Investigating the changing deformation mechanism beneath shallow foundations

The design of shallow foundations has traditionally used a mixture of plasticity-based solutions to find the ultimate limit state and either a factor of safety on the plasticity solution or a linear elastic solution to attempt to design for the serviceability limit state. The serviceability limit state is intrinsically linked to the deformation mechanism that occurs beneath the shallow foundation in service. A better understanding of these soil movements can pave the way for more rational design approaches. In this paper, small-scale experimental work is used to show that the deformation mechanism beneath strip and circular foundations continuously changes as the footing is displaced. The mechanisms observed at intermediate settlements, noted to be typical design points, are best described by a mixture of solutions. Linear mixes of idealised fields were analysed using an upper bound approach to determine the load–displacement behaviour of each mix. The envelope of lowest upper bounds indicated that the optimal mix of fields changes depending on the footing settlement. At typical design points for shallow foundations mixtures dominated by ellipsoidal cavity expansion mechanisms were found to be optimal for both axisymmetric and plane strain cases. Comparison of theoretical and experimentally measured predictions indicated that using linear mixes of fields gives a good approximation to the true behaviour and may be used for settlement-based design approaches in the future. This is the author accepted manuscript. The final version is available at http://www.icevirtuallibrary.com/content/article/10.1680/geot.14.P.226

Fundamental basis of single-point liquid limit measurement approaches

The liquid limit is defined as the point at which a clay’s behaviour changes from liquid to plastic. This transition is in reality gradual, rather than sudden. The definition of when this transition has been crossed must therefore be determined based on some arbitrary criterion. The percussion cup method of determining liquid limit in the manner suggested by Atterberg and subsequently standardised by Casagrande determines liquid limit as the water content at which 25 standard blows are required to cause closure of a standard groove. In order to speed up the determination of the liquid limit, a single-point method is defined in ASTM D4318, and in many other codes, to interpret liquid limit from groove closure at a different numbers of blows by assuming a relationship between water content and the number of blows required for groove closure. These methods differ considerably between different codes of practice currently in use worldwide. This paper examines the procedures for single-point determination of the liquid limit and offers some fundamental explanations that underpin the applicability of these procedures. This paper demonstrates that the variation in single-point liquid limit procedures suggested by various codes of practice can be attributed to the variability of liquid limit devices, rather than to variation in the soils being tested.This is the author's accepted manuscript. The final version is available from Elsevier at http://www.sciencedirect.com/science/article/pii/S0169131714004062?np=

The undrained strength - liquidity index relationship

A database of 641 fall cone tests on 101 soil samples from 12 countries has been analysed to determine the best mathematical relationship linking undrained shear strength with liquidity index. From the database, it is shown that the use of a linear relationship linking liquidity index and the logarithm of undrained shear strength that uses the commonly assumed 100-fold factor increase in strength from the liquid to plastic limit overpredicts the measured data of soil strength. The use of a factor of about 35 for the ratio between the strength at liquid limit and that extrapolated to the plastic limit is shown to be more realistic. Logarithmic liquidity index is examined and found to also correlate strongly with the logarithm of undrained shear strength; however, it is shown that no great statistical improvement is present compared with the semi-logarithmic formulation. When considering data of individual soils a power law fitting is statistically shown to be the preferred mathematical function.The authors gratefully acknowledge Dr Kevin Stone of the University of Brighton for providing a copy of the thesis by B. Kyambadde. Thanks to Professor Malcolm Bolton for his helpful comments. Thanks to Dr Sarah Allen for her help with the translation of the Swedish version of Atterberg’s original paper. The authors also thank all the reviewers of this paper for their insightful comments and helpful suggestions.This is the author accepted manuscript, which can also be found on the NRC Research Press website here: http://www.nrcresearchpress.com/doi/abs/10.1139/cgj-2013-0169#.U7awT_ldXH

Using tactile pressure sensors to measure dynamic earth pressures around dual-row walls

Following the destructive 2011 Tohoku earthquake and tsunami, dual-row retaining walls are being employed in Japan to protect coastal areas from these natural hazards. Dynamic centrifuge tests are conducted to investigate the system behaviour, in particular the earth pressures generated. High-frequency tactile pressure sensors can measure the dynamic earth pressures. By directly measuring the stresses generated during earthquake events invaluable insight can be gained into the dynamic behaviour. However, the processing and interpretation of the raw output can be challenging. This paper describes a processing scheme that can be used to reduce the apparent scatter in the data. This is applied to the earth pressure data collected using the system developed by Tekscan. The calibration and sensitivity correction process is detailed, with attention to the limitations of the assumptions made in the proposed method. The variation of the processed earth pressures with time is considered to better understand the wall behaviour. The distribution along the wall is considered at several instants to gain further insight. To help verify the earth pressure measurements, equilibrium-based methods are used to investigate the consistency between the recorded earth pressures and the outputs from other instruments that directly measure the wall response. </jats:p

Centrifuge testing of dual row retaining walls in dry sand: The influence of earthquake sequence and multiple flights

Multi-hazard threats from tsunami events preceded by large earthquakes have been put into sharp focus in recent times. Dual row retaining walls are soil-structure systems that can have large lateral capacity with a small horizontal extent, making them ideal for the next generation of coastal protection. However, the dynamics of their behaviour is a complex interaction problem. Centrifuge tests, with multiple earthquakes within a single flight as well as multiple flights with varied embedment ratio were conducted to elucidate the mechanics of these systems when founded in dry sand. The structural response shows permanent, plastic deformations during the early cycles dependent mainly on the PGA superposed with more elastic vibrations during prolonged shaking. The development of the soil stresses and stiffnesses mobilised is used to explain the overall system response. Finally, the recorded structural and soil behaviour during swing up and down are combined to show that the soil stress and strain state is effectively reset between flights. Overall, useful methods for judging the progressive response of a complex soil-structure system are presented which can help justify future comparisons between experimental datasets and understand the implications of practical dynamic design

Subsurface deformation mechanisms beneath a flexible pavement using image correlation

Flexible pavement structures are widely used in road construction, especially in circumstances where very high traffic volumes are not expected. These structures comprise of multiple layers of granular material, generally having decreasing strength with depth. Failure of these systems is typically observed as rutting on the ground surface, but the failure instigates at depth. In this research, for the first time, observations can be directly made of the progressive failure of these deeper layers under repeated wheel loading due to the combination of a new test apparatus, the Cambridge Accelerated Pavement Tester (APT), and Digital Image Correlation (DIC) technology. The use of a window allowing observation of soil displacements at depth during repeated wheel loading cycles allows the progressive failure to be observed and the changes in soil displacements and strains with different layer thicknesses to be quantified. It was observed that the critical failure mechanisms for thin and thick surficial layers are different, resulting in changes in the rates of surface rutting. Understanding these deformation mechanisms potentially allows savings to be made in road or airfield construction by using correctly-sized structural layers.non

Accuracy of distributed optical fiber temperature sensing for use in leak detection of subsea pipelines

Accurate and rapid detection of leaks is important for subsea oil pipelines to minimize environmental risks and operational/repair costs. Temperature-sensing optical fiber cables can provide economic, near real-time sensing of leaks in subsea oil pipeline networks. By employing optical time domain reflectometry and detecting the Brillouin scattered components from a laser source, the temperature gradients can be detected at any location along an optical fiber cable attached to the oil pipeline. The feasibility of such technology has been established in the literature along with a case study on a land-based pipeline. In this paper the accuracy of an optical fiber-based temperature sensing system is investigated. A mathematical model that simulates the process of temperature sensing is developed and the results are presented. An experimental investigation is carried out with two different laboratory setups to establish the spatial resolution and accuracy of the optical fiber cable detection system, and the experimental results are compared with predictions from the theoretical model. Based on these comparisons it has been established that the optical fiber cable detection system is capable of providing an accurate and rapid assessment of the location of a leak along a subsea pipeline. Furthermore, the sensing system can be used to give an indication of the scale of the oil leak using the temperature gradients detected by the system.The first author would like to acknowledge the support received under the UROP program from the Centre for Smart Infrastructure and Construction (CSIC) at the Department of Engineering, University of Cambridge.This is the accepted manuscript. The final version is available from ASCE at http://ascelibrary.org/doi/abs/10.1061/%28ASCE%29PS.1949-1204.0000189

A new macro-element model encapsulating the dynamic moment–rotation behaviour of raft foundations

The interaction of shallow foundations with the underlying soil during dynamic loading can have both positive and negative effects on the behaviour of the superstructure. Although the negative impacts are generally considered within design codes, seldom is design performed in such a way as to maximise the potential beneficial characteristics. This is, in part, due to the complexity of modelling the soil–structure interaction. Using the data from dynamic centrifuge testing of raft foundations on dry sand, a simple moment–rotation macro-element model has been developed, which has been calibrated and validated against the experimental data. For the prototype tested, the model is capable of accurately predicting the underlying moment–rotation backbone shape and energy dissipation during cyclic loading. Utilising this model within a finite-element model of the structure could potentially allow a coupled analysis of the full soil–foundation–structure system's seismic response in a simplified manner compared to other methods proposed in literature. This permits the beneficial soil–structure interaction characteristics, such as the dissipation of seismic energy, to be reliably included in the design process, resulting in more efficient, cost-effective and safe designs. In this paper the derivation of the model is presented, including details of the calibration process. In addition, an appraisal of the likely resultant error of the model prediction is presented and visual examples of how well the model mimics the experimental data are provided. The authors would like acknowledge the collaborative and financial support received through the European Community’s Seventh Framework programme (FP7/2007-2013) under grant agreement number 227887 (SERIES – Seismic Engineering Research Infrastructures for European Synergies).This is the accepted manuscript. The final published version is available at http://www.icevirtuallibrary.com/content/article/10.1680/geot.SIP.15.P.020

Discussion of “Reclaimed Lignin-Stabilized Silty Soil: Undrained Shear Strength, Atterberg Limits, and Microstructure Characteristics” by T. Zhang, G. Cai and S. Li

The discussers read with interest the recent paper by Zhang et al. (2018), which reports on the investigation of the effects of 0–12% lignin additive on some index and shear strength properties of a silty soil material. The use of the fall-cone device to study undrained shear strength variation with moisture content is pleasing to see and shows how the approach is useful for this purpose: namely the study of undrained strength variation. We wish to make the following comments regarding some of the underlying assumptions in the paper by way of offering some other explanations and interpretations for the results obtained

Bearing capacity and settlement of circular shallow foundations using a nonlinear constitutive relationship

The design of shallow foundations is dominated by issues of settlement rather than bearing capacity per se. The ability to predict the settlement of foundations at a given factor of safety is hence of key importance in design. In this paper, the energy method for a linear-elastic, perfectly plastic method utilizing the von Mises’ yield criterion with associated flow developed and reported by McMahon et al. in 2013 is extended to consider the nonlinear behaviour of soil. The energy method is used to investigate the load–settlement behaviour of shallow foundations by utilizing an ellipsoidal cavity-expansion mechanism and deformation fields within the boundaries of the classical Hill and Prandtl mechanisms. An elastic mechanism obtained from an analysis in ABAQUS was also investigated using this energy method. The upper-bound approach demonstrates that the cavity-expansion mechanism produces a better solution at small values of settlement, whereas at greater settlements the Prandtl mechanism is shown to produce a more optimal upper-bound solution. The first author would like to thank the financial support he received from the Cambridge Australia Trust (Poynton Scholarship) and the Principals of UK Universities (Overseas Research Students Awards Scheme) throughout his studies in Cambridge.This version is the author accepted manuscript. It can also be viewed on the NRC Research Press website at: http://www.nrcresearchpress.com/doi/pdf/10.1139/cgj-2013-027