Sw Jacobsz

University of Pretoria | UP · Department of Civil Engineering

A physical modelling approach was adopted to simulate the process of cave mining propagation at various horizontal to vertical stress state ratios, and image data was captured and used to characterize the resulting failure mechanisms. Image processing was integrated into the study to determine minor principal strains in models to identify where cri...

This paper presents the results of a centrifuge study in which swell of an expansive clay profile was induced in the centrifuge. A factor which has previously hindered such research is the time required to induce significant swell in-flight, within a reasonable time frame. In this study the use of an artificially fissured fabric together with geote...

In the design of onshore wind turbines constructed in swelling clays, the use of grouped-piled foundations is an attractive option. The large lateral forces to which the turbine is subjected, results in significant vertical cyclic loading of the piles. This study presents the results of a centrifuge test, conducted to analyse the response of scaled...

To investigate the time-dependent load-deformation behaviour of foundations under alternating loads in overconsoli-dated clay, centrifuge tests on rafts and piled rafts in Kaolin clay are carried out. The foundations are subjected to unloading and reloading processes as well as to groundwater lowering, simulating typical loading scenarios of struct...

In reinforced concrete design it is assumed that, due to the low tensile capacity of concrete, it is unable to resist significant tensile forces and steel reinforcing is required to resist the loads causing tensile stresses. Concrete would thus, for all design purposes, be considered cracked, affecting both the flexural stiffness and sectional prop...

Internationally, the stability of tailings dams is under scrutiny due to the relatively high failure rates and the disastrous consequences of these events. Tailings dams have traditionally been designed assuming drained analyses, but it has become apparent that the possibility of undrained failure needs to be considered. A possible mode of failure...

Reinforced-concrete bored piles were installed in an expansive clay profile near Vredefort, South Africa. Each pile was instrumented with 6 vibrating wire strain gauges (VWSGs) and 20 fibre Bragg gratings (FBGs). Static and cyclic lateral loads were applied to the pile heads to simulate the cyclic nature of loading on wind turbine foundations. This...

CALL FOR PAPERS: We are pleased to announce that the 9th International Conference on Modeling, Simulation and Applied Optimization will be held in Marrakesh, Morocco on April 26th-28th 2023 (www.icmsao.org/). ICMSAO provides a venue for engineers, mathematicians, and scientists from all over the world to share their latest research results in the f...

Recent global tailings dam failures have placed a renewed interest in the design and safety evaluation of tailings dams. With the introduction of the Global Industry Standard on Tailings Management it is becoming more common to evaluate the stability of tailings dams considering undrained shear strengths. In practice, this is typically done by perf...

Piled foundations are widely used to limit the movement of foundations in expansive clays. These piles are subjected to large uplift forces as the clay swells (and downdrag forces when it shrinks). The appropriate method to estimate these forces is not well understood, and estimates from various methods presented in literature result in large varia...

The Cone Penetration Test (CPT) is a useful tool for soil profiling due to its near continuous data measure-ments, low cost and repeatability. When conducting designs or safety evaluations of slopes, it is often of in-terest to understand the undrained response of the soil at large strains. This behaviour can be defined in terms of the Undrained Br...

Synopsis Several high-impact tailings dam failures around the world in recent years have placed a renewed focus on the stability of tailings dams and pointed to potential shortcomings in traditional drained design and safety evaluation procedures. A need to consider undrained shear strength in the design of tailings facilities has become apparent....

Soil liquefaction is a major concern for structures constructed with or on saturated, contractive sandy soils as it can result in sudden loss of strength and subsequent flow failure. The first step in designing against liquefaction is to assess the liquefaction potential of the soil. There are several methods available, however, many of these metho...

Expansive clays are problem soils that undergo significant volume changes with seasonal wetting and drying. These volume changes cause damage to civil engineering infrastructure and pose design challenges for deep foundations. A site underlain by highly expansive clay near Vredefort, South Africa was monitored over a period of 12 months. The soil m...

The development of wind farms across Africa is an attractive solution to meet the increasing electricity demand of a growing and widely dispersed population, whilst ensuring that such electricity provision meets global standards in carbon emissions. However, since vast portions of the continent are underlain by expansive soils, foundation design fo...

The mechanical properties of a high plasticity expansive clay from South Africa are reported. The experimental programme considered wetting after loading tests followed by one-dimensional consolidation and unloading of undisturbed and statically compacted samples. The results of this study illustrate that laboratory preparation of expansive clays d...

Tailings dams are large, often self-contained, storage facilities of mine residue. On self-contained tailings dams the tailings material itself is used to raise the containment embankments holding newly deposited residue. To develop the necessary strength, it is essential that material must dry out sufficiently. Despite substantial advancements in...

The tensile strength of soil is often neglected. Because of the presence of matric suction, unsaturated soils may have substantial tensile strengths with implications for a range of geotechnical problems, such as slope stability, bearing capacity, and the integrity of clay liners. Direct measurement of the tensile strength of soils is complex to ca...

The design of pile foundations to accommodate horizontal loads is a complex soil−structure interaction problem. Current small-scale laboratory testing of pile foundations is mostly focused on metal piles, which raises the question of the validity of results from these tests, and the principles derived, for the use of analysing and designing reinfor...

Geotechnical centrifuges are valuable instruments for physical modelling of complex geotechnical problems in a controlled laboratory setting. In comparison to full-scale testing, scaled models are cost effective to construct and instrument and, when tested in a geotechnical centrifuge at increased centrifugal accelerations, are capable of replicati...

In the August 2020 edition of the SAICE Magazine, we prove the utility of merging IoT together with low-cost, battery powered instrumentation for monitoring some of the largest geotechnical structures under construction in South Africa, namely tailings dams. The sensor readings are successfully transmitted using Sigfox over existing telecommunicati...

The need to tunnel closely beneath piles is increasing due to the development of urban areas. This poses a risk to the stability and serviceability of overlying structures (e.g. buildings, piers, piled embankments). The impact of tunneling on piles is usually assessed using a displacement threshold, yet this provides no information about the post-t...

The observation of an existing structure supporting a particular maximal load provides a direct constraint on the possible range of values its resistance capacity may take. The implied update of structural reliability allows monitoring and maintenance planning to be done from a risk optimal perspective. Existing proof load-based reliability updatin...

Soil-nails and anchors as means of lateral support in surface excavations require stability analyses as part of design. Generally, the acceptance criterion is some arbitrary ‘factor of safety’ which can be routinely computed using well-established principles of limit equilibrium analyses. These methods have been tested over many years and have been...

The article describes a study using discrete fibre optic sensing as a means of leak detection on water distribution pipes installed in unsaturated ground. A short length of pipe fitted with artificial leak points was installed, to which a fibre optic cable with fibre Bragg gratings was attached. An optical fibre with fibre Bragg gratings was also i...

Recognising the importance of the soil water retention curve (SWRC) in establishing unsaturated soil properties, this paper aims to provide a review of various commonly used laboratory measurement techniques. Following a brief overview of various methods, the results of several SWRCs measured at the University of Pretoria using different techniques...

ABSTRACT: Field-testing of piles in an expansive clay is required for the validation of the design criteria for wind turbine foundations in such soil profiles. The test programme includes lateral load tests to investigate the horizontal stiffness of the soil and vertical plug pull-out tests to examine the axial capacity of pile-soil interface. Thes...

Sinkholes are a common occurrence in Centurion, South Africa. The mechanisms associated with the propagation of a subsurface cavity to the soil surface during the formation of a sinkhole are poorly understood, resulting in overly conservative and potentially unrealistic methods for assessing the size of sinkholes. A series of deep-seated trapdoor e...

Working stress design (WSD) has long been used in geotechnical practice. However, in the past decade this has, to a significant extent, been replaced by limit state design (LSD) which is the basis for Eurocode 7, and presents a more robust approach in dealing with uncertainties. Factually, however, none of these approaches quantify the degree of un...

An integral bridge is constructed without joints between the superstructure and substructure. This reduces the maintenance costs over the life of the bridge. However, since there are no joints present, the bridge can be viewed as a single structural element. This means that when the deck moves as a result of a temperature variation and the abutment...

This paper explores the development of an expansive soil which can be repeatedly reproduced for use in centrifuge models to investigate soil-structure interaction problems involving expansive soils. The study considered two materials, namely a sand-bentonite mixture and a naturally occurring, highly expansive clay. For the natural material, two app...

Designing foundations of wind turbines in expansive soils present a challenge. Piled foundations are often used to mitigate some of the problems associated with construction on expansive soils. Swelling clays frequently occur in an unsaturated state in many arid and semi-arid areas of the world. Upon wetting, soil surrounding the pile shaft simulta...

The trapdoor problem is a useful model to understand the stress distribution around geo-structures. This paper focuses on evaluating the conditions of maximum arching (minimum loads on trapdoors) developing during the lowering of plane-strain active trapdoors in cohesionless granular materials. A parametric study using finite element analysis has b...

Sinkholes are common geohazards, frequently responsible for sudden catastrophic ground collapse. Development on these areas is affected by historic and ongoing geomorphologic processes that may result in surface movements. Centurion and Pretoria in the province of Gauteng, South Africa are large urban environments underlain to a significant extent...

Large areas of the economically important Gauteng Province of South Africa are underlain by extensively weathered dolomite bedrock in which sinkholes are a common occurrence. Deep trapdoor experiments were conducted in the geotechnical centrifuge to study cavity propagation towards the surface to obtain a better understanding of the sinkhole develo...

This article highlights some recent advances in the development of expertise and equipment facilitating the implementation of unsaturated soil mechanics in geotechnical engineering.

Changing climatic conditions worldwide are causing changes in moisture conditions of railway formations and slopes, thereby either strengthening or weakening them. Current railway formation design methods do not take into account the changing moisture conditions over the predicted life of a railway formation. These changes in soil strength are due...

Reinforced concrete is the most widely used construction material and thus effective condition assessment of reinforced concrete elements forms a significant part of structural health monitoring. An effective structural health monitoring system should be able to give the owner prior warning that structural elements are reaching conditions approachi...

The discrete element method, implemented in a modular GPU based framework that supports polyhedral shaped particles (Blaze-DEM), was used to investigate effects of particle shape on backfill response behind integral bridge abutments during temperature-induced displacement cycles. The rate and magnitude of horizontal stress build-up were found to be...

The Duplancic conceptual model is the industry accepted model of caving and is the framework within which most results from numerical modelling and cave monitoring are interpreted. The Duplancic conceptual model implies that the damage ahead of the cave back decreases continuously with increasing distance from the cave surface. Evidence from a vari...

Validation and calibration of numerical models is vitally important, particularly in the field of cave mining where our ability to monitor the caving rock mass is limited. As part of a project investigating caving mechanics, physical models of caving were tested in a geotechnical centrifuge (Cumming-Potvin et al. 2016b). This paper describes numeri...

The Duplancic conceptual model is the industry accepted model of caving and is the framework within which most results from numerical modelling and cave monitoring are interpreted. The Duplancic conceptual model implies that the damage ahead of the cave back decreases continuously with increasing distance from the cave surface. Evidence from a vari...

In South Africa leaking water distribution networks contribute to the formation of sinkholes in soil underlain by dolomite areas. By detecting the position of leaks in pipe networks as soon and as accurately as possible, maintenance action can be taken and the formation of sinkholes prevented. Local changes in the support conditions of a pipeline,...

ABSTRACT: The paper describes a tensiometer that can be manufactured in most workshops and saturated in geotechnical laboratories at a cost of approximately US$27 (components only). Suctions of up to 500kPa are routinely measured, with scope to increase the measuring range. The instruments are produced by fitting a porous ceramic filter of the desi...

The optimal placement of geogrid reinforcement in clay liners subject to differential settlement was investigated both numerically and with centrifuge modelling. Two unreinforced liners, a liner reinforced at the top-quarter depth, a liner reinforced at the bottom-quarter depth and a double reinforced liner were modelled in the centrifuge. Differen...

SYNOPSIS Namdeb's Southern Coastal mining operation is located in Southern Namibia, just north of the Orange River mouth. Diamonds were fi rst discovered in the area in 1928 and mining of diamondiferous onshore raised beaches has been the mainstay of Namdeb's production for nearly 90 years. The onshore beaches are now largely mined out but mining o...

Differential settlement and local voids below landfill liners can result in the liners distorting and cracking. By using geogrids as reinforcement the effect of the voids, or differential settlement can be mitigated. The geogrid both increases the stiffness of the system - reducing its distortion - and decreases the tensile stress at the crack tips...

Provided that inter-particle flow remains laminar, hydraulic conductivity tests can be carried out in a centrifuge to accelerate flow, allowing the hydraulic conductivity of relatively impervious materials to be measured within a reasonable time. It is well documented that the inter-particle flow velocity in the centrifuge increases linearly with a...

With past embankment failures on the heavy haul coal export railway line between Ermelo and Richards Bay in South Africa, a study was conducted focusing on the modelling of embankments in a geotechnical centrifuge. This was done in order to investigate the influence of static and cyclic loading on settlement, and to evaluate the potential failure o...

The objective of this study was to determine whether the contact stress distribution underneath a strip footing on dense cohesionless sand can be predicted as a function of the properties of the footing and that of the underlying sand. Together, the footing and the underlying sand form a foundation system which can be classified in terms of relativ...

Sinkholes can occur on land underlain by dolomite and cause substantial damage to buildings and even loss of life. More than four million people work or reside on dolomite land in South Africa and it is therefore important to be able to construct safely on dolomite land and to minimise the risk of damage to infrastructure and loss of life. Compacte...

The behaviour of geogrid-reinforced clay liners, subject to differential settle-ment, was investigated using finite element (FE) analyses. The optimal geogrid reinforce-ment strategy for a liner was determined by generating a Pareto front of maximum liner ten-sile strain versus total cost of reinforcement. To conduct the number of FE analyses requi...

The Duplancic model of caving is widely accepted in industry and is the framework within which most monitoring and numerical modelling results in caving mines are interpreted. As a result, the damage profile ahead of the cave back is often interpreted as continuously decreasing damage with increasing distance ahead of the cave back. Physical modell...

A centrifuge model study was carried out investigating the effects of the inclusion of geogrid reinforcement in a sand layer constructed over a thin, very soft clay layer to facilitate the construction of warehouse floors supporting product stockpiles. The use of geogrid reinforcement allowed larger surcharge loads to be applied for a given amount...

This paper describes a physical modelling programme which recreated block cave propagation in a geotechnical centrifuge. A brief description of the design of the test, along with the sample material and monitoring equipment is given. The samples tested showed caving behaviour and fragmentation distributions that one would expect from caving mines,...

A foundation system consists of not only a footing which distributes an applied load over an area of soil large enough to form a stable system, but also the supporting soil. Foundation systems are complex interactive sys-tems due to the variable material properties of the different constituents of the system. Neither the size nor the stiffness of t...

Finite element analyses of a clay liner subject to differential settlement, reinforced with geogrids at four positions, were conducted. The analysis allowed the definition of a function to predict the maximum tensile strain in the liner for any combination of stiffness of the four geogrids. Consequently, the function was used to determine the optim...

This paper discusses the modification of the Brazilian disk test so that it can be applied to measure the tensile strength of unsaturated soils. The study involved testing the tensile strength of unsaturated gold tailings pre-pared across a range of moisture contents. In order to monitor the initiation and propagation of cracking with-in each sampl...