Ebook: Proceedings of the 15th African Regional Conference on Soil Mechanics and Geotechnical Engineering

The Mozambican Geotechnical Society (SMG) organized with great pleasure the 15^th African Regional Conference on Soil Mechanics and Geotechnical Engineering in Maputo. The valuable contribution of the South African Geotechnical Chapter particularly in the review of the abstracts and papers is gratefully acknowledged.

The general theme of the conference was Resource and Infrastructure Geotechnics in Africa: Putting Theory into Practice. More than half of the papers submitted by authors are related to the construction of geotechnical works in Africa. Roads, airports, bridges, dams, railways, among other significant works were the subject of these papers. This signals a remarkable growth in the number of infrastructure projects that have been carried out or are under construction in Africa.

The increasingly specialized nature of the construction works and some very difficult local conditions demand a deeper knowledge of soil mechanics and geotechnical engineering and the involvement of large numbers of geotechnical engineers, as well as specialists of related areas such as geology, rock mechanics, subsurface investigation and field and laboratory testing. The drastic increase in the number of projects in the mining industry will also create additional opportunities and challenges for geotechnical engineers. The proper training of these individuals must be a priority in Africa. We hope that this conference has made a significant contribution towards this goal.

The 94 papers submitted to this Conference are presented in 8 sections namely Roads (17), Foundations (14), Lateral Support and Retaining Walls (11), Materials Testing (16), Site Investigation (20), Environmental Engineering (5), Slopes (3), Dams (2) and General (6). Three Keynote Lectures presented at the Conference on relevant issues for the African continent are included in this volume.

The Editors wish to thank the authors for their valuable work in the preparation of the papers and the members of the Organizing Committee and of the Scientific Committee for the assistance and engagement that made this publication possible.

The Editors

In this paper, the author looks back at the developments in everyday investigations, testing and analysis that have taken place in geotechnical engineering during his 40 year career in the industry to date. Demonstrating how the use of public information freely available on the internet can allow geotechnical practitioners to reduce early project risk, the author goes on to discuss and explore modern equipment and techniques that allow important information to be more-readily and less-intrusively recovered and processed; providing substantially better strength information and predictions of behaviour under load. The use of the computer to reduce human error and involvement in testing is discussed, alongside the obvious benefits now routinely possible through broader and more sophisticated and representative analysis techniques. Looking forward on the basis of past and recent technological progress, the author attempts to explore and predict the developments in geotechnical engineering that we might be likely to see over the coming 4 decades.

The principle of ideal grading, low plasticity and higher compactive effort producing higher density and higher bearing strength of the compacted material for satisfactory pavement performance has characterized pavement materials acceptance specification requirements and practices related to the temperate zone countries. Investigations of cases of premature distress and deteriorations of pavements in some tropical environments have revealed that in addition to selecting well-graded gravels and aggregates to produce high compaction densities and bearing strengths for design, serious attention should also be given to the influence of the nature, geo-chemical, chemical and mineralogical compositions of the materials, testing and geomechanical rating procedures, construction techniques, as well as pavement maintenance history and environmental conditions. For tropically weathered materials formed in diverse climatic and drainage conditions, there is the need for materials oriented approach that integrates relevant aspects of such fields as engineering geology, geomorphology, geochemistry, petrography, pedology, climatology, rock and soil mechanics, innovative roadway design and construction methods as well as cost-effective roadway management and maintenance strategies, etc. A key component of this approach would be the construction and instrumentation of road test sections in relevant climatic, geologic, soils and drainage conditions for long-term serviceability and structural integrity assessment and evaluation. The objective of this lecture is to highlight the key factors, characteristics and parameters useful for developing materials oriented paving materials acceptance specifications for lateritic and saprolitic soils.

After reviewing seismic performance of earth structures based on case histories in Japan and relevant model test results, advantages of using geosynthetics in improving their seismic performance are demonstrated. As one of successful applications, geosynthetics reinforced soil retaining walls are highlighted, focusing on several influential factors such as facing rigidity, arrangement and properties of reinforcements, and backfill and subsoil conditions. In addition, further applications of the reinforcement method using geosynthetics are introduced, which include combination with other reinforcement methods, application to bridge abutments and piers, and application to ballasted railway tracks.

Ghezala Earth Dam was built in the North West area of Tunisia, located west of Mateur City. First full filling of reservoir, of storage capacity close to hundred thousand of cubic meters, started in 1985 and lasted two years. The foundation of dam is essentially composed of a consistent clayey marl layer. At lower level of the dike's body a pipe, made up of reinforced concrete, has been designed for water evacuation from upstream side to downstream side. The pipe structure is composed of eleven sections of 15 m length each. During dam exploration, over twenty six years, a monitoring system permitted, in particular, the measurement of consolidation settlement which occurred in the soil foundation. This follow up made possible the observation of cracks which mainly occurred in central portion of the pipe of evacuation as result of differential settlements. In this context, a 2D numerical simulation of the dam foundation was performed by using the Mohr Coulomb behavior for the dam's material and the soil foundation. Numerical predictions of the evolution of consolidation settlement were compared to recorded ones. A good agreement between predictions and in situ records confirmed the effectiveness of adopted modeling for the studied case history.

Within the scope of the “Pretarouca Dam Construction” Project the injection of joints' compartments between the blocks that comprise the dam body was carried out, to ensure its water-tightness and to achieve a monolithic behavior of the entire structure. The works carried out comprised three different stages: washing and saturation of the joint compartments with water, filling up secondary circuits with bentonite (for preservation against deficient operation of non-return valves) and injection of the joints with cement grout, with special emphasis on the use of bentonite.

Engineered capping systems for landfills are an important tool in the containment strategy for waste disposal. The principal goal of an engineered landfill capping system is to prevent or control infiltration of precipitation, thereby controlling or preventing leachate production and to control the development and emission of landfill gas, thus preventing its emission into the atmosphere. The landfill cap is a composite system of soil layers and geosynthetic components. Steeply sloped sections of capping sealing systems are permanently subjected to shear stresses which require a detailed design of the stability of the sealing system against sliding keeping in mind individual construction stages, operating conditions as well as the long aftercare period or reuse. According to experience a so called “veneer reinforcement” is required in cases where the slope of a landfill cap exceeds an inclination of approx. 20°. The geogrid absorbs the “deficit resistance”, which cannot be provided by the sealing system itself due to insufficient interface friction. In this paper the characteristics of the veneer reinforcement design as well as construction details will be addressed based on two landfill case studies from South Africa and Germany. In addition to this results of a monitored landfill veneer reinforcement project will be presented, which allows the comparison of measured deformations in the reinforcement to those predicted by current design standards.

Soil cover systems are widely used for the containment of waste with the objective of limiting the ingress of precipitation and oxygen, and also providing a medium for vegetation growth. The positive aspect of soil covers is that they are a natural resource that supports vegetation used in land reclamation. In designing effective soil covers for the African region, it is important that the wide range of varying climatic conditions be taken into account in developing designs that are tailored to fit the local conditions. This paper argues the point that there are benefits for cover design in Africa to be based on a formalized design methodology, rather than based on generic designs

Geosynthetic clay liners (GCLs) are commonly used in geocomposite waste barrier systems as replacement for compacted clay liners. In recent years a number of problems associated with GCLs have been identified including panel shrinkage, desiccation cracking, chemical incompatibility, cation exchange and lack of hydration in geomembrane/GCL composite liner systems. The most important factor affecting the performance of a GCL is cation exchange in the bentonite. Leachate compatibility testing of the GCL is therefore necessary, initially by swell index testing, followed by hydraulic conductivity testing if the swell index test indicates promising results. The results of leachate GCL compatibility testing performed for four waste disposal facilities are presented and their influence on the respective barrier designs. It is concluded that there is a future for the use of GCLs in waste barrier systems provided that the compatibility of the GCL is confirmed through testing

A hazardous waste landfill can be regarded a complex system, with many components that contribute to the overall environmental performance of the facility. These include access control, waste containment, drainage systems, operations, gas management systems, and so forth. Of these components, the landfill liner is the first line of defense against long-term environmental contamination. While the requirements for and behavior of compacted clay liners (CCLs) are generally known, the geosynthetic components of hazardous landfill liners also require understanding and informed design. This paper considers issues currently considered in hazardous waste landfill liner design in South Africa, including sub-soil drainage requirements, desiccation, chemical compatibility issues, slope stability issues, and leachate collection requirements. It is noted that this paper cannot be considered to be exhaustive, given its brevity.

In the history of construction materials used by humankind, Geosynthetic Clay Liners (GCLs) are relatively new, having been in use internationally since the late 1980s (Heerten 2002)^[1]. However in the author's (and many others') experience the way they work, and their advantages and limitations are not well understood by designers who have not studied them in depth, or who do not work with them often (Heerten & Koerner 2009)^[1]. Their international success over the last 30 years however, has recently led to a modern South African manufacturing plant, and this paper adopts a practical approach and examines the construction, applications and performance of these useful water and gas tight lining materials. The paper highlights the equivalence of GCLs to compacted clay linings (CCLs) and the substantial cost savings that their use can show over ‘conventional’ materials such as CCLs in structures including landfill liners and containment structures of many kinds. Their constraints are also noted. In addition, an independent generic specification (GIGSA 1200W)^[2] for GCLs is presented as a model non-commercial example of how to specify and construct linings incorporating these products.

This paper explores the possibilities of using skirted foundation system to mitigate the liquefaction hazards, control the horizontal soil movement and decrease the pore water pressure underneath the foundation during the earthquake. This technique is investigated numerically using finite element analysis. Four stories reinforced concrete building resting on raft foundation is idealized as two-dimensional model with and without skirts. In the present study, two dimensional plane strain program PLAXIS, dynamic version is used. A series of models for the problem under investigation was run under different skirts depth below the foundation level. The results showed improved effectiveness in liquefaction mitigation due to confinement effect and decreasing horizontal soil movement. The presence of such skirts can modify and decrease the induced excess pore water pressure. In addition, the skirts can significantly reduce the foundation acceleration and the ground motion amplification is reversed.

This study aims at evaluating the performance of five methods based on CPT and static analysis in predicting the axial capacity of bored piles installed in some Sudanese soils. These methods were used for estimating pile capacity at six study sites located in Khartoum and White Nile States. Static pile load tests and CPT soundings were performed at adjacent points in each site to enable comparison of measured and estimated data. Pile capacities were determined using the Van der Veen method of load test results interpretation. Statistical analysis was conducted to evaluate the performance of the five methods according to established ranking criteria. Based on the evaluation criteria adopted, Touma and Reese method showed the best performance by achieving the closest agreement between estimated and measured pile capacities whereas the Dutch method showed the lowest overall performance. From comparison of predicted and measured values of the pile end bearing (Q_p) and skin friction (Q_s) capacity components obtained for an instrumented pile, the Touma and Reese method gave the best comparisons between measured Q_p and Q_s whereas the Schmertman method showed a quite reasonable estimate of Q_s. The other three methods revealed unrealistic and gross over-prediction or under-prediction of the two components of bored pile capacity.

A tank farm to contain four oil storage tanks was proposed for an area lying close to the Abonnema Wharf in Port Harcourt, Nigeria. The intended tanks would be 25.0m diameter and 13.5m high. For a tank diameter of 25.0m, two boreholes to depth of 30.0m and three cone penetration tests with pore pressure measurements were considered adequate for the subsurface investigation. The subsurface investigation in the tank site disclosed that the upper portion of the soil deposits in the area is composed essentially of Coastal Plain Sands sediments with few intrusions of Mangrove Swamp soils. Beneath the upper clayey deposits is a prevalent deposit of sands. The Coastal Plain Sands formation is composed of sandy, clayey sediments which have been laterized through the leaching of silica and the concentration of alumina and iron. The laterization is generally enhanced by tropical weathering and where the sediments are sufficiently dry, the weathering process produces iron oxide as cementing agent in the soil profile causing the soil to be brown or reddish brown. The project area is characterized by a thick layer of this soil. From the analyses of the results of the laboratory tests, high total and differential settlement of the tanks would be expected if the existing ground was not treated. Ground improvement techniques such as replacement of the weak soils with competent soils, preloading of the tank area with a surcharge and controlled loading of the tank foundation during water testing were considered. Evaluation of the feasibility of adopting pad or concrete ring or concrete raft foundation for the tanks was also carried out. In this paper, is presented the ground improvement technique adopted, the foundation type selected, the bearing capacity and settlement analyses and the results of the water testing of the tanks.

The rotational displacement of offshore shallow foundations on clay due to moment loading has been studied in the Niger Delta Environment. Wave characteristics were deduced from available meteorological and oceanographic data while moments were evaluated from horizontal forces which impact on circular piles of 1.0-2.0 m diameter. The rotational displacement on an equivalent square foundation breadth B ranging from 9.9 m to 17.73 m, typical of circular foundation diameters of 10-20 m, was subsequently evaluated. Undrained shear strength s_u, of the sub-seabed was obtained from both field and laboratory tests. It is observed that rotational displacement θ_m1, reduces with increase in foundation breadth B, and Poisson ratio for a given applied moment M. It also reduces as M/B ratio reduces with increasing μ. A dimensionless plot of the ratio of moments to undrained shear strength, foundation breadth and rotational displacement gave values of 18.66 and 37.33 at μ = 0 and 0.5 respectively.

The lateral response of suction caissons used as anchors for floating structures in the offshore Niger Delta has been investigated using the “Lumped Parameter Systems” model. In this process, the dynamic stability (horizontal vibration) of suction caissons used to anchor floating production facilities located deep offshore of the Niger Delta has been examined. Geotechnical conditions prevalent at Niger Delta Deep offshore have been used to determine dynamic soil parameters needed for analyses. Also, dynamic wave properties of the offshore environment which correspond to 100 years return period have served as inputs into the analyses. Results of analyses show that for a given wave condition, an increase in the mass of caisson whose height to diameter ratio is 2:1 causes a decrease in the horizontal amplitudes of vibration of the caissons. Results also reveal that continuous increase in the mass of caisson beyond certain limits does not significantly reduce vibrating amplitude. This is important because it provides information on the limiting mass and hence the size of caisson required in any particular situation. A most striking observation made is the fact that for a given wave steepness, several smaller units of suction caissons can be used rather than a single massive unit. Cases considered show reduction in amplitude by 67%, 41%, 32% and 23% respectively for increase in the number of caissons to 2, 4, 6, and 10.

LPG (liquefied petroleum gas or cooking gas) domestic consumption, despite the abundance of natural gas in Nigeria is still very low. Nigeria is the second most endowed nation in Africa with proven reserve of over 260 trillion cubic feet of natural gas either associated with petroleum or occurring on its own gas field, even as up to 2 million cubic feet is flared off daily as crude oil is drilled making Nigeria the largest gas flaring country in the world. The low gas consumption is a result of lack of adequate downstream LPG handling infrastructures to take bulk LPG gas from mother vessels to onshore locations where gas can be easily transported for domestic use.

In 2009, TREVI Foundations Nigeria was commissioned to design and construct all waterfront facilities for a major LPG terminal in coastal Lagos. Subsoil investigation data proved the site is underlain with recent organic clays and young sediments in shallow water ranging between 1.2m to 6.0m. The requirement was to design berthing and mooring facility capable of handling up to 60,000 metric ton LPG gas vessel with water intake channel, fire fighting facility and (water reservoir) pit. In all, the following were installed at this site: 28No. 1.5m bored piles seated at between 35m and 40m below the river bed to support the berthing and mooring dolphins, 10No. 600mm diameter open bottom steel cased piles to support the intake pit, 247No. mono fluid jet grouted (JG) columns acting as bottom plugs to between 11m and 13m below the river bed to seal the bottom of the pit and make excavation works easy in marine environment and 184 elements of precast 600mm wide by 300mm thick by 10-13.0m long reinforced concrete plates with joints grouted installed by vibro jetting to form the intake channel wall. TREVI proprietary software and PLAXIS 8.6 code were utilized for the design and simulation of the various foundation and structural elements. The project was completed in a record time of 5½ months, handed over to the client and is already in use.

The variation of hydrodynamic forces and moments on different circular pile sections of 1.0 to 2.0 m diameters in the offshore Niger Delta Environment has been investigated. Available meteorological and oceanographic data were used to obtain wave characteristics. Inertia coefficient, cm and drag coefficient, cd were obtained from Reynolds number, Re, while maximum inertia force, kim, maximum drag force, kDm, maximum inertia moment, Sim and maximum drag moment, SDm were evaluated from standard charts. The revealed variation of hydrodynamic forces and moments with pile diameter showed that the horizontal forces and moments increase as pile diameter and wave height increase. Drag dominated forces and moments were obtained for wave heights greater than 3m.

La communication présente les résultats d'analyse d'une base de données locale relative au nord Algérien, constituée de 39 projets de fondations sur pieux dans lesquels 54 essais de chargement vertical statique ont été menés. Le premier volet comporte une interprétation des essais de chargement des pieux, à la base des courbes normalisées de chargement. Il a été constaté que la charge critique correspond à un tassement assez proche de celui du tassement de référence, défini comme étant le rapport de la charge limite et la pente initiale de la courbe de chargement. En deuxième partie, une étude comparative des méthodes courantes de dimensionnement à la base des essais in-situ a été menée, en l'occurrence les méthodes pressiométrique et pénétrostatique.

The Gautrain Rapid Rail Link (GRRL) is one of ten Spatial Development Initiative (SDI) projects implemented by the Gauteng Department of Public Transport, Roads and Works (Gautrans) to stimulate economic growth, development and employment opportunities in the Gauteng Province. As part of this development initiative, Viaduct 5c (a 3.2 km long viaduct) crosses highly variable and sinkhole prone dolomitic ground conditions in Centurion, Pretoria. The original design for piers 39 and 40 of Viaduct 5c consisted of four large diameter (1.2m) bored piles designed to be end-bearing onto the dolomite bedrock. Due to severe difficulties encountered with the installation of large diameter end-bearing piles and the highly variable rock head and ground conditions encountered, an alternative founding solution at piers 39 and 40 was proposed. The alternative solution consisted of a grid of small diameter piles (micropiles) installed as soil reinforcement elements overlain by a compacted soil mattress. As bearing capacity failure of the footing is unlikely, settlement or differential settlement of the footing was the main concern. Due to the complex and variable nature of dolomitic profiles in general, settlement of the foundation was estimated using a 3D finite element model. The analysis indicated that the micropile/soil mattress system was effective in reducing the settlement of the foundation to an acceptable value. This paper presents the design methodology and analysis that was carried out for the two Piers.

A typical soil profile composed by relatively thick sand layer resting on very thick compressible clay layer is considered. In such geotechnical condition the construction of very high embankments induces unallowable consolidation settlement. Then a novel foundation system is suggested by installing floating rigid inclusions in the upper sand layer so that skin friction and tip components will develop along the inclusion as reaction to the concentrated vertical load applied on capped mini slabs. For the design it will be of interest to determine the optimum length of rigid inclusions and spacing to come up with negligible settlement at surface of the sub clayey layers. The analytical prediction of bearing of bearing capacity of rigid inclusions in sand formation is presented. The consolidation settlement of clayey layers is estimated.