Sandy Layers Research Articles

Dispersion of guided waves in stratified medium with a sandy layer

Dispersion analysis of acoustic guided waves, propagating in a stratified medium, containing an internal sandy layer in contact with the isotropic elastic substrate and the upper elastic isotropic layer, is developed by applying the modified Cauchy formalism coupled with the exponential fundamental matrices and the Thomson–Haskell transfer matrix method. The developed approach is suitable for analyzing dispersion properties of guided waves at the broad variation of the sandy parameter. Several phenomena, concerning anomalous dispersion of guided waves at varying sandy parameter, are observed, revealing the possibility of nondestructive evaluation of sandy layers within the upper crust.

Estimation of soil detachment capacity on steep slopes in permanent gullies under wetting-drying cycles

Permanent gullies, mainly distributed on granite weathering mantles in tropical and subtropical areas, are the major threat to the sustainable development for economy and ecosystem of southern China. Collapsing wall, with its stability influenced by the frequent wetting-drying cycles under the hot and rainy climatic conditions, is the most active part of permanent gully. The free surface along the collapsing wall provides a favorable condition for soil detachment by runoff scouring, however, little attention has been given to the detachment capacity of granite soils under the influence of wetting-drying cycles in permanent gullies. To quantify the impacts of wetting-drying cycles on soil erosion resistance in permanent gullies, the detachment capacity of granite soils (surface soil layer A; lateritic layer B; sandy layer C1; and detritus layer C2) that were collected from the collapsing wall was investigated under 0, 2, 5, and 10 cycles of wetting-drying. The impacts of wetting-drying cycles could be well quantified by soil pore size distribution that was calculated by soil-water characteristic curve (p < 0.01). The content of macro-porosity (>30 μm) increased constantly with increasing W-D cycles, while micro-porosity (<0.2 μm) decreased, especially for soils in the surface and lateritic layers (A and B). Soil erosion resistance of granite soils under the W-D cycles was determined by the integrated effects of soil porosity, and the tested soil detachment capacity soil detachment capacity in a range of 0.008–12.488 kg.m-2 s−1, increased with the increase of W-D cycles. In addition, the rill erodibility (Kr) was significantly influenced by soil bulk density, macro-porosity, and micro-porosity (R2 = 0.930, Nash-Sutcliffe efficiency = 0.910), demonstrating a power function relationship. Moreover, the critical shear stress (τc) had a significant and positive correlation with soil cohesion (R2 = 0.576, p < 0.010). Furthermore, soil detachment capacity could be well predicted by these measurable parameters including bulk density, macro-porosity, micro-porosity, soil cohesion, and shear stress of flowing water (R2 = 0.950, NSE = 0.980), under the theoretical foundation of WEPP model. The obtained results would facilitate the understanding of the formation mechanisms of the permanent gullies, as well as the development of soil conservation measures in southern China.

Soil erosion facilitates shrub encroachment in Patagonian herbaceous steppes

AbstractPrevious studies in northeastern Patagonia suggested that, mainly as a consequence of overgrazing by sheep, the herbaceous steppes are being replaced by shrub steppes dominated by Chuquiraga avellanedae as a consequence of shrub encroachment. The herbaceous steppe presents a thick sandy loam to loamy sand A‐horizon lying on a clay‐horizon while the shrub steppe presents the clay‐horizon at or near the soil surface. The thickness reduction of the remaining A‐horizon and the formation of desert pavements are evidence of soil erosion. As previous studies suggested that shrub encroachment co‐occurs with the erosion of the sandy A‐horizon, we hypothesized that shrub encroachment is favored by the presence of an clay‐horizon near the soil surface. We expected that C. avellanedae emergence and establishment would be higher as the clay horizon is closer to the soil surface and the A‐horizon is reduced. We performed field and greenhouse experiments to evaluate seedling emergence and establishment along a gradient of A‐horizon thickness. We found that the presence of a relatively thin sandy layer on the surface had a negative influence on both shrub emergence and establishment. The summer drought was not a critical period for the seedlings. Conversely, the highest seedling mortality took place during late winter in sandy soils, suggesting that these soils are more susceptible to frost heaving than clay soils. In conclusion, soil erosion in the northeastern Patagonian steppes would indirectly facilitate shrub encroachment due to differences in frost heaving susceptibility between sandy and clay soils.

Effect of the mineralogical composition on the elastoplastic hydromechanical response of Opalinus Clay shale

Opalinus Clay is the shale currently under investigation as the host formation for geological radioactive waste disposal in Switzerland. Its hydromechanical response has been widely studied, and the experimental results show a range of values whose dispersion needs to be clarified.This work aims to explain the dispersion in the literature results by correlating the hydro-mechanical response to the mineralogical variability of the tested specimens.Based on published microstructural studies, the Opalinus Clay shale is herein schematised as a sequence of two kinds of layers: the shaly (high in clay-mineral content) and the sandy (low in clay-mineral content) layers. The mineralogical composition, porosity and hydromechanical parameters are assigned to each layer type. The one-dimensional compressibility and the elastic response of combinations of layers are derived adopting an analytical solution for the stratified, transversely isotropic medium. The drained elastic properties are used to calibrate the approach, and the undrained elastic properties are derived and compared to literature data. Empirical correlations between the layer composition and the geomaterial strength are also drawn.It is shown that by adopting a layered structure with an alternation of two kinds of layers, most of the variability in the studied geomechanical properties of Opalinus Clay can be captured.

DRAINMOD modeling framework for simulating controlled drainage effect on lateral seepage from artificially drained fields

We demonstrated a DRAINMOD modeling framework to predict controlled drainage (CD) effect on the fate of water in artificially drained agricultural fields, which is key for determining the water quality benefits of the practice. To demonstrate this modeling framework, DRAINMOD simulated the hydrology of a subsurface drained grass field in Eastern North Carolina, U.S. under both free drainage (FD) and CD. Three scenarios were simulated for each water management: no lateral seepage (LS) and LS with constant and dynamic hydraulic head (Hr). For each scenario, predicted water table depth (WTD) and subsurface drainage were compared to observed values using Mean Absolute Error, Nash-Sutcliffe Efficiency, and Normalized Percent Error. Predicted water balance components for different scenarios were also investigated. Results clearly showed that LS was a significant component of the water balance for CD. Model predictions showed that 96% of the reduction in subsurface drainage due to CD could be attributed to LS (33.5 cm yr−1). The large values of LS predicted by the model were attributed to the presence of a permeable sandy layer in the soil profile, the shallow management depth of the drain outlet, and the small size of the experimental field plots. Agreement between predicted and observed WTD and subsurface drainage ranged from acceptable to excellent for FD with and without considering LS. In contrast, DRAINMOD simulations for CD yielded acceptable predictions only for the scenario considering LS with dynamic Hr. This study demonstrated the power of process-based simulation models, such as DRAINMOD, for interpreting and explaining data of experimental studies and underscored the importance of using a proper model calibration strategy for yielding reliable predictions. This study highlights the need for well-coordinated experimental and modeling research to further investigate how seepage affect CD performance for reducing drainage flow and nitrogen losses from artificially drained agricultural fields.

Experimental Deformation of Opalinus Clay at Elevated Temperature and Pressure Conditions: Mechanical Properties and the Influence of Rock Fabric

The mechanical behavior of the sandy facies of Opalinus Clay (OPA) was investigated in 42 triaxial tests performed on dry samples at unconsolidated, undrained conditions at confining pressures (pc) of 50–100 MPa, temperatures (T) between 25 and 200 °C and strain rates (dot{varepsilon }) of 1 × 10–3–5 × 10–6 s−1. Using a Paterson-type deformation apparatus, samples oriented at 0°, 45° and 90° to bedding were deformed up to about 15% axial strain. Additionally, the influence of water content, drainage condition and pre-consolidation was investigated at fixed pc–T conditions, using dry and re-saturated samples. Deformed samples display brittle to semi-brittle deformation behavior, characterized by cataclastic flow in quartz-rich sandy layers and granular flow in phyllosilicate-rich layers. Samples loaded parallel to bedding are less compliant compared to the other loading directions. With the exception of samples deformed 45° and 90° to bedding at pc = 100 MPa, strain is localized in discrete shear zones. Compressive strength (σmax) increases with increasing pc, resulting in an internal friction coefficient of ≈ 0.31 for samples deformed at 45° and 90° to bedding, and ≈ 0.44 for samples deformed parallel to bedding. In contrast, pre-consolidation, drainage condition, T and dot{varepsilon } do not significantly affect deformation behavior of dried samples. However, σmax and Young’s modulus (E) decrease substantially with increasing water saturation. Compared to the clay-rich shaly facies of OPA, sandy facies specimens display higher strength σmax and Young’s modulus E at similar deformation conditions. Strength and Young’s modulus of samples deformed 90° and 45° to bedding are close to the iso-stress Reuss bound, suggesting a strong influence of weak clay-rich layers on the deformation behavior.

Liquefaction Potential Analysis in Bucharest City as a Result of the Ground Shaking during Strong Vrancea Earthquakes

Bucharest, the capital of Romania with about 2.5 million inhabitants, is frequently struck by intense, damaging earthquakes (2–3 events per century). The Collaborative Research Center 461 (CRC-461) entitled: “Strong Earthquakes - a Challenge of Geosciences and Civil Engineering” was established in July 1996 and ended in December 2007, but some projects continued until 2010. It was funded by the German Research Foundation and involved the University of Karlsruhe which today belongs to Karslruhe Institute of Technology. The CRC aimed strategic research in the field of strong earthquakes with regional focus on the Vrancea seismic events in Romania. Between 1995–2007 several research works were done in Romania, with the support of several Romanian research institutes and the University of Bucharest. One of the research questions was to study the occurring of liquefaction during strong earthquakes within the shallow sandy layers in Bucharest. In suitable conditions, strong earthquakes can cause, under certain geologic conditions, liquefaction and therewith ground failure as sand boils, lateral spreading, or differentiated subsidence. In the present paper we analyze the liquefaction risk for Bucharest. For this purpose, at 10 representative sites in Bucharest, Seismic Cone Penetration Tests (SCPTu) were executed. An area-wide evaluation of the liquefaction probability in Bucharest was established. The factor of safety (FS) against liquefaction and the probability of liquefaction (PL) were computed from the obtained test-data. For the first time, maps of the liquefaction potential index (Li) for Bucharest were outlined. This map shows how severe the liquefaction phenomena might be during strong Vrancea earthquakes in Bucharest, amplifying the site effects. Keywords: hydrogeologic conditions, liquefaction probability, liquefaction hazard, Bucharest city, strong Vrancea earthquakes

Three thousand year paleo-tsunami history of the southern part of the Japan Trench

This study conducted a field survey and multiproxy analyses on sediment cores retrieved from the Kobatake-ike pond in Choshi City, Chiba Prefecture, Japan. Kobatake-ike pond is located at a high elevation (i.e., 11 m above present-day sea level) and faces the southern part of the Japan Trench. Three event sand layers were detected within the continuous mud and peat sequences of 3000 years. Based on the multi-proxy analyses, including mineralogical composition, diatom assemblages, and geochemical markers, these sedimentological events were associated with past tsunamis. The most recent event was a sandy layer and is attributed to the AD 1677 Enpo tsunami, which was reported by an earlier study conducted in the pond. Our results demonstrated that two older sand layers are associated with large tsunamis that struck the Choshi area in AD 896–1445 and in BC 488–AD 215. In addition, the age ranges of these events seem to overlap that of large earthquakes and tsunamis known from the central part of the Japan Trench. This implies a possible spatiotemporal relation of earthquake generations between the central and southern parts of the Japan Trench. However, since the error ranges of the ages of tsunami deposits at the southern and central parts of the Japan Trench are still large, further investigation is required to clarify the relations of large earthquakes in both areas.

Clayey soils from Boulgou (North Cameroon): geotechnical, mineralogical, chemical characteristics and properties of their fired products

In this study, clayey soils are characterized as backfill and potential fusible raw materials for engineering. Geotechnical tests (particle size distribution, Atterberg limits, densities) and chemico-mineralogical analyses (X-ray fluorescence, X-ray diffraction and Fourier transform infrared) were carried out on samples collected from field in the locality of Boulgou (North Cameroon) followed by the determination of mechanical properties of fired bricks (850–1200 °C). The excavations carried out have revealed a thick layer of clayey soils (~ 2 to 2.5 m) over a sandy layer. This clay formation presents some stratifications with limited influence variation in terms of mineralogy. Mean grain size distribution is dominated by sand (62–80%), clay (12–25%) and silt (6–9%). It corresponds to silty–clayey soils regarding their methylene blue values (3.3–5.6), with low plastic (13–22%) to high plastic (33%) characteristics. They are classified by USCS as clayey sand/silty clay, while one sample is a poorly graded sand (SP). Clay minerals observed are kaolinite (6–12%), montmorillonite (2–6%) and illite (8–10%). Those parameters associated with their fine nature and high compressibility make them suitable engineering applications for backfill material and barriers. From chemical analyses, relatively high contents in alkali and alkaline earth elements (~ 4 to 6%) to be used as potential fusible raw materials for fired bricks or gres ceramics were revealed. Bricks characteristics are varied: linear shrinkage (0.3 to 9%), bulk density (1.8 to 2.3 g/cm3), water absorption (19.7 to 1.3%) and flexural strength (0.3 to 17.4 MPa). Overall, the requirement for fired brick (< 20%) is met up by the water absorption values. As from 1050 °C, these soils are made suitable for brick-making with good characteristics (metallic sound, good cohesion and flexural strength).

Weiskopf model for sandy materials: Rayleigh – Lamb wave dispersion

Rayleigh – Lamb wave propagation in a heterogeneous medium containing an upper sandy layer in a perfect contact with isotropic elastic substrate is analyzed by applying the modified Cauchy sextic formalism. It is also proved that sandy material is essentially anisotropic and should be modeled as an anisotropic material with the cubic symmetry. The performed numerical analysis revealed essential peculiarities with respect to dispersion of Rayleigh – Lamb waves. The observed discrepancy in dispersion portraits of media with sandy layer and with isotropic layer, may be important for the nondestructive evaluation of presence sandy layers in the upper Earth crust.

Numerical analysis of earth embankments in liquefiable soil and ground improvement mitigation

The security and stability of soil structures is seriously affected by earthquake-induced liquefaction, and many earth embankments located in seismically active areas failed to meet the new seismic design code criteria, introduced following the 2011 off the Pacific coast of Tohoku Earthquake. This paper proposes an appropriate in-situ countermeasure, and the seismic performance of the liquefaction mitigation method by grouting is numerically evaluated. The elastoplastic constitutive model of cyclic mobility (CM) model can present the cyclic shearing and liquefaction properties of cohesionless sand and cohesive clay in embankments, by considering stress-induced anisotropy, over-consolidation, and soil structure. A numerical investigation, based on the water-soil coupled finite element and finite difference (FE-FD) methods is conducted to explain the dynamic behavior of embankment in fully saturated sandy deposits, during an earthquake, and long-term post-consolidation in a consistent manner. Special emphasis is placed on the growth of excess pore water pressure (EPWP), acceleration response, and ground deformation (settlement and lateral spreading) in liquefiable soil during earthquake and post-earthquake consolidation (approximately 50 years). A comparative analysis demonstrates that the implemented countermeasure is effective in reducing co-seismic and post-seismic liquefaction-induced deformation, regardless of whether the top sandy layer on both sides of the embankment is completely liquified during the earthquake loading. However, there is local failure on the embankment slope instead of deep-seated failure in the embankment-soil system when grouting the saturated sandy layer, and appropriate countermeasures should be adopted to protect the slope. Furthermore, the ground strengthening effect is not apparent when the reinforcement thickness exceeds 6 m.

The soil configuration on granite residuals affects Benggang erosion by altering the soil water regime on the slope

A permanent collapsing gully, locally called Benggang, formed on slopes with deep granite red soil and is a type of unique gully erosion widely prevalent in southern China. Three different soil configurations (SC), ie, red-transition-sandy (SC Ⅰ, the transition is the soil layer between the red soil and the sandy soil layer), transition-sandy (SC Ⅱ) or sandy (SC Ⅲ) are usually present in the soil profile of the Benggang slope. However, little attention has been paid to impacts of SCs on the triggering of Benggang erosion. In this study, we aimed to explore the relationships between soil water content (SWC) and triggering of Benggang erosion under different SC conditions. The soil properties of different soil layers were measured and the SWC at depths of 20, 40, 60, and 80 cm were monitored at 5-min intervals along a typical Benggang (SC Ⅰ) during 2016–2018. The SWC of Benggang slopes with different SCs were simulated by VADOSE/W model. Results showed that the red soil layer had a higher water retention capacity and shear strength than the sandy soil layer. Even if the SWC is higher (e.g., 0.42 cm3/cm3) at red soil layer or transition layer, the corresponding shear strength is greater than that of sandy soil layer with a lower SWC (e.g., 0.32 cm3/cm3). Relationships between shear strength and SWC of different soil layers indicate that Benggang erosion is triggered by an increase in the SWC in the deep sandy layer. Results also showed that differences exist in the SWC distribution among the different SCs. The SWC is higher in topsoil than in deeper soil in SC Ⅰ and SC Ⅱ, while in SC Ⅲ, the opposite trend is observed. These results revealed that the presence of the red soil or transition layer can reduce the infiltration of rainwater into the deep sandy layer, thus can reduce the possibility of collapse. Our results show that the SC affects the stability of the headwall, and results provide great significances to guide the mitigation of Benggang erosion.

Farmland heavy metals can migrate to deep soil at a regional scale: A case study on a wastewater-irrigated area in China

Heavy metal risks to human health in farmland of wastewater-irrigated areas have long been recognized. It remains to be shown whether farmland heavy metals from wastewater irrigation can migrate to deeper soil at a regional scale. In this study, nine soil cores deep to 30 m from three transects (A, B and C) of a linear wastewater reservoir and the adjacent farmland topsoils and wheat grains were sampled. Heavy metals including As, Cd, Cr, Cu, Pb and Zn in the soils and wheat grains were determined, and the grains’ health risks were assessed using the Target Hazard Quotient (THQ). Considerably high contents of heavy metals in both total and soluble forms were detected in deep soils, especially for the transect B where total As of 73.0 mg kg−1 at 29 m, Cd of 3.80 mg kg−1 at 13 m and Pb of 214 mg kg−1 at 30 m were detected. The silty clayey and silty layers of the transect B had higher contents of As, Cr, Cu, Pb and Zn compared with the sandy layers. Across the studied area, 19.5%–34.1% of the topsoil samples were contaminated by As, Cd, Cu, Pb and Zn, and 34.1% and 19.5% of the wheat grains were contaminated by Cd and Pb, respectively. Wheat grains from all the sampling sites had a combined target hazard quotient (TTHQ) value of >1, with As and Cd being the most important contributors. Our study revealed a wider and deeper risk of typical heavy metals originated from long-term wastewater irrigation in the sampling area, which may pose substantial health risks to the local residents via wheat grains and groundwater.

Noise of underwater explosions in the North Sea. A comparison of experimental data and model predictions.

An analysis is presented of sound measurements performed near two detonations of unexploded ordnance (UXO) in the North Sea, at distances ranging from 1.5 to 12 km. The charge masses of the detonations were 325 and 140 kg TNT equivalent. The objective of the measurements was to improve the accuracy of model predictions of the area where UXO detonations affect harbour porpoises in the North Sea. For the predictions, an explosion emission model is combined with a shallow-water propagation model. The prediction model was previously validated for distances up to 2 km. The measurements reported here allowed validation up to a distance of 12 km. The measured levels and spectra are well explained by the model calculations. The model results depend strongly on the sea sediment layering. The propagation of high-frequency components appears to be affected primarily by the silty top layer, while low-frequency components are affected also by deeper sandy layers. Measured and calculated noise levels are used to determine permanent-threshold-shift effect distances for harbour porpoises (Phocoena phocoena). Values ranging from 2 to 6 km are found for the two detonations.

Effect of Screw Piles Spacing on Group Compressive Capacity in Soft Clay

Screw piles are considered as one of the solutions in different complications projects. Performance of circular screw piles group subjected to compressive load and embedded in two layers; soft clay and compacted sandy soils is investigated in this study. The thickness of two layers soft clay and sand soil is 250 mm and 350 mm respectively which are prepared after compacted to sub layers in the container. By choosing different slenderness ratios of L/ D 25, 32 and 39, many model tests were achieved on screw piles group. Single and double helix plates were used in screw piles with constant of helix spacing 60mm. A square pattern of screw piles group with different spacing between piles (1.5Dh, 3Dh, 4.5Dh), where Dh is diameter of helix is adopted in this study. The results of this study showed that the ultimate capacity of piles group was increased by increasing the spacing between the piles until reached spacing 3Dh then begin to decrease when piles extended in sandy soil, while in soft clay the ultimate capacity increase with decrease piles spacing in two cases single and double helix. The increasing percentages for pile spacing 3Dh and 4.5Dh are (55.3% and 39.4%) respectively for L/D ratio 32 and (27.2% and 23.8%) respectively for L/D ratio 39. Also, it is recommended to use screw piles group in case of deep layer of soft clay laying on sandy layer due to high increase in compressive resistance capacity of group when extended in sandy layer.

Geophysical Characterization of Oil Spill Sites in Parts of Niger Delta Using Electrical Resistivity Tomography, Southern Nigeria

Geophysical survey using 2D resistivity tomography was acquired to characterize two hydrocarbon impacted sites of Ikarama and Kalaba. In the impacted sites six (6) 2D resistivity imaging profiles were carried out to understand the resistivity distribution as basis for interpreting the depth and lateral extent of contamination. Additionally, two (2) 2D resistivity imaging profiles were also carried out in the un-impacted sites to serve as control. Surface geophysical survey was acquired using ABEM SAS 1000 terammeter set with electrode constant separation of 6 step array system with a maximum electrode separation of five metres in a traverse of 100 meters. The 2D resistivity imaging data was interpreted using Res2Div. The results indicated the distribution of electrical resistivity within the subsurface. The resistivity sections were displayed in shades of rainbow colours. The values for the models range from 1.05 the resistivity models show a general increase of resistivity with depth relative to the control points. The 2D resistivity models correlate well with the borehole logs as top soil, clay, sandy clay and sand. Also, some of the 2D models indicate increases in resistivity with depth. They also show interaction of water interplaying with the contaminant plume. The resistivity range of 0.005 to 11.3 which occurred from the surface is interpreted as top soil. Immediately underlying this layer is a region of high resistivity with range of 9.4 to 78.1 which indicate petroleum contamination that may have seeped through the clay horizon to the sandy layer. This may be due to the fact that the invading crude oil may have exceeded the retention capacity of the porous medium hence allowing crude oil to spread to other zones of high hydraulic conductivity materials. In addition, it is also possible that the clay bodies were not localized. Superimposition of the main profile generated to monitor petroleum contaminant and the control profiles established a remarkably difference showing the main profiles increases of resistivity with depth, while the resistivity of the control profiles decreases with depth and possibly with materials highly resistive at the top. The study established that biodegradation increases as the depth decreases. The characterization of high resistivity with decrease in depth of the impacted sites were due to oil spill contamination whereas, decrease in resistivity with respect to depth is agreeable to non-petroleum site having materials that were restive at the top. The study pointed out hydrocarbon contamination of the both sites between shallow depths of 3m to 7m which may infiltrate to change the composition of near surface aquifers in the locality. Therefore, more geophysical studies is recommended in this area. Keywords: Electrical Resistivity Tomography, Oil spill, Geophysical Characterization DOI: 10.7176/JEES/11-2-08 Publication date: February 28 th 2021

Numerical simulation of liquefaction phenomenon considering infinite boundary

The response of liquefaction behaviour can be analyzed by incorporating the effects of the transient flow of the pore-fluid through the voids, which requires a two-phase continuum formulation for saturated porous media named ‘mixture theory’. The saturated soil system is modelled as a two-phase material based on the Biot's theory for porous media. Infinite elements have been incorporated in the solution algorithms to simulate infinite boundary. A computer code is developed in FORTRAN 90 considering fully coupled dynamic analysis to determine the responses of pore fluid and soil skeleton of an unbounded saturated sandy layer. The unconditionally stable generalized Newmark-Beta method is employed in the solution algorithm for evaluation of response at each time step. A generalized non-associative plasticity-bounding surface model Pastor–Zienkiewicz Mark III is considered to describe the inelastic behaviour of soils under earthquake loading. A parametric study is conducted to consider the effect of the material nonlinearity of the soil grain on liquefaction behaviour. A significant reduction in displacement and excess pore pressure is observed while considering unbounded domain. The salient feature of the solution algorithm is simulation of build up pore pressure and consequent reduction in mean effective stress. It was also observed that key parameters like permeability, earthquake intensity and shear modulus had significant effect on liquefaction response.

Effects of declining water levels on water-air interactions in cover collapse sinkhole

Water-air interactions caused by water level changes over a karst cavity can lead to sinkhole collapse. Declining water levels in a two-layered cover (clayey upper layer and sandy lower layer) above a karst cavity will lead to changes in the air pressure inside both the cover layers and the cavity. The change in air pressure can cause changes in the rate of water level decline and induce subsidence and collapse. In this study, a mathematical formula was derived and used to better understand the water-air interaction mechanism. Air velocity, vacuum variation in the cover, cumulative air intake, and elapsed time in the water level decline process are considered. A physical model was used to conduct laboratory simulations of the water-air interaction process and to determine the relationship between collapse and the water-air interaction. The main findings of this work are as follows: (1) Water level declines cause changes in air pressure both inside the cover and the cavity, and, conversely, the velocity of the water level decline was influenced by the variable air pressure; water-air interaction is obviously occurring. (2) With a low permeability cover layer, the declining water level mainly generates vacuum suction, but also erosion of the underlying high permeability sandy layer; vacuum suction and erosion can lead to subsidence or collapse of the cover. (3) Physical model simulations are useful for representing the phenomena of water-air interactions in karst cavities, and provide a practical way to understand the mechanism of collapse in karst terrain.

Experimental Geotechnical Characterization Campaign of the Matam Soil for Lithology: Application to the Study of Supports of the Balterdi Bridge

This paper presents a geotechnical study whose objective is to determine the lithology of the soil of Balterdi village (Matam, Senegal). A bridge-type structure with six supports is to be built on the studied site. The drilling program consisted of six core drillings (SC) each 30 m deep with sampling and six destructive drillings with pressuremeter tests (SP) every 1.5 m up to 30 m deep. A pair of core and pressuremeter tests was carried out under each support. From the results of the core drill holes and the geological model of the site, it can be seen that the lithology of the soil along the structure consists essentially of soft clay layers, becoming firm at depth over the first 18 m on average (with some incursions of sandy layers) resting on medium to compacted sand up to 30 m. Brown clayey fill is also present between piers P3 and P5. The presence of a watercourse along the structure is noted. The foundations will therefore be carried out in the water. The results of the pressuremeter tests confirm the homogeneity of the soil over the whole area. Indeed, the values obtained are almost similar for all the tests. According to the limit pressures obtained, the clayey-sandy formations encountered along the structure are respectively soft to firm and moderately compact to compact. Their limit pressures are high enough to allow good bearing capacities.

Soft soils: A study on their electrical resistivity values and geotechnical properties (porosity, SPT and particle size distribution)

Soft soils pose abundant engineering issues due to its low bearing capacity and shear strength. Comprehensive study on soft soil’s physical properties such as shear strength and ability to store water (porosity) could help in devising the optimum ground improvements and foundations techniques. Therefore, physical properties of soft marine clay in Nibong Tebal were thoroughly studied using 2-Dimensional Resistivity Imaging (2-DRI) method in conjunction with porosity measurements, standard penetration test values (SPT-n) and particle size distribution (PSD) analysis. The 2-DRI profile depicts three lithologies, which are unsaturated topsoil, saturated soft clayey soil and saturated sandy soil in the area. The soft soil extends up to 32 m in thickness where it overlies the sandy layer and could be correlated back to lithology profile from borehole record. Additionally, soil samples were collected at three locations along the survey line for porosity measurements via saturation porosimetry method. The samples demonstrate that the clay layer has a very large porosity range and signifies that the soil will compress tremendously under load. On the other hand, SPT-N values of the soft clay is also very low; thus, could be classed as very soft to soft cohesive soil with very low shear strength as compared to a higher range SPT-n values of the sandy layer. The PSD result also compliments the 2-DRI, porosity and SPT results to show distinct differences between topsoil and the soft clay layer in terms of the presence of fine grains. These results further indicate that the thick upper layer is not capable of bearing immense loads such as high-rise infrastructures due to the soil’s high porosity and low shear strength. Hence, the area must undergo ground remediations prior to any infrastructure developments on the land.