The combined effect of clay and moisture content on very small strain stiffness of compacted sand-clay mixture

The very small strain shear modulus (stiffness) of soils, Gmax, is one of the most important parameters for predicting ground movements and dynamic responses of geo-structures. In this study, the combined effect of clay fraction and moisture content on shear stiffness of an unsaturated sand-clay mixture at very small strains was investigated using bender elements. Compacted soil specimens were prepared at three different clay contents of 10, 20, and 30%, and at four different initial moisture contents of 3, 6, 9 and 12%. Bender element tests were carried out under isotropic and constant moisture content conditions and inside a modified triaxial testing system equipped with a pair of piezoelectric bender-extender elements. Gmax was calculated based on the velocity measurement of shear waves propagated through the specimen. The tests results showed that Gmax decreases approximately linearly with an increase in moisture content, and non-linearly with an increase in clay content. A basic empirical equation was derived from an examination of trends in evolution of Gmax with clay and moisture content. Additional empirical correlations were also derived for estimation of moisture content and degree of saturation based on the compression wave velocity measurements

Reliability of HCT-based Soil Water Retention Curves

The measurement of SWRCs using HCTs has been the subject of several recent studies. Consequently, there have been several design and experimental procedures developed. However, despite these developments, the accuracy, range and duration of HCT-based measurement is still largely characterized by uncertainties and inconsistencies, thereby, reducing the reliability of the obtained SWRCs. In this work, an experimental program is designed to address these uncertainties. SWRCs of reconstituted London clay were measured using the continuous drying method with evaporation rate control. The obtained SWRCs were analysed based on the maximum suction value recorded by HCTs (sₘₐₓ), the obtained air-entry value (sₐₑᵥ), the suction at inflection point (sᵢ), the water content at inflection point (wᵢ), and the slope of tangent to inflection point (mᵢ). A percentage uncertainty of ±4% was obtained for the saev and si values. Similarly, percentage uncertainties of ±6% and ±0.5% were obtained respectively for the mᵢ and wᵢ values. These results were further compared with parametric analysis of the reported SWRCs of the same soil in the literature. Given the observed tolerance ranges, cautions must be taken in selecting values for these parameters e.g. as input values in mathematical curve fitting equations for prediction of the entire SWRC, or in unsaturated constitutive modelling, to enhance reliability of the outputs

Centrifuge modelling of rainfall-induced landslides in unsaturated soil slopes

Landslides and debris flows are geohazards common to countries with mountainous terrains. A significant number of rainfall-induced landslides occurs in regions with tropical and subtropical climates where residual and colluvial soils are widespread. This study is in the context of the Innovative Training Network project MUMOLADE (Multiscale Modelling of Landslides and Debris Flows) which aims to study the multi-scale and multi-phase analysis of landslides and debris flows. The research presented here aims to investigate the initiation conditions of landslides in soil slopes under varied rainfall conditions. A physical modelling experimental investigation on the initiation of rainfall-induced landslides in unsaturated soil slopes was performed in this study using the geotechnical centrifuge at the University of Nottingham. To accomplish the objectives of the study, a testing apparatus was designed and built. The tests were conducted in a plane-strain centrifuge box with embedded systems for controlling rainfall and groundwater conditions. The container has a transparent Perspex window at one side which provided the ability to measure subsurface ground displacements using digital image analysis. Pore Pressure Transducers (PPTs) were used for measuring pore water pressures within the slope models during the tests. Initially, the study focused on the behaviour of a fine, uniform sand. Classification tests of the soil were performed in the laboratory, including the Soil Water Retention Curve (SWRC) determined using the tensiometer method. Sand slope models were built at a high inclination angle, about 1.5 times higher than the soil’s angle of friction, with suction providing the necessary cohesive component for stability at 1g. The simulation of two failure mechanisms was attempted in this study. The first mechanism involves the loss of suction due to wetting in a soil slope of an angle higher than the angle of friction due to wetting. The second mechanism involves the collapse of the saturated voids due to increased pore water pressure which reduces the effective stresses within the soil mass. The second mechanism was not able to be reproduced in the tests conducted in initially unsaturated soil slopes with a low GWT, since the models showed unexpected resistance to failure. On the other hand, soil slope models failed under a condition of an elevated (i.e. shallow) GWT. In order to study further the unsaturated soil conditions under increased gravity tests were performed in which drainage of the soil under these conditions was enabled and measurements of the retained water content and degree of saturation at different g-levels were made. Results indicated that even medium-grained soils retain an amount of water content in their mass and that the retained water content at each g-level strongly depends on the grain size distribution of the soil. The implication of these tests is that soil slopes retain their stability when gravity increases even at extreme conditions, i.e. at an angle 50% higher than the angle of friction. In order to overcome the scaling issue of the increased seepage velocity, slope models were built from a slightly finer soil which was scaled down by $\sqrt{N}$. These soil slopes were tested under similar boundary conditions at the sand slopes. The results indicated that the reduction of the permeability caused instability initiation under different combinations of rainfall intensity and duration. Slope centrifuge tests were simulated using limit equilibrium analysis in order to validate the experimental results. The analysis revealed the previous statement, that the elimination of suction was not sufficient to reduce the shear strength of the soil to a level that would cause instability in the slopes. Failure was attributed to the small increase of positive pore water pressures which were generated only to the slopes with reduced seepage velocity (i.e. in slopes made of $M_{90-10}$). Therefore, grain size scaling in soil slope models was found to provide the necessary conditions for the simulation of rainfall-induced landslides in centrifuge tests. Rainfall thresholds for the occurrence of rainfall-induced landslides were obtained and compared to existing data from the literature. The comparison showed that the centrifuge tests conducted in this study provided an over-estimated rainfall threshold

An Introduced Methodology for Estimating Landslide Hazard for Seismic andRainfall Induced Landslides in a Geographical Information System Environment

The demand for estimating landslide hazard has evolved during the last decade. Landslides are characterised among the most severe natural hazards, which can cause casualties, fatalities, harm or detriment in natural and man-made environment. In the first part of this paper the results of the research conducted on slope deformation due to seismic loading are presented. According to field observations deformation and displacement of natural and man-made slopes in strong earthquakes are common phenomena, even though they are associated to moderate magnitude seismic events. These permanent displacements are due to seismic loading, and are produced because the material, through which acceleration pulses have to travel before reaching the ground surface, has a finite strength, and stresses induced by strong earthquakes may overcome this strength limit and bring about failure. Many methods were developed in order to assess the earthquake induced ground displacements due to seismic energy flow. We applied the simplified Newmark’s model, in order to study the problem of slope stability estimation and induced permanent deformations. In the current paper, the outcome of the studies attached to slope stability estimation under static and dynamic conditions considering the factors controlling safety conditions is introduced. These principal factors were first introduced to an artificial neural network and the estimated factor of safety and displacement were subsequently implemented in a geographical information system. A software tool was developed in order to produce landslide hazard maps due to static and dynamic loading, implementing failure criteria. In the second part, the results of the investigation of slope hydrology conditions in slope stability are presented. In these cases the factor of safety decreases due to prolonged precipitation and eventually the slope may fail. A parametric study of the effect of suction zone in slope stability of unsaturated soils is examined. This study focuses on slope behaviour under rainfall conditions

Centrifuge modelling of rainfall-induced slope instability in sand and silty sand

Rainfall-induced instability in slopes is an important challenge around the world. This paper aims to provide a better understanding of the processes involved when modelling slopes in a geotechnical centrifuge by presenting data from tests with two types of soil under two rainfall conditions. Results show that slopes with pure sand do not fail under low and high rainfall intensity whereas a slope comprising the same sand but with 10% silt (i.e. A silty sand), failed under both low and high rainfall intensity. In case of the silty sand slope with low rainfall intensity, a failure occurred even though the relative intensity of the rainfall (i.e. Ratio of rate of rainfall to saturated permeability of the soil) was lower than that of pure sand with high rainfall intensity. The different results obtained for the two types of soil are explained by the difference in their permeability and the physical phenomena at grain scale

Centrifuge modelling of rainfall-induced landslides in unsaturated soil slopes

Landslides and debris flows are geohazards common to countries with mountainous terrains. A significant number of rainfall-induced landslides occurs in regions with tropical and subtropical climates where residual and colluvial soils are widespread. This study is in the context of the Innovative Training Network project MUMOLADE (Multiscale Modelling of Landslides and Debris Flows) which aims to study the multi-scale and multi-phase analysis of landslides and debris flows. The research presented here aims to investigate the initiation conditions of landslides in soil slopes under varied rainfall conditions. A physical modelling experimental investigation on the initiation of rainfall-induced landslides in unsaturated soil slopes was performed in this study using the geotechnical centrifuge at the University of Nottingham. To accomplish the objectives of the study, a testing apparatus was designed and built. The tests were conducted in a plane-strain centrifuge box with embedded systems for controlling rainfall and groundwater conditions. The container has a transparent Perspex window at one side which provided the ability to measure subsurface ground displacements using digital image analysis. Pore Pressure Transducers (PPTs) were used for measuring pore water pressures within the slope models during the tests. Initially, the study focused on the behaviour of a fine, uniform sand. Classification tests of the soil were performed in the laboratory, including the Soil Water Retention Curve (SWRC) determined using the tensiometer method. Sand slope models were built at a high inclination angle, about 1.5 times higher than the soil’s angle of friction, with suction providing the necessary cohesive component for stability at 1g. The simulation of two failure mechanisms was attempted in this study. The first mechanism involves the loss of suction due to wetting in a soil slope of an angle higher than the angle of friction due to wetting. The second mechanism involves the collapse of the saturated voids due to increased pore water pressure which reduces the effective stresses within the soil mass. The second mechanism was not able to be reproduced in the tests conducted in initially unsaturated soil slopes with a low GWT, since the models showed unexpected resistance to failure. On the other hand, soil slope models failed under a condition of an elevated (i.e. shallow) GWT. In order to study further the unsaturated soil conditions under increased gravity tests were performed in which drainage of the soil under these conditions was enabled and measurements of the retained water content and degree of saturation at different g-levels were made. Results indicated that even medium-grained soils retain an amount of water content in their mass and that the retained water content at each g-level strongly depends on the grain size distribution of the soil. The implication of these tests is that soil slopes retain their stability when gravity increases even at extreme conditions, i.e. at an angle 50% higher than the angle of friction. In order to overcome the scaling issue of the increased seepage velocity, slope models were built from a slightly finer soil which was scaled down by $\sqrt{N}$. These soil slopes were tested under similar boundary conditions at the sand slopes. The results indicated that the reduction of the permeability caused instability initiation under different combinations of rainfall intensity and duration. Slope centrifuge tests were simulated using limit equilibrium analysis in order to validate the experimental results. The analysis revealed the previous statement, that the elimination of suction was not sufficient to reduce the shear strength of the soil to a level that would cause instability in the slopes. Failure was attributed to the small increase of positive pore water pressures which were generated only to the slopes with reduced seepage velocity (i.e. in slopes made of $M_{90-10}$). Therefore, grain size scaling in soil slope models was found to provide the necessary conditions for the simulation of rainfall-induced landslides in centrifuge tests. Rainfall thresholds for the occurrence of rainfall-induced landslides were obtained and compared to existing data from the literature. The comparison showed that the centrifuge tests conducted in this study provided an over-estimated rainfall threshold

Barcode Mapping in Warehouses

Automation in warehouses has been improved in a very certain manner, combining sensors for perception of the environment and mapping of the warehouse. The most common characteristic, which makes the products and the pallet rack cells discriminative, are the barcodes placed on them. This means that the warehouse management system should successfully perceive all the necessary information of the detected barcodes, which also includes their position in the warehouse and build a barcode map of the environment. For this process a barcode reader is needed, with extended capabilities such as estimation of the 3-dimensional coordinates of the barcodes. The main idea of this research was the development of a system to be used in future work, placed on the roof of a forklift, which will be able to detect the barcodes and localize itself using an existing map of the warehouse and update new information in this map. However, the purpose of this project was the investigation of a suitable system for barcode mapping. The main challenge of this project was the development of a barcode reader, which fulfills all the referred capabilities and the comparison with a commercial reader in order to evaluate the performance of the system. In this project a barcode reader was developed using software libraries and a camera for industrial use. The performance of the system was compared with a commercial barcode reader. Moreover, an algorithm was implemented, for estimation of the position of each detected barcode, with reference to the position of the camera's lens.  According to the results of all the investigations the performance of the developed system was quiet satisfying and promising. The comparison of the two systems proved that the commercial barcode reader had a better performance than the implemented system. However, it lacked the ability to provide the required information for mapping also the flexibility for integration with other systems. Overall, the developed system proved to be suitable for integration with a warehouse management system for barcode mapping of the environment.

Reliability of HCT-based Soil Water Retention Curves

The measurement of SWRCs using HCTs has been the subject of several recent studies. Consequently, there have been several design and experimental procedures developed. However, despite these developments, the accuracy, range and duration of HCT-based measurement is still largely characterized by uncertainties and inconsistencies, thereby, reducing the reliability of the obtained SWRCs. In this work, an experimental program is designed to address these uncertainties. SWRCs of reconstituted London clay were measured using the continuous drying method with evaporation rate control. The obtained SWRCs were analysed based on the maximum suction value recorded by HCTs (smax), the obtained air-entry value (saev), the suction at inflection point (si), the water content at inflection point (wi), and the slope of tangent to inflection point (mi). A percentage uncertainty of ±4% was obtained for the saev and si values. Similarly, percentage uncertainties of ±6% and ±0.5% were obtained respectively for the mi and wi values. These results were further compared with parametric analysis of the reported SWRCs of the same soil in the literature. Given the observed tolerance ranges, cautions must be taken in selecting values for these parameters e.g. as input values in mathematical curve fitting equations for prediction of the entire SWRC, or in unsaturated constitutive modelling, to enhance reliability of the outputs