fill. In this paper, a series of the sedimentation-consolidation test, self-weight consolidation test and CRS test were conducted to clarify sedimentation and consolidation characteristics of dredged fill in the Songdo area, Incheon. In addition, the settlement of dredged fill was numerically simulated using the PSDDF program. The dredged soil obtained from the Songdo area was classified as low-compressible silt (ML) based on USCS (Unified Soil Classification System), and the final bulking factors were estimated to be 1.56 and 1.17 by Yano's method and the numerical simulation, respectively. This difference is attributable to relatively high reclaimed height and large permeability of dredged soil in this region.

Estimation of inelastic displacement demands of flexible-based structures on soft soils

This study aims to develop efficient tools for performance-based seismic design of soil-structure interaction (SSI) systems on soft soils. To simulate the SSI effects, linear and nonlinear 'equivalent fixed-base single-degree-of-freedom' (EFSDOF) oscillators as well as a sway-rocking SSI model were adopted. The nonlinear dynamic response of around 10,000 SSI models and EFSDOF oscillators having a wide range of fundamental periods, target ductility demands, and damping ratios were obtained under a total of 20 seismic records on soft soil sites. Based on the results of this study, a practical method is developed for estimating the base shear and maximum displacement demands of a nonlinear single-degree-of-freedom structure on soft soil deposits. In the proposed procedure, the effect of frequency content of ground motions is considered by normalising the period of vibration by the spectral predominant periods, while the nonlinear EFSDOF models are used to improve the computational efficiency.

Estimation of inelastic displacement demands of flexible-based structures on soft soils

This study aims to develop efficient tools for performance-based seismic design of soil-structure interaction (SSI) systems on soft soils. To simulate the SSI effects, linear and nonlinear 'equivalent fixed-base single-degree-of-freedom' (EFSDOF) oscillators as well as a sway-rocking SSI model were adopted. The nonlinear dynamic response of around 10,000 SSI models and EFSDOF oscillators having a wide range of fundamental periods, target ductility demands, and damping ratios were obtained under a total of 20 seismic records on soft soil sites. Based on the results of this study, a practical method is developed for estimating the base shear and maximum displacement demands of a nonlinear single-degree-of-freedom structure on soft soil deposits. In the proposed procedure, the effect of frequency content of ground motions is considered by normalising the period of vibration by the spectral predominant periods, while the nonlinear EFSDOF models are used to improve the computational efficiency.

Yüksek Hızlı Trenlerin Serbest Zemin Yüzeyinde Oluşturduğu Titreşimlerin Deneysel ve Analitik Olarak İncelenmesi

Normal 0 21 false false false EN-US X-NONE X-NONE /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Normal Tablo"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-parent:""; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin:0cm; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:"Times New Roman",serif; mso-ansi-language:EN-US; mso-fareast-language:EN-US;} Railway platform which is vibrated with higher frequency dynamic loads induced by high speed train cars transfer these vibrations energies to surrounding ground and close by structures are effected by them. Strong ground motion not only can give damage to the nearby buildings and their footings but also effect the human comfort by undesired vibrations. Extensive in-situ research are still required for both recent ongoing construction projects in soft soil deposits (Eskişehir-Adapazarı-Izmit) and for planned construction projects on highly populated areas (Edirne-Istanbul-Ankara-Kars) in Turkey. The aim of the study can be summarized as to determine comfort disturbance and devastating environmental vibration created by high speed trains on free field. Strong ground motions induced by high speed train measured and recorded at the inspection points selected at the site close to high speed train line passing through highly populated regions. Environmental effects of these vibration data compared and evaluated. However, the applicability of a well know half space solutions based on Bornitz’s analytical approach which takes account the loss of the amplitude of waves due to geometrical damping and material damping of Rayleigh waves are investigated. Bornitz’s solution tends to overlap vertical free field vibration with in-situ measured records.

Seismic microzonation of Kolkata

This paper presents the probabilistic seismic microzonation of densely populated Kolkata city, situated on the world's largest delta island with very soft alluvial soil deposit. At first probabilistic seismic hazard analysis of Kolkata city was carried out at bedrock level and then ground motion amplification due to sedimentary deposit was computed using one dimensional (1D) wave propagation analysis SHAKE2000. Different maps like fundamental frequency, amplification at fundamental frequency, peak ground acceleration (PGA), peak ground velocity (PGV), peak ground displacement (PGD), maximum response spectral acceleration at different time period bands are developed for variety of end users, structural and geotechnical engineers, land use planners, emergency managers and awareness of general public. The probabilistically predicted PGA at bedrock level is 0.12 g for 50% exceedance in 50 years and maximum PGA at surface level it varies from 0.095 g to 0.18 g for same probability of exceedance. The scenario of simulated ground motion revealed that Kolkata city is very much prone to damage during earthquake.

Field performance of a genetic algorithm in the settlement prediction of a thick soft clay deposit in the southern part of the Korean peninsula

Long-term settlement data of a thick soft clay deposit improved by vertical drains frequently shows different field settlement behaviors from laboratory-driven predictions by conventional theories due to the variability and uncertainty of the soil properties, modeling simplifications, and types of prediction methods. This paper presents the field application of a back-analysis method based on a genetic algorithm (GA) to evaluate the performance of a new settlement prediction method compared with conventional graphical settlement prediction methods, such as the hyperbolic method and the Asaoka method. The GA back-analysis method shows better flexibility in modifying surcharging plans and adaptability to multi-layered thick soft soil deposits at the early stages of post-construction settlement. Thus, this new settlement prediction method enables geotechnical engineers to subsequently modify the heights of surcharge fills in accordance with field settlement data monitored in the interim for rapid and cost-effective construction. The comparative results show that the GA back-analysis method is capable of superior field performance in settlement predictions compared with two conventional graphical methods, within a margin of less than 200mm in a thick soft clay deposit with multiple layers under complex loading conditions.

Electrokinetics to Modify Strength Characteristics of Soft Clayey Soils: A Laboratory Based Investigation

The effects of Electrokinetic (EK) processing on compressive strength characteristics of two types of soils were investigated using laboratory experimental models. Soils were collected from soft alluvial soil deposit (Soil S1) and basaltic soil deposit (Soil S2) in central Victoria, Australia. A layer of soil was placed in glass tanks (90cm×18cm plan area) and compacted to a known density and water content typical of field conditions. Using electrodes inserted into the soil, a direct current was passed across the soil under various voltage gradients (0.5, 1.0, 2.0V/cm) for periods of 7, 14, 30 and 60 days. Throughout the experimental duration, test samples were supplied with a continuous flow of water via the hollow core anode from external reservoirs. Unconfined compression (UC) tests were conducted on original (untreated) soils and EK processed soils.From the test results it is noted that, soil compressive strength increases with the increasing processing time and increasing voltage gradients, at various rates. Under certain voltage gradients and processing times, around 175% and 200% strength increases are observed. In general, stress increases of at least 30% or more are reported for both soils under all test conditions.It is apparent that the variation in strength can be attributed to several complex and interrelated processes that become active under EK processing. These may include, (i) Electroosmotic advection - When a soil is subjected to EK processing with an open electrode configuration, the water content of the soil varies predominantly due to the electroosmotic advection while natural drying and evaporation could also add to the decrease in water content to some extent, depending on the time and environmental conditions such as temperature and humidity. The test results show that with the decrease in water content, there is a corresponding increase in the strength. (ii) Electromigration - The electromigration of charged ions and their interaction with clay minerals can also affect the soil strength due to the variations in the DDL (Diffused Double Layer) ionic concentration and subsequent modifications in the soil structure. (iii) Ionic Diffusion and Aging - After the complete termination of EK processes, the ionic concentrations still continued to modify at a slower rate. This is considered to be due to the ionic diffusion. In this phase too, cementation bonds may continue to develop that could contribute to the increase in soil strength. During this period, two other processes, i.e. natural drying of soil and aging may continue that can also affect the variation of soil strength.

Rotational excitation of bridges supported on pile groups in soft or liquefiable soil deposits

This paper investigates the seismic response of reinforced concrete bridges founded on pile groups in soft or liquefiable soil deposits under simultaneous translational and rotational earthquake excitation. The pile cap rotational excitation results from pile bending under vertically propagating seismic S-waves. The response of the pile cap (in terms of displacement and rotation) defines the Foundation Input Motion (F.I.M.) computed through kinematic interaction analysis. The resulting superstructure demand due to the coupled translational and rotational excitation is compared to the demand developed due to translational only F.I.M. The results confirm earlier observations regarding the impact of pile bending on rigid body rotations of bridges founded with cast-in-drilled hole single piles. It is also shown that the excitation mechanism of pile groups in soft soils is non-uniform along the pile length, thus exciting higher modes of vibration and inducing out-of-phase translational and rotational response of the pile cap.

Investigation of the soil amplification factor in the Adapazari region

Turkey is located on the highly active Eurasian plate. A very active strike slip fault, the North Anatolian Fault Zone (NAFZ), crosses Turkey from east to west; earthquakes occurred on this fault on August 17, 1999 (Kocaeli) and November 12, 1999 (Duzce). Regional geology and subsoil conditions can significantly change the characteristics of ground motion. For this reason, determining soil amplification during an earthquake, especially for soft soils, is a very important topic for researchers. In this study, one dimensional ground response analyses were performed for selected Adapazari sites using the August 17, 1999 Kocaeli earthquake strong ground motion record with SHAKE2000 software. Soil characteristics and depth to engineering bedrock at the selected sites are different and the observed level of structural damage at these sites during the Kocaeli earthquake was also different. Calculated soil surface response spectrums at these sites were compared with the recommended design spectra of the Turkish Earthquake Code and the Eurocode 8. According to one dimensional ground response analyses, the calculated response spectra of the selected sites exceed the recommended design spectra of the Turkish Earthquake Code and the Eurocode 8. Calculations show that higher amplification ratios occur at higher periods due to soil behaviour. Results of this study indicate that local geological conditions may amplify ground motion at some periods and, due to this amplification, the calculated response spectra may exceed the recommended design spectra. Therefore, it is clear that local site conditions must be considered for earthquake-resistant engineering designs on soft alluvial soil deposits.

Performance of a RC Building using Site Specific Ground Motion Parameter of Salt Lake Sector-V Region, Kolkata, India

This work presents the performance of a G+3 storey RC building using site specific response spectra at Maximum Considered Earthquake (MCE) condition of Salt Lake sector V, Kolkata, India, situated on the world’s largest delta island with very soft and thick soil deposit in the surficial layer. A comparison has been made with the codal response spectra [IS 1893 (Part-I):2002] and site specific response spectra of the performance of a G+3 storey building designed according to IS codes and analyzed by nonlinear static procedure. The results of simulated nonlinear behavior of the building during MCE earthquake depicts that the building without masonry infill is safe if it is designed as per IS 1893 (Part I):2002 (assuming soft soil) response spectra as well as using site specific response spectra. However, the building with masonry infill (except ground floor) may not be safe due to soft storey effect during code specified as per MCE.

Numerical analysis of vertical drains accelerated consolidation considering combined soil disturbance and visco-plastic behaviour

Soil disturbance induced by installation of mandrel driven vertical drains decreases the in situ horizontal hydraulic conductivity of the soil in the vicinity of the drains, decelerating the consolidation rate. According to available literature, several different profiles for the hydraulic conductivity variation with the radial distance from the vertical drain, influencing the excess pore water pressure dissipation rate, have been identified. In addition, it is well known that the visco-plastic properties of the soil also influence the excess pore water pressure dissipation rate and consequently the settlement rate. In this study, a numerical solution adopting an elastic visco-plastic model with nonlinear creep function incorporated in the consolidation equations has been developed to investigate the effects of disturbed zone properties on the time dependent behaviour of soft soil deposits improved with vertical drains and preloading. The employed elastic visco-plastic model is based on the framework of the modified Cam-Clay model capturing soil creep during excess pore water pressure dissipation. Besides, nonlinear variations of creep coefficient with stress and time and permeability variations during the consolidation process are considered. The predicted results have been compared with V<TEX>$\ddot{a}$</TEX>sby test fill measurements. According to the results, different variations of the hydraulic conductivity profile in the disturbed zone result in varying excess pore water pressure dissipation rate and consequently varying the effective vertical stresses in the soil profile. Thus, the creep coefficient and the creep strain limit are notably influenced resulting in significant changes in the predicted settlement rate.