Shallow Clay Research Articles

Assessment of small-strain characteristics for vibration predictions in a Swedish clay deposit

Environmental vibrations induced by human activities such as traffic, construction or industrial manufacturing can cause disturbance among residents or to vibration sensitive equipment in buildings. In Sweden, geological formations of soft clay overlying a stiff bedrock are soil conditions prone to ground vibrations that are encountered both in urban areas and along parts of the national railway network. This paper presents an extensive investigation of the small-strain soil properties for the prediction of environmental ground vibrations in a shallow clay where the bedrock is situated at 7.5 m depth. The small-strain properties are estimated using available empirical correlations, bender elements, seismic cone penetration tests, seismic refraction and inversion of surface wave dispersion and attenuation curves. The results are synthesised into a dynamic layered soil model which is validated by measurements at the soil's surface at source-receiver distances up to 90 m in the frequency range 1–80 Hz. Analyses of uncertainties in the estimated values of wave speeds and material damping are performed by model investigations, indicating that surface wave tests overestimate the damping compared to bender element tests. The properties of the topmost unsaturated part of the soil is found to have a significant influence on the response at large distances, caused by critically refracted P-waves resonating in the top layer.

Inhabited subsurface wet smectites in the hyperarid core of the Atacama Desert as an analog for the search for life on Mars

The modern Martian surface is unlikely to be habitable due to its extreme aridity among other environmental factors. This is the reason why the hyperarid core of the Atacama Desert has been studied as an analog for the habitability of Mars for more than 50 years. Here we report a layer enriched in smectites located just 30 cm below the surface of the hyperarid core of the Atacama. We discovered the clay-rich layer to be wet (a phenomenon never observed before in this region), keeping a high and constant relative humidity of 78% (aw 0.780), and completely isolated from the changing and extremely dry subaerial conditions characteristic of the Atacama. The smectite-rich layer is inhabited by at least 30 halophilic species of metabolically active bacteria and archaea, unveiling a previously unreported habitat for microbial life under the surface of the driest place on Earth. The discovery of a diverse microbial community in smectite-rich subsurface layers in the hyperarid core of the Atacama, and the collection of biosignatures we have identified within the clays, suggest that similar shallow clay deposits on Mars may contain biosignatures easily reachable by current rovers and landers.

An overview of the Muara Laboh geothermal system, Sumatra

The Muara Laboh geothermal system is a liquid-dominated, fracture-controlled reservoir showing some characteristics of both intrusion-related and fault circulation systems. The developed reservoir has moderate to high-temperature (230–310 °C) with a long NNW outflow (160–230 °C) extending to the Sapan Malulong boiling chloride springs. Reservoir fluids generally have low salinity (∼400-1600 ppm Cl) with benign chemistry and low non-condensable gas (NCG) content (∼0.5 to 2 wt% in steam). The proven reservoir is divided into distinct SW and NE sectors. The principal deep upflow zone (270 to 310 °C) is located in the SW and is associated with Patah Sembilan volcano and satellite vents (Anak Patah Sembilan). The SW upflow is characterized by relatively low salinity (∼400-600 ppm Cl), whereas fluids in the NE emerge from a second upflow along faults and fractures at 240–250 °C (1200–1600 ppm Cl) and ascend to an initial-state steam cap near the Idung Mancung fumarole. The shallow NE part of the geothermal system is hosted mainly by Quaternary to Miocene Age andesitic to rhyolitic rocks, whereas the deeper SW part of the system occurs in a Mesozoic to Cenozoic Age plutonic complex and its host rocks that are cut by younger dikes. Rock porosity and permeability trends can be related to reservoir age, volcanic deposit type, and alteration history. Fracture permeability is high, whereas matrix porosity (<1-7%) and permeability are moderate to low and decrease with depth. The volcanic sequence is capped by tuffs dated at ∼34,000 to 41,000 years BP, incapsulated in debris flows. This sequence may have formed during and shortly after likely sector collapse episodes of the Patah Sembilan volcano. The shallow clay cap of the system wraps around the crater, suggesting that it was excavated during crater formation.Permeable fracture patterns and their origins differ somewhat between the deep SW and the shallow NE reservoir. In the deep SW reservoir, fluids ascend along fractured margins of stock and dike intrusions. Steeply dipping intrusions and open fractures strike from WNW to NNW, subparallel to the main Great Sumatra Fault trend, with a secondary set of N to NE fractures and intrusions parallel with the inferred maximum horizontal stress direction (SHMax). In the shallower NE steam cap and outflow, N to NE steeply dipping fractures dominate, with bounding N to NNW faults. Bulk rock alteration includes shallow argillic and transitional clay zones overlying phyllic and propylitic zones. High-T propylitic alteration (secondary amphibole) and rare potassic alteration (secondary biotite) are associated with intrusion. Vein paragenesis in the SW reservoir indicates that portions of the initially permeable propylitic zone have been sealed by calcite and quartz ± prehnite. Late calcite veins suggest ingress of steam-heated bicarbonate waters contributed to resealing of the system. These relationships suggest that the SW system underwent extensive boiling, fluid loss, and inflow of dilute bicarbonate waters, possibly related to sector collapse and partial excavation of the topseal of the system (now Patah Sembilan crater).

Assessment of Strength Development in Stabilized Soil with CBR Plus and Silica Sand

This paper investigates the potential use of a nano polymer stabilizer, namely CBR PLUS for stabilization of soft clay and formulation of an optimal mix design of stabilized soil with CBR PLUS and silica sand. The highway settlements induced by the soft clay are problematic due to serious damages in the form of cracks and deformation. With respect to this, soil compaction and stabilization is regarded as a viable method to treat shallow soft clayey ground for supporting highway embankment. The suitability of stabilized soil was examined on the basis of standard Proctor compaction, California Bearing Ratio (CBR), unconfined compression, direct shear and permeability falling head tests. Furthermore, the chemical compositions of the materials were determined using X-Ray Fluorescence (XRF) test. The objectives of this paper are (i) to stabilize the compacted soil with CBR PLUS and silica sand in the laboratory; and (ii) to evaluate the strength and CBR of the untreated and stabilized soil specimens. It was found that the optimal mix design of the stabilized soil is 90% clay, 1% CBR PLUS, 9% silica sand. It is further revealed that, stabilization increases the CBR and unconfined compressive strength of the combinations by almost 6-fold and 1.8-fold respectively. In summary, a notable discovery is that the optimum mix design can be sustainably applied to stabilize the shallow clay without failure.

A Deep Alteration and Oxidation Profile in a Shallow Clay Aquitard: Example of the Tégulines Clay, East Paris Basin, France

The oxidation profile of a surficial clay aquitard was studied on a 35-meter borecore from the Albian Tégulines Clay near Brienne-le-Château (Paris Basin, France). Mineralogical, geochemical, and petrophysical data showed evidences of gradual oxidation taking place down to a depth of 20 m. Below 20 m, the clay material was nonplastic and nonfractured, and it inherited reduced redox conditions from bacterial sulfate reduction that occurred after sediment deposition. Above 20 m, the clay material was plastic. Up to a depth of 10-11 m, only rare yellowish aggregates of glauconite attested to limited oxidation, and pore water chemistry was unmodified. The 5–11 m depth interval was characterized by intensive pyrite oxidation, calcite dissolution, and formation of sulfate and iron hydroxide minerals. The upper 2-3 m was ochrous and entirely oxidized. These mineralogical changes were mirrored with pore water chemistry modifications such as an increase of alkalinity and sulfate concentration in the upper part of the profile. The presence of siderite at ~11 m evinced the reactivity of Fe(II) in the structure of clay minerals with dioxygen from meteoric waters that infiltrated into the Tégulines Clay through vertical fractures.

Assessment of strength development in stabilized soil with CBR PLUS and silica sand

Abstract This paper investigates the potential use of a nano polymer stabilizer, namely CBR PLUS for stabilization of soft clay and formulation of an optimal mix design of stabilized soil with CBR PLUS and silica sand. The highway settlements induced by the soft clay are problematic due to serious damages in the form of cracks and deformation. With respect to this, soil compaction and stabilization is regarded as a viable method to treat shallow soft clayey ground for supporting highway embankment. The objectives of this paper are: i) to stabilize the compacted soil with CBR PLUS and silica sand in the laboratory; and ii) to evaluate the permeability, strength and California bearing ratio (CBR) of the untreated and stabilized soil specimens. The suitability of stabilized soil was examined on the basis of standard Proctor compaction, CBR, unconfined compression, direct shear, and falling head permeability tests. Furthermore, the chemical composition of the materials was determined using X-ray Fluorescence (XRF) test. It was found that the optimal mix design of the stabilized soil is 90% clay, 1% CBR PLUS, 9% silica sand. It is further revealed that, stabilization increases the CBR and unconfined compressive strength of the combinations by almost 6-fold and 1.8-fold respectively. In summary, a notable discovery is that the optimum mix design can be sustainably applied to stabilize the shallow clay without failure.

Stabilization of compacted clay with cement and/or lime containing peat ash

Soft clay is problematic due to its settlement, swelling and strength issues, when applied as road embankment material. This paper investigates the possibility of the use of cement and/or lime for improvement ground of shallow clay to support highway embankment. A novel approach to stabilize the clay is to use peat ash as a supplementary material in the compacted and stabilized soil. It is worth noting that research on the application of peat ash as a pozzolanic material in stabilizing soft clay is relatively scarce. The objectives of this research are: (i) to stabilize the compacted clay with cement and/or lime, peat ash and silica sand in the laboratory and (ii) to evaluate the effect of binder dosages on short-term and long-term strength of stabilized soil. To this end, the stabilized soil specimens with the highest maximum dry density were chosen for further evaluation under laboratory unconfined compression tests. In addition, the chemical compositions of materials and microstructure of the stabilized clay were examined using X-ray Fluorescence and Scanning Electron Microscope (SEM), respectively. It was found that, the optimal mix design of the stabilized soil is 14% cement, 12% peat ash and 5% silica sand. SEM analysis suggests that the cementitious products were increased with cement and peat ash dosages and clogged the pore spaces. It was further revealed that, partial replacement of cement with 12% peat ash in the optimal mix design resulted in maximum unconfined compressive strength. In summary, a notable discovery is that the partial replacement of cement with 12% peat ash in the optimal mix design can be sustainably applied in order to stabilize the clay without failure. Meanwhile, it has been observed that cement content higher than the lime content enables a better homogeneity of the stabilization, enhancing the strength development in the stabilized soil.

Evaluation of deep groundwater development for arsenic mitigation in western Bangladesh

Groundwater contamination by arsenic frequently occurs in western Bangladesh. Integrated hydrogeological studies were carried out by the Japan International Cooperation Agency (JICA) in the Jessore, Jhenaidah and Chuadanga districts to assess the possibility of supplying safe drinking water from deep aquifers. The subsurface geology of up to 300 m in depth was classified into 5 formations (viz. A to E formations in descending order). Thick clay facies are found in C formation in the Jessore district, however, clay facies are absent in the Jhenaidah and Chuadanga districts. The clay layer separates deep aquifers from shallow aquifers, and controls vertical groundwater flow. The results of core sample analysis showed that high arsenic contents of more than 30 ppm were found not only from shallow clay but also even from deep clay below 200 m. However, the arsenic concentrations in groundwater were generally below 0.05 mg/L in the deep aquifers. The simulation study using a vertical 2-D groundwater model indicates that deep groundwater will not be contaminated by arsenic in shallow groundwater when the piezometric heads of the deep aquifers are higher than the shallow aquifers. However, the simulation results indicate that overexploitation of the deep aquifers will cause arsenic contamination in deep aquifers due to the downward movement of contaminated shallow groundwater when no sorption takes place in the sediments. These results suggest that groundwater management and control of groundwater pumpage in deep aquifers are crucial for sustainable supply of arsenic safe deep groundwater in western Bangladesh.

Thirty-year land elevation change from subsidence to uplift following the termination of groundwater pumping and its geological implications in the Metropolitan Taipei Basin, Northern Taiwan

Several levelling routes in the metropolitan Taipei Basin have been repeatedly conducted during the past decades, mainly in order to monitor the anthropogenic ground subsidence due to massive pumping of groundwater. We analysed the levelling data released from government and investigated the rate of ground level change from 1975 to 2003, which postdate the massive groundwater exploitation in Taipei area. Based on the contour maps created from the levelling data of 406 benchmarks, the overall subsidence rate in the Taipei Basin gradually decreased since 1975, and around 1989 the basin switched to slight uplift throughout a large part of the basin. Three mechanisms are proposed to be responsible for the observed land elevation changes, including shallow soil compaction, deformation within aquifer, and tectonic subsidence. The trend of the ground level change in 1975–2003 essentially demonstrated the effects of natural recharge to previously depleted aquifers, and is explained by the hydro-mechanical coupling of aquifer materials, i.e., elastic rebound, to the rising piezometric level. The rate of shallow soil compaction is estimated about 1–8 mm/yr throughout the basin according primarily to the shallow clay thickness. Asymmetric tectonic subsidence related to the Shanchiao Fault was estimated to be 1.75 mm/yr and 0.9 mm/yr in the western part and the central part of the basin, respectively. By subtracting the components of the soil compaction and tectonic subsidence from the surface land elevation change, the rebound of aquifer strata was estimated to be about 6.7 cm and 16 cm in western margin and Central Taipei, respectively. The amount of rebound is approximately 10% in magnitude comparing to the amount of previous anthropogenic subsidence due to massive groundwater pumping, totally about 2 m.

Changes in aboveground primary production and carbon and nitrogen pools accompanying woody plant encroachment in a temperate savanna

AbstractWhen woody plant abundance increases in grasslands and savannas, a phenomenon widely observed worldwide, there is considerable uncertainty as to whether aboveground net primary productivity (ANPP) and ecosystem carbon (C) and nitrogen (N) pools increase, decrease, or remain the same. We estimated ANPP and C and N pools in aboveground vegetation and surface soils on shallow clay and clay loam soils undergoing encroachment by Prosopis glandulosa in the Southern Great Plains of the United States. Aboveground Prosopis C and N mass increased linearly, and ANPP increased logarithmically, with stand age on clay loam soils; on shallow clays, Prosopis C and N mass and ANPP all increased linearly with stand age. We found no evidence of an asymptote in trajectories of C and N accumulation or ANPP on either soil type even following 68 years of stand development. Production and accumulation rates were lower on shallow clay sites relative to clay loam sites, suggesting strong edaphic control of C and N accumulation associated with woody plant encroachment. Response of herbaceous C mass to Prosopis stand development also differed between soil types. Herbaceous C declined with increasing aboveground Prosopis C on clay loams, but increased with increasing Prosopis C on shallow clays. Total ANPP (Prosopis+herbaceous) of sites with the highest Prosopis basal area were 1.2 × and 4.0 × greater than those with the lowest Prosopis basal area on clay loam and shallow clay soils, respectively. Prosopis ANPP more than offset declines in herbaceous ANPP on clay loams and added to increased herbaceous ANPP on shallow clays. Although aboveground C and N pools increased substantially with Prosopis stand development, we found no corresponding change in surface soil C and N pools (0–10 cm). Overall, our findings indicate that Prosopis stand development significantly increases ecosystem C and N storage/cycling, and the magnitude of these impacts varied with stand age, soil type and functional plant traits

Low‐cost geophysical investigations of a paleochannel aquifer in the Eastern Goldfields, Western Australia

Compressional (P) wave and shear (S) wave seismic reflection techniques were used to delineate the sand and gravel aquifer within a highly saline clay‐filled paleochannel in the Eastern Goldfields of Western Australia. The seismic refraction and gravity methods were also used to investigate the paleochannel. The unsaturated loose fine‐grained sand up to 10 m in depth at the surface is a major factor in degrading subsurface imaging. The seismic processing needed to be precise, with accurate static corrections and normal moveout corrections. Deconvolution enhanced the aquifer and other paleochannel reflectors. P‐wave reflection and refraction layer depths had good correlation and showed a total of six boundaries: (1) water table, (2) change in velocity (compaction) in the paleochannel sediments, (3) sand and gravel aquifer, (4) red‐brown saprolite and green saprolite boundary, (5) weathered bedrock, and (6) unweathered bedrock. P‐wave explosive and hammer sources were found to have similar signal characteristics, and the aquifer and bedrock were both imaged using the hammer source. The deep shots below the water table have the most broadband frequency response for reflections, but stacking clear reflections was difficult. The S‐wave reflection results showed high lateral and vertical resolution of the basal saprolite clay, the sand and gravel aquifer, and very shallow clays above the aquifer. The S‐wave reflection stacking velocities were 10–20% of the P‐waves, increasing the resolution of the S‐wave section. The gravity data were modelled to fit the known drilling and P‐wave seismic reflection depths. The refraction results did not identify the top of bedrock, so refraction depths were not used for the gravity modeling in this highly weathered environment. The final gravity model mapped the bedrock topography beyond the lateral extent of the seismic and drilling data.

Seismic reflection and ground‐penetrating radar imaging of a shallow aquifer

In 1995 and 1996, researchers associated with the US Air Force’s Phillips and Armstrong Laboratories took part in an extensive geophysical site characterization of the Groundwater Remediation Field Laboratory located at Dover Air Force Base, Dover, Delaware. This field experiment offered an opportunity to compare shallow‐reflection profiling using seismic compressional sources and low‐frequency ground‐penetrating radar to image a shallow, unconfined aquifer. The main target within the aquifer was the sand‐clay interface defining the top of the underlying aquitard at 10 to 14 m depth. Although the water table in a well near the site was 8 m deep, cone penetration geotechnical data taken across the field do not reveal a distinct water table. Instead, cone penetration tests show a gradual change in electrical properties that we interpret as a thick zone of partial saturation. Comparing the seismic and radar data and using the geotechnical data as ground truth, we have associated the deepest coherent event in both reflection data sets with the sand‐clay aquitard boundary. Cone penetrometer data show the presence of a thin lens of clays and silts at about 4 m depth in the north part of the field. This shallow clay is not imaged clearly in the low‐frequency radar profiles. However, the seismic data do image the clay lens. Cone penetrometer data detail a clear change in the soil classification related to the underlying clay aquitard at the same position where the nonintrusive geophysical measurements show a change in image character. Corresponding features in the seismic and radar images are similar along profiles from common survey lines, and results of joint interpretation are consistent with information from geotechnical data across the site.

Anisotropy detected in shallow clays using shear-wave splitting in a VSP survey : Douma, J; Den Rooijen, H; Schokking, F Geophys ProspectV38, N8, Nov 1990, P983–998

A shear-wave VSP was carried out in an uncased 0.3 m diameter borehole drilled to a depth of 120 m in the north of The Netherlands. The upper 80 m of the sequence, consisting of a glacial till and sands and clays of Pleistocene age, was studied. Shear-wave splitting and therefore anisotropy was identified at various geophone depths for one source offset. Hodograms showed a consistent polarization of the fast shear-wave component over a large depth interval. Under the assumption that the anisotropy was caused by planar discontinuities with common normals, this polarization direction gives the strike of the fissures in this interval

Rb-Sr evidence for punctuated illite/smectite diagenesis in the Oligocene Frio Formation, Texas Gulf Coast

The µ -size fractions of a suite of upper Oligocene Frio Formation shale samples taken from below 10,500 ft in a well on the Texas Gulf Coast fall on a Rb-Sr isochron corresponding to an age of 23.6 ± 0.8 m.y. The isochron shows an initial 87 Sr/ 86 Sr ratio of 0.7088 ± 0.0004. This suggests that regularly interstratified illite/smectite simultaneously equilibrated soon after burial over a range of burial depths at 7,000–8,000 ft shallower than at present. Oxygen-isotope values for these clays are also consistent with equilibration under shallower burial conditions. Such a pattern conflicts with the prevailing idea of a gradual illitization of smectite with progressive burial. A new model of episodic or “punctuated” diagenesis is proposed, whereby sudden diagenesis is initiated by a change in pore-water chemistry. Clay reaction occurs over a range of temperature at this time, such that the deeper clays are transformed completely to 80% illite, and the shallow clays are transformed less completely. Subsequent pore-water chemical changes inhibit further clay reaction, and the mineralogic gradient and the isotopic signature formed at 23.6 m.y. B.P. are “frozen in.” Seven to eight thousand feet of additional post-Oligocene burial has not resulted in the creation of younger illite/smectite. Consequently, there is no evidence that illite/smectite diagenesis is currently taking place in the Frio Formation.