Guidelines for sinkhole and subsidence rehabilitation based on generic geological models of a dolomite environment on the East Rand, South Africa

A sound understanding of the various factors influencing and associated with the formation of sinkholes or subsidences on dolomite land is essential for the selection of appropriate rehabilitation methods. The investigation and rehabilitation of numerous sinkholes and subsidences located on dolomite in the East Rand of South Africa, created an opportunity to develop a broad based understanding of different karst environments, their susceptibility to sinkhole and subsidence formation and best practice rehabilitation methods. This paper is based on the guidelines developed whereby the geological model of the sinkhole or subsidence is used to recommend an appropriate rehabilitation method. Nine typical geological models with recommended rehabilitation methods are presented in this paper.http://www.elsevier.com/locate/jafrearsci2017-05-31hb2016Geolog

On the cubic law and variably saturated flow through discrete open rough-walled discontinuities

Fracture flow is fairly well documented with the widespread application of, for instance, the cubic law and assumed smooth parallel plate model. Geometrical intricacies such as aperture, roughness and infill do however significantly influence the validity of the cubic law with even its application to smooth parallel systems being contestable. Rock mechanical discontinuity surveys provide valuable information regarding the discontinuity geometry that can likely contribute to the evaluation of flow through individual fractures with variable properties. The hydraulic aperture is available for the transmission of flow, while normal and shear stresses alter discontinuity properties over time. In this, numerous advances have been made to better accommodate deviations of natural discontinuity geometry to that of smooth parallel plates and at partial saturation. The paper addresses these advances and details conditions under which the cubic law, even in local form, fails to adequately estimate the hydraulic properties. The role of roughness in open discontinuities is addressed in particular, as contact areas and high amplitude roughness cause most extensive deviation from the cubic law. Aperture of open fractures still governs hydraulic properties, but inertial forces control flow in very rough fractures, in which instances the applicability of the cubic law should be revisited. Open questions are finally posed,assessment of which will contribute significantly to the understanding of flow through individual discontinuities as well as fracture networks.The Water Research Commission of South Africa (www.wrc.org.za) for funding of project K5/2052 [68] on Multidisciplinary Vadose Zone Hydrology, as well as project K5/2326 on Quantification of Unsaturated Flow in the Fractured Intermediate Vadose Zone by means of Geotechnical Centrifuge (to be published by the WRC in 2016).http://www.elsevier.com/locate/ijrmms2017-11-30hb2016Geolog

Review of engineering, hydrogeological and vadose zone hydrological aspects of the Lanseria Gneiss, Goudplaats-Hout River Gneiss and Nelspruit Suite Granite (South Africa)

Please read abstract in the article.Water Research Commission of South Africa (WRC) project K5/2052 on Multi-disciplinary Vadose Zone Hydrologyhttp://www.elsevier.com/locate/jafrearscihb201

Hazard of sinkhole formation in the Centurion CBD using the simplified method of Scenario Supposition

A large part of the land south of Pretoria is underlain by dolomite from the Chuniespoort Group of the Transvaal Supergroup. In South Africa, dolomite rock has a notorious reputation for the formation of sinkholes and subsidences. Thousands of people reside and work in the Centurion area, where numerous sinkholes have occurred, causing damage and in some instances loss of property and even lives. Centurion has rapidly densified over the last 40 years, with an increase not only in the number of people, but also in the density of waterborne services. This paper proposes draft guidelines for the allocation of an Inherent Hazard Class for percussion boreholes, referred to as the ‘Simplified Method of Scenario Supposition’. This method was then used to classify the Centurion CBD and surrounding [email protected]://www.journals.co.za/ej/ejour_civileng.htmlam201

The influence of the geological origin on soil volume change through collapse settlement

Partially saturated soils are often dense with a high bearing capacity and will subsequently only suffer small amounts of compression under normal foundation loads. However, when wetted under load many such soils undergo a marked and sudden increase in settlement, the phenomenon that is known as collapse settlement. It was originally assumed that the collapse phenomenon is largely restricted to loose aeolian deposits, with the result that most of the research and work dealt almost exclusively with such deposits. However, the collapse phenomenon has since been identified in a number of different transported soils as well as residual soils, most notably the residual granitic soils associated with the Basement Complex of South Africa. Recent studies suggest that the geological origin of the material plays an important role in the collapse mechanism of the particular material. This paper highlights the differences between the collapse process in transported and residual soils by focusing on the geological origin of the material.http://www.elsevier.com/locate/jafrearscihb201

Mechanical and hydraulic properties of residual dolomite and wad

The properties of residual dolomite, sometimes termed wad, are variable and uncertain. It represents the insoluble residue after dissolution of dolomite and is commonly found in the Neoarchaean Malmani Subgroup of the Chuniespoort Group (South Africa). This study comprised triaxial tests, crumb tests, X-Ray diffraction and fluorescence spectroscopy, foundation indicators, stereo-microscope imagery and permeability testing of the represented formations outcropping in the northeastern portions of South Africa. Results concur that residual dolomite is not typically dispersive, has low density that can be below that of water, mostly grades in the silt fraction, has high plasticity indices with low to high liquid limits, and has hydraulic conductivities in the order of 1x10-6 m/s. This new knowledge database contributes to our understanding of the flow through these systems and to how ingress scenario subsidences and sinkholes can possibly occur.The South African Water Research Commissionhttp://www.gssa.org.zapm2020Geolog

Revised test protocols for the identification of dispersive soils

Dispersive soils are prevalent in many areas of South Africa, and the presence of these soils has always posed a problem on road construction sites. The use of dispersive soils in roadway embankments and structures can lead to serious engineering problems if the soils are not accurately identified before use and appropriate mitigation measures taken. The current identification methods include the pinhole, double hydrometer, crumb and chemical tests, which are commonly used in combination to obtain the most reliable outcome. These laboratory tests, however, have not always been entirely consistent, whether used in combination or individually, and it is possible that the reason lies in the actual testing procedures. This paper discusses the observations made during a detailed investigation into the current methods used for testing and identification of dispersive soils. The test methods were thoroughly analysed and shortcomings identified. The differences resulting from different test techniques are examined and solutions to overcome the problems proposed. The paper concentrates mainly on the modification of the physical tests. The recommended solutions and process of identification are also proposed.http://www.journals.co.za/ej/ejour_civileng.htmlam201

The identification and treatment of poor durability Karoo dolerite base course aggregate – evidence from case studies

The Karoo Supergroup covers approximately 57% of South Africa’s surface area and the sedimentary rocks therein generally do not yield acceptable pavement aggregates. The Karoo Dolerite Suite (intrusions) present in these sedimentary units have successfully been used as pavement aggregate sources, but numerous cases of premature pavement failure due to alleged rapid degradation of the dolerite have been reported. Durability tests are included in basic or mafic igneous rock aggregate specifications, but rapid pavement failures continue to occur. A study was recently undertaken to identify cases where degradation of Karoo dolerite was the cause of pavement failure. A secondary objective of the study was to determine if any observed degradation could have been identified using currently specified or alternative testing methods. Three such case study sites are presented in this article and the properties of their materials compared to those from five non-problematic dolerite materials. It is shown that the poor performance of the case study materials was likely due to the poor durability of the materials, manifesting as a reduction in resistance to abrasion and attrition. The identification of the observed poor durability could not have been performed accurately using only the currently specified test specifications. Alternative tests that allow an accurate differentiation to be made were, however, identified and, based on the results, tentative limits set. Additionally it was shown that modification of problematic Karoo dolerite base course materials, by applying lime at a rate less than the initial consumption of lime, can be successful in preventing further rapid pavement failures.am2016Geolog

Lugeon tests at partial saturation : experimental and empirical contributions

Implications of improved understanding of variably saturated flow are numerous, especially given the complexity, heterogeneity, and anisotropy of the intermediate fractured vadose zone. One such an implication is the quantification of water movement for engineering purposes, as flow-through unsaturated discontinuities cannot be quantified through commonly applied saturated approaches. This paper presents an experimental study using geotechnical centrifuge modelling to investigate flow behaviour during Lugeon tests, through the fundamental concept of a smooth, clean, open fracture, at partial saturation. The study comprised flow visualisation experiments conducted on transparent replicas of horizontal and inclined fractures, with water injected under a consecutive series of ascending and descending pressures. Findings from the research proved that preferential flow occupied the minority of cross-sectional area despite the hydraulic pressure. Furthermore, the observed behaviour of these preferential pathways indicated non-linear flow. Deviation from linearity occurred at small fluxes and is likely as a result of inertial effects due to fluid bending at the inlet source into the fracture. To assess these non-linear results, the Forchheimer relationship was used to predict the flow rate at the imposed hydraulic heads. As the width of the fracture could not be used as input into the equation, due to the lack of saturation across its width, the width of the flow path was used instead. This resulted in the predicted results comparing well with the measured flow rates, and indicates that the Forchheimer relationship can potentially be used to describe unsaturated flow in discrete, open fractures.The South African Water Resource Commission (http://www.wrc.org.za) for their financial support by projects K5/2326, together with the National Research Foundation (NRF).http://link.springer.com/journal/6032020-02-01hj2018Geolog

A qualitative model study on the effect of geosynthetic foundation reinforcement in sand overlying very soft clay

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 of settlement by allowing load to be spread over the footprint area of the geogrid due to the tension membrane effect and its associated benefits. A lower-bound calculation of the load at which the tensile reinforcement starts to function is presented. Upper-bound solutions were found to overestimate allowable surcharge loads, as they are associated with unacceptable deformations. The reinforcing effect of the geogrid was found to have been fully mobilised at 5% vertical compression of the clay layer.The National Research Foundation (NRF)http://www.journals.co.za/ej/ejour_civileng.htmlam2016Civil Engineerin