Geotechnical Applications

Various methods are available to evaluate the settlement of shallow foundations on cohesionless soil. Fifteen numbers of such commonly adopted methods have been selected for the settlement calculations. These methods have been categorized as empirical (nine numbers), semi-empirical (two numbers), and theoretical (four numbers). The settlements of foundations with different footing sizes have been calculated using these methods on improved sandy soil (with the stone columns or vibro compaction) and on unimproved soil. The calculated settlement values are compared with the plate/footing load test results carried out on corresponding plate sizes and full scale RC footings. Among various studied methods, empirical method proposed by Terzaghi and Peck, semi-empirical method proposed by Schmertmann and a theoretical method proposed by Janbu appeared to predict reasonably accurate and reliable settlement values. The stiffness of soil does not appear to be nonlinear with strain and stress level as observed based on the actual settlement values at different stress levels. Almost all the methods overestimated the footing settlements at lower as well as higher stress levels. Though semi-empirical and theoretical methods are preferred over empirical methods due to lack of theoretical study in later, it is also suggested to appropriately select the geotechnical design parameters rather than trying to select the best method for the settlement analysis.

Many empirical and semi-empirical methods are existing to predict bearing capacity and settlement, but majority of them are inconsistent and not user friendly. In this work, a genetic algorithm approach is used for predicting bearing capacity and settlement of shallow foundations on C-soil, φ-soil and C-φ soil, separately, based on those input parameters which can be easily find out from simple experiments. The development and verification of the genetic models were done using a large database containing about 832 datasets from 167 soil investigation reports. The equation developed for bearing capacity and settlement thus obtained can be used in prediction of new cases that were not used for the development of the genetic model. The results of the model obtained were compared with various conventional equations available for calculating bearing capacity and settlement and was found to be superior. The correlation of predicted data with actual field measurements was determined and it was found out that the genetic algorithm approach have high degree of accuracy. Parametric study was also done to evaluate the effect of varying each of the input parameters on the corresponding output.

This work deals with the deformation analysis of combined footings resting on geosynthetic and stone column improved ground and subjected to symmetrically placed column loads. Simple mathematical model has been proposed which idealizes a combined footing as a beam. Just below the footing, there is the presence of a granular fill layer which has been modeled as a Pasternak shear layer. Geosynthetic layer, idealized as an elastic extensible membrane, has been placed in between this granular soil layer. There is an existence of a soft soil layer below geosynthetic-reinforced granular fill layer improved by stone columns. Stone columns and the soft soil layer have been represented by Winkler springs and Kelvin-Voigt body respectively. Nonlinear behavior of granular fill, stone columns and the soft soil has been considered in the analysis employing hyperbolic constitutive relationships. Governing differential equations have been derived and solution of these has been obtained with the help of appropriate boundary and continuity conditions. Finite difference method has been adopted for solving the model of footing-geosynthetic-reinforced granular fill-stone column reinforced soft soil system. Parametric study has been conducted to study the influence of configuration of stone columns. Five column loads have been considered. These results have been presented in the form of non-dimensional charts which can be used for deciding the optimum configuration of the stone columns in such soil-footing system. It has been seen that both these parameters, i.e., diameter and spacing of stone columns significantly influence the response of combined footing. The deflection of footing has been found to reduce with reduction in spacing to diameter ratio (s/d) of stone columns. The ratio, s/d has been found to possess an optimum value of 2.5–3.

In this present study, the nonlinear behavior of a single hollow steel pile having an outer diameter of 0.114 m and length of 3 m was investigated under vertical excitations of rotating machine. Forced vibration tests were performed in the field to determine the dynamic responses of the single pile for four different eccentric moments. First time-acceleration responses of the single pile were measured for different frequencies and finally from that the frequency-amplitude response curves have been obtained for different excitation forces. It is observed from the field test results that the measured frequency-amplitude response curves exhibit nonlinear behavior of the soil-pile system by showing the decrement in resonant frequencies and disproportional increment in resonant amplitudes with the increase of excitation intensities. Additionally, the inverse analytical method of Novak is used to quantify the variation of stiffness, damping, and effective mass of the pile for different excitation intensities using the measured response curves. It is observed that the calculated damping values are increased with the increase of excitation forces. However, the effective mass and average stiffness values are decreased with the increase of excitation forces. The frequency-amplitude response curves are also back-calculated using the theory of vertical vibration with the calculated parameters of the soil-pile system. The back-calculated response curves are compared with the field dynamic test results and it can be seen from the comparison curves that the dynamic nonlinear response obtained from the theoretical analysis have a very close match with the field test response curves.

Structures resting on pile foundation are generally subjected to lateral loads and moments acting on the pile head in addition to vertical loads. Horizontal forces originate due to wind, traffic, earth pressure, water wave, and seismic forces or their combination. Analysis and design of piles subjected to lateral forces and moments is very important for ensuring the stability and serviceability of structure. In designing laterally loaded piles, the pile head deflection is very important which depends on soil type, pile installation, pile flexibility (or pile stiffness), loading condition, and type of fixity of pile with pile cap. The flexural behavior of a pile is a function of the interaction between the soil and the pile and is governed by the properties of both. In the present study, lateral load behavior of single piles in layered soils, i.e., alternate layer of clay and sand is conducted. The analysis is carried out considering fixed-headed pile and floating tip at the base. Since soil response is complex idealized models are used for the analysis. Analysis based on generated nonlinear P-Y curves representing the soil behavior using OASYS ALP v19.2 has been elucidated. Flexural response of the pile along with lateral pile capacity, moment and length of fixity is estimated considering all the analysis with different pile diameters.

The present study tries to predict the settlement of shallow foundation on granular soil using a mathematical model. The application of feed-forward neural networks with back propagated algorithm is followed for the same. For the development of ANN model, 193 in situ tests data were collected from the literature. The inputs required for the development of model were the foundation pressure, width of footing and the standard penetration number. The predicted settlement using this model was found to compare favourably with the measured settlement. Further the results of sensitivity analysis indicated that the width of foundation has highest impact on the predicted settlement in comparison to other input variables. The present study confirms the ability of ANN models to predict a complex relationship between the nonlinear data as in present case.

The study pertains the behaviour of two closely spaced strip footings resting on the surface of the semi-infinite clay soil medium. The effect of interference on characteristic behaviour like bearing pressure, settlement and tilt are observed using two dimensional plane strain finite element analysis. The foundation soil medium is modelled using Mohr–Coulomb soil model that follows linear-elastic perfectly plastic behaviour using finite element analysis software PLAXIS. The soil domain is discretized into 15-noded triangular elements with fine meshing in vicinity of the footings and coarser meshing towards boundary of the domain. Parametric study is performed by varying the clear spacing between the footings and depth of the footings; their effect on the load-settlement characteristic, variation of settlement and bearing pressure with clear spacing are studied. The results are presented in terms of non-dimensional efficiency factors defined as the ratio of settlement/bearing pressure of interfering footings to that of the isolated footing. It is found the interference affect the performance of isolated footing. The bearing pressure is found to decrease with decrease in clear spacing between the footings and in contrast, the settlement is found to increase compared to that of the isolated footing. The results may help in finding the minimum spacing between the footings for better performance.

Ring footings are widely used as foundation for water tanks, television antennas, silos, chimneys, oil storage tanks, etc. This paper presents an experimental study to investigate the cyclic as well as static behavior of model ring footing and circular footing resting on coir geocell-reinforced sand. The parameters studied are coir geocell width, and depth of embedment of geocell. The studies have shown that, with the provision of geocell-reinforced sand cushion, there is substantial reduction in settlement of both ring and circular footings due to modified stress distribution. The beneficial effect in terms of increased load carrying capacity and reduced settlement is related to the width and depth of placing geocell mattress.

Load-displacement behaviour and vertical uplift capacity of horizontal anchor plate embedded in layered sand deposits have been investigated by conducting laboratory model tests. Strip and circular plate anchors are used for this purpose. The layered sand deposits have been prepared with local sand having relative densities 25% (loose sand upper layer) and 65% (medium dense sand with bottom layer), respectively. The medium dense sand layer is always kept closer to the anchor plate and the loose sand layer is kept close to the top surface in the layered sand system. The experiments have been conducted for different embedment ratios with different relative thickness of the dense sand layer (Hdense/H). The maximum displacement of the anchor plate experienced at failure and the uplift capacity of the anchor plate have been found to increase with an increase in the thickness of the dense sand layer (Hdense) at any particular embedment depth (H). The present uplift capacity of the anchor plate determined experimentally is compared well with the available numerical solution.

As structures get taller and the load demand on the foundation soils increases, it has become imperative to reconsider conventional geotechnics, and calibrate and validate our theoretical predictions with ground conditions. A prototype footing load test was conducted on a 2 m × 2 m size concrete footing for a better assessment of the load-settlement behavior at a site in Noida. This paper presents the test results and compares them to results from conventional methods of analysis. A numerical simulation of the test conditions has also been performed using finite element method to validate the field results with mathematical simulation. It has been found the results of the field test are in good agreement with theoretical predictions, thus enhancing the factor of reliability of our foundation analysis.

Pile foundations are often subjected to lateral forces in addition to vertical loads. The lateral forces acting on piles may arise due to wind pressure, seismic waves, wave action in offshore structures, and lateral earth pressure in earth retaining structures. Lateral displacement at the pile head is an important criterion for a successful design of pile that supports lateral load. In this paper, results observed from model tests conducted on laterally loaded piles embedded in medium dense sand are presented. The effect of variation in pile embedment length to diameter (L/D) ratio and flexural stiffness on lateral load carrying capacity of single pile was studied by varying pile embedment length and pile diameter. Stainless steel pipes have been used as model piles. The piles were kept free headed with floating tips. Various value of pile length to diameter ratio (L/D = 10, 15, 20, 25, 30, and 35) were employed in the model testing. Pile group tests were also performed for comparison with single pile. The observed results are presented in the form of lateral load displacement curves and design charts are prepared for lateral load displacement response of piles in cohesionless soils.

This paper presents the results of numerical studies carried out to investigate the effect of excavation on the adjacent circular footing. To obtain the response of the circular footing, series of numerical analyses were performed on the footing by incorporating varying parameters such as the footing location from an excavation, varying density of soil and depth of excavations. In the numerical analysis, the footing and sheet pile walls were idealized as a linear elastic materials and nonlinear behaviour of soils using a Mohr–Coulomb model. The paper mainly discusses on the effect of excavations to an adjacent circular footing embedded in medium and dense sandy soil. It is observed from the results that the settlement and bending moment of the footing increases with increase in depth of an excavation. The location at which the effect of excavation on the footing is less significant as termed a safe location or critical location of the footing from excavations. The critical location of the footing based on settlement criteria is 1.5 m from an excavation faces in case of the footing in medium sand and 0.5 m in dense sand.

Fly ash is a waste product of thermal power plants, can be used as embankment material, stabilization of soft soils, road sub-base constructions, and other geotechnical fields. In this study, fly ash is used as soil stabilizer for an embankment slope. This paper discusses the shear strength parameters of the soil stabilized with fly ash. The soil has been mixed with 10, 20, 30, and 40% fly ash by dry weight for conducting modified Proctor compaction test and direct shear test. The experimental results indicate that the dry density and cohesion value of soil decreases where as the angle of internal friction increases with increase in the percentage of fly ash. The analysis of slope stability of native soil and stabilized soil has been studied by Fast Lagrangian Analysis of Continua (FLAC) slope software. Parametric study has been done to calculate the factor of safety by considering different slope height at constant slope angle under summer and rainy season. The analysis revealed that the slope with native soil was stable up to 12.0 m height under both summer and rainy season. With further increase in height, the slope becomes critically stable and failed under rainy season at 14.0 m height. The addition of fly ash enhances the strength and provides resistance to slope instability under both the conditions up to 14.0 m height. It has been found from the analysis that the factor of safety increases with increase in percentage of fly ash at a particular height. 30% fly ash is obtained as optimum amount as stabilizer for a slope of certain height.

The study area for this paper is coastal Karnataka in India, which has laterites and lateritic soils. The soil stratification in this area mainly consists of lithomargic clay, which is a product of laterization, sandwiched between the hard and porous weathered laterite crust at the top and the hard granite or granitic gneiss underneath. This lithomargic clay, locally called as ‘shedi soil’ behaves as dispersive soil and is also highly erosive. In the first stage of this study, laboratory erosion studies are conducted by using the hole erosion test apparatus on controlled shedi soil samples. Erosion observed in the HET is accelerated due to slaking irrespective of dispersive nature of the soil. Erosion problems were also dealt with using a stabilizer, calcium lignosulfonate and resulted in high increase in its erosion resistance. In the second stage of this study, slope stability studies of excavated slopes in lateritic formations are conducted considering intensity of rainfall, ponding and seepage, apart from the usual geotechnical parameters. The slopes steeper than 60° are not stable in the case of shedi soil considered here.

Seepage becomes an inevitable concern when dealt with earthen dams, as the impounded water seek paths of least resistance through the dam and its foundation. Seepage, if uncontrolled, can erode soil from the embankment or foundation causing progressive or rapid erosion and piping of the embankment or foundation soils. Out of the several approaches, increasing the length of the flow path by using sheet piles has proved to be effective in seepage reduction. This study examines the effect of using multiple sheet piles of varying length positioned at different distances from the upstream and downstream of a homogeneous earthen dam on the variation of exit gradient, uplift pressure, porewater pressure, and flux along the toe of the dam. The analysis is done using a Finite Element modeling with the aid of SEEP/W module in Geo-Studio 2007 software. The results of the numerical analysis gives an insight about the effect of varying geometrical arrangement of the sheet piles on the reduction of exit gradient, uplift pressure, porewater pressure, and flux along the downstream portion of the dam during the reservoir filling condition. The study highlights the arrangement of the sheet piles to obtain the maximum reduction of exit gradients and porewater pressures during the rise-up condition.

Tailings dams are used to impound waste tailings generated by the mining industry. The tailings material is fine-grinded particles of sizes similar to clay particles however they behave as cohesionless material. The tailings dam under consideration is a zoned dam raised in six stages using the centre line method and having three different types of fill materials. The zones, from upstream to downstream, are consisted of (i) impervious material (ii) compacted tailings and (iii) pervious random fill. An inclined chimney drain and a connecting horizontal filter are provided to keep most of the dam section on the downstream side dry. The main objective of this study is to perform static and pseudo-static analyses by the Finite Element Method, using the Strength Reduction Technique, and comparing the results with those from various Limit Equilibrium Methods, such as Bishop Simplified, Janbu Simplified, Janbu Modified, Spencer, Corps of Engineers-1, Corps of Engineers-2, Lowe Karafiath and Morgenstern-Price for both circular and non-circular failure surfaces. The results are presented in the form of normalised plots considering the following cases: (i) Stagewise (Without Tailings) (ii) Stagewise (With Tailings) (iii) Considering circular failure surface, and (iv) Considering non-circular failure surface.

A container yard is being developed near Mumbai by creating around 90 ha of land through reclamation in sea over soft, compressible marine clay. The thickness of the marine clay varies from 4 to 18 m over the reclamation area underlain by Basaltic rock. The strength of the soft clay varies from very soft to stiff clay, and susceptible to large settlements and shear failure. This stratum cannot support the reclamation fill by its own, hence ground improvement is essential to improve the strength and stiffness properties so that the finished reclamation is able to satisfy the serviceability criteria. Stability analyses of perimeter bund confining the marine reclamation were examined for all probable surfaces including circular, non-circular, and sliding failures as per BS 6031. The analyses show that overfill sections beyond the permanent reclamation cope line are required along with a layer of tension geotextile in order to achieve stability during filling to support the full design load until the cope line. Once the surcharge is removed, the overfill is cut back to the final geometry and revetment is provided along the perimeter bund.

Recently, use of geosynthetics has gained widespread popularity and has been increasingly used as reinforcing element in several engineering practices of earth retention structures like reinforced earth walls, reinforced slopes, etc. Often the required soil type of requisite characteristic soil properties considered for design are not available locally, hence the available soil type has to be used. With that under consideration, this study deals with the analysis of a steep soil slope embankment reinforced with geogrids under different soil backfill properties. The steep slope is basically designed to widen and elevate the existing natural ground in order to provide road pavement over it. Initial design consideration included conventional limit equilibrium method along with classical earth pressure theories. The design is further modeled and analyzed meticulously according to the staged construction sequence undertaken at site, in a geotechnical finite element analysis software, i.e., PLAXIS. This study mainly investigates the effects of use of different types of soil backfill on stability of slope. Effect of variation of soil parameters, i.e., cohesion and angle of internal friction on steep slope embankments of different heights (6, 12, 18, 24, and 30 m) are studied through this analysis. Since very steep and very high slope embankments were considered for the study, tiered structure, i.e., provision of berm was considered. The effect of width of berm was also considered as a parameter and corresponding effect on the stability of slope was studied. The observed trends of results were further compared with available literatures and previous studies. The results suggested that with increase in soil parameters like cohesion (c′) and angle of internal friction (Φ) the factor of safety for the global stability further increased. From the study it was observed that the most optimum soil type was found to be the soil with low cohesion and high angle of internal friction, because upon increasing the cohesion of soil, although increase in global stability was observed, it also led to increase in horizontal displacements and axial forces acting over the geogrids. From the results it was also observed that use of tiered structure or provision of berm increased the factor of safety for global stability marginally and reduced the forces and horizontal displacements significantly on the lower tier. It was concluded that in case of non-availability of recommended granular soil, the available soil fill be utilized only for low height embankments. For high embankments, only recommended granular soils should to be used.

Back-to-back reinforced retaining walls are mostly used in approach embankments of bridges and flyovers. In the internal stability check for mechanically stabilized earth (MSE) walls, earth pressure theory is used to obtain the tensile forces in the reinforcement. However, tensile forces calculated from this method are found to be very conservative (by a factor of two) in comparison to the actual field measurements. The objective of this study is to examine the mobilization of reinforcement tensile forces at different levels within back-to-back MSE wall at working stress condition. Tensile forces in the reinforcements with reinforcement connected in the middle (i.e., reinforcement extending from one wall to the other) are also obtained. Parametric study is carried out with the stiffness of reinforcement varying from 500 to 50,000 kN/m and the ratio of spacing between two walls to wall height (W/H) varying from 1.4 to 2.0 to investigate their effect on tensile forces at every level of the reinforcement. Charts are also proposed showing the variation of the maximum tensile forces along the height of the wall.

In this paper, a case study is considered to study the behavior of a 3.6 m high MSE wall reinforced with geogrid reinforcement. Reinforcement spacing is 0.6 m. A limit analysis has been carried out using LimitState:GEO (which used discontinuity layout optimization DLO) to study the performance of a full-scale wall. The maximum reinforcement tension at each level was computed and the results are compared with the measured values. Computation of load coming on reinforcement under different surcharge load is done by analytical approach using MATLAB. In this paper analytical model (current AASHTO Simplified Method) are also prepared with different surcharge loading. The computed values of maximum load on reinforcement are finally compared with the measured values. The results show the LimitStateGEO analysis gives a better estimation of the maximum reinforcement tension compared to the measured values.

This paper reports about the numerical analyses conducted to investigate the instability induced due to toe cutting of simple homogeneous slopes during possible construction and widening of roads. A simple homogeneous hill slope made up of c-φ soil and overlying a rigid bedrock foundation material has been analyzed using Morgenstren-Price limit equilibrium method (Morgenstern and Price 1965) with the aid of Geostudio SLOPE/W 2007. Toe cutting analyses have been conducted for a wide variation of geotechnical, hydraulic and seismic parameters. The stability of the slope is assessed by the evaluated factor of safety which defines the mobilization of the shear strength along a particular slip line. Analyses are done in three stages. In the first stage, simplified hill slopes are analyzed solely under self-weight of the slope. In the second stage, pseudo-static analysis has been conducted. In third stage, water table variation is incorporated in the toe cutting analysis. Results are presented in graphical form and based on present study discussions are made regarding the critical horizontal extent of vertical toe cutting of hill slopes depending upon the slope type and different geotechnical parameters.

Seepage through and below earth dam plays an important role in determining the stability of earthen embankment. Stability analysis during rapid drawdown is an important aspect in earthen embankment. Reducing porewater pressure through improved drainage could possibly ensure longer life of embankments. Introduction of filters, drains, clay blankets, clay core, and flatter side slopes are the potential remedial measures to avoid the failure of embankment, due to seepage. Out of these provision of an embankment core is an economic solution. The embankment core is usually installed at the center of the embankment, has significance in controlling the seepage through embankment body. In the present investigation stability analysis of an embankment using clay core under tidal fluctuation has been studied. The current study has been done by seepage and stability analysis of a typical section of an embankment with base width of 17 m and side slope 2(H):1(V) made of homogeneous soil, under rapid drawdown condition. The study has been done by finite element method using SEEP/W software varying thickness of thin clay core of embankment. Pore pressure variations, Flownet, and Phreatic surface have been obtained for the numerical models by SEEP/W software. In this study the pore pressure development of the embankment, by a mixed clay core of different thicknesses, is investigated and evaluated. The stability of the slope during water level drawdown conditions has also been analyzed, considering the variation of porewater pressures calculated from the transient seepage analysis. It has been observed that, factor of safety against slope failure, decreases when river side slope of the embankment is subjected to drawdown condition. The paper brings out the effect of variation on thickness of thin clay core on improvement of stability of earthen embankments through numerical modeling.

The present study is conducted to investigate the effectiveness of relief shelf to reduce the lateral thrust on rigid non-yielding retaining wall, through a small-scale physical model test in laboratory. The height of the model retaining wall was 700 mm, and five earth pressure sensors were used to measure the lateral pressures along the height of the retaining wall with and without relief shelves. A maximum surcharge pressure of 50 kPa was applied on the backfill. From the results of the study, it is found that relief shelves are effective in reducing total thrust on wall. A parametric study is also carried out on 8 m high wall having 3 relief shelves by varying the width of shelves, through numerical analyses using FLAC3D. The study reveals that relief shelves having width factor ranging 0.3–0.8 can reduce total thrust on the wall in range of 11–26%. Among all the cases of retaining wall with relief shelves analysed in the present study, retaining walls with shelf width of 1.4 m (width factor of 0.7) exhibited substantial reduction in lateral thrust, without leading to excessive deflection of relief shelves and backfill surface settlement.

This paper presents the stability analyses of non-homogeneous slopes under different loading conditions. A non-homogeneous soil slope with two different soil layers is considered. Special cases are created, varying the height of the layers, to account for the non-homogeneity of the earth slope. A water table is considered in this study to account for the seepage forces. Pseudo-static earthquake force is considered, taking into account both the forces in the horizontal and vertical directions. A rigorous limit equilibrium method of slices, i.e. Morgenstern-Price method is used to analyze the stability of the slope. Finite element shear strength reduction technique is also used for displacement calculations and comparison with limit equilibrium method. Safety factor, critical slip surfaces and displacements with different loading conditions are studied and compared.

Stability of rock slope along the stretch of Badrinath National Highway (NH-58) of Garhwal Himalayas in Uttarakhand, India is very much essential to protect the infrastructure and livelihood in the area nearby. Every year this part of the slope has been facing severe landslides due to intensive rainfall causing substantial damages. The chosen area is also highly sensitive to earthquake according to Geological survey of India. Due to the severe earthquake and intensive rainfall, stability of rock mass is under continuous depletion. Rock slope in this area is highly jointed and possess a joint orientation which is highly vulnerable to seismic force. In this study, finite element slope stability analysis of this rock slope has been carried out in commercial software PHASE2. As per the earthquake zonation map of India, the site is located in seismic zone V, and hence, dynamic stability of slope has been performed considering pseudo static and time-response method. Initially, the rock slope is considered to be devoid of any joints and is modeled as a continuum mass using equivalent Mohr–Coulomb shear strength criteria. In the second model, the discontinuities are applied in the continuum model by providing interface elements as joints in between the rock walls. Results from the analysis accentuate the fact that the of rock slopes is stable under high seismic force.

Failure of rock is an important problem arising during underground construction of shafts, tunnels etc. It has been determined that the peripheral stress developed around a circular tunnel causes rock failure which makes it essential to recognize these failures patterns under these circumstances. Prediction and quantification of failure mode is very complex and difficult. Therefore, it is important to carry out comprehensive study in laboratory to determine rock failure pattern which is significant to stabilize engineering rock masses and to recognize the suitability of support system as per nature of engineering work. Uniaxial compressive strength, Brazilian tensile strength and triaxial strength tests were conducted on three different rock samples namely Migmatitic Gneiss (MG), Phyllitic Quartzite (PQ), and Quarzitic Phyllite (QP), collected from the vicinity of Rohtang tunnel in Higher Himalaya, to understand the nature of failure patterns. Test results shows axial splitting along with shearing along plane and multiple fracture patterns in phyllites and gneiss whereas gneiss also shows multiple fracture failure patterns with increase in comprehensive strength of rock under triaxial conditions. In case of Brazilian tensile strength tests failure pattern are generally of central multiple type. This paper highlights the failure modes of gneiss and phyllite rocks under different loading conditions using ISRM and BIS suggested methods.

Instances of leakage from the joints between the panels of a diaphragm wall, during construction, are not uncommon and several cases have been reported in literature, along with measures taken for repair and rectification. This paper deals with one such case wherein, following repairs to stop the leakage, a parametric study was carried out using PLAXIS, considering various scenarios of subsoil disturbance, both the decrease in the subsoil strength as well as likely extent of zone of disturbance. The back-analysis was based on the data from the inclinometers, installed in the diaphragm wall panels and cone penetration tests carried out to establish the subsoil condition, pre and post leakage. This enabled to establish the most probable combination for assessment of the BM developed in the wall for ascertaining its structural adequacy. Based on the study it was concluded that the disturbed zone is limited to about 5 m beyond the face of the D’Wall, defined by Case A. The computed deformation for the lower Es values (Cases B and C) and 10 m disturbed zone (Case D), is much in excess of the observed deformation, hence not applicable.

One of the important issues of excavation and tunneling in urban areas is the assessment of its impact on foundations of neighboring structures due to ground movements in particular on pile foundations. Excavation and tunneling operations cause lateral soil movements which induce additional lateral loads on piles and they are known as passive piles. They are subjected to additional bending moment and shear force which can lead to serviceability problems or even failure of piles itself. The present study focuses on 1 g model tests to determine the response of single pile adjacent to a tunnel and a combined effect of excavation and tunneling. The effect of various parameters related to the pile, tunnel, and soil on the response of pile to passive loading was analyzed in this study. The following parameters viz. pile length to diameter ratio, distance between pile and retaining wall, distance between pile and tunnel springline, position of pile with respect to retaining wall and tunnel are varied to understand the response of piles and also to identify the factor which influences the response most. The lateral deflection of pile head was measured. Analysis of the test results showed that the shorter pile (L/d = 10) deflects more when compared to a long pile (L/d = 20) in case of excavation, tunneling and combined case.

The design of deep excavations requires careful consideration of the strength and stability of the various structural elements and soil properties at all stages during the construction process. In addition, the ground movements induced by the excavation need to be carefully studied and controlled, to ensure no damage to nearby properties and services and to keep these within serviceable limits. The performance of a deep excavation depends on the method of construction as well as precise assessment of the local ground conditions. Making reliable predictions of performance often presents a considerable challenge looking at various factors and uncertainties involved. This paper presents a case history on the use of temporary support of excavation using contiguous bored piled wall technique for deep basement excavation in Chennai. Case history presents an excavation of up to 9.3 m deep for the construction of a basement for a commercial building. The site stratification is sedimentary formation (Sandy Clay underlain by Stiff Clay) and the rock is at very deep depths beyond even shoring pile socketing extents. The geotechnical challenge is to design and construct a deep basement up to 9.3 m depth where SPT ‘N’ values are less than 10 up to 15 m below Existing Ground Level (EGL), high ground water table, close proximity of high-rise structures and adjacency of underground metro line to the shoring system. Finite element analysis using PLAXIS was used to analyse this staged excavation. Based on the finite element analyses, temporary strut supports were used to provide lateral support for contiguous piles. Monitoring works all along the shoring line was carried out using total station on regular intervals, and these readings were compared with the analysis results.

Jack-up rigs are primarily used for carrying out offshore operations related to drilling for hydrocarbons. Geotechnical assessment is a pre-requisite for deployment of any jack-up rig at a new offshore site. When deployed for operations, jack-ups are supported by their legs which initially penetrate below the seafloor on loading. Majority of jack-up units have individual/independent foundation, called ‘spudcans’, at the bottom of their legs. This paper presents a case of deployment of a jack-up at a site off the east coast of India where the jack-up had a ‘punch-through’ during preloading. Punch-through is a sudden and uncontrolled penetration of the spudcan often causing structural damage of the unit. It may occur when the load becomes equal to bearing capacity for the spudcan while within a soil layer having relatively higher bearing capacity than the soil underlying that layer. The deployment of the rig was carried out on the basis of preliminary soil investigation report from the consultant on-board the geotechnical vessel deployed for soil investigation. The case is analysed and discussed. Computer program ‘MAHAJACK’ developed by the author for carrying out leg-penetration and punch-through analysis for foundation of jack-up rigs has been used for the analysis.

This article presents a systematic approach for the reliability analysis of slopes in strain-softening soils based on the first order reliability method (FORM). The performance function is based on the Spencer method modified to take strain-softening into account in terms of the average residual factor RF over a potential slip surface estimated based on a simple progressive failure model available in the literature. The shear strength parameters, peak and residual, are assumed as normally distributed random variables and the reliability analysis is performed on the probabilistic critical slip surface. For the residual factor RF, bounded by 0 and 1, a generalized beta distribution has been assumed. Results obtained from an illustrative example indicate that a significant reduction (25%) occurs in the value of the minimum reliability index when RF is considered as a random variable compared to when RF is considered as a deterministic parameter. A FORM based sensitivity study also reveals that, amongst the five random variables, residual factor has the most dominating influence on the estimated reliability index and thus justifies its inclusion as one of the random variables.

Sensitivity (St) of cohesive soils is defined as the ratio of peak undisturbed shear strength (su) to the remoulded shear strength (sur) at the same water content. In offshore geotechnics, sensitivity is an important design parameter for clays especially where strength of remoulded clays is an important input in foundation analysis and design, for example, in design of some anchors in deep waters, assessment of lateral and vertical pile capacity in the zone disturbed by jack-up spudcan penetration. In many instances in offshore projects, results of laboratory or field tests for remoulded shear strength of clays are not available and in such cases empirical correlations are used which correlate either the remoulded strength or the sensitivity directly with other index parameters measured or derived in laboratory. Presently such correlations have not been derived or investigated for the fields of western Indian offshore and the available correlations in literature may not be applicable for the clayey soils of Western Indian Offshore. This paper discusses the results of evaluation of published correlations, for the soil data from top 30 m of soil profile from various fields of Western Indian Offshore. Results of attempts to develop new correlations for the specific application for the soils of Western Indian offshore are also presented.

Monopod bucket foundations supporting offshore wind turbines have to resist high overturning moment due to water waves and wind currents acting laterally on the structure above the seabed level. Hence, the stability of the bucket foundation system is governed by the lateral load-displacement behaviour. In this study, the drained behaviour of monopod bucket foundation under monotonic eccentric lateral loads has been investigated using finite element analysis. The influence of bucket dimensions and wind turbine self-weight on the lateral load response of the foundation system was also studied. The ultimate lateral load capacity of the bucket foundation has been observed to decrease with the increase of load eccentricity, but it increases marginally with self-weight and noticeably with the foundation size. For a preliminary design of the bucket foundation, the permissible loading state is presented as a lateral load-overturning moment interaction diagram for different geometries.

Offshore jacket platforms are being extensively used for the exploration of oil and natural gas. Nowadays these structures are being suggested for supporting offshore wind turbines. The jacket structures are fixed to sea bed using piles. These piles penetrate into the soil and transfers structural loads to the soil. The soil structure interaction of the piles should be studied for the safety of the structure. In the present study, the soil structure interaction (SSI) of the jacket structure is studied by developing a MATLAB code and performed static analysis using stiffness method. Structural elements are modeled using three dimensional beam elements. The modulus of sub-grade reaction of the surrounding soil is estimated using Vesic equation. The spring stiffness of the surrounding soil along the length of the piles of the jacket is estimated using Newmark distribution. Wave force on the structural elements of the jacket structures is estimated using Morison’s equation. The deflection of the topside and at the sea bed is presented in this paper. The effect of the marine growth on the static and dynamic response is also studied and concluded that the response of the structure and the natural period of the structure are increased when the marine growth is present.

Stiffness degradation studies for monopile-supported offshore wind turbines (OWTs) are usually limited to the geotechnical domain, largely ignoring the dynamic loads from the wind and the waves. This paper makes use of a time-domain approach, coupling aerodynamic and hydrodynamic loads, to investigate the influence of stiffness degradation in clay on the response of a monopile-supported OWT in a water depth of 20 m. p-y curves are used to represent the soil–structure interaction (SSI) in the lateral direction and a suitable degradation method is applied to consider the effects of cyclic loading. It is observed that the influence of stiffness degradation wanes with increasing number of load cycles. OWT’s being highly dynamic structures; the debilitating effects of stiffness degradation cannot be entirely discounted.