Seismic Earth Pressure Development in Sheet Pile Retaining Walls: A Numerical Study

The design of retaining walls requires the complete knowledge of the earth pressure distribution behind the wall. Due to the complex soil-structure effect, the estimation of earth pressure is not an easy task; even in the static case. The problem becomes even more complex for the dynamic (i.e., seismic) analysis and design of retaining walls. Several earth pressure models have been developed over the years to integrate the dynamic earth pressure with the static earth pressure and to improve the design of retaining wall in seismic regions. Among all the models, MononobeOkabe (M-O) method is commonly used to estimate the magnitude of seismic earth pressures in retaining walls and is adopted in design practices around the world (e.g., EuroCode and Australian Standards). However, the M-O method has several drawbacks and does not provide reliable estimate of the earth pressure in many instances. This study investigates the accuracy of the M-O method to predict the dynamic earth pressure in sheet pile wall. A 2D plane strain finite element model of the wall-soil system was developed in DIANA. The backfill soil was modelled with Mohr-Coulomb failure criterion while the wall was assumed behave elastically. The numerically predicted dynamic earth pressure was compared with the M-O model prediction. Further, the point of application of total dynamic force was determined and compared with the static case. Finally, the applicability of M-O methods to compute the seismic earth pressure was discussed

Laboratory evaluation of electrokinetic dewatering of dredged marine sediment as an option for climate change adaption

The climate change affects the coastal infrastructure including ports. This effect is through changes in the tides, waves, wind and coastal erosion. As a result, sedimentation in harbours and coastal area increases and therefore there is a need for more regular dredging as well as adaption to climate change to reduce the vulnerability. More frequent dredging means higher amount of dredging sediments need to be disposed or treated. One of the methods to be proposed to reduce the impact of high amount of dredging and reducing the environmental wastes as a by-product of dredging is to reuse or reproduce the dredged sediments. Electrokinetic stabilization is one of the environmentally friendly methods to dewater and strengthen the engineering properties of the soils and dredged sediments. This study investigates the effect of electrokinetic stabilization to improve the engineering properties of the dredged mud as an alternative option to reduce the environmental impact and use of a sustainable method for climate change adaption. Two laboratory designs are tested to determine the most efficient electrokinetic dewatering configuration and to examine the potential use of this method for dewatering and improving dredged mud. Electrokinetic stabilization is a promising method to dewater and expedite the settlement of the dredged marine sediments. However, the placement of electrodes can affect the power consumption and the efficiency of the technique and the resistivity of the soil. Some studies in the literature determine the best electrode configuration to optimize the electrokinetic stabilization. However, a few studies examined the electrode placement for electrokinetic dewatering and sedimentation. This study investigates the effect of electrode placement based on the efficiency of the method depending on power consumption versus dewatering, soil electrical resistivity, the settlement of the sediments, and treatment time. To reduce the energy expenditure first a constant voltage of 20 V is applied and the variation of electric current during the electrokinetic stabilization is monitored. Once the electric current approached zero, the voltage is increased to 30 V. Using constant voltage for both cases of electrode placement (anode on top, cathode at the bottom; anode at the bottom, cathode on top), it was observed that higher efficiency based on dewatering and power consumption is obtained when the cathode is placed on top

Advances in Instrumentation and Monitoring in Geotechnical Engineering

[Extract] Geotechnical instrumentation to monitor the performances of earth and earth-supported structures is increasingly becoming popular. Verification of long-term performances, validation of new theories, construction control, warning against any impending failures, quality assurance, and legal protection are some of the many reasons for geotechnical instrumentation. They are not only used in field situations, but in laboratories too. With the recent advances in materials and technology, and the need for more stringent performance control, there had been significant developments in the recent past in instrumentation and monitoring techniques

Laboratory and field investigations in granular soils to correlate relative density, relative compaction and grain size

An attempt has been made to correlate relative density and relative compaction based on laboratory and field test data. In this investigation, 185 sandy soil samples were tested in the laboratory. The experimental investigations include classification tests, maximum and minimum density tests, and standard and modified Proctor and in-situ density tests. The values of the dry unit weight of these samples obtained by performing different tests fall between 14.7 kN/m(3) and 20.8 kN/m(3). Based on these results, linear and multivariate regression analyses were carried out to (a) relate relative compaction and relative density, (b) relate maximum (e(max)) and minimum void ratios (e(min)), and (c) express e(max) and e(min) in terms of median grain size (D-50) and uniformity coefficient (C-u). Experimental and predicted values varied +/- 5%, with a 95% confidence interval for the relation between relative compaction and relative density, and for other relations the variation was +/- 10%. The proposed equations were validated using a new data set which had not been used for the development of the correlations. Proposed equations were also compared with equations presented by various other researchers. Validation of the proposed equations suggests that these equations may be used for future prediction of the respective variables

Drainage issues and stress developments within hydraulic fill mine stopes

The mining industry plays a vital role in the Australian economy. The large voids created in the process of underground mining are filled with minefills, with hydraulic fill being one of the most popular backfills used in Australia and worldwide. Barricade failures within the drives have claimed many lives worldwide. Drainage and stress developments within the hydraulic fill are the two main issues that need to be addressed in an attempt to provide a safe working environment in the mines. Drainage relates to the pore water pressure developments within the stope, and proper understanding of the stress developments within the stope is necessary to quantify the loadings on the barricade more realistically. This paper summarises the research carried out at James Cook University over the past five years through four PhD projects in these two areas, namely drainage and stress developments. This includes extensive experimental studies on hydraulic fills and porous barricade bricks and numerical modelling using FLAC. A new vertical stress reduction factor (alpha) is proposed that quantifies the stress reduction taking place within the fill due to arching

Geotechnical testing

The vane shear test is a popular in situ test used in the geotechnical engineering practice to assess the undrained shear strength of cohesive materials in a saturated state. The cone penetration test, vane shear test and dilatometer test are the most popular in situ tests for a soft soils. However, it is generally recognized that a field vane shear test has many advantages over other in situ tests as well as some an laboratory tests. Mayne et al. categorized all the in situ tests as 'old' and 'new' methods, with a seismic cone penetration tests with pore pressure measurements and seismic dilatometer tests falling under the new methods. In situ testing and laboratory testing on samples recovered during a site investigation are considered the main weapons available to the practicing engineer to assess geotechnical design parameters for any project

A general expression for geosynthetic strain due to deflection

Giroud [Geosynthetics International, 2, No. 3, 635–641, 1995] presented expressions for the strain within a geosynthetic layer assuming it to take the shape of a circular or parabolic arc. A more realistic situation would be one in which the deflected shape consists of both circular and parabolic arcs. This technical note presents an expression for the geosynthetic strain for a combination of parabolic and circular shapes of the deflected geosynthetic, which is commonly observed in many field applications of geosynthetics. The special simplified cases of this general equation, expected in field situations, are discussed in detail. The variation of geosynthetic strain with relative deflection, defined as a ratio of rut depth (maximum deflection, r) to the initial length (L 0) of the geosynthetic, is presented to show the application limits of special simplified cases of the general equation. It is observed that the geosynthetic strain significantly depends on the shape of the deflected geosynthetic, especially at large rut depths. It is therefore recommended that the deflected geosynthetic shape considered must be exactly the same as the one observed or expected in the field application under consideration, especially at large strain levels. It is also shown that the approximate expression presented by Giroud holds for all possible shapes considered in this note, for small strains

Basic soil mechanics

This chapter provides an overview of the topics in soil mechanics that are relevant to soft clay engineering and ground improvement. An assessment of vertical stress increase due to external loads is a key point in soft clay and ground improvement designs. The vertical normal stress at a depth below a long embankment along the centerline could be assessed using influence factors proposed by Osterberg, which is discussed in several soil mechanics books. As excess pore water pressure dissipates, the effective stress increases by the same magnitude until the additional load is fully transferred to the soil grains. A soil classification system is a systematic way to group soils of similar behavior and describe soils without any ambiguity. The person who identifies the soil at the site or the one who does the tests in the laboratory is often different to the one who carries out the designs and analysis

Maximum and minimum void ratios and median grain size of granular soils: their importance and correlations with material properties

Synopsis: The relationships between maximum and minimum void ratios of granular soil with various percentages of fine contents have been presented. The maximum and minimum void ratios are functions of soil properties such as grain size distribution, uniformity coefficient, angularity, and percentage of fine contents. Based on the existing results, it appears that the difference between the maximum and minimum void ratios, not maximum void ratio or minimum void ratio alone, is the controlling parameter for compressibility, relative density, and strength of granular soils. In spite of some scatter, the difference between the maximum and minimum void ratio bears a unique relationship to the median grain size. Several correlations relating the median grain size with the strength and compressibility are presented