K.J. Northmore

Classification of artificial (man-made) ground

Abstract The legacy inherited from anthropogenic processes needs to be addressed in order to provide reliable and up-to-date ground information relevant to development and regeneration in the urban environment. The legacy includes voids as well as anthropogenic deposits (artificial ground). Their characteristics derive from former quarrying and mining activities industrial processes creating derelict ground, variably consolidated made ground, and contaminated groundwater and soils. All need to be systematically assessed to inform the planning process and provide the basis for engineering solutions. Site-specific investigation needs to be conducted on the back of good quality geoscientific data. This comes from ‘field’ survey, remotely sensed data interpretation, historical maps, soil geochemical sampling, and geotechnical investigation. Three-dimensional and, in the future, four dimensional, characterization of superficial deposits is required to reach an understanding of the potential spatial lithological variability of artificial ground and the geometry of important surfaces, i.e. the boundary conditions. The classification scheme for artificial ground outlined in this paper and adopted by the British Geological Survey, will help in achieving this understanding.

The engineering properties and behaviour of the brickearth of south Essex

Abstract Brickearth occurs in south Essex where it overlies Quaternary terrace gravels and London Clay of Eocene age. In places it is as much as 8 m in thickness. Quartz is the most abundant mineral and feldspar occurred in most specimens examined. Of the clay type minerals, mica is generally more abundant than mont- morillonite which, in turn, is more abundant than kaolinite. Calcium carbonate occurs as grains, concretionary nodules and thin tube fillings. The moisture content normally varies between 12 and 25% and the soils are of low to high plasticity. Clayey, silty and sandy brickearths are recognized, with most belonging to the silty variety. They are uniform to well sorted with most samples having a positive skewness. The porosity of the brickearths is around 40% with a mean dry density of 1.68 Mg/m3. Brickearth can have a collapsible fabric. In the case of the brickearths from south Essex this was assessed in terms of collapse indices and the oedometer test. About half the samples tested in the oedometer were metastable. As would be expected, the degree of compressibility of these brickearths is high. The undrained shear strength of these brickearths showed that they ranged from very soft to very stiff. There is a general tendency for shear strength to decrease with depth as the soil near the ground surface has a ‘crust-like’ nature.

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.

The metastability of some gull-fill materials from Allington, Kent, UK

Samples of infill material from gulls in the Hythe Beds at Allington, Kent, England, were tested to determine their geotechnical index properties and their susceptibility to collapse when wetted under load in an oedometer. The soils accumulated during late Devensian, late Pleistocene, times within a periglacial environment. They probably represent, at least in part, an overlying layer of brickearth type material that gradually subsided into the gulls as the ice that occupied them melted with the onset of warmer conditions. The soils resemble, both in colour and their index properties, brickearth found elsewhere in Kent and in south Essex. All the gull-fill sample material had silt contents in excess of 75%, low dry densities (1.4–1.7 Mg m−3), medium to high porosities (35–47.5%) and carbonate contents below 0.5%. The results from the oedometer collapse tests showed that one of the samples was non-collapsible and that the others were conditionally collapsible. When these results were compared with several empirical collapse criteria, all derived from basic geotechnical soil properties, only some of the criteria showed reasonable agreement with the oedometerresults.

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 Short History of Engineering Geology and Geophysics at the British Geological Survey

Engineering geology in the British Geological Survey (BGS) began, in a formal sense, with the creation of the Engineering Geology Unit in 1967. Virtually since its inception, despite changing research priorities and economic drivers, the survey and research work carried out by BGS engineering geologists can be conveniently divided into four broad research areas: engineering geological mapping and urban geoscience, geotechnical properties of soils and rocks, engineering geophysics and geohazards. Since the late 1960s engineering geologists have undertaken innovative research initiatives and continue to play an important role in ensuring the delivery of BGS research.

Some geotechnical properties of the Claygate ‘Beds’ and Bagshot ‘Beds’ of south Essex

Abstract The Claygate Beds and Bagshot Beds are the uppermost formations of Eocene age in south Essex, the former resting on the London Clay. The Claygate Beds consist primarily of silts and clays with subordinate sands, whilst sands are more common in the Bagshot Beds. The paper describes the geotechnical properties of these two sedimentary units. The results form part of a larger study of the engineering geology of south Essex. The sands, particularly in the Bagshot Beds, are fine grained. All the sands are uniformly sorted and negatively skewed. Quartz is the principal minerals in these beds and in the fine material, mica, montmorillonite, kaolinite and chlorite figure in that order of relative abundance. The geotechnical properties of the silts and clays of both formations are generally similar. However, the plasticity of the Claygate Beds tends to decrease somewhat from the lower to the upper division and the range of plasticity is greatest in the upper division. This may be due to the greater variation in quartz content and montmorillonite content in the latter division than in the two others. The clays in both formations tend to have normal activity whilst that of the silts is both normal and active. The values of the undrained shear strength parameters, especially in the sandy material, are influenced by the amount of cement present, and the degree of interlocking of grains and the degree of compaction. The undrained shear strength of the silts and clays suggests that they range from soft to very stiff.