Matthys A. Dippenaar

Partially to fully saturated flow through smooth, clean, open fractures: qualitative experimental studies

Fractures are both rough and irregular but can be expressed by a simple model concept of two smooth parallel plates and the associated cubic law governing discharge through saturated fractures. However, in natural conditions and in the intermediate vadose zone, these assumptions are likely violated. This paper presents a qualitative experimental study investigating the cubic law under variable saturation in initially dry free-draining discrete fractures. The study comprised flow visualisation experiments conducted on transparent replicas of smooth parallel plates with inlet conditions of constant pressure and differing flow rates over both vertical and horizontal inclination. Flow conditions were altered to investigate the influence of intermittent and continuous influx scenarios. Findings from this research proved, for instance, that saturated laminar flow is not likely achieved, especially in nonhorizontal fractures. In vertical fractures, preferential flow occupies the minority of cross-sectional area despite the water supply. Movement of water through the fractured vadose zone therefore becomes a matter of the continuity principle, whereby water should theoretically be transported downward at significantly higher flow rates given the very low degree of water saturation. Current techniques that aim to quantify discrete fracture flow, notably at partial saturation, are questionable. Inspired by the results of this study, it is therefore hypothetically improbable to achieve saturation in vertical fractures under free-draining wetting conditions. It does become possible under extremely excessive water inflows or when not free-draining; however, the converse is not true, as a wet vertical fracture can be drained.RésuméLas fracturas son rugosas e irregulares, pero pueden expresarse mediante un modelo sencillo de dos placas paralelas lisas y la ley cúbica asociada que rige la descarga a través de fracturas saturadas. Sin embargo, en condiciones naturales y en la zona vadosa intermedia, es probable que estos supuestos sean transgredidos. Este trabajo presenta un estudio experimental cualitativo que investiga la ley cúbica bajo saturación variable en fracturas discretas de drenaje libre inicialmente secas. El estudio comprendió experimentos de visualización de flujo realizados en réplicas de placas transparentes paralelas y lisas con condiciones de entrada de presión constante y caudales diferentes sobre la inclinación tanto vertical como horizontal. Las condiciones de flujo fueron alteradas para investigar la influencia de los escenarios de afluencia intermitente y continua. Los hallazgos de esta investigación demostraron, por ejemplo, que no es probable que se produzca un flujo saturado laminar, especialmente en fracturas no horizontales. En las fracturas verticales, el flujo preferencial ocupa la minoría del área transversal a pesar del suministro de agua. El movimiento del agua a través de la zona vadosa fracturada se convierte así en una cuestión del principio de continuidad, por lo que el agua se debería transportar teóricamente hacia abajo a caudales significativamente más altos dado el muy bajo grado de saturación de agua. Las técnicas actuales que pretenden cuantificar el flujo discreto de fracturas, especialmente en la saturación parcial, son cuestionables. Inspirándose en los resultados de este estudio, es hipotéticamente improbable lograr la saturación en fracturas verticales bajo condiciones de humectación de drenaje libre. Se hace posible bajo entradas de agua extremadamente excesiva o cuando no se drena libremente. Sin embargo, lo contrario no es cierto, ya que una fractura vertical mojada se puede drenar.ResumenLes fractures sont à la fois rugueuses et irrégulières, mais elles peuvent être représentées par un modèle conceptuel simple constitué de deux plaques parallèles lisses et par la loi cubique associée, qui régit le débit le long des fractures saturées. Cependant, dans des conditions naturelles et dans la zone vadose intermédiaire, ces hypothèses sont probablement non valides. Cet article présente une étude expérimentale qualitative qui analyse la loi cubique pour des saturations variables dans des fractures discrètes initialement sèches, à drainage libre. L’étude comprenait des expériences de visualisation de l’écoulement réalisées sur des répliques transparentes de plaques parallèles lisses, avec comme conditions d’entrée une pression constante et des débits divers pour une inclinaison aussi bien verticale qu’horizontale. Les conditions d’écoulement ont été modifiées afin d’étudier l’influence de scénarios de flux entrants intermittents et continus. Les résultats de cette recherche ont montré, par exemple, que l’écoulement laminaire saturé n’est. probablement pas atteint, spécialement dans les fractures non horizontales. Dans les fractures verticales, l’écoulement préférentiel occupe la plus petite partie de la section transversale malgré l’alimentation en eau. Le mouvement de l’eau à travers la zone vadose fracturée devient par conséquent une limite du principe de continuité, selon lequel l’eau devrait en théorie être transportée vers le bas pour des débits significativement plus élevés étant donné le très faible degré de saturation en eau. Les techniques habituelles qui ont pour but de quantifier l’écoulement au sein d’une fracture discrète, notamment à saturation partielle, sont discutables. D’après les résultats de cette étude, il est. donc par hypothèse improbable d’atteindre la saturation dans des fractures verticales pour des conditions mouillantes de drainage libre. Cela devient possible avec des apports extrêmement excessifs d’eau ou lorsque la fracture est. peu perméable. Cependant, le contraire n’est. pas vrai, étant donné qu’une fracture verticale mouillée peut être perméable.摘要断裂是粗糙和不规则的,但可以通过两个平滑平行的板块简单的模型概念及控制通过饱和断裂补给的相关立方定律来表示。然而,在自然条件下和在中间包气带,这些假设可能被改变。本文介绍了初始干旱自由排水离散断裂中多变饱和状态下调查立方定律的定性试验研究。研究包含在平滑平行板块透明复制品上进行的压力恒定、流入条件下及纵向和横向上不同流量条件下的水流可视化试验。改变水流条件以调查了断续和连续通量方案的影响。例如,本研究的发现证明,不可能取得饱和层状流,尤其是在非横向断裂中。在纵向断裂中,尽管存在着供水,优先流占据了截面面积的一小部分。因此,通过断裂的包气带水运移成了连续性原理的一个问题,而假定饱和程度非常低的情况下,水理论上以相当高的流速被输送到下面。旨在量化离散断裂流、尤其是部分饱和条件下的离散断裂流的目前技术是有问题的。受到本研究的启发,因此,假设在自由排水湿润条件下纵向断裂不可能达到饱和。在水流入极端过多的情况下或者在没有自由排水的的情况下,有可能达到饱和。然而,逆向是不会出现的,因为湿润的纵向断裂可排出水来。ResumoFraturas são acidentadas e irregulares mas podem ser expressadas por um conceito de modelagem simples de duas placas paralelas suaves e lei cúbica associada ao regimento da descarga através de fraturas saturadas. Entretanto, em condições naturais e em zonas vadosas intermediárias, essas suposições são comumente violadas. Esse trabalho apresenta um estudo experimental qualitativo que investiga a lei cúbica sob saturação variável em fraturas discretas de drenagem livre inicialmente secas. O estudo compreendeu os experimentos de visualização do fluxo conduzidos em réplicas transparentes das placas paralelas suaves com condições de entrada de pressão constante e diferentes taxas de fluxo em ambas inclinações verticais e horizontais. As condições de fluxo foram alteradas para investigar a influência dos cenários de entrada de fluxos continuo e intermitente. As descobertas dessa pesquisa provam, por hora, que o fluxo laminar saturado não é comumente atingido, especialmente em fraturas não horizontais. Em fraturas verticais, os fluxos preferenciais ocupam a menor parte da área seccional ao invés do armazenamento de água. O movimento de água através da zona vadosa fraturada por consequência se torna uma questão de princípio da continuidade, através do qual a água deveria teoricamente ser transportada descentemente para taxas de fluxo maiores significantemente dado o baixo grau de saturação da água. Técnicas atuais que visam quantificar o fluxo de fratura discreto, notavelmente em saturação parcial, são questionáveis. Inspirados pelo resultado desse estudo, é hipoteticamente improvável se atingir saturação em fraturas verticais sob condições úmidas de drenagem livre. Isso se torna possível sob condições de entradas de água extremamente excessivas ou quando não possuem drenagem livre. No entanto, a discussão não é verdadeira, visto que a fratura vertical pode ser drenada.

Assessing geotechnical centrifuge modelling in addressing variably saturated flow in soil and fractured rock

The vadose zone typically comprises soil underlain by fractured rock. Often, surface water and groundwater parameters are readily available, but variably saturated flow through soil and rock are oversimplified or estimated as input for hydrological models. In this paper, a series of geotechnical centrifuge experiments are conducted to contribute to the knowledge gaps in: (i) variably saturated flow and dispersion in soil and (ii) variably saturated flow in discrete vertical and horizontal fractures. Findings from the research show that the hydraulic gradient, and not the hydraulic conductivity, is scaled for seepage flow in the geotechnical centrifuge. Furthermore, geotechnical centrifuge modelling has been proven as a viable experimental tool for the modelling of hydrodynamic dispersion as well as the replication of similar flow mechanisms for unsaturated fracture flow, as previously observed in literature. Despite the imminent challenges of modelling variable saturation in the vadose zone, the geotechnical centrifuge offers a powerful experimental tool to physically model and observe variably saturated flow. This can be used to give valuable insight into mechanisms associated with solid–fluid interaction problems under these conditions. Findings from future research can be used to validate current numerical modelling techniques and address the subsequent influence on aquifer recharge and vulnerability, contaminant transport, waste disposal, dam construction, slope stability and seepage into subsurface excavations.

Porosity reviewed : quantitative multi-disciplinary understanding, recent advances and applications in vadose zone hydrology

AbstractPorosity—one of the most basic mechanical properties of a medium—has implications in a vast range of disciplines and used for a similar vast range of applications. These include, for instance, the storage and flow of water; the compressible component of earth materials, which can be subjected to consolidation under loading; the variable parameter in the swelling and shrinkage of clays; and possibly a governing parameter in the formation of wetlands and perched water tables. This review notes the relevance of a fourfold quantification of porosity for vadose zone studies, viz. (1) type (matrix or structure), (2) scale (submicro to macro scale), (3) connectivity, and (4) water saturation. This is followed by a review of recent advances in the quantification and description of porosity in porous media (visual and remote sensing methods, porosimetry, geometrical approaches, empirical estimations, densest packing simulations, etc.), the applications to quantification of hydrological parameters, and a brief glimpse into the significance of porosity in a temporary hillslope wetland underlain by Archaean Lanseria gneiss in South Africa. Final comments are made regarding areas where quantification of porosity is problematic.

Towards hydrological and geochemical understanding of an ephemeral palustrine perched water table “wetland” (Lanseria Gneiss, Midrand, South Africa)

AbstractWetland delineation is commonly based on terrain unit, soil form, soil wetness and vegetation indicators which along with the shallow groundwater or proximate surface water are often absent in many ephemeral inland South African areas due to, for instance, prolonged dry periods (seasons to years) which mask these indicators, as well as disruption of surface materials due to construction, agricultural activities and field fires. Furthermore, many “wetlands” in South Africa comply with the four indicators, but the notable absent requirement for wetlands is missing, namely the shallow groundwater table, as many of these systems form in hillslope seeps, catenas or from perched water tables. A 200-m-long excavation through one such a system is being studied in Midrand (Gauteng, South Africa). The site is underlain by Lanseria Gneiss and is waterlogged after long and intense periods of intense rainfall. Frequent downslope soil profiling, horizon-specific laboratory analyses for grading and Atterberg Limits, X-Ray Diffraction and X-Ray Fluorescence Spectroscopy data are interpreted together with soil percolation tests to generate a conceptual model of the system. The results are discussed in terms of the need to consider these temporary systems that do not have a shallow groundwater table and that are not in direct contact with surface drainage features as a possible special type of wetlands, notably in arid regions where they play a very important role in biodiversity and should, therefore, be protected.

How We Lose Ground When Earth Scientists Become Territorial: Defining “Soil”

This short review aims to provide some examples where terrestrial earth sciences—including, for this purpose, geology, engineering geology, soil science, pedology, hydrology, hydrogeology, geography, geomorphology and environmental sciences—do not interact sufficiently to ensure optimal results. Apart from different terminologies (for instance, “soil”), different specialist fields claim expertise over certain disciplines or portions of the Earth’s crust without a proper understanding of the influence of other specialist fields. This severely limits interdisciplinary understanding, and it is recommended that a better understanding of interdisciplinary definitions is required to ensure a wider audience. Although this is not a technical article in its own right, this forms part of a study to link vadose zone hydrology for the disciplines as noted earlier, thereby creating hopefully a platform for easier communication.

Partially saturated flow from sand into a discrete smooth open vertical fracture at the soil–rock interface: experimental studies

The two proposed conceptual models explaining partially saturated flow from soil into fractured rock in the intermediate fractured vadose zone have not been confirmed due to the difficulty involved in observing the soil–rock interface in situ. To address that challenge, this paper presents a series of newly developed physical experiments using a geotechnical centrifuge model of sand overlying a single dry clean smooth vertical fracture. The model shows the development of a perched water system and a saturated wetting front that progresses transversely along the interface, while breaching through the interface occurs via multiple point sources. The dominant flow mechanisms within the fracture comprise droplets, discontinuous rivulets with droplet formation, and continuous rivulets. A maximum drainage area of 30% of the width of the fracture contributes to the flow in the model, and this drainage area decreases with depth due to the merging of oscillating rivulets in the upper regions of the fracture. The presence of evidence supporting both conceptual models shows that a combined conceptual model is required to accurately explain partially saturated flow at the soil–rock interface.

Towards a multi-faceted Vadose Zone Assessment Protocol: cemetery guidelines and application to a burial site located near a seasonal wetland (Pretoria, South Africa)

Cemeteries are generally considered low-risk landfills and are, therefore, not adequately governed in terms of minimum requirements for engineering geological and hydrogeological investigations. With the decay of human bodies, the bulk of the contaminant load is typically present within one year of burial and decreases over time. Further controls include adsorption of pathogens and particulates to soil in the vadose zone, and eventual breakdown in changing aerobic and anaerobic subsurface environments. A case study is presented where a cemetery has been active for decades and contains in excess of 18,000 human bodies. With recent expansion, water seepage into newly excavated burial pits was identified, and all further development has ceased. Trial pitting and detailed soil profile descriptions are used to infer hydrological interaction at the site and to address possible contamination pathways. Future work is recommended with respect to water quality, although valuable insight is provided into the consequences of improper ground investigation prior to development. Final comments are made regarding provisional guidelines in the forms of a cost-effort-risk screen and a multi-faceted Vadose Zone Assessment Protocol.

Aquifer vulnerability using recharge, depth to groundwater, soil type and slope to classify the vadose zone (Molototsi and Middle Letaba quaternary catchments, Limpopo Province, South Africa)

Abstract The aquifer vulnerability of the Molototsi (B81G) and Middle Letaba (B82D) quaternary catchments was assessed to determine the influence of the vadose zone on the groundwater regime. The aquifer vulnerability was assessed by developing a new method, which evaluates the vadose zone as a pathway for pollutants using the following four parameters: recharge, depth to water table, soil type (saturated vertical hydraulic conductivity) and slope (RDSS). Recharge was estimated using the chloride mass balance method, and the depth to the water table was measured in the field using dipmeter. The seepage behaviour (soil type) was determined as hydraulic conductivity from in situ infiltration and percolation testing (SABS 0252-2:1993 and double ring infiltrometer). The slopes were determined with the digital elevation method using ArcGIS software. The four parameters were overlaid using weighted sum, weighted overlay and raster calculator to produce the vulnerability map. Different weightings were attributed in the methods and the best selected. The results obtained indicated high vulnerability on the lower and upper parts of both catchments. The benefits of the method described are (a) the easy quantification of the parameters through fairly simple methods and (b) the exclusion of arbitrary index values.

Vadose Zone Characterisation for Hydrogeological and Geotechnical Applications

Rapid urbanization is resulting in increased vertical development and use of anthropogenic materials. Geotechnical site investigation is well established in assessing ground conditions, and moisture specifically is a standard descriptor in soil profile logging and is addressed through a variety of laboratory tests. However, changing moisture conditions, occurring in the vadose zone between land surface and the groundwater table, results in highly variable conditions. Noting the presence and variability in moisture is not sufficient to ensure longevity of engineering structures and protection of water resources. Water at partial saturation is proposed to impact the infrastructure and the vadose zone moisture budget as: (A) perching above lower permeability (lower-k) materials; (B) perching as waterlogged lower-k materials above capillary barriered higher-k materials; both resulting in (C) possible imbibition into less saturated low-k materials; or, under further wetting, resulting in (D) lateral interflow under a hydraulic gradient; (E) gravity-driven percolation breaching capillary barriers and resulting in translatory downward flow; or (F) unsaturated fracture flow. All these mechanisms combine to result in complex moisture implications on infrastructure during project lifecycle, as well as on recharge and contaminant transport rates above the phreatic surface. These are further exacerbated by anthropogenic materials (e.g. made ground) replacing natural materials and infringing on the natural and pre-development subsurface water cycle, as well as climate change, and more elaborate engineering development. Contrary to saturated systems, unsaturated systems result in alternating wetting-drying cycles causing continuous changes in effective stress and redox conditions. The paper addresses some key findings and examples from experiments and case studies.

On the Differing Role of Contact Obstacles on Variably Saturated Flow in Vertical and Horizontal Fractures

A series of flow visualization experiments is developed in order to observe the different role that contact obstacles have on variably saturated flow in a vertical fracture compared to a horizontal fracture. The model fractures consist of a single rough-walled quartzite fracture wall mismatched with a second transparent smooth replica wall. Results drawn from this research show that contact obstacles result in longer flow paths of between 4 and 14%, with longer flow paths observed in horizontal fractures. Furthermore, contact obstacles result in different unsaturated flow mechanisms when present in vertical compared to horizontal fractures under low flow rates. Contact obstacles enhance the variably saturated flow process in vertical fractures, but act as choke points in horizontal fractures.