OpenUCT :: Browsing by Author "Kalumba, Denis"

Browsing by Author "Kalumba, Denis"

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Open Access
A Comparative Study on Shear Strength Testing of Single and Multi-layer Interfaces using Large Direct Shear Apparatus
(2021) Muluti, Shade; Kalumba, Denis; Sobhee-Beetul, Laxmee
Geotechnical structures such as composite liner systems in landfills consist of multiple interfaces, which include a broad range of geosynthetics in conjunction with soil, rocks and any other related materials. This results in the introduction of many interface planes into the structure, which can potentially create instability especially along the slope and ultimately result in failure. To date, many laboratories use single interface testing instead of multi-layer interface testing to determine geosynthetic shear design characteristic values that are used in the design of structures such as landfill liners. A topic of discussion remains the preferred interface testing configuration and only a few studies have substantiated and quantified the significance of varying the different interface shear testing configurations. This study, therefore, aimed to evaluate and compare the effects of the use of the two interface test configurations on the shear strength of soil/geosynthetic and geosynthetic/geosynthetic interfaces. Furthermore, it was intended to identify the test configuration that provides the most critical shear strength results, while also understanding the fundamental mechanisms responsible for the shear strength observed. In this study, three geosynthetics were used: geotextile (GTX), geomembrane (GMB) and geosynthetic clay liner (GCL), which generally constitute the critical interface components of a lining system in a modern South African landfill liner. Two soils were utilised as a part of the materials required for the investigation and they were: river sand and red clay. The laboratory tests were conducted under saturated conditions in accordance with the ASTM D5321 and ASTM D6243 standards, using a 305 mm x 305 mm large direct shear box. The tests were carried out over a range of applied normal pressures of 50, 100, 200 and 400 kPa. A constant shear rate of 1.0 mm/min was used in the interface tests that did not involve GCLs or clay specimens and therefore no excess pore pressure was anticipated at the interface. On the other hand, for all other interface tests involving either clay or GCLs samples, a shearing rate of 0.1 mm/min was utilized. The results showed that nonlinear behaviour of the shear stress versus shear displacement responses was exhibited in both the single and multi-layer interface tests, regardless of the normal stress applied. However, it was noted that with an increase in normal stress applied, the deviation in mobilized shear stress between the two test configurations increased, with single interface tests yielding higher shear stress values compared to multi-layer interface tests. In single interface tests, the high shear stresses could be related to the clamping that confined each of the test specimens during shearing to one end of the shear block. On the other hand, only the top and bottom test specimens were clamped in multi-layer interface tests, thus allowing failure to have occurred at the weakest of the available interfaces. Moreover, for single interface tests, peak strengths were generally 9% lower for the range of normal stresses considered, whereas Large Displacement (LD) strengths were generally 24 % lower for the single interface tests, compared to the peak and LD strength values for multi-layer interface tests. This was particularly observed at low normal stresses between 50 and 200 kPa, and it could probably have been caused by the rigid clamping of the geosynthetics which results in some tensile strains in the geosynthetics. In addition, it was observed in multi-layer interface tests that a transfer of shear stresses within the system could have occurred, which could have led to higher overall shear resistance of the composite. As a result, single interface tests yielded a conservative estimate of the peak and LD shear strengths for the tested interfaces compared to multi-layer interface tests. This may be attributed to higher displacement along with the critical interface in single interface tests than in multi-layer interface tests. To allow the investigator to observe the displacement, as well as the possible transfer of shear stresses within the system during the shearing of the various geosynthetics, it was recommended that real-time monitoring of the displacement mobilization should be carried out in multi-layer interface tests during shearing.
Open Access
A laboratory investigation on the shear strength characteristics of soil reinforced with recycled linear low-density polyethylene
(2018) Nolutshungu, Lita; Kalumba, Denis
Since the development of plastics in the 1930’s, plastics have increasingly become widely used for packaging in the commercial market place. With this application being for immediate disposal, the amount of plastic waste generated presents a challenge in the disposal thereof. The risks associated with non-biodegradable products on humans and animal life, pressure on existing landfills and the increasing costs thereof have necessitated the development of alternative options for waste management over the years. Research has resulted in various forms of treatments and recycling processes adopted and implemented as environmentally and economically viable solutions. The use of this recycled material in various applications, such as soil reinforcement addresses the need for engineering solutions with a multifaceted approach which strike a balance between environment, economy and equity. This has been the driving force behind research on the use of alternative materials in engineering design. This study aimed to present an investigation into the use of recycled Linear Low-Density (LLDPE) as reinforcement in Cape Flats sand. To understand the implication of the main aim of the investigation, a review of literature on soil reinforcement theory, various forms of reinforcement material and previous studies was conducted. The selected material for testing was in the form of pellets and flakes produced during the recycling process. Triaxial tests were done on samples where the concentration of the inclusions and compaction effort was varied. The test data presented showed that both pellets and flakes affected the shear strength by plotting Mohr’s circles and the relationship between shear stress and normal stress, which revealed changes in the shear strength parameters. The friction angle was increased by 3.35% at an optimum pellet concentration of 5%. Inclusion of the flakes, however, resulted in a maximum improvement in cohesion of 295% at 0.25% concentration. A discussion on the stress- strain relationship gave an indication on the effect on the stiffness. This showed that the peak shear stress was reached at higher strains when the flakes and pellets were included, compared to the unreinforced sand. Improvements by up to 25% were recorded from the initial 6% strain at peak shear stress of unreinforced sand. In concluding the study, Slide7.0 was used to conduct a 2D finite element analysis using Bishop’s method to analyse the practical application of LLDPE flakes and pellets for slope stability. The optimum shear strength parameters were used in the model, which resulted in an improved global factor of safety meeting the minimum requirement of 1.25.
Open Access
A study on ground improvement using a combination of stone and concrete columns
(2018) Pudaruth, Yogendra; Kalumba, Denis; Sobhee-Beetul, Laxmee
Stone column is a cost-effective ground improvement technique that is typically employed for low-rise buildings and road embankments. This technique mainly uses naturally occurring materials as its load transferring medium. However, stone columns have some constraints because of the loose interactions between their aggregates which can lead to uncontrolled settlements, especially in soft soils. As a result, their performance is usually improved by the inclusion of geosynthetics either in layers or as a confinement. However, there was a lack of studies that used a binder within the stone column aggregates with a view to limit the bulging/lateral spreading of its aggregates in such soils. In this study, the upper portion of the stone columns was replaced by different grades of unreinforced concrete. The length of the concrete, as well as the depth of the soil beneath the columns, were varied. The effects of these different variables, when the resulting column was subjected to an applied load, were investigated. The optimum configuration of the above was identified and its resulting change in performance when it was combined with a reinforced bedding layer was studied. Considering application/installation procedures on site, it was best deemed to install and test a geosynthetic-reinforced bedding layer on top of, rather than within, the stone column. It was observed that increasing the grades of concrete did not have any consistent influence on the performance of the resulting columns when there was a considerable layer of soil beneath them. The hybrid stone columns (combination of stone and concrete) performed better than the normal stone column and even to a full concrete column of the same length in several cases. Physical modelling revealed that the bulging length ranges from 2.0-2.4D (D is the diameter of the column). Test results for the optimum hybrid stone column yielded a maximum load improvement factor of 3 to 6 folds (200% to 500% increase in bearing capacity) depending on their respective configuration compared to the unreinforced soil. The improvement factor was further increased to 9.9-fold (nearly 900% increase in bearing capacity) when the optimum hybrid stone column was tested in combination with a reinforced bedding layer. The findings from this research can be used to enhance and promote the stone column ground improvement technique while still providing an economical advantage as well.
Open Access
An Experimental Study on the Infiltration Potential of Stormwater Ponds in Zeekoe Catchment, Cape Town, South Africa
(2022) Mavundla, Kgomoangwato; Kalumba, Denis; Armitage, Neil; Okedi, John
In early 2018, the city of Cape Town, with a population of approximately 3.8 million, was at risk of running out of water from the six large reservoirs to the east of the city. This was due to the worst drought in almost a century, which occurred between 2015 2017, causing the city to be declared a disaster area. Although alternative water resources had been identified, they had not been developed. This has now become imperative as droughts are expected to recur in the future. This study investigated the prospect of using existing stormwater ponds in the Zeekoe catchment area as infiltration cells transferring detained stormwater into the underlying aquifer storage zone to enhance the available groundwater resource. The Zeekoe catchment is a 89 km2 area within the 630 km2 Cape Flats Aquifer (CFA). Based on hydrogeological data and aquifer parameter interpretation, it is considered to have good storage characteristics that can support groundwater development for water supply. Bouwer (2002) highlights how infiltration tests in the field can be useful for estimating desired volumetric recharge rates within a certain area. The hydraulic conductivity of the unsaturated layer is an essential non-linear function of soil-water content and has been generally recognised as the most important transport property to describe the ability of soil to permit water movement. A series of in-situ infiltration experiments were conducted at three representative stormwater ponds using a Double Ring Infiltrometer (DRI) to determine the rate of water recharge. Infiltration data was interpreted using both the Green-Ampt and Horton methods to determine the hydraulic conductivity and infiltration decay constants. A total of 18 core samples retrieved from the in-situ infiltration test locations were analysed in the laboratory to determine the ed hydraulic conductivity through constant-head permeability (CHP) tests. The physical and hydraulic soil parameters gathered from field and laboratory tests were used as inputs for a finite element numerical modelling software (HYDRUS 2-D) to estimate the range of recharge rates for the study area. Based on field infiltration test results, the hydraulic conductivity was found to be 0.3 19.9 cm/hr; typical for silty sands to fine sands. Hydraulic conductivities estimated in the laboratory were greater than the field hydraulic conductivity by 103%. This could be attributed to entrapped air under field conditions which reduces the effective cross-sectional area available for water to flow. From the HYDRUS 2-D simulations, the period required for the wetting front movement from the pond surfaces to the water table ( 5.5 m below the surface) was 15 140 hours. Hydraulic conductivities estimated using the pedotransfer function (PTF) of the built-in software, Rosetta-Lite, were also greater than the field hydraulic conductivity values by 118%. For an actual test pond, the infiltration rates would be expected to be slower, and recharge times would be greater because the HYDRUS 2-D simulations did not consider layers of low permeability suggestion that the field saturated hydraulic conductivity could be taken as roughly 0.5 times the laboratory hydraulic conductivity was thus considered reasonable. This means that ponds in the central area of the catchment would be suitable for artificial recharge with an estimated infiltration rate of around 20.6 cm/hr which could provide a mean annual groundwater yield of 29 33 Mm3 . A more extensive survey would aid in assessing local conditions that may impede groundwater flow.
Open Access
An Investigation into the Effects of Asperities on Geomembrane/Geotextile Interface Shear Characteristics
(2020) Adeleke,Daniel; Kalumba, Denis; Nolutshungu, Lita; Oriokot, Johnny
Geomembranes are often utilized as fluid barriers in geotechnical applications such as landfills. Due to their relative impermeability and chemical resistance characteristics, they are usually used alongside other geosynthetics like geotextiles in landfills to constitute base, side-slope, and cover liner systems. Uniquely, within the side-slope liner composite system, which consists of multiple geosynthetics interfaces, the geomembrane/geotextile (GMB/GTX) interface is known to have relatively low shear strength. In an effort to mitigate sliding failure occurrence at the GMB/GTX interface, asperities have been incorporated into GMB manufacturing to increase the shear characteristics. Presently, many GMB with various asperities properties is now available because of asperities proven advantage. Challengingly, only a few studies have substantiated and quantified the importance of varying asperities properties (height, density, and shape) on the GMB/GTX interface. Therefore, this study was aimed at investigating the effects of asperities variation on GMB/GTX interface shear characteristics and mechanism, as well as to identify the asperity parameters combination which optimizes the GMB/GTX interface shear strength. The GMB/GTX interface shear tests were conducted according to ASTM D5321, under saturated conditions with the “305 mm by 305 mm” direct shear box at applied normal stresses; 25 kPa to 400 kPa. In this research, the two common geotextile polymers (polypropylene and polyester) in South Africa were used at the GMB/GTX interface. Also, the geomembranes used had their asperity height varied from 0 mm to 2.02 mm, while the asperity density and shape were varied from 0 to 663 spikes per 10000 mm2 , and conical to hook-cone asperity shape, respectively. GMB/GTX interface shear results showed that with a 70 % increase in the geomembrane asperity height at constant asperity density, friction angle increased by 25 %. Also, an average increase of 25 % in the friction angle was observed as asperity density was doubled at constant asperity height. However, the friction angle was not significantly affected by changes in asperity shape from conical to “hook-cone” shape. Therefore, among identified asperities and roughness features, asperity height together with surface roughness affect the GMB/GTX interface shear parameter more dominantly. These outcomes present a better explanation of the “fibre/asperity” interaction at the GMB/GTX interface and identified asperity properties which optimised surface interaction. The optimised interaction produces efficient shear mechanism that would ultimately lead to a stable and durable landfill liner system.
Open Access
An investigation into the use of plastic chips to reduce collapsibility in loess-like soils at Mount Moorosi Village, Lesotho
(2021) Venter, Jason; Kalumba, Denis; Chebet, Faridah
Mount Moorosi Village, located within the Senqu River Valley in Lesotho, has experienced significant damage to its buildings over the last several years, due to severe cracking. The cause of these cracks has been attributed to the local soil, which presents itself as an ideal building and founding material, while it may in fact be to the contrary. A previous investigation by Damane (2019), into the underlying soil profile of Mount Moorosi Village concluded that the village was underlain by a top layer of silty-sandy loess, a windblown soil of at least 3 m in thickness, with the majority of its grains falling within the siltsized particle range. The material demonstrated significant warping potential as well as a high hydrocollapse potential. To attempt to mitigate the hydrocollapse potential of the soil, linear low-density polyethylene (LLDPE) pellets, made from recycled plastic waste, were blended with the soil at concentrations of 3, 6 and 9 % by mass, before being subjected to pycnometer, oedometer and triaxial tests, to determine the specific gravity of the blended soil as well as the settlement and shear strength parameters, respectively. Test results showed that the addition of the LLDPE chips, at 6 % concentration by mass, resulted in the maximum improvement in both the settlement and shear strength parameters. The soil's collapse and hydrocollapse coefficients were reduced by 54 and 40 % respectively, with the overall classification of the soil going from ‘severe' to ‘moderately severe'. The cohesion of the soil was improved by 8 and 400 % for field and saturated moisture contents, respectively. The internal angle of friction of the soil was increased by 97 and 150 % for field and saturated moisture contents, respectively. Collapse and settlement calculations, conducted on Settle3D, showed that the soil's overall settlement was reduced by 34 and 54 % for field moisture and saturated soil conditions, respectively. The total settlement of the strip foundations with their current dimensions, however, was still outside of the acceptable ranges. The total settlement was lowered to within acceptable range of under 50 mm when the width of the strip footings was widened to 750 mm, and their depth increased to 200 mm.
Open Access
An investigation into the volume change characteristics of loess like soil in Mount Moorosi Village in Lesotho
(2019) Damane, Monica; Kalumba, Denis; Sobhee-Beetul, Laxmee
The Mount Moorosi village is situated in the Senqu River Valley of southern Lesotho, within the Stormberg landform. The integrity and aesthetic appearance of nearly all the structures in this area are undermined by recurrent cracks. At present, no apparent institutionalised effort had been conducted to investigate the source of this problem. The crack patterns were associated with the possible volume change of the underlaying loess like soil. This soil has caused a disastrous failure to brittle civil engineering structures in various parts of the world. Its behaviour is attributed to sand and silt particles bonded by minerals, which become active upon saturation and induce hydrocollapse settlement. This study characterised the volume change properties of the underlaid deposits in Mount Moorosi. The research utilised representative samples from trial pits in the study region to perform laboratory experiments such as the Atterberg limits, wet sieving, sedimentation, free swell, x-ray diffraction, scanning electron microscope and slaking. The consolidated undrained tests and hydrocollapse potential were also determined from the GEOCOMP triaxial and Global Digital System oedometer, respectively. Results revealed that Mount Moorosi is generally underlaid by a more than 3 m thickness of low plasticity (9 to 17 %) silty-sandy loess. The material had significant warping (up to 27 mm) in linear shrinkage that illustrated potential inducement of detrimental stresses to the superimposed structures during drying. The identification and quantification of the mineralogy composition clearly evidenced the passive minerals (quartz, feldspar and mica) to be predominant (86 %), while the active phases (kaolinite, carbonates, sulfates, halides, the oxides and hydroxides of aluminum and iron) were subordinate (14 %), which substantiated potential soil settlement upon wetting. Furthermore, the micrographs depicted structures that synergistically enhanced the collapse properties of the tested deposits. These included the porous clays, silts bonded by clay and silts coated with clay, which all rendered a metastable fabric. A comparison of the stressstrain graphical plots from the consolidated undrained tests at the field and saturated moisture contents indicated a drastic reduction (up to 73 %) in deviator stress at saturated water content. This was attributed to the augmentation of the interparticle spaces, caused by a rise of up to 337 kPa in pore water pressure. Shear strength parameters obtained from Mohr’s failure envelopes were also decreased by up to 80 %. The hydrocollapse index measured from the oedometer tests ranged from 10 to 15 % at a vertical stress of 200 kPa. It indicated severe settlement problems for structures constructed on this soil. This was caused by the loss in shear strength of the soil under the saturated conditions and a high slaking mechanism that reached a maximum rating of 4. Generally, the mineralogy composition, morphology, saturated shear strength, slaking and hydrocollapse index collectively indicated the possibility of soil volume decrease. In fact, the check for serviceability limit state demonstrated a settlement that exceeded the tolerable value of 50 mm. The cracks observed on structures were, therefore, related to soil settlement. This study recommends further research on suitable ground techniques to minimise settlement, thereby improving the durability of structures. Moreover, investigations should be conducted to understand the pressure induced by warping during shrinkage.
Open Access
An Investigation of the Effects of Specimen Gripping Systems on Shear Stress at the Geosynthetic/Geosynthetic Interface in Landfill Applications
(2019) Sikwanda, Charles; Kalumba, Denis; Nolutshungu, Lita
The use of geosynthetics has rapidly increased in nearly all geotechnical related fields as they allow for innovations, improved performance and cost effectiveness in projects. However, when geosynthetics are installed on sites, particularly on landfill slopes, their interface interaction against the adjacent materials becomes the critical section where shear failure is likely to occur. For this reason, their shear strength behaviour is determined in the laboratory at anticipated site conditions, mainly using a direct shear device to obtain design parameters. These laboratory tests are preferably conducted in accordance with ASTM-D5321 and ASTM-D6243 standards. The direct shear equipment, however, requires the use of an appropriate gripping system for shear to take place in the desired interface. Otherwise, tensile failure within the tested geosynthetics will be generated, resulting in obtaining design parameters which do not represent the actual field performance of the tested geosynthetics. This could lead to unsafe, cost ineffective, etc. design of projects with the respective geosynthetic materials. To date, many laboratories use a variety of gripping systems in a direct shear device to determine the shear design characteristics of geosynthetics and the preferred system remains a topic of concern. As a consequence, there is a large variability in the test results obtained, thus, difficulties in their interpretations. In this research, the effects of two commonly used gripping systems in a direct shear device, namely the nail plate (NP) and sandpaper (SP), have been investigated using a landfill case liner. This liner consisted of the three different classes of geosynthetics which are popularly installed in a landfill i.e. geotextile, geomembrane and geosynthetic clay liner. The results revealed that there exists a dissimilarity in the mobilized shear strength at geosynthetic interface when the NP is used as compared to the utilization of the SP due to the specimen engagement with the respective gripping systems. The exact difference, however, was not established as it varied depending on the interface tested. This highlighted the need to standardize the geosynthetic gripping systems in a direct shear device as it would capture these variations, increase result reproducibility and ease their interpretations.
Open Access
Analytical and numerical study of dolomite sinkholes in Centurion South Africa
(2014) Avutia, Daniel John; Kalumba, Denis
Sinkholes encompass the withdrawal of shallow sediment into deep hollow compartments located in karstic stratum. These subsequent surface openings associated with karst geology have inhibited multiple infrastructure developments. Sinkholes are triggered by the alteration of the existing groundwater level which erodes weathered altered dolomite (WAD) residuum into karst cavities. Substantial literature has explored the stability of sinkholes, with reliance on limit analysis and empirical data, to quantify the strength of the porous karst residuum. In this study, the appraisal of sinkhole propagation was facilitated with the geological data acquired along the Gautrain route through Centurion, South Africa. Sinkhole development was analysed through analytical theories and the application of numerical methods. The analytical study conceptualized the ‘angle of draw’ of dolomite overburden layers into cavities, with Terzaghi’s arching in soil equation. The analytical results illustrated constant vertical drawdown in the WAD and incremental cavity propagation in the frictional chert residuum.
Open Access
Application of a discrete element model to the analysis of granular soil recovery in an offshore tubular vibrocore
(2015) Wegener, Sam Bryant; Kalumba, Denis; Lai Sang, Johnny; Raubenheimer, Gert
As the human need for ocean resources accelerates, offshore geotechnics continues to grow and become ever more relevant. Seabed soil sampling is crucial in deep-water engineering projects or geological studies where a detailed knowledge of the seabed geology is required. Deep-sea vibrocoring is a relatively new offshore sampling technique. The system consists of a vertical, tubular core barrel with a sharp cutting edge at its lower end vibrated into the seabed by a high-frequency, low-amplitude vibratory motor. In the past, success of a coring operation has been judged primarily by the length of the recovered core. More recently, studies have given focus to the problems associated with achieving soil specimens in which the in-situ sedimentary structure is preserved. In practice, the core recovery ratio - defined as the ratio between the sampled length of core sediment and the length of core barrel penetrated into the soil - is frequently less than unity. Literature suggests that the physical processes governing the dynamic interaction between core barrel and soil are poorly understood. Through review of relevant literature, and the execution of both physical testing and numerical modelling, this study aimed to a) Develop a calibrated 30 discrete element model of a given vibrocore-soil system, and b) Investigate the soil mechanics phenomena influencing the disturbance and recovery of vibrocore soil samples.
Open Access
Comparative Assessment of Single & Double Interface Shear Strength Properties: A Case Study of a Landfill Project in the Western Cape Province, South Africa
(2022) Sylivery, Victor; Kalumba, Denis; Mawer, Byron
Geosynthetics are now stipulated inclusions in municipal solid waste (MSW) landfill's lining systems as they provide a competent hydraulic barrier, limiting environmental pollution and protecting public health. While these inclusions have considerable cost and technical benefits, they have presented several new potential interfaces for shear failure, particularly on side-slopes and basins of conventional hole-in-ground type MSW landfills. In the laboratory, interface shear strength properties of either soil-geosynthetic or geosynthetic-geosynthetic are determined through a single interface testing configuration as per standard testing practices like ASTM D5321 & D6243. However, there is little data on interface shear strength tests conducted through multiple-interface configurations. In addition, conducting single interface shear tests in the laboratory consumes time, effort, resources, and budget, especially when a multi-layered soilgeosynthetic lining system has been proposed. Therefore, this work aimed to ascertain the appropriateness of multiple-interface shear testing in the laboratory by comparing shear strengths mobilised at peak and at large displacements (LD) for single and double testing configurations. Moreover, the interface shear results obtained were applied on a typical side-slope and basal lining system design through the limit equilibrium method (LEM) to understand the implications on factors of safety (FS). In this study, single interface testing configurations constituted only two lining components, either soil-geosynthetic or geosynthetic-geosynthetic. Multiple-interface testing configurations involved direct shearing of three lining components: geosynthetic-geosynthetic-geosynthetic, or soil-geosynthetic-geosynthetic, or soil-geosynthetic-soil, also referred to as double interfaces. The investigated MSW landfill components formed part of the proposed basal lining system for the MSW landfill cell in the Western Cape Province in South Africa. They included three nonwoven geotextiles, two smooth-sided geomembranes, a synthetic cuspate drain, sand, gravelly sand (GS), and leachate collection stone (LCS). All tests were conducted at applied normal pressures of 150, 300, & 450 kPa under saturated conditions with fresh tap water at a 1mm/min shearing rate as recommended by ASTM D5321. The critical interface assessment revealed that failure within the lining components would likely occur at the same interface(s) regardless of the testing configuration. Specifically for this research, both testing configurations, i.e., single and double, involving smooth High-Density Polyethylene (HDPE) geomembrane with nonwoven geotextile combinations depicted the weakest shear resistances. Additionally, this work found that the peak strengths of the critical interface were highest for a double interface testing configuration. Conversely, the LD strengths obtained were either equal or less for a single interface testing configuration. Ultimately, the assessment of both maximum and minimum FS using single and double interface shear results revealed that the former was conservative as it produced lower values of FS. As a result, interface direct shear testing through a double interface configuration enabled an understanding of the dynamics of shear strength transfer among the lining components proposed for the basal lining system of the landfill cell in the Western Cape Province in South Africa.
Open Access
Comparison of shear strength properties of textured polyethylene geomembrane interfaces in landfill liner systems
(2019) Buthelezi, Sanelisiwe Nonhlanhla Precious; Kalumba, Denis
The interface shear strength between geomembranes and geosynthetics is a critical factor governing the stability of slopes that incorporate geosynthetics. In order to better characterise the shear properties of geomembranes, a wide-range of shear strength friction measurements were conducted. This dissertation presents the results of a study that examined interface shear strength parameters of textured high density polyethylene geomembranes (HDPE) and textured low linear density polyethylene (LLDPE) geomembranes sheared against different geosynthetics; geotextiles, geogrid and geosynthetic clay liners (GCLs), typically used in South African landfill base liners and capping systems. Tests were performed using a modified 305 x 305 mm x 100 mm large direct shear box over a range of normal pressures of 25, 50, 100, 150, 200 and 300 kPa. Shear rates of 0.1 mm/min and 1 mm/min were used for geomembrane/ GCL and geomembrane/ geosynthetic interfaces respectively. Results indicated that LLDPE and HDPE geomembranes sheared against various geosynthetic combinations produced different friction characteristics which resulted in varying performance patterns. HDPE geomembrane surfaces mainly experienced conventional linear failure envelopes when sheared with different geosynthetics. However, LLDPE geomembrane interfaces showed that the linear failure envelopes did not always give the best representation of the shear stress and normal stress relationship for sheared interfaces. These geomembrane shear strength envelopes could be described as linear until a critical confining stress in the range of 100 kPa to 150 kPa was attained, therefore making the failure envelopes bilinear. A comparison of the linear and bilinear failure envelopes showed that a bilinear failure envelope was a more appropriate approximation over large normal stress ranges. A bilinear relationship resulted in higher interface friction angles and low apparent adhesion parameters being achieved at normal stresses less than 100 or 150 kPa. While low interface friction angles and large adhesion values were produced above the critical confining pressure. Although HDPE geomembrane interfaces indicated larger stiffness and rigidity at early shear, it was observed that LLDPE geomembrane/ geosynthetic interfaces presented larger factor of safety values when compared to HDPE geomembranes sheared against majority of the geosynthetics. These results were produced when friction parameters generated from this study were applied to practical design examples of landfill base liner and capping systems. From these observations several practical recommendations were generated to assist professionals to choose suitable materials during design.
Open Access
Correlating standard penetration test and dynamic probe super heavy penetration resistance values in sandy soils
(2011) MacRobert, Charles; Kalumba, Denis; Beales, Patrick
This paper presents a statistical method used to develop an empirical equivalence between the Standard Penetration Test (SPT) and the Dynamic Probe Super Heavy (DPSH) in sandy material. Penetration resistance values from the two tests are often taken as equivalent for design purposes, as the same drive energy is used in both. SPT and DPSH resistance values from different geological depositional and weathering environments were examined. The data came from the following areas across southern Africa: Matola in Mozambique, Gope in Botswana, Umdloti and Cape Town in South Africa, and Illha de Luanda in Angola. It was apparent that energy losses were greater in the DPSH test than in the SPT, leading to higher resistance values in the former. The SPT is carried out within a borehole, whereas the DPSH is continuously driven into the soil. The dynamic force applied to the DPSH rods causes soil to fill the small air annulus around the rods, exerting a frictional resistance. The different geological settings of the test sites revealed that, although different factors cause the friction, the equivalence varied in a similar manner. Hence a single correlation formula is suggested to determine equivalent SPT values from raw DPSH resistance values.
Open Access
Direct shear and direct simple shear tests: a comparative study of the strength parameters and their dependence on moisture and fines contents
(2016) Babalola, Zainab; Kalumba, Denis; Chebet, Faridah
Shear strength of soil is characterized by cohesion, angle of internal friction and dilatation. The first two parameters mentioned primarily define the soil's ability to resist shear stress underspecified load. These parameters can be determined by tests conducted either in the laboratory or the field for use in design of geotechnical structures. Some of the tests in the laboratory to determine the shear strength of soil, include triaxial, ring shear, torsional shear, direct shear and direct simple shear. Direct shear test is the most widely used geotechnical shear device due to its simplicity, however, the test suffers from stress inhomogeneity. Direct simple shear apparatus was developed because of the shortcomings in the direct shear test. In these two tests, different shearing conditions are applied to soil samples. For the direct shear test, shearing occurs at a predetermined center of the specimen which may not be the weakest plane of the soil while indirect simple shear, the entire specimen distorts without the formation of single shearing surface. The mode of shearing established in the direct simple shear device is similar to that which occurs around the shaft of a pile. In contrast to the extensive geotechnical application of direct shear test, limited information exists on direct simple shear test. This thesis endeavours to establish the relationship between the two tests by undertaking extensive testing to obtain a better understanding of direct simple shear test as used for testing local soils and to determine a correlation with the results from direct shear tests. A series of shear tests were undertaken on Klipheuwel sand, Kaolin clay and composite of the sand and clay using universal shear device. The soils were mixed with water in percentages of 5,10, 15, 20 and 25%, and the composite with clay percentages of 10, 25, 50 and 75%, to investigate the impact of water and clay on the shear parameters determined from the two tests. The results showed that direct shear test gives higher shear strength when compared to direct simple shear test under the same soil condition. The addition of water, and clay, generally reduced the internal friction angle of sand for both tests. Furthermore, increase in cohesion was observed with the addition of water to Kaolin clay for the direct simple shear test and the reverse was true in the direct shear test. The correlation factors developed in this study for the direct simple shear test could be used to refine the results from the direct shear test.
Open Access
Effect of grading and grain size on the friction characteristics of a sand/geotextile inteface
(1998) Kalumba, Denis; Scheele, F
Geofabrics are incorporated in geotechnical engineering structures for various reasons and functions. This study addresses the reinforcement function whereby geotextiles are utilised as reinforcing elements in reinforced slopes and fills. It particularly focuses on the soil/geotextile interface behaviour. Geotextile reinforcements transfer a majority of the shear stress from the soil to the reinforcement and vice versa by friction. This interfacing ability manifested by the soil/geotextile frictional contact is very important in the performance of reinforced soil structures, and depends on the physical characteristics of the backfill as well as the geotextile. In this investigation, the interaction behaviour of geotextiles with sand is evaluated by conducting extensive laboratory interface tests both in direct shear and pull-out. A comprehensive test program was established to include a needle punched non-woven geotextile interacting with sands of different grading, grain size distributions and grain shapes namely; Cape Flats, Klipheuwel and Munich sands. The respective responses were primarily presented in terms of shear stress/horizontal displacement and pUll-out resistance/front displacement relationships; showing the frictional performance of the geotextile in these sands of different physical characteristics. Interface shear strength in both test methods was determined using Mohr-Coulomb's law. The ensuing shear strength values were compared with each other and with the direct shear strengths of the respective sands used in this investigation Specific emphasis and detailed analyses went into the pull-out experiments in which local displacements of the geotextile specimens were measured as the test progressed. The measurements enabled the study of the stretching characteristics of the geofabric in the different sands. Applying an extrapolation procedure to approximate the constantly changing deformation modulus of the geotextile as it stretched in the respective sands, allowed the back-prediction of the pull-out force/displacement relationship, and thus enabled the study of skin friction distribution along the geotextile specimen during pull-out. The effect of the grading and grain size on the development of the interface shear stress, the peak values, and the type of interface failure could be demonstrated. The analysis of the skin friction along the geotextile specimen led to the development of a generalised shear stress distribution graph which, if validated in further research, may be adopted in practical design situations. In a design example, it was shown that the assumption of interface shear parameters based on direct shear tests provides too optimistic a factor of safety. This study recommends the use of interface shear parameters derived from pull-out tests. The in-depth analysis of the tests in a variety of sands showed clearly that the shear stress is not uniformly distributed over the embedment length of the reinforcement. This skin friction drops dramatically from a peak value near the loaded end to zero at the free end in all investigated confinements.
Open Access
Evaluation of the electrical density gauge for in-situ moisture and density determination
(2015) Lekea, Angella; Kalumba, Denis; Chebet, Faridah
Densification of soil during construction of earth structures is achieved through the process of compaction by application of mechanical energy to obtain the required engineering properties of the soil for a particular project such as hydraulic conductivity, soil strength and compressibility. These properties are dependent on attainment of high compaction densities normally achieved at specific moisture contents for a given compactive effort. The optimum moisture content and maximum dry density for a particular soil is determined by means of Proctor tests in the laboratory. A relative compaction index is then used to correlate the laboratory values with the field compaction values obtained using in-situ tests. The Sand Cone (SC) and Nuclear Density Gauge (NDG) are the common field tests used to the dry density and moisture content of the soil for purposes of quality control of the compaction process. The sand cone is a laborious test that involves excavation of part of the compacted layer and requires a 24-hour waiting period to obtain the moisture content of the soil through the laboratory oven method. The NDG on the other hand is less laborious, however it uses a radioactive source that is a potential health hazard and therefore requires strict handling, storage and maintenance of the equipment to maintain safety standards. The Electrical Density Gauge (EDG) is an alternative in-situ test that is quicker, safer and easier to maintain since it uses electric current to measure the compaction characteristics of the soil. The objective of the study was to determine the repeatability, accuracy and applicability of the EDG on South African soils by comparing its measurements for dry density and moisture content in the laboratory and in the field to the results from the sand cone and oven method. In the laboratory, a clean sand and a clayey sand were tested at the optimum moisture content and at ± 3% of the optimum moisture content. The soils were compacted to 200 mm using the RT74 rammer and the compaction values first tested using the EDG then followed by the sand cone test at the centre of the EDG test spot. The moisture content of the excavated sample from the sand cone test was determined using the oven method. For the field tests, the compaction characteristics of a sandy gravel and three uniformly graded sands were tested in-situ using the EDG followed by the sand cone test. Overall, the EDG measurements were repeatable based on test-retest comparison of the paired measurements. EDG results for moisture content were consistent with the values obtained from the laboratory oven method especially in the uniformly graded sands. However, the density measurements differed from the results of the sand cone test, which was considered the reference test for determination of field soil density. It is recommended that the EDG calibration relationship for bulk density be revised in order to improve the accuracy of the density measurements.
Open Access
Experimental study of shear behaviour of high density polyethylene reinforced sand under triaxial compression
(2017) Wanyama, Paul; Kalumba, Denis; Chebet, Faridah
Soil reinforcement is an ancient technique which involves the addition of tensile elements like plastics in the soil to increase its engineering properties like shear strength, settlement, cohesion and bearing capacity. In consideration of this, a series of triaxial tests were undertaken to investigate the reinforcing effect of High-Density Polyethylene (HDPE) plastic material in Cape Flats sand, predominant in the Western Cape region of South Africa. Plastic strips of various lengths were randomly included to the soil at different concentrations to form a homogenous soil-plastic composite specimen prepared at varying compactive effort. Using a split mould, cylindrical specimens of 50 mm diameter and 100 mm height were prepared using the dry tamping technique. The test specimens were compacted to achieve target average dry densities of the composite sample. The plastic strip reinforcement parameters comprised of 7.5 mm to 30 mm lengths, and concentrations of 0.1 % to 0.3 % by weight of dry sand. Triaxial compression tests were performed using confining pressures of 50 kPa, 100 kPa, 200 kPa, 300 kPa and 400 kPa at a shear rate of 0.075 %/min, and to a maximum strain of 10 %. Laboratory results favourably suggest that there is an improvement in the soil shear strength properties due to these inclusions. The friction angle increased up to a peak value on varying plastic strip length and concentration, beyond which further addition of plastic material led to a reduction in the friction angle. The greatest friction angle was reported at plastic strip length and content of 15 mm and 0.2 % respectively. Additionally, the results indicate that a higher compactive effort leads to a greater increase in friction angle of the soil. The existence of a critical confining stress was observed from triaxial test results on soil-plastic composites. This threshold limit was influenced significantly by the plastic inclusions, and the range of confining stresses. Consequently, a bilinear failure envelope was reported in reinforced samples while unreinforced specimens realised a linear relationship. The Mohr-Coulomb failure line above the critical confining pressure almost paralleled the unreinforced linear relationship. An embankment model was developed using Slide Modeler software and the factor of safety of slope was analysed with unreinforced and reinforced backfill subjected to static and dynamic loading. It was observed that the safety factor increased due to polyethylene strip inclusions. Therefore, the proposed technique will find potential practical applicability in low-cost embankment or road construction.
Open Access
Geotechnical considerations for onshore wind turbines : adapting knowledge and experience for founding on South African pedocretes
(2013) Warren-Codrington, Charles John; Kalumba, Denis
Wind energy has been placed at the centre of the South African Government's Renewable Energy Independent Producer Programme (REIPPP) with the purpose of addressing electricity capacity deficits and poor service delivery. In doing so, substantial wind farm development has been proposed for the Western, Eastern and Northern Cape of South Africa, with several projects already underway. Wind energy, from a technological standpoint, is regarded as a mature form of renewable energy. However, much of the wind turbine geotechnical experience was gained in the temperate climate of the Northern Hemisphere, where soil conditions differ significantly from those of South Africa. Simultaneously, although mature, wind energy is a novel field in South Africa. Therefore, this study sought the need bridge the gap between local South African soil conditions and international wind energy experience. It was against this backdrop that the following study was initiated, which aimed to provide insight into the site-specific geotechnical design of foundations for wind turbine structures. In doing so, this major objective was divided into four minor objectives, each contributing a major theme to the study, the key points of which are summarised below. It should be noted that the following study was limited to three-blade wind turbines mounted on conical tubular steel towers with shallow foundations.
Open Access
Geotechnical engineering design of a tunnel support system - a case study of Karuma (600MW) hydropower project
(2017) Ongodia, Joan Evelyn; Kalumba, Denis
Tunnels have been built since 2180 B.C., through the stone age. They became popular worldwide since the eighteenth century, as transportation, military, mining, conveyance, storage and flood control structures. Due to the increasing world population, urbanization and industrialization, the construction of underground tunnel structures are preferred as they limit interferences with existing surface uses of the land and water bodies. Although underground tunnels are a common flexible construction alternative, they are high hazard risk structures. The risks are mostly related to ground conditions. Tunnels buried at depth disturb in-situ conditions, cause ground instability and ultimately failure. Widespread tunnel failures, though not publicly advertised because of their adverse implications, have claimed human lives, cleared cities, cost 100 million United States dollars' worth in financial losses and year-long project delays. As such, stability of the structures is crucial to prevent the catastrophes thereby reducing societal outcries. Permanency of underground structures is ensured by provision of adequate resistance to any impeding failure of the ground surrounding deep underground excavations. The effectiveness of the ground-support interaction depends on geology, material properties, geotechnical parameters, loads of the surrounding ground mass and mechanism of the interaction. Using actual project information, the factors influencing stability, structural resistance as well as methods to select the required support are explored in this dissertation. The study used typical geological data of an underground tunnel component of Karuma, a proposed 600MW hydropower project in Uganda. It doubles as the largest hydropower project and first underground construction, to date. The project is located along the River Nile in a sensitive ecosystem neighboring both a major national park and the Great Rift Valley system in East Africa. The instability problem at Karuma was assessed using scientific and universal tunneling practice. Typical site data formed input for the geotechnical engineering design of the tunnel support based on analytical, observational and empirical methods. The study demonstrated that all methods were independent and dissimilar for the same geotechnical engineering challenge of the underground structure. The most comprehensive method was the one based on geotechnical engineering principles and rock mechanics theory. The outcomes of the different approaches in this study were unique functions of their underlying scientific philosophies. The study proposes that in designing adequate support systems to resist forces causing failure of underground tunnels, excavations buried in the ground should encompass several methods. The most conservative design should be chosen to ensure permanency.
Open Access
Ground improvement using PET bottle waste as a potential reinforcement material for granular columns: an experimental approach
(2019) Sobhee-Beetul, Laxmee; Kalumba, Denis
Out of the many ground improvement methods aimed at ameliorating the weak engineering properties of certain soils, granular columns are often preferred due to their cost effectiveness and environment friendliness. Despite their high usage in other countries, this technology remains rather unpopular in South Africa. Therefore, this study was undertaken to extend the associated knowledge of the granular column within the local context. In line with the need to develop environmentally sustainable construction technologies, the study incorporated Polyethylene Terephthalate (PET) bottle waste as a reinforcement material for these columns. Several laboratory experiments were conducted to improve the load carrying capacity and settlement characteristics of a local fine silt. A wet silt bed (prepared at optimum moisture content or liquid limit) was created within a steel cylindrical tank. An ordinary granular column (OGC) or reinforced granular column (RGC) was then installed within the tank and a compressive vertical load was applied to the prepared sample up to a settlement of 50 mm. Reinforcement for the columns was used in different forms and arrangements. The stresssettlement characteristics were electronically captured and subsequently analysed. Posttesting, the deformation of the column was physically modelled by vacuuming out the column material to create an empty opening. A prepared wet mix of plaster of Paris and sand was then poured into the empty hole until it was filled to the top. Once set, the casted column was removed from the tank and its circumference was measured at different intervals up along the length of the column. This process was repeated after each test and these measurements were later used for determining the respective maximum bulging. The study confirmed that the inclusion of granular columns generally improved load carrying capacity, as well as reduced settlement in weak soils such as fine silts. Also, certain conditions of reinforcing of the columns further improved their performances. From the results, it appeared that reinforcing a column with a concentration of 0.1 % of randomly mixed fibres, and installed in a base soil at liquid limit, produced the largest percentage improvement of 244 % in load carrying capacity. Furthermore, the diameter of maximum bulging was reasonably low and was measured as 144 mm, compared to 150 mm for an OGC which was tested under similar conditions. The outcome of this study considerably extended the understanding of the reinforcement of granular columns using PET bottle waste. Since the concept of reinforcing granular columns with waste is new, several areas were identified for future research to further increase knowledge pertaining to this ground improvement method.