David Boardman

Graphicacy Revisited: mapping abilities and gender differences

Abstract A review of research carried out during the past decade indicates that the map‐drawing and map‐using abilities of young children had previously been underestimated. Experiments have shown that primary school children are able to draw from memory simple sketch‐maps of the area around their home and to recognise features on aerial photographs and large‐scale plans of the same area. Research undertaken with secondary school pupils has provided a greater understanding of the difficulties they experience in understanding contour patterns as representations of relief, and in correlating Ordnance Survey maps with aerial photographs. Among younger children there is little difference between the mapping abilities of boys and girls, but, as they grow older, boys consistently perform better than girls of the same age in map‐drawing and map‐reading tasks.

Case study of a loess collapse field trial in Kent, SE England

Loess soils undergo collapse due to bond weakening under loading and, especially, wetting, and consequently constitute a major engineering geology hazard. To understand better the relationship between wetting and volume reduction in loess, a field collapse test was conducted at a ‘brickearth’ quarry, where a 5.0 × 5.0 × 1.5 m deep sample was isolated, flooded in a controlled manner and subjected to a surface stress of up to 210 kPa for 10 days. Geotechnical instrumentation, consisting of piezometers and rod extensometers, was complemented by geophysical instrumentation (resistivity arrays, shear wave transducers and a resistivity probe) to provide evidence of changes in interparticle bonding during the collapse process. Laboratory index and oedometer testing, together with SEM study of samples removed from the site, complemented the site monitoring. The field collapse test eliminated many problems associated with laboratory testing, notably small volumes of material and sample disturbance. This paper presents the geotechnical findings on ‘large-scale’ loess performance and relates them to the results of shear wave velocity and resistivity monitoring. The different behaviour of two distinct soil strata and the importance of the degree of saturation to soil fabric changes are demonstrated. The results identify how the soil in situ and oedometer samples respond under similar applied stresses.

Electrical resistivity monitoring of a collapsing meta-stable soil

Amodel of electrical conduction through clay-coated, silt-sized quartz-grains inter-connected by clay-bridges (e.g. brickearth) is developed. Underpinned by SEM studies of brickearth, the model predicts resistivity to be proportional to the size of the quartz-grains, where the resistance afforded by clay grain-coatings and clay-bridges is comparable. The model accommodates resistivity that increases through bridge breakage and decreases through bridge compression. The resistivity of in-situ undisturbed brickearth was found to be in the range 15 to 35 ohm-m. At such low values we demonstrate that electrical flow is dominated by conduction within clay-coatings and their interconnecting clay-bridges, rather than in mobile pore-water. A small electrode array, buried at shallow depth beneath the load plate (1.0 m by 1.0 m) of a field collapse experiment, monitored resistivity to a depth of 1.5 m over a 260 hour period. While the water level beneath the load plate remained below 1.0 m depth, the resulting 3D inverted resistivity models detected water injected immediately beneath the plate; recording rapid increases, in stages over 90 minutes, in the depth interval 0.45 to 0.75 m directly under the plate, during what appears to be collapse. These increases are attributed to breaking of clay-bridges weakened by injected water.

Shear wave velocity monitoring of collapsible loessic brickearth soil

Metastable loessic brickearth comprises a stiff fabric structure with inter-particle interactions different to those normally associated with clay-sized or silt-sized particle fabrics. Laboratory samples loaded near in situ moisture contents exhibited little consolidation and relatively high shear wave velocities, which changed in response to sample flooding. In situ hydro-collapse caused non-monotonic changes in the velocity of shear waves through loessic brickearth that was subjected to simple flooding and to flooding while under additional surface loading. Hydro-collapse in situ resulted in an overall reduction of up to 50% in the shear wave velocity. A conceptual model of brickearth structure based on SEM images is presented to explain the process of collapse and its effect on shear wave velocity. These indicate a transition from a relatively low-density, high-stiffness fabric to the higher-density, lower-stiffness fabric during structural collapse of the loessic brickearth. The collapse process disrupts clay bridge-bonds that hold individual and aggregated clay-coated silt sized particles in an open packed structure, and which are absent in a more closely packed collapsed structure. These studies provide information for geohazard research and the development of shear wave velocity and other geophysical tools to assess soil collapse potential in situ.

The mineralogy and fabric of ‘Brickearths’ in Kent, UK and their relationship to engineering behaviour

Mineralogical and petrographical investigation of two loessic brickearth profiles from Ospringe and Pegwell Bay in north Kent, UK have differentiated two types of brickearth fabric that can be correlated with different engineering behaviour. Both sequences comprise metastable (collapsing) calcareous brickearth, overlain by non-collapsing ‘non-calcareous’ brickearth. This study has demonstrated that the two types of brickearth are discretely different sedimentary units, with different primary sedimentary characteristics and an erosional junction between the two units. A palaeosol is developed on the calcareous brickearth, and is associated with the formation of rhizolithic calcrete indicating an arid or semi-arid environment. No evidence has been found for decalcification being responsible for the fabric of the upper ‘non-calcareous’ brickearth. Optically-stimulated dates lend further support for the calcareous and ‘non-calcareous’ brickearth horizons being of different age or origins. The calcareous brickearth is metastable in that it undergoes rapid collapse settlement when wetted under applied stresses. It is characterised by an open-packed arrangement of clay-coated, silt-sized quartz particles and pelletised aggregate grains (peds) of compacted silt and clay, supported by an inter-ped matrix of loosely packed, silt/fine-grained sand, in which the grains are held in place by a skeletal framework of illuviated clay. The illuviated clay forms bridges and pillars separating and binding the dispersed component silt/sand grains. There is little direct grain-to-grain contact and the resultant fabric has a very high voids ratio. Any applied load is largely supported by these delicate clay bridge and pillar microfabrics. Collapse of this brickearth fabric can be explained by a sequence of processes involving: (1) dispersion and disruption of the grain-bridging clay on saturation, leading to initial rapid collapse of the loose-packed inter-ped silt/sand; (2) rearrangement and closer stacking of the compact aggregate silt/clay peds; (3) with increasing stress further consolidation may result from deformation and break up of the peds as they collapse into the inter-ped regions. Smectite is a significant component of the clay assemblage and will swell on wetting, further encouraging disruption and breaking of the clay bonds. In contrast, the ‘non-calcareous’ brickearth already possesses a close-packed and interlocking arrangement of silt/sand grains with only limited scope for further consolidation under load. Minor authigenic calcite and dolomite may also form meniscus cements between silt grains. These have either acted as “scaffolds” on which illuviated clay has subsequently been deposited or have encrusted earlier-formed grain-bridging clay. In either case, the carbonate cements may help to reinforce the clay bridge fabrics. However, these carbonate features are a relatively minor feature and not an essential component of the collapsible brickearth fabric. Cryoturbation and micromorphological features indicate that the calcareous brickearth fabric has probably been developed through periglacial freeze–thaw processes. Freezing could have produced the compact silt/clay aggregates and an open porous soil framework containing significant inter-ped void space. Silt and clay were remobilised and translocated deeper into the soil profile by water percolating through the active layer of the sediment profile during thawing cycles, to form the loosed-packed inter-ped silt matrix and grain-bridging meniscus clay fabrics. In contrast, the upper ‘non-calcareous’ brickearth may represent a head or solifluction deposit. Mass movement during solifluction will have destroyed any delicate grain-bridging clay microfabrics that may have been present in this material.

Physicochemical effects on uncontaminated kaolinite due to electrokinetic treatment using inert electrodes

To determine the consequences of applying electrokinetics to clay soils, in terms of mechanisms acting and resulting effects on the clay, tests were conducted in which an electrical gradient was applied across controlled specimens of English China Clay (ECC) using ‘inert’ electrodes and a ‘Reverse Osmosis’ water feed to the electrodes (i.e., to mimic electrokinetic stabilisation without the stabiliser added or electrokinetic remediation without the contaminant being present). The specimens in which electromigration was induced over time periods of 3, 7, 14 and 28 days were subsequently tested for Atterberg Limits, undrained shear strength using a hand shear vane, water content, pH, conductivity and zeta potential. Water flowed through the system from anode to cathode and directly affected the undrained shear strength of the clay. Acid and alkali fronts were created around the anode and cathode, respectively, causing changes in the pH, conductivity and zeta potential of the soil. Variations in zeta potential were linked to flocculation and dispersion of the soil particles, thus raising or depressing the Liquid Limit and Plastic Limit, and influencing the undrained shear strength. Initial weakening around the anode and cathode was replaced by a regain of strength at the anode once acidic conditions had been created, while highly alkaline conditions at the cathode induced a marked improvement in strength. A novel means of indicating strength improvement by chemical means, i.e., free from water content effects, is presented to assist in interpretation of the results.

Creation of artificial loess soils

Loess is a loose, open-structured metastable soil of aeolian origin, predominantly composed of 20–60 µm quartz particles bonded together by clay particles and, in some cases, carbonate compounds. When dry it can withstand high overburden stresses, whilst upon saturation it collapses creating potentially enormous engineering problems. The mechanisms controlling this metastable behaviour involve the disintegration of inter-particle clay and chemical bonding and variations in the pore water pressures (i.e. suctions) during saturation.

Chemical performance of stabilized/solidified zinc-contaminated soil

Stabilization/solidification has been found to be a relatively sustainable and economical method for managing risks associated with contaminated land. The technology has been employed extensively over the last three decades, particularly for the containment of heavy metals. However, because contaminants are not removed, there is a need to validate the long-term effectiveness of the technique. This requires knowledge of the containment mechanisms, the kinetics of contaminant release (equilibrium and disequilibrium conditions), and identification of the reactive surfaces that induce containment. In the present study, cement-stabilized zinc-contaminated kaolin clay containing 1% humic acid was tested to evaluate chemical performance in the long term. Time-dependent (chemical kinetics) and pH-dependent (equilibrium conditions) and equilibrium porewater leaching were evaluated, using LeachXS and geochemical speciation modelling using ORCHESTRA (embedded in LeachXS). Results showed that zinc was effectively contained within the waste form matrix under the prevailing chemical conditions, with immobilization increasing with hydration. Presence of humic acid increased the availability of zinc at an early stage, but this decreased at later stages and was comparable with results for organic-free matrices.

Time dependent leachability and potential environmental implications from stabilization/solidification treatment of chromium contaminated clay

Requirements for sustainable waste management and environmental protection have yielded recent EU regulations, which actively encourage the use of source control remedial approaches such as stabilization/solidification (S/S). S/S has proven particularly viable for heavy metal containment, relying on interactions between introduced reagents and contaminated media to induce containment. However, there is a need to validate long-term performance of the method to prevent accidental releases and subsequent environmental pollution. This requires evaluation of leaching controlling mechanisms for treated contaminated media to address time and equilibrium pH dependent leaching characteristics. The selection of a suitable suite of leaching tests to evaluate leaching is essential to ensure confidence in the applied techniques, and can be used to expedite durable and effective mix design and application. This paper presents findings from leaching evaluations for CEM II (Portland limestone cement) treated chromium contaminated kaolin clay. Leaching tests were undertaken at 28, 150, and 300 days of sample curing, for time dependent, equilibrium, and porewater leaching characteristics. Evaluations were also undertaken for influences of sulfate and organic content (as humic acid) on chromium containment. Chromium was found to be contained effectively within the stabilized contaminated matrix, with leaching and availability comparable to those observed for the stabilized uncontaminated specimen. The primary solubility controlling mineral was found to be chromium hydroxide, and increasing hydration improved chromium containment. Increased organic content resulted in an initial increase in chromium availability at 28 and 150 days of hydration, a trend also observed for an increase in sulfate content. However, after hydration for 300 days, the availabilities for chromium were comparable for all matrices, and comparable to measures observed for leaching from the uncontaminated clay.

Chemical Leaching Assessment and Durability Evaluation of Cement Stabilized Zinc Contaminated Kaolin Clay

Sustainable redevelopment to meet increasing urban expansion has mandated the increased redevelopment of contaminated land. Current national / international environmental regulations, and increased disposal costs for landfilling, has necessitated a need for sustainable and cost efficient in situ remediation alternatives. Stabilization / Solidification (S/S) techniques offer a viable solution for the amelioration of unacceptable environmental risk, particularly effective in heavy metal remediation. However, uncertainties over long term effectiveness and durability, and previously low landfill disposal costs had limited its utilization. Effective characterization of the stabilized material and evaluation of its leaching and containment properties are required, to ensure long term effectiveness and improve design confidence. Zinc contaminated kaolin clay, stabilized with CEM II Portland Limestone cement, was tested using a suite of leaching tests designed for monolith specimens. Evaluations of leaching for zinc and selected master species (Al, Si and Ca) were undertaken, at different intervals of hydration (28, 150 and 300days). Results show that zinc is successfully contained and partitioned within the cement stabilized kaolin clay, but does not exhibit amphoteric leaching. Increased hydration time increases the stabilized materials chemical durability regardless of kaolin degradation, but does not modify pH dependent leaching trends.