The deformation history of southern England, and its implications for ground engineering in the London Basin

Structures in the basement beneath the London Basin affect the geology of relevance to geotechnical engineering within London. Unfortunately, the basement beneath London is covered by Cretaceous and Tertiary sediments. It is cut by major faults linked to the compressive phases of the Hercynian and Alpine Orogenies and to the regional extension that occurred during the Mesozoic between these compressive events. Evidence is presented that movement on basement fractures beneath London played a major role in the distribution and deformation of sediments within the Basin, causing local folding and faulting significant to engineering works. Basement rocks are exposed in SW England where the type and orientation of these fractures (faults and joints) can be examined in outcrop. This study, complemented by seismic sections in the southern UK, enable the architecture of this fault network within the basement to be determined. Understanding the fracture system in the basement provides a basis for (i), interpreting the lateral facies variations of sediments in the Basin and hence provides a means for predicting from a ground investigation the likely presence, activity or influence on site of such structures at depth and (ii), understanding the extent of local, steeply inclined and sub-horizontal planar zones of shearing when encountered on site.Thematic collection: This article is part of the Geology of London and its implications for ground engineering collection available at: https://www.lyellcollection.org/cc/london-basin

An Investigation into Ground Movement on the Ventnor Landslide Complex, UK Using Persistent Scatterer Interferometry

Analysis of ground movement rates along the coastline and upper sections of the Ventnor landslide complex was carried out utilizing Persistent Scatterer Interferometric Synthetic Aperture Radar methods using Sentinel-1 SAR data from 2015 to 2019 (four years). Results were compared with rainfall data, historical ground investigation records and monitoring surveys carried out at Ventnor to relate observations to geology, geomorphology and rainfall. Decomposition of InSAR viewing geometries to vertical and horizontal aligned well with previous ground-based studies. Subsidence of −9.8 mm a−1 at the Lowtherville Graben and heave of +8.5 mm a−1 along the coastline south of Ventnor Park were observed. Decomposition to east-west geometry results showed an eastward displacement of approximately 12.4 mm a−1 along the coastline south of Ventnor Park, and a westward displacement of −3.7 mm a−1 throughout built up sections of Ventnor town, indicating the landslide was displacing more in an eastern direction than vertically. The cause of this movement was investigated by using publicly available intrusive boreholes paired with Persistent Scatterer Interferometry, and a new ground model spanning east-west parallel to the coastline was presented. No evidence of significant ground movement was observed along heavily protected sections of the coastline, suggesting coastal defences comprised of concrete aprons and rip rap appear to be an effective coastal management/landslide stabilisation tool when compared to rip rap alone. The mechanism of this increased stability is likely due to the combination of toe weighting and reduced toe erosion. A lag of approximately 13–20 days was observed between high rainfall events and subsequent peaks in ground displacement, which was shorter than a 29 day lag observed in a previous study. Similar observations of prolonged rainfall resulting in prolonged displacements were also observed. The study demonstrates the capabilities of the PSI methodology in identifying the same ground movements that conventional methods provide. By providing detailed analysis of ground deformation of the Ventnor landslide, we demonstrate small ground movements, validated with existing ground movement surveys. Similar methodology can be applied to coastal landslides in urban environments worldwide, providing a relatively cheap and rapid resource for coastal landslide monitoring.

Geological map of Athens Metropolitan Area, Attica (Greece): A review based on Athens Metro ground investigation data

The ground investigations for the construction of Athens Metro –including over 60.000 m of sampling boreholes and geological mapping of the underground tunnel face–, planned and carried out under the supervision of ATTIKO METRO S.A., offer important geological data that enrich and locally modify our knowledge for the geology of Athens Metropolitan Area (AMA). On the basis of these data, this paper presents the Geological Map of AMA as well as a revised tectonostratigraphic scheme for the area and geological profiles along several sections of the Athens Metro lines. The geological map is a synthesis of the geological data obtained from the ground investigations with the already published geological maps and includes a Mesozoic rock assemblage as well as the Neogene-Quaternary Athens Basin. The following basic conclusions can be drawn from the interpretation of these data: (a) The Athens Unit, the basement of AMA, is divided into four formations (from bottom to top), the Lower Athens Schist, the Upper Athens Schist, the Athens Sandstone-Marl Series and the Crest Limestone. (b) Ultrabasic rocks (serpentinite) constitute the basement of Athens Unit. (c) Serpentinite bodies at the eastern border of Athens Basin, have undergone almost complete metasomatism to listwanite along their tectonic contacts with Alepovouni Marble on top and Kessariani Dolomite at their base. (d) The limestone outcrops at the western border of Athens Basin (e.g., Karavas hill) form tectonic windows of Pelagonian Upper Cretaceous limestone underneath the Athens Schist and not klippen of Crest Limestone on top of it. The revised geological map also includes the Attica-Evia Fault, which is the dominant structure of the broader area, locally mapped by two sampling boreholes across the planned metro line 4.

MONITORING CRITICAL INFRASTRUCTURE EXPOSED TO ANTHROPOGENIC AND NATURAL HAZARDS USING SATELLITE RADAR INTERFEROMETRY

Synthetic Aperture Radar Interferometry (InSAR) is a remote sensing technique very effective for the measure of smalldisplacements of the Earth’s surface over large areas at a very low cost as compared with conventional geodetictechniques. Advanced InSAR time series algorithms for monitoring and investigating surface displacement on Earth arebased on conventional radar interferometry. These techniques allow us to measure deformation with uncertainties of 1mm/year, interpreting time series of interferometric phases at coherent point scatterers (PS) without the need for humanor special equipment presence on the site. By applying InSAR processing techniques to a series of radar images over thesame region, it is possible to detect line-of-sight (LOS) displacements of infrastructures on the ground and therefore identifyabnormal or excessive movement indicating potential problems requiring detailed ground investigation. A major advantageof this technology is that a single radar image can cover a major area of up to 100 km by 100 km or more as, for example,Sentinel-1 C-band satellites data cover a 250 km wide swath. Therefore, all engineering infrastructures in the area, suchas dams, dikes, bridges, ports, etc. subject to terrain deformation by volcanos, landslides, subsidence due to groundwater,gas, or oil withdrawal could be monitored, reducing operating costs effectively. In this sense, the free and open accessCopernicus Sentinel-1 data with currently up to 6-days revisit time open new opportunities for a near real-time landmonitoring. In addition, the new generation of high-resolution radar imagery acquired by SAR sensors such as TerraSARX,COSMO-SkyMed, and PAZ, and the development of multi-interferogram techniques has enhanced our capabilities inrecent years in using InSAR as deformation monitoring tool. In this paper, we address the applicability of using spaceborneSAR sensors for monitoring infrastructures in geomatics engineering and present several cases studies carried out by ourgroup related to anthropogenic and natural hazards, as well as monitoring of critical infrastructures.

Hydrogeophysical characterization of aquifers in Upper Denkyira East and West Districts, Ghana

Hydrogeophysical assessment of aquifers in the Upper Denkyira East and West Districts of Ghana has been carried out for groundwater potential and protective capacity of the overburden rocks determination. The data for the study were obtained from the Regional office of Community Water and Sanitation Agency (CWSA), Cape Coast. A total of thirty-seven VES probed using a Schlumberger array with maximum current electrode spacing (AB/2) of 100 m at each point using the Abeam Terameter was used. The data were interpreted using the partial curve matching and WINRESIST computer iteration program techniques. The thickness and resistivities of the various overburden layers, basement resistivity, reflection coefficient and longitudinal conductance were used for the assessment of the groundwater potential and the protective capability of the overburden layers. The study revealed three-to-six layers with an average of four (4) layers including laterite (0.34–3.57 m); clay (0.64–8.84 m); sandy-clay (0.67–27.09 m) and slightly-to-highly weathered bedrock which includes phyllites and granite (3.09–86.89 m) that show high level of heterogeneity of electrical resistivity of the geologic materials within the study area. The weathered layers serve as the aquiferous zones. About 21.6%, 62.2%, 2.7% and 13.5% showed high, medium, low and very low potential for groundwater, respectively, with higher groundwater potentials at the western and south-eastern corner of the study area underlain by the Birimian and Tarkwaian formations, respectively, indicating that the two formations have similar groundwater potentials in the study area. The lowest groundwater potential was observed at the point underlain by the granitic intrusion. The assessment of the protective capacity of the aquifer showed very good (5%), good (27%), moderate (19%), weak (22%) and poor (27%) indicating that the study area is overlain mostly by materials of different protective capacities. This study presents information on the aquifer protective capacity evaluation by using geophysical technique and it has revealed that the Birimian formation has a good aquifer protective capacity than the Tarkwaian formation and has also confirmed the effectiveness of using geophysical techniques in ground investigation.

Study of Ground Investigation Data along the Brunei Temburong Bridge Alignment

The Brunei Temburong Bridge (BTB) is an iconic project connecting Brunei Muara and Temburong districts in Brunei over a length of 30 km. The bridge crosses from Brunei Muara across Brunei Bay to Temburong. A key aim of the BTB project was the development of Temburong district, being prior the more isolated district, because of not having a direct land link to the other districts. This study reviews the geology of soil materials and sedimentary rocks along the bridge alignment based on 164 boreholes drilled for ground investigation. Standard Penetration Test, Rock Quality Designation and Fracture Index data were evaluated to interpret soil materials and sedimentary rocks along the alignment. Borehole data was used to generate lithological cross sections along the alignment providing an overview of geology. The geology encountered is extremely heterogenous, with low strength sediments encountered at depth of up to 80 m and more e.g., at the Temburong section. Rock quality designation vs. Fracture Index did not show a strong correlation likely due to the extreme heterogeneity in particular for the Brunei Bay and Temburong sections. The study demonstrates the importance of a closely spaced investigative drilling and sampling program for major construction programs which was successfully achieved for this project. Other investigative techniques such as geophysical methods could be highly useful for future projects. Considering economic and environmental reasons these techniques could be very useful to map the top of the bedrock even under highly heterogeneous ground conditions.

Development Mechanism of Moraine Debris Flows in Parlung Zangbo Basin, Southeast Tibet, China

<p>Located in the north of the Great Bend of Yarlung Zangbo River, the Purlung Zangbo basin is the largest distribution area of marine glaciers in China. Here is one of the most serious sections for geological disasters in the Sichuan-Tibet transportation corridor, which poses a serious threat to the planning and construction of the Sichuan-Tibet Railway and Expressway and other important infrastructures. In order to assess the risk of geological disasters along the proposed Sichuan-Tibet Railway, more than 80 typical moraine landslides, glacier collapsed debris flows, and glacial lake breakouts in the basin were selected through remote sensing interpretation and field verification, which were carried out ground investigation and geotechnical properties tests in detail. The main types, material sources and key influencing factors of the moraine geological hazards are analyzed by in-situ tests of large-scale straight shear, double ring penetration, matrix suction, dynamic penetration and indoor sample testing methods such as consolidation compression, triaxial shear, and OSL dating, combined with regional temperature, rainfall, multi-stage remote sensing images and geological mapping data. Finally, the starting mechanism and disaster scale of loose moraine are preliminarily simulated by combining the water and thermal product index method and FLOW 3D numerical analysis software. The results show that the physical and mechanical properties of moraine have good statistical relations with its water content, void ratio and clay content. The natural density and compression modulus are negatively linear related to the void ratio, while the vertical permeability coefficient and the free expansion ratio show a quadratic relationship with clay content, and also the shear strength and compressive strength are binary relationship with void ratio and moisture content. The permeability, consolidation degree and free surface characteristics are the key parameters affecting the stability of moraine slope. Moraine located in three depositional positions (the front of modern glacier tongue, the middle reaches of glacial valley and the bank slope of main river ) has different starting modes, which are "shoveled-scraped and migration type", "erosion and blocking-burst type", and "unloaded and permeated type". When it intersects with the main traffic lines, different countermeasures and application should be adopted: avoidance by bridges or tunnels, slope reinforcement, and cutting or subgrade. </p>

Geoengineering problems and solutions in the Murge karst (Apulia, SE Italy)

<p>The Murge area extends for about 6000 km<sup>2</sup> and is represented by a NW–SE karst plateau whose elevation decreases by means of fault-bounded displaced blocks toward NE, from about 680 m a.s.l. (High Murge) down to the Adriatic sea (Low Murge and Apulian Adriatic shelf). Geologically, it consists of a 3 km-thick Cretaceous carbonate succession of well-bedded limestones and dolomites, locally covered by thin Late Pliocene-Quaternary deposits, namely calcarenites and subordinate sands and clays. From the end of Cretaceous up to Pliocene, the Murge area experienced a long period of exposure which favored the development of karst processes responsible for the genesis of surface and underground features, like swallow holes, dolines, dry valleys, poljes and caves. In particular, the main cave and conduit systems develop in the first tens of metres from the ground, involving entirely the epikarst or subcutaneous zone. </p><p>Many are the geoengineering problems in this area due to complexity of the karst landscape and of the underground drainage system. Some of them regard the hydrogeological and hydrological aspects, involving aquifer pollution and groundwater contamination, or flash floods related to clustered rainfall and the related sudden runoff; other problems result from rock or soil failure mechanisms through occurrence of collapse and suffosion sinkholes. In the Murge karst, collapse sinkholes occur both as natural phenomena, linked to karst caves, but they can be also induced by anthropogenic cavities, consisting of excavation by man for shelter, cultural proposes or where rocks were mined to be used as building material. In particular, the occurrence of sinkholes results typically from sudden collapses of the roof of underground voids, and have been at the origin of casualties and severe damage. Subtle and gradual suffosion sinkholes develop where seepage erosion occurs in sandy soils whose grains settle into voids in the underlying carbonate rocks. This mechanism induces differential settlements and rotations of foundations, leading to instability of buildings and other structures.</p><p>Numerous remedial and preventive solutions dealing with geoengineering aspects in karst can be adopted due to the complex peculiarities and high variability of the Murge landscape. Thus, the selection of appropriate measures to predict and remediate future damage scenarios becomes very important and require i) the development of detailed geological and engineering geological models, and ii) careful understanding of the geological hazards, and of their likely effects.</p><p>The main difficulties for planning and monitoring actions are linked to the implementation of integrated methods capable of exploring and modeling carbonate rock masses and their structural uncertainty in the karst environment. Multi‐technique geophysical methods, integrated with geotechnical surveys (including borehole drillings) can be adopted for passing from a conceptual geological model to an observational engineering-geological model, constrained by data from site-specific ground investigation and laboratory tests. The next step is then constituted by assessing analytical and numerical models which usually require considerable simplifications of the engineering geological model and can be used as general guidelines for designing mitigation and remediation measures.</p>