D.C. Entwisle

Benefits of a 3D geological model for major tunnelling works : an example from Farringdon, east-central London, UK

In the design of major construction works, the better the ground conditions are known, the more control there is on the assessment of risks for construction, contract and personnel, and ultimately on final costs. Understanding of the ground conditions is usually expressed as a conceptual ground model that is informed by the results of desk study and of dedicated ground investigation. Using the GSI3D software, a 3D geological model (a model composed of attributed solid volumes, rather than of surfaces) can be constructed that exactly honours geologists’ interpretations of the data. The data are used in their true 3D position. The 3D model of faulted Lambeth Group (Palaeogene) strata in the area of the proposed new Crossrail Farringdon underground station, in central London, has several types of benefit. These include allowing optimum use of available ground investigation data, including third party data, with confidence. The model provides an understanding of the local geological structure that had not been possible using other commonly used methods: in particular, it shows the likely distribution of numerous water-bearing coarse deposits and their faulted offsets, which has potentially significant effects on groundwater control. The model can help to focus ground investigation, constrain design and control risk.

3D geological models and their hydrogeological applications : supporting urban development : a case study in Glasgow-Clyde, UK

Urban planners and developers in some parts of the United Kingdom can now access geodata in an easy-to-retrieve and understandable format. 3D attributed geological framework models and associated GIS outputs, developed by the British Geological Survey (BGS), provide a predictive tool for planning site investigations for some of the UKs largest regeneration projects in the Thames and Clyde River catchments. Using the 3D models, planners can get a 3D preview of properties of the subsurface using virtual cross-section and borehole tools in visualisation software, allowing critical decisions to be made before any expensive site investigation takes place, and potentially saving time and money. 3D models can integrate artificial and superficial deposits and bedrock geology, and can be used for recognition of major resources (such as water, thermal and sand and gravel), for example in buried valleys, groundwater modelling and assessing impacts of underground mining. A preliminary groundwater recharge and flow model for a pilot area in Glasgow has been developed using the 3D geological models as a framework. This paper focuses on the River Clyde and the Glasgow conurbation, and the BGSs Clyde Urban Super-Project (CUSP) in particular, which supports major regeneration projects in and around the City of Glasgow in the West of Scotland.

PREDICTING SUBGRADE SHEAR MODULUS FROM EXISTING GROUND MODELS

Abstract Trackbed stiffness is the primary control on rail performance, the subgrade providing the majority of the stiffness for ballasted-track. Using lithological data derived from linear geological maps, examples are presented showing the variability of small strain shear modulus along selected lengths of the ‘East Coast Main Line’. Generalised, effective stress-controlled relationships relating small strain shear modulus and density to lithology are used to develop models of subgrade shear modulus from which ground stiffness may be assessed. The models show results for ground, ranging from gravel through sands to weaker clay-rich soils.

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.

Environmental geology maps of Bath and the surrounding area for engineers and planners

Abstract In March 1984 the British Geological Survey commenced an environmental geological study of parts of west Wiltshire and south east Avon for the Department of the Environment. The objective of this study was to collect the available geological data relevant to the area, and to present them as a series of thematic maps accompanied by a descriptive report and a database/archive of the data used. The output is intended to be used by land-use planners. It is designed simultaneously to be understandable by people not trained in geology and yet to contain detailed information required by specialists concerned with the environment and its development. The 14 maps which were produced describe themes which include solid lithostratigraphy, drift deposits, the inferred distribution of Great Oolite Freestone, the inferred distribution of fuller’s earth, groundwater, ground conditions in relation to groundwater, geotechnical properties of bedrock and superficial deposits, landslipped and cambered strata, distribution of slope angle, and mining. Although the task was primarily a desk study, it was found necessary to carry out a small amount of field survey to re-interpret the foundered strata to the north-west of Bath. This re-survey has been presented in a style consistent with the mapping of the rest of the study area. A fifteenth map showing the result of the re-interpretation of the foundered strata was produced as a supplement to the main report. This paper describes the methods and the results of this study and comments on the implications of the technique for land-use planning.

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.

Methodology for creating national engineering geological maps of the UK

In the United Kingdom (UK) geological maps traditionally have been attributed with lithostratigraphical map units. However, without significant supplementary information, these maps can be only of limited use for planning and engineering works. During the middle part of the 20th century, as development of the science of engineering geology began to accelerate, engineering geological maps started to appear in various forms and at various scales to meet the challenge of making geological maps more suited to land-use planning, engineering design, building, construction and maintenance. Today, engineering geological maps are routinely used at various scales as part of the engineering planning, design and construction process. However, until recently there had been no comprehensive, readily available engineering geological map of the UK to provide the broad context for ground investigation. This paper describes the recently published (2011) 1:1 000 000 scale engineering geology superficial and bedrock maps of the UK. It describes the methodologies adopted for their creation and outlines their potential uses, limitations and future applications. Supplementary material: Engineering Geology (Bedrock) Map of the United Kingdom, Engineering Geology (Superficial) Map of the United Kingdom and Extended Key for the Engineering Geology Maps of the United Kingdom are available at http://www.geolsoc.org.uk/SUP18528.

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.

A GIS for the planning of electrical earthing

When creating an electrical earth for a transformer with vertically driven earthing rods, problems can arise either because the ground is too hard or because the ground is too resistive to achieve the required earthing resistance. To assist in the planning of earthing installations a geographic information system (GIS) layer has been created. In its simplest form it consists of a colour coded map that indicates the most likely earthing installation: a single vertically driven rod (indicated by dark green); multiple vertically driven rods (indicated by light green); a horizontal trench, where a rod installation is unlikely (indicated by yellow); for difficult ground, a specialist installation (i.e. drilling; indicated by red). However, the GIS can be interrogated to provide site-specific information such as site conditions, likely depth of installation and quantity of earthing materials required. The GIS was created from a spatial model constructed from soil, superficial and bedrock geology that has been attributed with engineering strength and resistivity values. Calculations of expected earthing rod resistance, rod or trench length, and all possible combinations of ground conditions have been compared with the ‘likely’ conditions required for each of the four proposed installation scenarios to generate the GIS layer. The analysis has been applied to the electrical network distribution regions of Western Power Distribution, in the English Midlands, and UK Power Networks, which covers East Anglia, London and the SE of England. Because the spatial model that underlies the GIS has been constructed from national databases the analyses can be extended to other regions of the UK.