Layer Overlain Research Articles

Numerical Modeling of the Ultimate Bearing Capacity of Strip Footings on Reinforced Sand Layer Overlying Clay with Voids

The presence of underground voids within a failure zone usually results in a reduction in the bearing capacity of footings. This paper presents results for the ultimate bearing capacity ratio, qu/γB, of a strip footing on top of a sand layer overlying a clay layer with voids, with and without the placing of geotextile reinforcement at the interface between the sand and clay layers. Using the finite difference software FLAC 2D, the bearing capacity ratio of the strip footing was calculated for voids with different depths and horizontal distance for two configurations: parallel and symmetrical. The effect of parameters on the ultimate bearing capacity ratio was also investigated, including the undrained shear stress ratio of the soil, the thickness of the top layer and the size, location, height, width and spacing of the voids, with and without placing of geotextile layers at the interface between the sand and clay layers. It was found that the influence of a void on the ultimate bearing capacity ratio of the strip footing vanished when the void was located outside the failure zone beneath the footing and increased further with reinforcement until it reached a constant limit value.

Numerical Study on the Behaviour of Geocell-Reinforced Sand Layer Overlying Soft Clay Subgrade

Numerical Study on the Behaviour of Geocell-Reinforced Sand Layer Overlying Soft Clay Subgrade

Influence of asphalt thickness on performance of geosynthetic-reinforced asphalt: Full-scale field study

In this study, a series of controlled traffic loadings was conducted on unreinforced and geosynthetic-reinforced full-scale asphalt overlays. Unlike the common objective of using paving interlayers to mitigate the development of reflective cracks, the main purpose of adopting geosynthetics for this study was to render an increased roadway structural capacity. The project involved instrumented test sections constructed during the rehabilitation of an in-service roadway in Texas, USA. The rehabilitation involved repairing the pre-existing pavement, placing tack coat, installing a geosynthetic interlayer (except in the unreinforced section), and finally constructing a 75 mm-thick asphalt overlay. This overlay comprised a 50 mm-thick, dense-graded (TY-D) layer overlain by a 25 mm-thick, thin-overlay mixture (TOM) layer. Controlled traffic loadings were conducted, which involved driving standard and light axle loads directly above asphalt strain gauges that had been installed at mid-depth of the pre-existing asphalt layer. Comparison of tensile strains among the different test sections revealed significantly smaller tensile strains in the geosynthetic-reinforced sections compared to those obtained in the unreinforced section. Consequently, and even though geosynthetic interlayers have often been adopted to minimize reflective cracking in asphalt overlays, the field monitoring results generated in this study demonstrate that they also provide added roadway structural capacity.

Centrifuge modelling of ultra-thin high strength steel fibre reinforced concrete pavements

Ultra-thin continuously reinforced concrete pavement (UTCRCP) is an innovative pavement type that consists of a 50 mm high strength steel fibre reinforced concrete (HS-SFRC) layer overlain on a pavement substructure. The thickness results in a flexural stiffness significantly smaller than for conventional concrete pavements. In this paper, the conceptual understanding of the response of UTCRCP to traffic loading was investigated using centrifuge modelling. Simplified pavement models were subjected to a bidirectional moving axle load. The results indicated that axle loading, and not single wheel loading, should be used to investigate the response of UTCRCP as there is significant interaction in substructure deformation caused by the wheels on the ends of an axle. Due to the flexural toughness of the highly reinforced concrete layer, a gap forms between the ultra-thin HS-SFRC overlay and its substructure. Brittle, cemented bases between the HS-SFRC overlay and subgrade should be used with caution, as the flexible nature of the layers above and below the stabilised layer may result in rapid degeneration of the brittle layer.

Improved Prediction of Peak Resistance for Spudcan Penetration in Sand Layer Overlying Clay

AbstractA reliable assessment of peak resistance is the key aspect of geotechnical design for spudcan installation in sand-over-clay soil profiles. The most recent failure stress–dependent method h...

Laboratory and Numerical Studies on the Performance of Geocell Reinforced Base Layer Overlying Soft Subgrade

The use of geocells for support of foundations on weak subgrades has been gaining popularity in recent years. Detailed studies were performed in this work through laboratory tests and numerical simulations to investigate the influence of different parameters on the geocell supported foundations on soft subgrades. A series of large-scale laboratory plate load tests were performed on the geocell-reinforced base layer overlying soft subgrade. The soft foundation soil was simulated using blocks of Expanded Polystyrene (EPS) in the laboratory tests for obtaining consistent subgrade conditions between the tests. The numerical analyses were performed in three-dimensional finite difference numerical program Fast Lagrangian Analysis of Continua in Three Dimensions (FLAC3D). The numerical models were calibrated against the data obtained from laboratory tests. This paper deals with investigating the influence of different parameters such as pocket opening size, height and size of the footing on the performance of geocell-reinforced base layer overlying soft subgrade. The provision of three-dimensional cellular reinforcement (geocells) in the base layer has substantially increased the bearing capacity of the pavement section and the optimum aspect ratio of geocell pocket was found to be close to unity. From the parametric study, it is observed that the potential benefits of geocell reinforcement increases with reduction in weld spacing and height of geocell while the influence decreases with increasing loading area for constant height of geocell layer.

Influence of Throughflow Effects Combined with Internal Heating on the Onset of Convection in a Fluid Layer Overlying an Anisotropic Porous Layer

Influence of Throughflow Effects Combined with Internal Heating on the Onset of Convection in a Fluid Layer Overlying an Anisotropic Porous Layer

Rutting Behavior of Geocell Reinforced Base Layer Overlying Weak Sand Subgrades

In this study, a series of large scale repeated model load tests are performed on geocell reinforced and unreinforced base layers overlying weak sand subgrades. The weak sand subgrades are prepared at 30% relative density (RD) through pluviation (sand raining) technique in a test tank of dimensions 1m × 1m × 1m (length × width × height). Two different base courses consisting of 75% RD sand and a granular base material have been tested. The 75% RD sand base course is also prepared by pluviation technique, while the granular base course is prepared in 5 layers, each of 50mm thick, by static compaction.A repetitive load of 0.97kN and 9.7kN was applied on the prepared base layer through a 150mm diameter plate to replicate the traffic load equivalent to a contact pressure of 550kPa. Loading was applied through a graphical user interfaced multi-purpose test software along with the help of a hydraulic power unit, hydraulic service manifold and sophisticated double acting linear dynamic 100kN capacity actuator which is connected to a 3.5m high, 200kN capacity reaction frame. Four different tests are conducted on both the base courses (75% RD sand and granular base) with and without reinforcement overlying the weak sand subgrade separately. There is a considerable amount of improvement observed for different number of cycles and plate settlements on quantification of traffic benefit ratios (TBR), cumulative plastic deformations (CPD) and rut depth reduction (RDR) for geocell reinforced base courses. However, geocell reinforced granular base course have shown a better improvement comparatively.

Defining near-surface groundwater flow regimes in the semi-arid glaciated plains of North America

The dominant transport mechanisms controlling the migration of contaminants in geologic media are advection and molecular diffusion. To date, defining which transport mechanism dominates in saturated, non-lithified sediments has been difficult. Here, we illustrate the value of using detailed profiles of the conservative stable isotope values of water (δ2H and δ18O) to identify the dominant processes of contaminant transport (i.e. diffusion- or advection-dominated transport) in near-surface, non-lithified, saturated sediments of the Interior Plains of North America (IPNA). The approach presented uses detailed δ18O analyses of glacial till, glaciolacustrine clay, and fluvial sand core samples taken to depths of 11–50 m below ground at 22 sites across the IPNA to show whether transport in the fractured and oxidized sediments is dominated by advection or diffusion. Diffusion is by far the dominant transport mechanism in fine-textured lacustrine and glacial till sediments, but lateral advection dominates transport in sand-rich sediments and some oxidized, fine-textured lacustrine and glacial till sediments. The approach presented has a number of applications, including identifying dominant transport mechanisms in geomedia and potential protective barriers for underlying aquifers or surface waters, constraining groundwater transport models, and selecting optimum locations for monitoring wells. These findings should be applicable to most glaciated regions of the world that are composed of similar hydrogeologic units (i.e. low K clay till layers overlain by higher K coarse-textured aquifers or weathered clay till layers) and may also be applicable to non-glaciated regions exhibiting similar hydrogeologic characteristics.

Distal “impact” layers and global acidification of ocean water at the Cretaceous-Paleogene boundary (KPB)

The KPB sections at Hojerup in Denmark, Agost and Caravaca in Spain and El Kef in Tunisia and (elsewhere in the world) consists of a very thin reddish biogenic calcite-poor smectite-rich “impact” layer overlain by a thicker smectite-rich marl. The massive amount of impact-generated atmospheric CO2 at KPB would have accumulated globally in the ocean surface, leading to acidification and CaCO3 undersaturation. These chemical changes would have induced a low biocalcification of calcareous plankton and a high dissolution of their shells. The biocalcification/dissolution crises may have played a significant role for the low abundance of biogenic calcite in the “impact” layer of the marine boundary clays at Hojerup, Agost, Caravaca and El Kef (and elsewhere in the world). Experimental data and observations indicate that the deposition of the “impact” layer probably lasted only a few decades at most.

The conspicuous red “impact” layer of the Fish Clay at Højerup (Stevns Klint, Denmark)

The marine Cretaceous-Paleogene boundary (KPB) section at Hojerup-Fish Clay consists of a very thin red smectite-rich carbonate-poor (“impact”) layer overlain by a thick black marl. Similar red layers are found in the KPB sections at Agost in Spain and El Kef in Tunisia.

Effect of a Freshwater Layer Overlying Salt Water on the Regime of Replacement in Confined Coastal Aquifers

A mathematical model of fresh groundwater flow from a semi-infinite confined aquifer into a sea (pool, trench, and the like) filled with salt water and having a freshwater layer above its horizon. To study this problem a mixed boundary-value problem of the theory of analytical functions is formulated and solved with the use of the Polubarinova-Kochina method. The obtained exact analytical relationships and numerical calculations are used to perform a detailed hydrodynamic analysis of the effect of the freshwater layer and other physical parameters of the model on the character and the extent of displacement.

Sporangia containing Scylaspora from the Lower Devonian of the Welsh Borderland

ABSTRACT. Bulk maceration of Early Devonian (Lochkovian) deposits from the Welsh Borderland has yielded eight specimens of a new type of sporangium characterized by its elongate shape and distinctive spores. The specimens have been examined using scanning electron, transmission electron and light microscopy. The elongate sporangia occur isolated and are fragmented to varying degrees. They contain trilete spores that possess a proximal surface with shallow murornate ornament and a distal surface that is laevigate. The spores belong to the dispersed spore genus Scylaspora, and this is the first report of such spores in situ. Ultrastructural studies demonstrate that the spore walls are bilayered with a lamellate inner layer overlain by an essentially homogeneous outer layer. There is little or no associated extra‐exosporal material. To date this is the earliest reported example of lamellate wall ultrastructure in trilete spores. Models of spore wall development are suggested in the light of evidence provided by spore wall ultrastructure. Detailed comparisons of the characters preserved in the fossils (morphological, anatomical and ultrastructural), with those in other fossil and extant plants, currently shed little light on the phylogenetic relationships of these specimens, primarily due to the paucity of comparable data. It is proposed that the plant is probably of vascular status, but in the absence of evidence for vascular tissue, it must be classified as rhyniophytoid.

A Waterflood And Chemically Improved Oil Recovery Evaluation For the Pembina Bear Lake Unit

Abstract The Pembina Cardium reservoir is the largest conventional oil reservoir discovered thus far in Canada, and has been extensively waterflooded since the late fifties. Operating waterfloods are now reaching their economic limits and improved oil recovery techniques are being considered to extend the life of the reservoir. This report investigates the technical feasibility of applying a chemical flood to the Pembina Bear Lake reservoir. The investigation has two major components. First, a chemical system was designed that lowered the brine/oil interfacial tension sufficiently to mobilize waterflood residual oil. Second, the effectiveness of the chemical system was tested using corefloods at reservoir conditions. Through phase behaviour studies an optimum chemical composition was determined: 5 wt% Petrostep B100 + 2 wt% nbutanol + 0.5 wt% NaCl in Bear Lake Brine. Injection of a slug of this chemical composition led to an incremental recovery of up to 48.5% OOIP Bear Lake Crude in homogenous rock material. Using real reservoir core the average incremental oil recovery was only 15% OOIP. The low oil recovery during the chemical floods in reservoir cores is a direct result of the extreme heterogeneity in the conglomerate rock. Introduction The Bear Lake reservoir consists of several deposited sandstone layers overlain by a high permeability conglomerate layer. During waterflooding, the conglomerate acts as a thief zone through which most of the water channels. The waterflood residual oil saturation has been estimated to be approximately 50%, reducing the oil recovery during waterflooding even further. As a result large quantities of oil are left behind due to poor sweep and poor displacement. This kind of problem can be addressed in several ways. One option is to place a blocking agent into the conglomerate zone to force more water into the sandstone layers. The sandstone layers would be better swept, improving the oil recovery. However, a large amount of waterflood residual oil would be locked in the conglomerate layer and lost from future production. Therefore, it was decided to look at a chemical flooding option to lower the waterflood residual oil and improve the sweep efficiency at the same time. Lowering the interfacial tension between reservoir brine and oil to the point of micro-emulsification has been proven to be tremendously effective in reducing residual oil saturations. The interfacial tension can be lowered by injecting a properly selected surfactant solution into the reservoir. Increasing the viscosity of the injection brine with polymer addition enhances the oil recovery by improving the sweep efficiency and displacing emulsified oil. In this work a micellar-polymer process was investigated for application in the Pembina Bear Lake Unit. Theory Micellar-polymer flooding relies on the injection of a micellar surfactant solution to lower interfacial tension (IFT) to ultralow levels, lower than 10−3 mN/m. Ideally, microemulsions are formed during micellar-polymer flooding which help to solubilize and transport residual crude oil. This reduction in the interfacial tension reduces the capillary forces which trap residual oil in place in porous media, making displacement of the oil easier.

Centrifuge Testing of Remediation of Liquefaction at Bridge Sites

Five centrifuge models were tested to begin to quantify the effectiveness of liquefaction remediation techniques and to optimize the required volume of soil to be treated to reduce liquefaction consequences to a tolerable level. Detailed models of a bridge site including a river channel, flood plains, and bridge abutments were tested in the large centrifuge at the University of California, Davis. The models consisted of a deep, level, liquefiable sand layer overlain by a sloping, less-permeable clay layer with a free face at the river channel. Densification of a limited region of sand and in-ground water barriers were evaluated as possible remediation treatments. While spinning at a centrifugal acceleration of 30 g, the base of each model was shaken with realistic earthquake base motions. Base motions, pore pressures in the treated and untreated zones, profiles of ground deformations, settlements, and lateral sliding are compared. Settlements were controlled satisfactorily, but lateral sliding of the surface clay layer was not adequately controlled by the limited remedial treatments.

Diagenesis of the sedimentary rocks enclosing coaly layers in Gavatha area, Lesvos Island, based on silica polymorph’s transformations

A Tertiary non-marine stratigraphic sequence composed of carbonates (limestone), siliceous carbonates, coaly layers overlain by pyroclastic rocks and lavas, outcrops in the Gavatha area of northwestern Lesvos Island. Pure carbonates consist almost completely of calcite, the siliceous carbonate sediments of quartz, opal-CT and calcite, the shales of quartz, opal-CT, K-feldspar, smectite-illite and calcite, and the coaly layers of organic matter, quartz, opal-CT, feldspars and pyrite. Geochemical data indicate that smectite-illite, feldspars and associated elements (La, Zr, Y, Ba, Ce) are the products of alteration of volcanic rocks in a subtropical area.

Modeling heat, mass, and species transport in polar firn

A finite-element model for simulating multi-dimensional air flow with heat, mass and chemical species transport through firn is discussed. The model is applied to an investigation of near-surface layering effects on ventilation rates. Field measurements of permeability at Summit, Greenland, are presented that show that permeability varies by at least a factor of 10 over the top 3 m, with the surface windpack having much lower permeability, in general, than the underlying firn. The effect of a lower-permeability surface layer is to decrease the air flow in the underlying firn, yet there is still sufficient air flow in the top meters of the firn so that ventilation must be considered for species transport. Channeling, or increased air flow in a layer overlain by a less-permeable layer, can occur even if the microstructure of each layer is isotropic. Conventional estimates of chemical transport due to diffusion alone are likely to underestimate transport, while estimates of ventilation that consider the firn as a homogeneous half-space may overestimate ventilation effects at the near-surface. Effects of firn layering are important for ventilation and must be considered for accurate assessment of firn–air transport mechanisms.

Modeling heat, mass, and species transport in polar firn

A finite-element model for simulating multi-dimensional air flow with heat, mass and chemical species transport through firn is discussed. The model is applied to an investigation of near-surface layering effects on ventilation rates. Field measurements of permeability at Summit, Greenland, are presented that show that permeability varies by at least a factor of 10 over the top 3 m, with the surface windpack having much lower permeability, in general, than the underlying firn. The effect of a lower-permeability surface layer is to decrease the air flow in the underlying firn, yet there is still sufficient air flow in the top meters of the firn so that ventilation must be considered for species transport. Channeling, or increased air flow in a layer overlain by a less-permeable layer, can occur even if the microstructure of each layer is isotropic. Conventional estimates of chemical transport due to diffusion alone are likely to underestimate transport, while estimates of ventilation that consider the firn as a homogeneous half-space may overestimate ventilation effects at the near-surface. Effects of firn layering are important for ventilation and must be considered for accurate assessment of firn–air transport mechanisms.

Geophysical Surveys of Leaking Tailings Impoundments

Alcoa of Australia's bauxite treatment plant at Kwinana discharges alkaline liquor to impoundments constructed on Quaternary formations of the Perth Basin. Contaminated groundwater has been detected outside some of these impoundments. The impoundments are constructed with an impermeable basal clay layer overlain by a permeable, thin sand blanket. Wet tailings slurry is poured in, the fine residue fraction is dried and stacked higher than the dyke walls allowing access of tracked vehicles and geophysical equipment to the impoundments. Leakage is initially detected from monitoring bores placed outside the impoundments. Approximate leak locations can then be detected from water level lows in piezometers placed within the semi-confined aquifer of the sand blanket. The mise-a-la-masse technique uses current electrodes placed within the impoundment and within conductive, leaking water at some distance away. A current pathway is provided between the impoundment and the conductive groundwater via the leakage zone and this can be mapped as voltage highs using potential electrodes within piezometer holes near the base of the impoundment. In some cases the detection of these leakage points is complicated, but not impossible, by multiple leaks, PVC liners placed above the basal clay layer and the strongly varying conductivity within the impoundment. The mise-a-la-masse anomalies only give leak location with differences in intensity between anomalies not being diagnostic of leak properties.

The Lower Cretaceous Chinkeh Formation: A Frontier-Type Play in the Liard Basin of Western Canada (1)

The Lower Cretaceous Liard basin in western Canada covers an area of 9500 sq km (3668 sq mi) but is relatively unexplored despite its size. The present-day expression of the basin, which formed during the latest Cretaceous to early Tertiary, trends north-south and is delineated by the outcrop of the coarse-clastic Upper Cretaceous Dunvegan Formation. The lowermost Cretaceous unit, herein named the Chinkeh Formation, is up to 32 m (105 ft) thick and unconformably overlies older Paleozoic strata. The Chinkeh Formation contains four major lithotypes: (1) conglomeratic breccia interpreted as debris-flow or talus deposits, (2) interbedded coal, carbonaceous shale, rooted sandstone, and conglomerate interpreted as nonmarine valley fill or channel deposits, (3) conglomeratic lag related to marine transgression, and (4) upward-coarsening sandstone interpreted as abandoned shoreline deposits. The lower part of the Chinkeh Formation consists locally of an angular-chert breccia overlain by medium- to coarse-grained sandstone and conglomerate with coal and rooted beds. The uppermost Chinkeh Formation consists of a basal conglomeratic layer overlain by an upward-coarsening, fine- to medium-grained, well-sorted marine sandstone, with bioturbated shale. Sediment was recycled repeatedly with the dominant sediment source being from the east with perhaps some western sources. Cretaceous strata in the Liard basin have good petroleum source-rock and reservoir potential, and hydrocarbons may be present in sandstone of the Chinkeh Formation. Potential play types include stratigraphic traps formed by incised-valley deposits and shallow-marine sandstone pinching out laterally into marine shales of the Garbutt Formation. A potential structural play may occur along the Bovie fault zone where reservoirs may abut against a shale seal on the eastern side of the fault. Most of Lower Cretaceous strata falls within the hydrocarbon generation window, with increasing maturity to the southwest. Pyrobitumen is locally abundant and constitutes 30% of the bulk rock volume in one sandstone. Potential source rocks include the lowermost Garbutt Formation and underlying Triassic oad and Grayling formations. The Chinkeh Formation sandstone has porosity values of 8-18%.