Sandy Layers Research Articles

Numerical investigation into the effects of geometrical and loading parameters on lateral spreading behavior of liquefied layer

Numerical simulation of liquefaction-induced lateral spreading in gently sloped sandy layers requires fully coupled dynamic hydro-mechanical analysis of saturated sandy soil subjected to seismic loading. In this study, a fully coupled finite element model utilizing a critical-state two-surface-plasticity constitutive model has been applied to numerically investigate the effects of surface/subsurface geometry on lateral spreading. Using a variable permeability function with respect to excess pore pressure ratio is another distinctive feature of the current study. The developed code has been verified against the results of the well-known VELACS project. Lateral spreading phenomenon has been the focus of an extensive parametric study using the developed code. Numerical modeling of three different geometrical forms of surface and subsurface of liquefiable layer indicates that the amount and direction of lateral spreading are noticeably affected by the geometrical conditions. Also, parametric studies by the developed numerical model show that the effects of layer height, maximum ground acceleration, and loading frequency on the level of lateral spreading are significantly different for the examined geometrical condition.

The Discussion about Improving the Bearing Capacity of Single Pile for Bored Pile in Sandy and Silt Layer

The bored pile which have the unique advantage in having no noise, no shake, no upheaval, no sides way in the process of construction has became into one of the most widely used pile foundations in high heavy construction in Inner Mongolia region. Through an engineering project of bored piles in Inner Mongolia region, this article takes an analysis for bearing behavior of the bored piles in sandy and silt layer in this region. It also discusses the influence construction technology on pile-end ultimate resistance and the reasonable range of reduction factor of the ultimate resistance. When the soil layers structure is similar, it could improve bearing capacity of single pile for bored pile by utilizing the supporting layer and substratums optional regrouping. Utilize traditional single pile bearing capacity theory formula calculating and the static load test results it takes an analysis, finding out the approach of improving the bearing capacity of single pile for bored pile in sandy and silt layer. So we could find the optimization design for pile foundation bearing capacity of bored pile and thus promote the improvement in bored piles theory research and engineering application level in Inner Mongolia region and provide theory and engineering design experience.

Investigation of groundwater resources in the Komadugu Yobe Valley (Lake Chad Basin, Niger) using MRS and TDEM methods

Groundwater resources quantification and management is a key issue for agricultural development in the Komadugu Yobe (KY) River valley region in the semiarid part of the Lake Chad Basin. To improve the knowledge of available groundwater resources in this poorly-documented area, a geophysical survey across the river valley was conducted near the town of Diffa, southeast Niger. The goal was to estimate the hydrogeological properties of Quaternary formations to a depth of ∼100m. Numerical modeling showed that sedimentary deposits composed of thin clayey, loamy, and sandy layers could not be accurately resolved in detail by non-invasive geophysical methods due to the limited spatial resolution of the methods. The use of the Time Domain Electromagnetic (TDEM) method alone was not sufficient to estimate aquifer parameters and the Magnetic Resonance Sounding method (MRS) was used to supplement the geophysical dataset. A twelve kilometer long profile (117 TDEM and 11 MR soundings) was surveyed across the valley to evaluate changes in hydrogeological properties of the Quaternary aquifer from the middle of the river valley to the surrounding sandy plain area. Our results show that the Quaternary aquifer below the KY valley differs from its surroundings and it can be described as follows: (a) the thickness of the KY aquifer ranges from 30 to ∼60m and is limited at depth by electrically conductive clay layer, (b) at a depth of ∼70–80m, TDEM soundings show a continuous conductive substratum (resistivity is ∼2Ωm), attributed to Pliocene clayey formations, (c) the KY valley aquifer may have a larger volume of unconfined groundwater than the surrounding plain aquifer area (MRS water contents of 20–25% and ∼15% respectively), (d) the KY aquifer transmissivity estimated from MRS data is higher than values derived from pumping tests conducted outside the river valley. This study confirms that the Komadugu Yobe valley aquifer represents a significant resource for future agricultural development. In addition, our study shows that the aquifer is not protected by shallow clayey layers; therefore, precautions against contamination should be taken to preserve the quality of this resource.

Electrical resistivity imaging of contaminant zone at Sotubo dumpsite along Sagamu-Ikorodu Road, Southwestern Nigeria

Electrical resistivity method was used to investigate the effect of leechate contamination of groundwater at Sotubo solid waste dumpsite. Both Vertical Electrical Sounding (VES) and Constant Separation Traverse (CST) technique were carried out using Schlumberger and Wenner electrode configuration, the estimated apparent resistivities (ρ) were interpreted using partial curve matching technique and computer iteration. The inferred lithology includes topsoil, sandy layer, limestone and sandy-clay with resistivity of 90, 421.5, 907.8, 143.0 Ωm and thickness of 1, ~3.8, ~29.0 m and infinity, respectively. The pseudosection revealed the leechate had a thickness of 19.8 to 25.0 m with ρ of 0.17 to 32.5 Ωm in the NW part of profile 1. In contrast, apparent resistivity of the leechate is less than 5.95 Ωm in profile 2. The leechate has infiltrated potential aquifers such as the sandstone layer. Aquiferous layer exceeding the depth of the contaminated zone can be drilled with screened borehole in order to avert groundwater pollution. Key words: Leechate, contamination, ERI, Sotubo, groundwater

Experimental study on the development of collapse of overhanging layers of gully in Yuanmou Valley, China

The collapse of overhanging layers of the heads and sidewalls of gullies accelerates gully development . Thus, the development features of the collapse of overhanging layers must be understood. In this study , the process of collapse of overhanging layers in man-made soil bodies containing different soil layers was investigated by flow scouring experiments. A soil body composed of a lower sandy layer and an upper layer of clay soil was evaluated, and a field soil profile was modelled in a test tank with an open side. The results revealed that the occurrence of the collapse of overhanging layers could be attributed to the development of a scour hole in the open side; the greater the flow rate, the lower the proportion of on -wall runoff. On -wall runoff played a more important role in the development of scour holes than off-wall runoff. The shape of scour holes formed under different flow conditions was similar. Also, the scour hole is not apparently related to the flow rate, implicating that the collapse is more dependent on rainfall (runoff) time rather than rainfall (runoff) intensity in the studied region; this is substantially different from other soil erosion types that are closely related to rainfall intensity. Moreover, the maximum length (S-L) and height (S-H) of the scour holes increased gradually with accumulated flow and scouring time; in addition, the height of the hanging soil body (H-D) gradually decreased, and a linear relationship between SL and HD was observed (H-D = -0.359; S-L + 117.3; R-2 =0.861; P = 0.003). After the seventh scour test, the scour hole triggered the collapse of the upper clay layer. The cantilever model was modified to predict the stability of the overhanging soil body. It provided a satisfactory explanation of the static balance of the weight of the soil as well as the tensile strength. After the seventh scour test, the measured and predicted threshold values of H-D, S-L, and T-C were nearly equal, and a stable balance state was attained. However, when a 10 cm tension crack appeared in the overhanging soil body, a stable balance state was obtained during the sixth scour test. (C) 2013 Elsevier B.V. All rights reserved.

Assimilation of terrigenous organic matter via bacterial biomass as a food source for a brackish clam, Corbicula japonica (Mollusca: Bivalva)

Corbicula japonica collected from the Kitakami River estuary, northeastern Japan, showed lower δ34S values in soft-body parts (+1.7 to +11.0‰) than the ambient seawater sulfate sulfur (+21‰), and this value gradually decreased at successive sites up to 15.8 km upstream from the river mouth. Previous study using carbon and nitrogen isotopes suggests that the bivalve nonselectively assimilates particulate organic matter of marine and terrestrial origin by filter feeding. This pattern in δ34S values may indicate a considerable contribution of a food source derived from terrigenous organic matter, which has low δ34S values close to 0‰, and the bivalve has been reported to have cellulase and hemicellulase activities. Unique fatty acids (iso 17:0 and anteiso 17:0 acids), both characteristic of sulfate-reducing bacteria, were observed in the salt-free, soft-body parts of the bivalve. The concentration of monounsaturated fatty acids, possibly derived from bacteria, was also high. Trace amounts of a polyunsaturated fatty acid (20:5ω3) specific to dinoflagellates were detected. In Corbicula habitats, reductive sandy layers with ample sulfides that were sporadically intercalated into the oxidative sandy sediment were often observed. The stable isotopic signatures of sediment sulfides (acid-volatile sulfide) and associated pore-water sulfates were −8.9 to +8.6‰ and +22.4 to +26.3‰, respectively, indicating the existence of bacterial sulfate-reducing activity and thiobios biomass. These isotopic signatures of the sediment, with the fatty acid composition of the bivalve, confirm the importance of a food source derived from bacteria belonging to the thiobios in the substrate sediments via pedal feeding, rather than direct digestion of terrigenous organic matter, in this estuarine ecosystem.

Liquefaction phenomena associated with the Emilia earthquake sequence of May–June 2012 (Northern Italy)

Abstract. In this paper we present the geological effects induced by the 2012 Emilia seismic sequence in the Po Plain. Extensive liquefaction phenomena were observed over an area of ~ 1200 km2 following the 20 May, ML 5.9 and 29 May, ML 5.8 mainshocks; both occurred on about E–W trending, S dipping blind thrust faults. We collected the coseismic geological evidence through field and aerial surveys, reports from local people and Web-based survey. On the basis of their morphologic and structural characteristics, we grouped the 1362 effects surveyed into three main categories: liquefaction (485), fractures with liquefaction (768), and fractures (109). We show that the quite uneven distribution of liquefaction effects, which appear concentrated and aligned, is mostly controlled by the presence of paleo-riverbeds, out-flow channels and fans of the main rivers crossing the area; these terrains are characterised by the pervasive presence of sandy layers in the uppermost 5 m, a local feature that, along with the presence of a high water table, greatly favours liquefaction. We also find that the maximum distance of observed liquefaction from the earthquake epicentre is ~ 30 km, in agreement with the regional empirical relations available for the Italian Peninsula. Finally, we observe that the contour of the liquefaction observations has an elongated shape almost coinciding with the aftershock area, the InSAR deformation area, and the I ≥ 6 EMS area. This observation confirms the control of the earthquake source on the liquefaction distribution, and provides useful hints in the characterisation of the seismogenic source responsible for historical and pre-historical liquefactions.

Geo-Electric and 3D-Imaging of Groundwater Distribution along Flood Plain Deposits of River Niger at Jebba, North-Central Nigeria

Twenty five profiles of Vertical Electrical Sounding were obtained at Jebba area, North-central Nigeria with the aim of ascertain the nature of groundwater distribution in the area. The area is characterized by arid climate of low rainfall and high evaporation. The old alluvial plains surface is covered with a silty-sand and gravel which are subject to new reclamation. The first sedimentary cover unit < 0.5 m is draped by silty-clay causing some of the area under analysis to be water-logged. It is underlain by a sandy layer more than one meter thick beneath the surface. The geo-electric cross-sections produced from the interpreted twenty five VES data using iterative computer program revealed three to four geo-electric layers. The top most layer ranged between 5 ?m-1722 ?m and is suggestive of lateritic clay with a thickness of 0 m-1.5 m. Layer two has thickness of 1.5 m-28 m with an average apparent resistivity of 30 ?m-122 ?m and is suggestive of weathered basement layer. Layer three has a geo-electric layer of thickness between 28 m-45 m with an average apparent resistivity of between 50 ?m-350 ?m which is suggestive of partly weathered/fractured basement and fourth layer suggestive of the fresh basement. The geo-electric sections suggested two system aquifer, weathered layer aquifer system and the partly weathered/fractured basement aquifer system. These aquifer units possess largely good hydraulic characteristics and groundwater potential. This study enables the delineation of shallow aquifers, their subsurface disposition and identified promising areas for elaborate groundwater development in the area. An integration of such hydro-geophysical study with lithologic logs/drilling data would enhance accurate delineation of aquifers of the study area. Another high point of this study is the application of the leapfrog hydro software to prepare 3D Imaging of the sub-surface and to delineate areas high groundwater potentials.

地下水圧および飽和度による粘着力の変化を考慮した切羽安定評価法

A ground water has a great influence on the tunnel face stability. This paper describes an evaluation method of tunnel face stability affected by ground water pressure and variation of sandy layer's cohesion due to a degree of saturation. A rigid plastic finite element analysis (RPFEA) was applied to simulate the face collapse in the study. The method pays attention only to the plastic state and uses the upper bound theorem. Body forces in the analysis were self-weight of soil and water-pressure. The subject is a railway tunnel of Tohoku-Shinkansen, constructed in soft ground by the shotcrete tunneling method, or the currently so-called NATM. At the construction site, the ground water level is high throughout the whole section, and the ground near the tunnel face is sandy soils with a clayey layer, whose position at the face varies by the face advance. The ground water flows both in the lower and upper sandy layers separated by the clayey layer. Therefore, deep well method was applied to lower the ground water level, since the ground water level normally has significant influence on the stability of the tunnel face. However, it is difficult to lower the ground water level in the upper sandy layer. Therefore, the authors analyzed the tunnel face stability by measurements of the groundwater level, results of face observation and RPFEA. As a result, it has been known that the face stability is affected strongly by value of soil cohesion c and it is necessary to estimate strictly the value of soil cohesion c. For this reason, the authors studied on the relation between degree of saturation and cohesion of sandy layer, and applied this relation to the evaluation of tunnel face stability. Furthermore, they developed the indicator for selection of auxiliary method to stabilize the tunnel face by the evaluation method.

Preliminary Geophysical Investigation to Delineate the Groundwater Conductive Zones in the Coastal Region of Akwa Ibom State, Southern Nigeria, around the Gulf of Guinea

This paper presents the first work of its kind within the confines of the study area. It unravels the distribution of the layers of conductive sand and their depths of interaction between freshwater from fresh sands and saltwater within the conductive layers in the coastal region of Akwa Ibom State (Nigeria) around the Gulf of Guinea. Vertical electrical sounding (VES) data whose fidelity was achieved by constraining the data by the available nearby logged borehole information during interpretation was the method applied. In the western region of the study area, the ferruginized and saline water layer is found within the depth range of 22 to 75 m deep. In the northern zone, conductive sandy layer is found within 50 to 210 m and in the eastern zone, the saline and ferruginized sandy layer is found within the depth of 88.5 m and above. Generally, the horizontal and vertical cross sections of the subsoil and the flow regime from water table depths have been delineated. With these information, water can be tapped in the area with caution and the flow direction determined can be used as input parameter in detailed contamination study.

Integrated Geophysical and Geochemical Methods for Environmental Assessment of Municipal Dumpsite System

Leachate originating from open refuse dumpsite systems can be delineated through an integration of qualitative and quantitative methods. This study was designed to examine extent of leachate and pollution from one of the numerous open refuse dumpsites in Lagos metropolis. Qualitative assessment was determined using electrical resistivity tomography (ERT), vertical electrical sounding (VES) and induced polarization geophysical methods. Both ERT and VES methods revealed persistent low resistivity (1 - 20 Ωm) of leachate to the depth above 35 m. The two methods were projected to produce 3-D view of the site which shows a NW-SE flow pattern of the leachate and possibly, the groundwater. IP values observed over the polluted zone was -2.9 - 8 ms, indicating a sandy layer. Quantitative assessment was achieved by analysis of geochemical substances in the water samples taken from wells and boreholes in the precinct of the dumpsite. Here, we examine the macroelements, salts (sulphates, nitrates and chlorides), heavy metals, radioactive metals contents and physical parameters of the water samples. The analyses reveal the presence of these substances in the water and their strong correlations justified the provenance as the same. As part of the quantitative evaluation, physical parameters (pH, TDS, DO, salinity, total hardness, turbidity, electrical conductivity EC and temperature) of the water samples were also determined. The samples pH plotted in the acidic domain unsuitable for human consumption. Leachate flow direction was generated from the decreasing concentration of measured parameters (geochemical elements and physicals properties) in NW-SE direction which agrees with similar flow pattern deduced from ERT results.

Efficiency of Constructed Wetland Vegetated withCyperus alternifoliusApplied for Municipal Wastewater Treatment

The treatment of municipal wastewater from Yazd city (center of Iran) by constructed wetland vegetated with Cyperus alternifolius was assessed. Two identical wetlands with a total working volume of 60 L and 10 cm sandy layer at the bottom were used. First wetland (W1) was control and had no Cyperus alternifolius plant. Second wetland (W2) had 100 Cyperus alternifolius shrubs with 40 cm height. Influent wastewater was provided from Yazd's septic tanks effluents and after a 4-day retention time in wetlands, reactors effluent was sampled for parameters analysis. Results show that chemical oxygen demand (COD), NO3 −–N, NH4 +–N, and PO4 −3–P in W1 were reduced to 72%, 88%, 32%, and 0.8%, and in W2, these parameters were removed in values of 83%, 81%, 47%, and 10%, respectively. In both wetlands, the highest and lowest removal efficiencies were related to COD and phosphorus, respectively. Also, the removed phosphorus can be released to stream when the soil saturated or influent phosphorus decreased and when the plant died. After a 4-day-retention time, the W2 wetland showed a statistically significantly lower COD and NH4 +–N in comparison with W2 wetland.

東京低地と沖積層

Chuseki-so is a valley-fill deposit that originated during the latest Pleistocene and Holocene, spanning the period from the last glacial maximum (LGM) to the present. This study describes its stratigraphic framework and sedimentary processes with particular reference to Holocene sea-level change, in order to understand its topographic and depositional evolution and geotechnical properties in the Tokyo and Nakagawa lowlands. Chuseki-so has been subdivided into two formations: the Nanagochi Formation and the Yurakucho Formation in ascending order. These formations can be clearly distinguished on the basis of physical properties: the Yurakucho Formation is very soft with a high water content, while the Nanagochi Formation is slightly compacted with a lower water content. Previous studies have suggested a falling sea level at the final stage of the Nanagochi Formation, corresponding to the Younger Dryas cold event. This led to the emergence of the deposits and physical differences due to escaping of pore water. However, recent studies seem to suggest that the sea level did not fall during the Younger Dryas. During the main stage of the Nanagochi Formation, from the end of the LGM until the Younger Dryas Event, sea level rose quickly, particularly during the Bølling/Allerød interstadial (B/A), and rapid sedimentation characterized by alternations of sand and mud occurred under a warmer and wetter climate. A new idea proposes that at the stage of the Nanagochi Formation, pore water of mud layers escaped easily into sandy layers immediately after sedimentation (Tanaka et al., 2006). The Yurakucho Formation, on the other hand, consists of very soft marine muddy and partly sandy sediments, and the main stage is thought to have been formed by rapid sedimentation in an inner bay environment during the Holocene. Topographic and paleogeographic changes closely related to the Holocene Jomon Transgression, have been investigated by reconstructing the migration of oyster reefs, which are good markers of paleoshorelines, throughout the wide inner bay during the rising stage of sea-level. After the highest stage of sea level, or thereabouts, the delta front began to prograde rapidly toward the south with the delta front migrating about 30-40km, reaching the present northern Tokyo Lowland 2000-3000 years ago. The NE Japan Megaearthquake of March 11, 2011, damaged large coastal areas of eastern Japan due to liquefaction and related phenomena. The northeast coast of Tokyo Bay in the Tokyo Lowland was damaged severely. Liquefaction occurred throughout the reclaimed land, which is composed of soft sandy soil dredged from the bay. The reclaimed sandy soil is underlain by new sandy delta front deposits and very soft thick marine valley-fill clays. It is essential to clarify the mechanisms of liquefaction in relation to the properties of soft ground (Chuseki-so), and the effects of shaking caused by strong earthquakes and subsequent sloshing for a long period.(View PDF for the rest of the abstract.)

Phosphorus fertilization and the saturation of bases for formation of seedlings of Swietenia macrophyla

Production of quality seedlings with high survival rate in the environment for reforestation purposes still faces many obstacles, especially with respect to mineral nutrition. In this study, we aimed to evaluate the response of mahogany (Swietenia macrophyla) seedlings to the supply of P and base saturation in a Typic Hapludox (Oxisol) with sandy layer surface. We used a completely randomized experimental design with four phosphorus rates (0, 50, 100 and 150 mg dm–3 of P), with Arad phosphate rock source, with two base saturation levels (natural at 25% and corrected to 45%) and three replications. We assessed the following parameters: soil chemical attributes, stem diameter, plant height, root dry matter, aerial part dry matter, total dry matter, and aerial part dry matter/root dry matter ratio. The content of P in the soil increased with P supply levels, in the absence and presence of liming, respectively. Liming improved soil attributes and the growth of mahogany seedlings. The combination of liming with phosphorus fertilization positively influenced plant height and aerial part and total dry matter. In the absence of liming, P doses as from 100 mg kg–1 negatively influenced the growth of mahogany seedlings.

Causes for the Formation of Waterlogged Land in the Black Soil Region of Northeastern China

Waterlogged land characterized by low productivity impact the productivity of the adjacent land and increase soil erosion. Gangzhongwa, one type of waterlogged lands, is widely distributed in Jiusan cultivated lands in Heilongjiang province China. Observation of soil profiles and measurements of soil physical properties were carried to investigate the factors related to the waterlogged land formation. The results showed Gangzhongwa waterlogged land is formed due to three reasons: firstly, the water table of groundwater is shallow and the soil profile texture varies regularly from clay loam layer, and sandy loam layer to aquitard layer; secondly soil infiltration is significantly correlated with sand content and gravel content; and thirdly the subsurface runoff makes the soil profile saturated where the surface clay loam layer is thin but the sandy layer is thick with larger infiltration rates. This study presents some theoretical insights for the Gangzhongwa waterlogged land conservation plan.

Liquefaction Survey and Case Analysis in Wenchuan Earthquake

Total 163 survey points distributed in 104 liquefied and non-liquefied sites were selected in the Wenchuan earthquake liquefaction database, where 50 survey points were from intensity VI region. Borehole data show that pure sandy layers could hardly be identified in Wenchuan earthquake affected areas, instead gravel soil layers mingled with sand silty or pebble prevalently exist. Songbai village which suffered severe liquefaction was chosen for case study in this paper. Through MASW and high density electrical method testing, three soil layers can be distinguished from the testing cross section. The discontinuity of resistivity of the first layer by the high density electrical method indicates the location of sand ejection. The grain size distribution curves of sand ejecta were different from drilling soil samples, which explanation need further investigation.

Depositional dynamics in the El'gygytgyn Crater margin: implications for the 3.6 Ma old sediment archive

Abstract. The combination of permafrost history and dynamics, lake level changes and the tectonical framework is considered to play a crucial role for sediment delivery to El'gygytgyn Crater Lake, NE Russian Arctic. The purpose of this study is to propose a depositional framework based on analyses of the core strata from the lake margin and historical reconstructions from various studies at the site. A sedimentological program has been conducted using frozen core samples from the 141.5 m long El'gygytgyn 5011-3 permafrost well. The drill site is located in sedimentary permafrost west of the lake that partly fills the El'gygytgyn Crater. The total core sequence is interpreted as strata building up a progradational alluvial fan delta. Four macroscopically distinct sedimentary units are identified. Unit 1 (141.5–117.0 m) is comprised of ice-cemented, matrix-supported sandy gravel and intercalated sandy layers. Sandy layers represent sediments which rained out as particles in the deeper part of the water column under highly energetic conditions. Unit 2 (117.0–24.25 m) is dominated by ice-cemented, matrix-supported sandy gravel with individual gravel layers. Most of the Unit 2 diamicton is understood to result from alluvial wash and subsequent gravitational sliding of coarse-grained (sandy gravel) material on the basin slope. Unit 3 (24.25–8.5 m) has ice-cemented, matrix-supported sandy gravel that is interrupted by sand beds. These sandy beds are associated with flooding events and represent near-shore sandy shoals. Unit 4 (8.5–0.0 m) is ice-cemented, matrix-supported sandy gravel with varying ice content, mostly higher than below. It consists of slope material and creek fill deposits. The uppermost metre is the active layer (i.e. the top layer of soil with seasonal freeze and thaw) into which modern soil organic matter has been incorporated. The nature of the progradational sediment transport taking place from the western and northern crater margins may be related to the complementary occurrence of frequent turbiditic layers in the central lake basin, as is known from the lake sediment record. Slope processes such as gravitational sliding and sheet flooding occur especially during spring melt and promote mass wasting into the basin. Tectonics are inferred to have initiated the fan accumulation in the first place and possibly the off-centre displacement of the crater lake.

Sand volcanoes induced by the April 6th 2009 Mw 6.3 L’Aquila earthquake: a case study from the Fossa area

This paper presents the study of some liquefaction features occurred near the Fossa village due to the April 6, 2009, Mw 6.3 L’Aquila earthquake (Central Italy). Our investigation is based on trenching and coring campaigns as well as sedimentological analyses and datings. The geometrical elements of the sand volcanoes on the surface, of the dike used to rise up and of the probable sandy source at depth are presented. A sandy pockets level found at less than 1 m of depth, interpreted as possible evidence for a paleo-liquefaction event is discussed. Sedimentologic and morphoscopic analyses both provided the necessary elements and parameters to link the ca. 4 m deep sandy layers to the 2009 sand blows on the ground surface as well as to the paleo-liquefaction layer and defined the main characteristics of the deposits sealing the sands that experience liquefaction at depth. Finally a tentative correlation between the paleo-liquefaction layer and the 1461 AD or the 1703 AD local earthquakes is suggested based on the available age constraints.

Engineering Site Characterisation Using 2-D and 3-D Electrical Resistivity Tomography

Land reclaimed from water bodies or its equivalent (swamp, flood zones and abandon river channel) usually contain certain mechanically unstable geological formations (e.g. peat and clay) which are inimical to the foundation of engineering structures. Concerted effort must therefore be ensured to determine depth to competent layer across the proposed site as against conventional geotechnical test which is a point test. Complementary non invasive geophysical methods must be carried out for any engineering construction so as to ensure stability of building construction and avoid the devastating effects of building collapse rampant of late. Thus, electrical resistivity tomography and induced polarisation methods, constrained by percussion drilling, were used for the present study. The results of the 2-D and 3-D DC resistivity tomography using 64 multi-electrode systems with gradient electrode configuration along five parallel profiles with smallest electrode separation of 4.0m on a total length of 252 m was interpreted using Earth Imager Inversion software. The subsurface was characterised by Peat/Clay sandwich between upper and lower Clayey-sand/Sandy-clay and Sand. The presence of Peat and Clay were correlated to the cause of chronic differential settlements of some engineering structures within the area. The depths to the sand material were observed between 17-47 m. At shallow depths, the sand materials were underlain by mobile clay/peat materials, which correlates with the depth to competent layer (Sand) achieved by percussion drilling in the range of 20-24 m. This lithological sequence in which the near surface sand deposits are underlain by clay/peat saturated with water, gives a sense of instability to the overlaying sandy layer. To buttress this finding, residential and other public buildings surveyed in the entire area are marked with various degrees of distress, ranging from multiple cracks, sinking of building, partial collapse to complete collapse. It should be noted that the depth to the sand layer achieved during the research varied along the respective profiles. Thus application of 2-D resistivity tomography has revealed both the lateral and vertical variation in depth to competent sand layer within the study area as opposed to point information from CPT or Percussion drilling. This provides a useful guide to the site engineers in designing appropriate foundation structure.

Application of Electrical Resistivity Tomography in Mapping Subsurface Hydrocarbon Contamination

The subsurface soil around Baruwa community was reportedly contaminated by hydrocarbon not because the area falls within oil-producing community but it suffers from inceasant leakage from petroleum pipeline that supply petroleum products to hydrocarbon terminar located within the area. Thus, to ascretain the extent of contamination of the subsurface soil around the area, an integrated geophysical methods involving vertical electrical sounding (VES) and electrical resistivity tomography (ERT) supported with induced polarization (IP) and spontaneous potential (SP) methods were carried out. The hydrocarbon contaminated layers were marked out beneath each VES point by high resistivity ranging between 943&Omegam and 4749&Omegam at a depth of 1 to 35.44m below the surface. Similarly, ERT result shows that the subsurface soil around the investigated area has been contaminated at a shallow depth of about 2m downward with resistivity value above 1000&Omegam. IP and SP data were Integrated to identify the sandy contaminated layers from clayey layer. IP value of 0 to 10mV/V and SP values of &saquo10mV were obtained over the sandy formation. The work shows that hydrocarbon leaking from the pipeline laid a few meters beneath the earth surface actually flow both upwardly (possibly due to seasonal variation in the water table which is usually close to the surface during rainy season) to the surface and downwardly at greater depth into the subsurface, through a porous medium - sandy layer. This may probably accounts for the reason while most of the handdug wells in the area are reportedly contaminated with hydrocarbon products.