Overlying Soil Layer Research Articles

Progresses of weathered bedrock ecohydrology in the Earth’s critical zone

<p indent="0mm">Weathered bedrock is crucial in the Earth’s critical zone, which connects soil moisture dynamics and shallow groundwater recharge, and therefore plays an essential role in the ecohydrological cycle on the terrene surface. The exchangeable water stored in weathered bedrock is an essential source of water that is different but parallel to soil water or groundwater. This water in the weathered bedrock, namely, rock moisture, is much greater than that in the soil, which is a pivotal and stable water source for plants, especially in dry years or arid and semiarid areas. When rainfall occurs, the infiltration and lateral flow of water are not limited to the soil layer. The rainwater that infiltrates by piston flow and preferential flow can pass through the fractured or weathered bedrock layer and collect at the fresh bedrock interface to form groundwater. Then, it flows laterally to adjacent river channels, where it also plays an important role in the distribution of evapotranspiration and runoff. Moreover, after weathered bedrock water storage accumulates to its maximum in the rainy season, the additional rainwater becomes base flow in the form of runoff or fissure flow. However, research on the ecohydrological cycle of the terrestrial surface layer has long focused on vegetation water absorption, surface soil moisture, and groundwater replenishment. Little attention has been given to the ecohydrological changes in the weathered bedrock layer and their external effects, and review articles are relatively scarce. Therefore, to clarify the latest research progress on weathered bedrock ecohydrology, the relevant documents collected in the Web of Science core collection database and the China National Knowledge Infrastructure (CNKI) journal database in the past 30 years were retrieved. The developments of the current focuses of research and potential research directions were quantitatively visualized using VOSviewer and bibliometrics, and the potential influential pathways of weathered bedrock ecohydrology were then drawn. The results show that weathered bedrock ecohydrological research is interdisciplinary and has a rich theoretical foundation of geology, water resources, environmental science ecology, and engineering. The number of published articles has gradually increased annually, and major mainstream Earth science journals wish to report these new findings; however, there is room for expansion and discussion in the international arena. The research area in this field mainly uses a shallow overlying soil layer, such as karst, as the platform and focuses on the karst hydrological model, the distribution of hydrological resources, the ecological risk, the lithologic characteristics of vegetation, and the interaction between bedrock and groundwater. Unfortunately, less attention has been given to areas with relatively thick soil layers, such as the hilly valley region. Moreover, ecohydrological effects of weathered bedrock include not only differences in the characteristics but also external disturbances, such as soil water redistribution, groundwater recharge, excretion regulation, and plant growth regulation. Future research might focus on the following: Water storage and its dynamic changes in the bedrock weathered layer, outflow characteristics, the process and mechanism of organism interactions, the process and mechanism of exchange with groundwater recharge, the process of water exchange with the soil-bedrock interface, the action of the vegetation root system and plant growth, and the framework and application of the ecological hydrological model of the atmosphere-vegetation-soil-weathered bedrock-groundwater continuum in the Earth’s critical zone. Furthermore, potential influential pathways of weathered bedrock ecohydrology might be regulating seasonal water shortage by inhibiting soil water infiltration, groundwater runoff, and replenishment; expanding bedrock fissure water storage by increasing animal activity; and transporting water and nutrients by participating in mycorrhizal hyphae. This study might provide a reference for deepening critical zone science, expanding ecohydrology research, and providing rational management for vegetation restoration.

STUDY ON THE INFLUENCE OF DRY DENSITY AND PARTICLE SIZE FRACTAL DISTRIBUTION ON THE RADON CONTROL PERFORMANCE OF OVERLYING SOIL IN URANIUM TAILINGS POND.

In order to explore the influence of dry density and particle size fractal distribution on the radon control performance of overlying soil in uranium tailings pond, overlying soil samples with different particle size fractal dimension and dry density were prepared for radon exhalation experiments. According to the principle of radon generation and diffusion, a set of radon measuring device was designed independently. In addition, the radon concentration on the surface of different overlying soil layers was measured by local static method, and the radon exhalation rate was further calculated. The relationship between radon exhalation law, dry density and fractal dimension of overlying soil was studied by correlation analysis method. Furthermore, a piecewise linear function between radon exhalation rate, dry density and fractal dimension was constructed. Then, the average value of the experimental data was compared with the calculated results. Results show that: at the same dry density and different fractal dimension, the radon exhalation rate decreased with the increase of fractal dimension. When the fractal dimension increased to a certain value, the radon exhalation rate tended to be stable. At the same fractal dimension and different dry density, the radon exhalation rate decreased with the increase of dry density. The calculated results of piecewise linear function were relatively close to the experimental data, which verified the practicability of the formula.

Experimental study of the comprehensive technology of grouting and suspension under an operating railway in the cobble stratum

The construction technology of shield tunnelling under an operating railway in an adverse stratum is complex and risky. The existing research results mostly focus on the settlement control of the tunnel under the existing structure, but they are not adequate to provide a systematic reference for shield tunnelling under an operating railway with strict requirements on the settlement control. Based on the background of the Luoyang Rail Transit Line 2 shield passing through the Luoyi Railway, the comprehensive technology of grouting and suspension (CTGS) has been proposed, and field test and numerical simulation are used to study the effects of its application. The results show that the comprehensive technology of grouting and suspension has a significant effect on the rail settlement control of shield tunnelling under the operating railway in a cobble stratum. Using the grouting method to grout subgrade soil can improve the strength of the overlying soil layer. Using the suspension method to suspend the railway tracks can directly transfer the load to the pile foundation to avoid the subgrade bearing excessive load. CTGS combines the grouting method with the suspension method, which can be used as the preferred technology to control the influence of subgrade soil settlement on railway track surface. Compared with the similar technology, although the project cost is slightly higher, CTGS significantly ensures railway safety during the construction process, which can help keep the railway operation free from interference during the construction period and has obvious comprehensive benefits. CTGS has an important reference value for the safety control of shield tunnelling construction with the risky source under adverse geological conditions such as a sandy cobble stratum.

Effects of bedrock erosion by tillage on architectures and hydraulic properties of soil and near-surface bedrock

Tillage erosion provides an opportunity for the occurrence of bedrock erosion in hilly croplands with shallow soil layer, influencing the hillslope architecture to some extent. In this study, tillage simulation, hydraulic tests and dye-staining tracing were performed to identify the variation in architectures and relevant hydraulic properties of the soil and near-surface bedrock caused by tillage-induced bedrock erosion. Results show that tillage-induced bedrock erosion played an important role in shaping the rock fragment architecture. Under the condition of no-soil cover, the coarse rock fragment content increased from 29.8% to 38.2% when tillage depth increased from 2 to 6 cm. The field-saturated hydraulic conductivity (Kfs) ranged from 18.63 to 23.78 m d−1 by the Guelph method and 24.26 to 98.46 m d−1 by the single ring (SR) method, but effects of tillage depth on Kfs did not show a clear pattern. Under the condition of overlying soil layer, the contents of medium, coarse and total rock fragments in soil increased significantly with tillage depth (P < 0.05). Correspondingly, the Kfs for the soil increased significantly with tillage depth (P < 0.05), implying that the higher content of rock fragments due to the greater tillage depth created the more water flow channels. With respect to the near-surface bedrock, the results of dye-staining tracing show that bedrock fragmentation by tillage tended to promote the development of fracture-preferential flow. The infiltration data derived from the SR method show that the Kfs increased by 33.3% to 50.0% after bedrock fragmentation by tillage compared with that for the control treatment, corresponding to the results of dye-staining tracing. These results suggest that tillage-induced bedrock erosion exerts positive effects on infiltration in the soil and near-surface bedrock by increasing preferential channels.

Study on the Loess Immersion Test of Metro Line 2 in Xi’an, Shaanxi Province, China

With the implementation of China’s western development and “One Belt, One Road” initiative, there are more and more projects in the collapsibility loess area, and the collapsibility loess problem encountered in the construction is becoming more and more prominent. In this paper, the collapsibility loess of the south extension section of Xi’an Metro Line 2 is investigated, and its collapsibility characteristics are studied through a large-scale site immersion test. The test site was a 15 m diameter circular test pit, which took 35 days for water injection and 60 days for observation after water stopped. The test results showed that the maximum self-weight collapsibility of the soil layer in the test pit is 32 mm, and the deformation amount is 10.05 mm in Q3 and 9.55 mm in Q2. The maximum deformation amount of 32 mm is less than 70 mm in the shallow marker; it may be caused by the paleosol layer as a bridge to provide a support to the overlying soil layer. The shape of the sphere of influence after immersion resembles a trumpet, slightly protruding outwards from the paleosol. The scope of influence between the infiltrated and saturated zones gradually increases with depth, and the saturated zone is generally smaller than the infiltrated zone. The research results of this paper can provide technical support and reference for the construction of Xi’an Metro Line 2 and other related projects in the region.

Analysis on construction deformation characteristics of Da-Puling tunnel by monitoring measurement

Tunnel monitoring and measurement is the guarantee of tunnel construction safety and implementation of dynamic parameter design; relying on the surface settlement, vault settlement and convergence deformation in the tunnel during the construction of Da-Puling tunnel on Guangxi expressway, based on the monitoring data, the stress situation and deformation characteristics of the tunnel are analyzed; the results show that the construction parameters of Da-Puling tunnel are basically reasonable, and the portal section is the key to the control of the whole construction parameters, the settlement of the tunnel vault in the V-Class surrounding rock of the portal section is about 4 times of the convergence deformation, which is mainly caused by the gravity of the overlying soil layer. The settlement of the tunnel vault in the IV-class surrounding rock is close to the convergence deformation, and the stress release deformation in the loose area of the tunnel surrounding rock is the main one. The monitoring and analysis in the construction process provides reliable data for design and construction in time, and ensures the smooth progress of tunnel construction.

Study on the Influence of Bedrock Thickness on Deformation and Failure of Overlying Soil Layer in Thin Bedrock Coal Seam Mining

The water-blocking action of the clay Cenozoic overburden layer is an important factor that affects safety of thin bedrock coal seam mining. The change of thin bedrock thickness has an important influence on the deformation and failure of overlying soil layer. In this work, similarity simulation experiment is used and two indexes of failure height and crack density in soil layer are selected to analyze the influence of thin bedrock thickness on the deformation and failure of overlying soil layer. The experiment results show that the failure height of soil layer increases with the decrease of the bedrock thickness. The change of failure height is small when the bedrock thickness is greater than 30 m (mining thickness is 6 m), while when the bedrock thickness is less than 30 m, the failure height of soil layer increases rapidly with the decrease of the bedrock thickness. The main effect of the decrease of the thin bedrock thickness on the deformation and failure of the overlying soil layer results in the increase of shear/tensile cracks. The shear/tension crack density is 0.04 strip/m when the bedrock thickness is 70 m, while the shear/tension crack density increases to 0.14 strip/m when the bedrock thickness reduced to 20 m.

Molybdenum isotopic behavior during intense weathering of basalt on Hainan Island, South China

The behavior of molybdenum (Mo) isotopes during chemical weathering was investigated through analysis of saprolite and soil samples from two weathering profiles developed on Neogene basalts of Hainan Island in southern China. The Penglai and Nanyang profiles exhibit strong (mean ∼60 wt.% kaolinite) and extreme (mean ∼85 wt.% kaolinite) degrees of weathering, respectively. Our results show that light Mo isotopes are selectively retained in residual minerals, including kaolinite and hematite, whereas heavy Mo isotopes are preferentially leached out. Relative to the Mo-isotopic composition of the underlying unaltered parent basalt –0.38 ± 0.05 ‰ (2σ); vs. NIST 3134), the δ98/95Mo values of the overlying saprolite and surface soil at Penglai are modestly lower (–0.81 to –0.39‰) and higher (–0.35 to –0.12‰), respectively. In this profile, both δ98/95Mo and total organic carbon (TOC) decrease with depth, and their positive correlation in the soil layer (r = +0.99, p(α) < 0.01, n = 6) implies that heavier Mo isotopes are fractionated into organic matter. In the more intensely weathered Nanyang profile, the δ98/95Mo of the underlying unaltered parent basalt is lower –0.56 ± 0.05‰ (2σ) than that at Penglai, and the weathered samples show a larger range of relatively lower values (–1.39 to –0.33‰), which may reflect adsorption of Mo by Fe oxides. Throughout the weathering profile, Fe2O3 correlates positively with Mo concentration (r = +0.86, p(α) < 0.01, n = 32) but negatively with δ98/95Mo (r = –0.69, p(α) < 0.01, n = 32), implying that adsorption onto Fe-oxides played a key role in Mo mobility and isotopic fractionation. Extremely negative δ98/95Mo values (to –1.39‰) in the middle saprolite were likely linked to hydrothermal activity, as indicated by unusual enrichments of trace metals (Ni, Cu, Mo), low Hf/Y ratios, and a positive Eu/Eu* anomaly. Furthermore, the overlying soil layers of both profiles are characterized by enrichment of heavier Mo isotopes and higher Mo concentrations relative to their parent rocks. This may have been caused by additional inputs of Mo from atmospheric precipitation (average δ98/95Mo = +0.94 ± 0.14‰, 1σ, n = 3). Based on mass balance modeling of Mo isotopes and concentrations through the integrated profile, we infer Mo inputs from volcanic ash in the basalt-derived soils of northern Hainan Island. This study highlights the range of potential factors controlling Mo retention and isotopic fractionation as a function of variable weathering intensity, which may help to explain the large range of Mo isotopic compositions observed in river catchments at a global scale.

Bedrock erosion due to hoeing as tillage technique in a hilly agricultural landscape, southwest China

AbstractTillage on hillslopes may not only induce severe soil erosion, but may also cause bedrock erosion under certain conditions. Yet, little is known about bedrock erosion by tillage in a hilly agricultural landscape, southwest China. The aim of this study is to quantify the translocation of rock fragments derived from bedrock fragmentation by hoeing under different conditions, including slope gradient, hoeing depth and soil‐covered thickness using a gravel tracing method. The reliability of the gravel tracing method was confirmed by the bedrock dyeing tracing method. Hoeing depth is a significant factor affecting the translocation rate of rock fragments (Qr). Meanwhile, under the condition of overlying soil layers (0.06−0.10 m thick), the values of Qr were significantly smaller with a reduction of 20.7−25.6%, compared with rock fragmentation by hoeing for bare bedrock. However, slope gradient was found to have insignificant effects on Qr. Fractured bedrock moved as individual small fragments, which was mainly controlled by the hitting force of the hoe, while soil moved in the shape of lumps, which was dominated by both drag force of the hoe and gravity. This study suggests that hoeing into soil‐covered bedrock can diminish bedrock erosion while providing soil matrix for shallow soil layers. Our work presents a quantitative assessment of bedrock erosion by hoeing and an underlying insight into characteristics of bedrock erosion by tillage operations in hilly agricultural regions with mudstone and shale, southwest China. © 2020 John Wiley &amp; Sons, Ltd.

Preliminary Investigation on the Causes of Odd Vibration of Buildings in Guilin City—a Study on the Resonance between Buildings and the Underlying Soil Layer

AbstractA residential building in Guilin City, China, underwent an up‐and‐down vibration with an amplitude of 2 cm. By eliminating several causes such as earthquake, wind and construction, in combination with the unique karst geological conditions of Guilin, it was inferred that the effect of turbulent karst water is the main contributor to the occurrence of this vibration. Therefore, a geophysical survey was undertaken on the vibration zone and its surroundings. The resutls suggest that the soil in the upper part of the karst grooves shows conspicuous traces of disturbance and the water content has low resistance. A comprehensive analysis indicates that there is a karst strong runoff zone beneath the area. According to the water level and water temperature data collected automatically by the instrument, it can be concluded that the groundwater level had changed significantly twice during this period, so it was determined that the denudation of groundwater strong runoff causes the overlying soil layer to collapse, forming a soil‐water soft‐flow material with mixed phase. Since the building in this area was supported by friction piles, the groundwater‐soil soft‐flow material with mixed phase has intense plasticity, and the up‐and‐down vibration resulted in the change of the friction piles, which caused the up‐and‐down vibration of the building structure.

Study on the Influence of Bedrock Thickness on Deformation and Failure of Overlying Soil Layer in Thin Bedrock Coal Seam Mining

Study on the Influence of Bedrock Thickness on Deformation and Failure of Overlying Soil Layer in Thin Bedrock Coal Seam Mining

Response of pavement foundations incorporating both geocells and expanded polystyrene (EPS) geofoam

The suitability of geocell reinforcement in reducing rut depth, surface settlements and/or pavement cracks during service life of the pavements supported on expanded polystyrene (EPS) geofoam blocks is studied using a series of large-scale cyclic plate load tests plus a number of simplified numerical simulations. It was found that the improvement due to provision of geocell constantly increases as the load cycles increase. The rut depths at the pavement surface significantly decrease due to the increased lateral resistance provided by the geocell in the overlying soil layer, and this compensates the lower competency of the underlying EPS geofoam blocks. The efficiency of geocell reinforcement depends on the amplitude of applied pressure: increasing the amplitude of cyclic pressure increasingly exploits the benefits of the geocell reinforcement. During cyclic loading application, geocells can reduce settlement of the pavement surface by up to 41% compared to an unreinforced case – with even greater reduction as the load cycles increase. Employment of geocell reinforcement substantially decreases the rate of increase in the surface settlement during load repetitions. When very low density EPS geofoam (EPS 10) is used, even though accompanied with overlying reinforced soil of 600 mm thickness, the pavement is incapable of tolerating large cyclic pressures (e.g. 550 kPa). In comparison with the unreinforced case, the resilient modulus is increased by geocell reinforcement by 25%, 34% and 53% for overlying soil thicknesses of 600, 500 and 400 mm, respectively. The improvement due to geocell reinforcement was most pronounced when thinner soil layer was used. The verified three-dimensional numerical modelings assisted in further insight regarding the mechanisms involved. The improvement factors obtained in this study allow a designer to choose appropriate values for a geocell reinforced pavement foundation on EPS geofoam.

On the ecology and biology of two endogean millipedes (Diplopoda: Julida: Julidae) endemic to the Western Rhodope Mtn., Bulgaria

Mountain slopes covered with stone debris have been of special interest for zoologists since the pioneer work of Juberthie et al. (1980) who defined and described the Milieu Souterrain Superficiel, most popularly referred to in English as the Mesovoid Shallow Substratum (MSS). Today this particular habitat is classified as one of the several types of MSS, namely the colluvial MSS. In a number of recent publications, the MSS is considered as one of the Superficial Subterranean Habitats (SSHs) within a broader concept of the subterranean domain. According to a widely accepted idea, the SSHs are different in their nature but are all characterized by the absence of light which is regarded as the main factor responsible for the occurrence of troglomorphic fauna in these habitats, along with epigean species that are able to live in such conditions. The present study focuses on two julid millipedes — Typhloiulus orpheus Vagalinski, Stoev &amp;amp; Enghoff, 2015 and a yet undescribed genus and species of the tribe Typhloiulini — occurring in the transitional layer of fine rubble between the soil stratum and the MSS in limestone taluses in the Western Rhodope Mtn. It was revealed that the spatial distributions of the two species in the studied sites were remarkably confined and follow the same pattern, which is suggested to reflect narrow microhabitat specialization. This assumption is further supported by certain traits in the morphology and biology of the two diplopods. It can be concluded that both T. orpheus and the new genus and species are essentially stenotopic endogean elements with strict requirements for an aphotic, mesophilous, limestone environment. Given the specific ecological conditions, which differ from both the overlying soil layer and the underlying colluvial MSS, together with the existence of certain narrowly adapted species, it is worth considering the recognition of the intermediate “microvoid” breakstone layer as a separate shallow subterranean habitat.

Study on Bias Characteristics of Multi Arch Tunnel Under Different Side Overburden Thickness

The entrance and exit of the Qifengshan tunnel are shallow and the pressure has a certain offset, the overlying soil layer is thin, and the stability is poor. After the excavation disturbance, the upper soil layer tends to slide. The excavation of black holes often causes large deformation, collapse and instability of surrounding rock. In this paper, combined with the geological conditions of the multi arch tunnel and the mechanical characteristics of surrounding rock, the theoretical analysis and numerical simulation are used to study the deformation law of the buried depth, side cover soil thickness and the stress and displacement of the surrounding rock after excavation. The results show that the bias stress of the tunnel under the vertical direction increases with the depth of the buried tunnel. The bias stress of the tunnel in the vertical direction decreases with the increase of the buried depth. The shallower the buried depth is, the more obvious the bias effect is. When the depth of burial exceeds 20m, the increased amount of bias stress is gradually reduced after the bias is applied. The vertical deformation characteristics of the rock mass caused by tunnel excavation with different thickness of the tunnel are analyzed. The vertical displacement is gradually reduced with the increase of the thickness of the side covering layer. When the thickness of the overlying soil is more than 21m, the vertical displacement is less affected by the thickness of the side cladding soil, basically only by the surrounding rock itself.

Stability analysis on veneer cover system for landfill considering the effect of internal seeper

A drainage layer can easily cause clogging during rainfall, and a retained seeper may cause a strong uplifted-floating effect on the overlying soil layer, thereby inducing serious disasters. This paper proposes two failure modes (i.e., direct-slip failure and uplifted-floating failure) that may be encountered simultaneously by the cover soil layer of landfills. Corresponding analytical models and computing methods are established. Results demonstrate that both failure modes can potentially occur on the cover system but exhibit different performance levels. The immersion ratio (λ) of the drainage layer is an important factor influencing the safety factor (FS) of both failure modes. By increasing the λ-value, the Fs-values of both failure modes decrease evidently, but the decrease rates are vastly different. Moreover, the effects of other parameters such as slope length and slope angle on the FS of the two failure modes are also discussed in this study. Results indicate that these parameters exhibit different effect degrees on the occurrence of the two failure modes. Finally, the Fs-values of the direct-slip failure calculated in this study are compared with the results of others', and indicated that the reliability of this study is high. The results of this study can optimize the design parameters of the landfill final cover system and render its stability evaluation more accurate.

Finite Element Analysis of Overlying Soil Failure Based on Fault Dip Angle

When the strong earthquake occurs, the deformation response and rupture of the overlying soil are very complicated. In this paper, the influence of fault dip angle on the surface deformation and rupture of overlying soil is analyzed by means of finite element numerical simulation. When the vertical dislocation of the fault is about 3∼5% of the thickness of the overlying soil layer, the surface rupture occurs only when the fault dip angle is 45°. When the vertical displacement of the fault increases to 10% of the thickness of the overlying soil layer, surface rupture occurs under three working conditions of 45°, 70° and 90° of fault dip angle. With the increase of the dip angle from 45°, 70° to 90°, the surface equivalent strain is gradually smaller, the deformation of the overlying soil is smaller, and the deformation width of the upper wall will change from about 2 times that of the lower wall to the same as the lower wall. The deformation and rupture of the overlying soil first begin with the fracture of the soil mass at the interface between the fault bedrock and the soil. With the increase of the dislocation quantity, the surface also will appear 1 breakpoint when the fault angle is 45° and 70°, the ground surface will appear 2 breakpoints when the fault angle is 90°, and at last overlying soil is broken through.

Analysis of Factors Influencing Stability of Overlying Soil Layer on Thin Bedrock Based on Crack Development‐Closure Test

The water‐blocking properties of the clay layer at the bottom of the Cenozoic overburden in China are an important factor influencing the safety of thin bedrock coal seam mining. Clay has remolding properties that are unlike the nonreversible characteristics of cracks in brittle rock, and failure cracks in clay can reclose or continue to expand under the influence of different external factors. In this work, the soil layer on top of thin bedrock is the research object, and the influences of the particle composition, water content, soil layer thickness, and crack width on the crack development‐closure state of soil layer are analyzed by the orthogonal test method. Visual analysis shows that the order of influence of each factor on the stability of soil layer is the crack width, particle composition, soil layer thickness, and water content. The stability of soil layer decreases with increasing crack width and sand content and decreasing soil layer thickness; in addition, soil layer stability decreases first and then increases with increasing water content. Further variance analysis shows that the crack width and particle composition are key factors that impact the stability of soil layer and that the soil layer thickness has some influence, while the water content has little effect on the stability of soil layer. In addition, the crack will reclose when the sand content in soil is less than 50% and the crack width is less than or equal to 1.0 mm, and the soil layer is prone to further failure when the sand content in soil is more than 50% and the crack width is greater than or equal to 3.0 mm; when the soil layer thickness is 15.0 cm, its stability is better than when the soil layer thickness is 10.0 cm or 5.0 cm.

Field experiments and numerical analysis on the ground vibration isolation of wave impeding block under horizontal and rocking coupled excitations

In this paper, the effectiveness of a semi-analytical BEM in studying the vibration mitigation of wave impeding block (WIB) in layered soil under horizontal loads is verified by field tests, subsequently the performance of WIB in mitigating the vibrations of ground subjected to horizontal-rocking excitations were investigated by numerical analysis using the verified BEM. Two types of layered half-space model under the horizontal-rocking excitations (site A: the stiff soil ground with relatively soft overlying soil layer, site B: the soft soil ground with relatively stiff overlying soil layer) are considered. Test and numerical results indicate that WIB can achieve desired vibration isolation effectiveness in layered soils under both the horizontal and horizontal-rocking coupled excitations. With the decrease of the embedded depth, the performance of WIB for vibration mitigation improves in layered ground under horizontal excitation. For horizontal-rocking coupled excitation, WIB reduces ground vibration by 50–70% at zones over one diameter of WIB (10 m) away from the vibration source; at zones within one diameter of WIB, peak AR value occurs, and the peak values are beyond 1.0 (2.0 in site A and 12.5 in site B at normalized distance D = 0.75) in radial direction. WIB behaves generally better in site A than in site B.

Model test on partial expansion in stratified subsidence during foundation pit dewatering

Partial expansion was observed in stratified subsidence during foundation pit dewatering. However, the phenomenon was suspected to be an error because the compression of layers is known to occur when subsidence occurs. A slice of the subsidence cone induced by drawdown was selected as the prototype. Model tests were performed to investigate the phenomenon. The underlying confined aquifer was generated as a movable rigid plate with a hinge at one end. The overlying layers were simulated with remolded materials collected from a construction site. Model tests performed under the conceptual model indicated that partial expansion occurred in stratified settlements under coordination deformation and consolidation conditions. During foundation pit dewatering, rapid drawdown resulted in rapid subsidence in the dewatered confined aquifer. The rapidly subsiding confined aquifer top was the bottom deformation boundary of the overlying layers. Non-coordination deformation was observed at the top and bottom of the subsiding overlying layers. The subsidence of overlying layers was larger at the bottom than at the top. The layers expanded and became thicker. The phenomenon was verified using numerical simulation method based on finite difference method. Compared with numerical simulation results, the boundary effect of the physical tests was obvious in the observation point close to the movable endpoint. The tensile stress of the overlying soil layers induced by the underlying settlement of dewatered confined aquifer contributed to the expansion phenomenon. The partial expansion of overlying soil layers was defined as inversed rebound. The inversed rebound was induced by inversed coordination deformation. Compression was induced by the consolidation in the overlying soil layers because of drainage. Partial expansion occurred when the expansion exceeded the compression. Considering the inversed rebound, traditional layer-wise summation method for calculating subsidence should be revised and improved.

Numerical Analysis on Stabilizing Mechanism of Soil Nails in Steep Fill Slopes Subjected to Rainfall Infiltration Using a Hypoplastic Model

Soil nailing has been widely used to stabilize slopes across the world over the last decades and is currently one of the predominant methods used for upgrading fill slopes in Hong Kong. The behavior of the soil nails in fill slopes has been investigated by experimental and numerical methods. However, the influence of rainfall infiltration on the reinforced fill slopes by soil nails was not investigated in existing studies. In addition, the stabilizing mechanism of the use of soil nails in fill slopes under rainfall condition is not yet understood thoroughly. The objectives of this study, therefore, are to explore the response of steep fill slopes to rainfall infiltration and the mechanism of soil nails in improving slope stability and reducing slope deformation. A series of two-dimensional numerical simulations where a hypoplastic constitutive model was used to model the strain-softening behavior of fill slopes was carried out. The numerical results show that a potential slide is developed in the unreinforced fill slope and thus slope failure may occur initiating from the toe of slope. Due to the presence of soil nails, both shallow failure and deep-seated failure are prevented in the soil nail reinforced slopes. The soil nails greatly limit the displacement of slope and the formation of potential slide in the slope. The tensile forces in the soil nails are mobilized through the effective interaction among soil nails, nail heads and surrounding soil when a potential slide is being initiated. The nail heads transfer the earth pressure of the overlying soil layer to the soil nails in question, causing the decrease of soil stress around the nail heads. The results also demonstrate that the interface friction between nail and soil can influence the stabilizing effect. Therefore, it is necessary to ensure sufficient interface friction to make the soil nails effective in transferring earth pressure and reducing slope deformation. However, once the friction coefficient between nail and soil reaches a threshold value, the interface friction will no longer be the key factor for the slope stability.