Overlying Soil Layer Research Articles

Nonlinear seismic response analysis of slopes considering the coupled effect of slope geometry and soil stratigraphy

SUMMARY To investigate the effects of slope geometric parameters and soil stratigraphic properties on the topographic amplification of ground motions, a large number of 2-D horizontally layered slope models are constructed. First, the linear and nonlinear seismic responses of a slope model are compared, and the result shows that the nonlinear characteristics of soils should be considered when studying the amplifying effect of slope topography on ground motions. Then, the nonlinear seismic responses of these slope models are analysed from four aspects: the maximum shear strain in the slopes, the effects of geometry and stratigraphy on the seismic response, the distance between the maximum topographic amplification indicators and the slope crest, and the influence range of slope topography behind the slope crest. The results indicate that the amplifying effect of slope topography on ground motions increases with increasing slope height or decreasing average shear wave velocity of the overlying soil layers. Besides, the variation of the topographic amplification effect with slope gradient is significantly influenced by soil stratigraphic properties. The distance between the maximum topographic amplification indicators and the slope crest is mainly in the range of 0–60 m, and the influence range of slope topography behind the slope crest is mainly in the range of 0–150 m. Subsequently, approximate relations are derived based on regression analyses of simulation results, which can provide meaningful references for the seismic design and seismic retrofitting of engineering structures behind the slope crest. Finally, the effects of slope geometric parameters and soil stratigraphic properties on ground motion modifications are further evaluated according to the prediction curves provided by the approximate relations.

Transmission characteristics and the factors influencing stable oxygen isotopes in precipitation, soil water, and drip water in Remi Cave, Western Hunan, China

Understanding the modern transport processes of δ18O between different waters in cave systems can provide a basis for the paleoenvironmental interpretation of records of stalagmite δ18O. We conducted monthly in-situ monitoring of local precipitation, soil water at three different soil depths, five drip water sites and environmental parameters in Remi cave, Western Hunan Province, Central China, from December 2020 to January 2024. During the monitoring period, due to differences in geological structure and ventilation effect at different locations within the cave, cave air temperature and relative humidity gradually remained constant from the cave entrance to the deep parts of the cave. On intra-annual timescales, precipitation δ18Op were depleted in the wet season and enriched in the dry season, which were mainly controlled by changes in moisture sources, upstream convective activity, and rainout effects. With the increasing thickness of the soil layer above the cave, soil water δ18Osoil gradually became negative and leveled off, possibly because shallow soil water responded sensitively to local year-round precipitation, accompanied by surface evaporation. While, deep soil water with more negative δ18O, may be mainly replenished by heavy precipitation during the wet season, but the effect of water retained in the soil from preceding years couldn’t be excluded and would be needed to verify by conducting more monitoring studies on the variation of δ18Osoil covering the long period. The mean δ18O values of drip water at sites D1 and D2 near the cave entrance were heavier than that of shallow soil water and precipitation, possibly due to evaporation from the thin overlying soil layer, together with ventilation effect. There were seasonal variations of δ18Od values at site D5 in the deep parts of the cave, which may be associated with the seasonal variation of drip rate at this site. Due to the smoothing effect of soil layer and epikarst zone, there were no seasonality in δ18Od values at sites D3 and D4, in the middle and deep parts of the cave. The mean δ18Od at these two sites were lighter than amount-weighted annual mean δ18Op, and close to the deep δ18Osoil and the wet-season amount-weighted mean δ18Op values. This suggests that the thick soil layer overlying the cave raises the precipitation threshold, causing these sites to be mainly recharged by heavy rainfall in the wet season. Therefore, the δ18O of stalagmites growing under these drip sites may mainly inherit the wet-season δ18Op signal. The short monitoring period of this study (three years) and the lack of modern calcite δ18O data, mean that caution is need in interpreting stalagmite δ18O on a timescale longer than that of this study.

Experimental study on self-burrowing dual anchor soft probe

This study focuses on the development and testing of a bio-inspired self-burrowing dual anchor soft probe for potential geotechnical applications. Dual anchor refers to the form of movement in soils in which some bivalve molluscs adopted by alternately generating anchoring effects in the soil through shell expansion and fluid-filled feet. By mimicking this mechanism, this study used pneumatic artificial muscles as soft actuators and developed an autonomous burrowing probe. The structure and the performance of the actuators and the probe were investigated and optimized. The burrowing-out process of the dual anchor probe was not a simple upward movement. Instead, it rose in the inflation phase and slipped downward in the deflation phase. The difference between the two was a stride in one single step. In the sands with relative densities of 30%, 50%, and 80%, the total slips accounted for 18.8%, 19.6%, and 26.9% of the total upward movements, respectively. Thus, the entire movement process showed a reciprocating upward trend. The burrowing process could be divided into a restricted stage and a free stage according to whether shear failure occurs in the overlying soil layer. When the soil density was high, the initial stage of burrowing was in a restricted stage. The amount of rise and slip were at a low level and increased slowly as the number of cycles increased. When the burrowing process was in the free stage, the increase was basically stable at a high value and accompanied by small slips.

Prediction of surface settlement caused by synchronous grouting during shield tunneling in coarse-grained soils: A combined FEM and machine learning approach

This paper presents a surrogate modeling approach for predicting ground surface settlement caused by synchronous grouting during shield tunneling process. The proposed method combines finite element simulations with machine learning algorithms and introduces an intelligent optimization algorithm to invert geological parameters and synchronous grouting variables, thereby predicting ground surface settlement without conducting numerous finite element analyses. Two surrogate models based on the random forest algorithm are established. The first is a parameter inversion surrogate model that combines an artificial fish swarm algorithm with random forest, taking into account the actual number and distribution of complex soil layers. The second model predicts surface settlement during synchronous grouting by employing actual cover-diameter ratio, inverted soil parameters, and grouting variables. To avoid changes to input parameters caused by the number of overlying soil layers, the dataset of this model is generated by the finite element model of the homogeneous soil layer. The surrogate modeling approach is validated by the case history of a large-diameter shield tunnel in Beijing, providing an alternative to numerical computation that can efficiently predict surface settlement with acceptable accuracy.

Preliminary Investigation of Sudden Ground Subsidence and Building Tilt in Balitai Town, Tianjin City, on 31 May 2023

A short-term rapid subsidence event occurred in the Bi Guiyuan community in Balitai Town, Tianjin City, leading to the tilting of high-rise buildings and the emergency evacuation of over 3000 residents. In response to this incident, InSAR (Interferometric Synthetic Aperture Radar) technology was swiftly employed to monitor the subsidence in the area before and after the event. Our observations indicate that the region had maintained stability for 8 months prior to the incident. However, over the course of the 15-day event, the ground experienced more than 10mm of subsidence. By integrating the findings from an InSAR analysis with geological studies, we speculate that the rapid subsidence in the region is related to the extraction of geothermal resources. It is suspected that during drilling operations, the wellbore mistakenly penetrated a massive underground karst cavity. Consequently, this resulted in a sudden rapid leakage of drilling fluid, creating a pressure differential that caused the overlying soil layers to collapse and rapidly sink into the cavity. As a result, short-term rapid subsidence on the ground surface and tilting of high-rise buildings occurred.

Evaluation of soft soil foundation reinforcement effect of prefabricated building based on BP neural network

The use of traditional reinforcement methods in construction sites often causes problems such as pore water pressure, which can not effectively form a solid foundation. Aiming at this problem, the evaluation model of soft soil foundation reinforcement effect of prefabricated buildings is established based on BP neural network, combined with the geological characteristics of soft soil and the elements of foundation reinforcement; The L-M algorithm is used to optimize the slow convergence problem of BP neural network, and finally its evaluation effect is verified through practical application. The results show that the strengthening effect of 1550 kN⋅m/m2 is better than that of 2000 kN⋅m/m2 with the more times of tamping for marine and river facies, and there is a positive correlation between the times of strengthening and the effect. At the same time, similar qualitative conditions also show that the greater the burial depth, the worse the reinforcement effect. When the overlying soil layer is soft, the shallow buried soil layer can be reinforced by laying a cushion to improve the overall reinforcement effect. The laws reflected in the final model output data are the same as those reflected in the construction, and the accuracy of the proposed model is up to 87%, indicating that the model has superior performance in the reinforcement effect evaluation.

A Study on the Influence of Double Tunnel Excavations on the Settlement Deformation of Flood Control Dikes

This article combines numerical simulation and field monitoring methods to study the stability of the overlying Liuyang River embankment in the tunnel crossing between Huaqiao Station and Rice Museum Station of Changsha Metro’s Line 6. Using AutoCAD, 3Dmine, and COMSOL Multiphysics, a calculation model of the entire subway tunnel section crossing the flood control embankment under the coupling of fluids and solids was established. The process of tunnel-crossing the embankment and the variation in spatial displacement and plastic strain in different geological layers were analyzed from the perspective of time evolution and spatial distribution. The research results show that during the process of crossing the embankment, the deformation of the east bank is greater than that of the west bank, and crossing the west bank is the relatively riskier stage of the entire project. Moreover, during the process of crossing the embankment, the overlying soil layer will produce a plastic strain zone, and only a small amount of plastic strain is generated in the surrounding sandstone layer of the tunnel walls. In terms of the magnitude of plastic strain, the plastic strain area produced by the leading tunnel’s surrounding rocks is larger than that of the following tunnel. As the excavation progresses, a funnel-shaped settlement displacement gradually forms during the passage of the leading tunnel, and this settlement funnel gradually expands during the passage of the following tunnel, with the maximum settlement point transitioning from directly above the leading tunnel to the middle position between the two tunnels. Using the jitter filter algorithm and the adjacent average method to process the field monitoring data, the results show that the monitored deformation results well match the simulated settlement results.

Geochemical behaviors of uranium and thorium during weathering and pedogenesis of carbonate rock: constraint from their speciation.

During weathering and pedogenesis of carbonate rock with poor-uranium (U) and thorium (Th), U and Th present the characteristics of strong leaching (especially U) and significant residual enrichment, the cause of which is still unclear. In this paper, a weathering profile developed by dolomite in karst area of Guizhou province in southwest China was selected, which showed zonation characteristics of bedrock (Y), powdery rock (Yf), and soil layer (T1 to T12) from the bottom to up. Through the determination of the occurrence speciation of U and Th in Y and weathering profile, combined with mineralogical, geochemical characteristics, and element mass balance calculation, the constraints of U and Th speciation on the geochemical behavior of U and Th during the weathering of carbonate rock were revealed. The results proved that U and Th in Y preferentially existed in acid insoluble phase, for example, the contents of U and Th in Y were 0.90 mg·kg-1 and 0.28 mg·kg-1, respectively, while those in acid insoluble matter were 2.34 mg·kg-1 and 2.57 mg·kg-1, respectively, but because the mass percentage of acid insoluble matter was extremely low (0.95%), the mass percentages of U and Th in the acid soluble phase in the whole rock were absolutely superior (96% of U and 86% Th). The U and Th in the acid soluble phase of Y were mainly adsorbed on the crystal surface of carbonate minerals or existed in the cement, and the U and Th in the carbonate lattice only accounted for a small proportion. From Y to Yf with the initial dissolution, U and Th released from the surface of carbonate minerals and cements were in carbonate-rich alkaline environment, and these portions of U and Th were leached out, resulting in strong loss of U and Th in the Yf (the loss rates are 83% of U and 65% of Th, respectively). From the Yf to the overlying soil layer T1, the carbonate components were completely dissolved, and the U and Th released from the carbonate lattice showed different behaviors, where U was completely leached and Th tended to stay in the weathered residue. Thus, in the soil layer T1 formed by Y or Yf , the residual U was the inheritance of the U in the acid insoluble phase of Y; For Th, it not only inherited the Th of acid insoluble phase of Y, but also superimposed the Th from carbonate lattice in Y. On the other hand, during the evolution process from Y to Yf and to soil layer T1, with the dissolution of carbonate, the acid insoluble phase also showed a significant tendency of chemical weathering. However, the U and Th in the Y acid insoluble phase were not leached with the decomposition of the acid insoluble phase but were redistributed among the residual phases. For the geochemical behaviors of U and Th in the evolution of soil profile (T1~T12), they were subjected to the occurrence speciation of U and Th in T1 and the change of U and Th occurrence speciation with the upward direction of soil profile. The U and Th released from the carrier minerals were mainly redistributed among the residual solid phases, which weakened the intensity of their further loss. This study deepens the understanding of the geochemical behavior of radionuclides in karst environment and provides reference for the treatment of radioactive pollution in karst areas.

Discussion on Blasting Vibration Velocity of Deep Rock Mass Considering Thickness of Overlying Soil Layer

The thickness of the overlying soil layer has a certain influence on the blasting vibration response of deep rock mass. The vibration wave velocity of the overlying soil layer during the construction of deep blasting is measured in this paper. Based on the measured data, parameters k and α of the Sadowski equation are used to characterize the influence of the comprehensive geological conditions of the site on the vibration wave propagation. The model of blasting vibration velocity of deep rock mass is established according to the existing blasting theory, and the calculation accuracy of the model is verified according to the field blasting parameters. The new model is used to simulate different overlying soil thicknesses, and the safe allowable distance under different soil thicknesses is calculated. The calculated results show that with the increase of the thickness of the overlying soil layer, the blasting vibration velocity decreases and the attenuation velocity decreases gradually. The research results reveal the reduction effect of overlying soil thickness on blasting vibration to some extent. In the area with overlying soil layer, the safe allowable distance of blasting vibration safety can be appropriately reduced to increase the land utilization rate, which has important reference value for the blasting design and safety prediction of deep rock mass.

Seismic Behavior of Arch Bridge Abutments under Sandy-Gravel Foundations with Different Grouting Depths: Insights from Shaking Table Tests

For arch bridges built on deep soil sites, significant seismic amplification effects were observed in the overlying soil layer from past earthquake events, and these effects can lead to more serious structural damage. Grouting-strengthening technology has recently been recommended as an innovative geotechnical method for strengthening soils to retain the seismic amplification phenomenon under strong ground motions, and it has exhibited advantages in improving the lateral resistance of soils. However, grouting reliability depends on the soil type, jet pressure, and engineering costs, and there has been little research on the differences in the natural period and seismic capacity caused by grouting behavior. To fill this research gap, based on a practical engineering prototype, shaking table tests were performed for semigrouting and all-grouting cases in sandy-gravel soil with a simple bridge abutment to investigate the difference in the seismic capacity of the two soil–foundation–abutment systems. The acceleration response difference between the grouting zone and the surrounding soil, characteristic periodic variation of the response spectrum of the soil and abutment, grouting depths, and seismic energy dissipation mechanism of the soil were analyzed. The results show that the acceleration amplification factor of the all-grouting foundation was smaller than that of the semigrouting foundation. However, the acceleration amplitude of the all-grouting foundation was larger owing to the higher seismic input energy. Significant differences in the lateral resistance mechanism between semigrouting and all-grouting foundations were observed. The grouting foundation realizes consistent acceleration and displacement through load transmission and soil–structure interaction mechanisms. The results demonstrate the suitability of soil thicknesses for significantly reducing the seismic demands of abutments and provide guidance for foundation treatment methods in relevant engineering projects.

Ground Surface Deformation Caused by Pipe Jacking Construction in a Soft Soil Area: An Experiment-Based Study

Soft soil has the characteristics of high compressibility, large void ratio, and strong structure. Therefore, it is more likely to cause surface subsidence or even surface cracking and collapse when laying oil and gas pipelines in soft soil areas through the pipe jacking method. In this study, test soil was pressurized using a flexible loading bladder, and variable formation losses brought on by pipe jacking were mimicked by varying the loading bladder’s water injection level. The surface deformation brought on by pipe jacking construction is related to the four parameters of stratum loss rate, overburden load, soil disturbance, soil tension fissures, and horizontal deformation. The findings demonstrate that the surface deformation brought on by the excavation gradually reduces and starts to stabilize after the thickness of the overlying soil layer on the pipe jacking surpasses 1.5 times the diameter of the pipe jacking. The constructed settling tank is broader the deeper the jacking pipe is submerged. Further ground surface settling will be exacerbated by the weight above the jacking pipe. The maximum ground surface deformation value will decrease with an increase in the overlying load when the overlying load is high (0.018 MPa).

Do soft soil layers reduce the seismic kinematic distress of onshore high-pressure gas pipelines?

AbstractOnshore high - pressure gas pipelines constitute critical infrastructure that usually cross seismic - prone regions and are vulnerable to Permanent Ground Deformations (PGDs) due to active seismic faults. In design, it may not be feasible to avoid fault rupture areas due to various technical, economical and topographic reasons. Moreover, the presence of soil layers affects the PGDs resulting from a tectonic fault, which in turn may alter the seismic demand on the pipeline. The current study investigates numerically the impact of soft soil layers on the seismic kinematic distress of onshore gas pipelines. For this purpose, a decoupled numerical modeling approach is adopted, consisting of two separate finite - element models for the simulation of soil response and pipeline distress, respectively. Soil non - linearities are taken into account utilizing the Mohr - Coulomb constitutive model with isotropic strain softening. An extensive parametric analysis is performed considering different faulting mechanisms and fault dip angles, as well as soil geometry and mechanical properties. Consequently, the maximum absolute values of both tensile and compressive pipeline strains are correlated with the seismic intensity level (i.e., in terms of bedrock offset which is associated with earthquake magnitude via simple relationships). The paper concludes with a set of design charts and tables for the preliminary seismic design of onshore high - pressure gas pipelines. These charts and tables predict with reasonable accuracy pipeline deformations, in terms of strains, for different magnitude, fault type, dip angle, sand type, and varying overlying soil layer thickness.

Influence analysis of overlying soil layer to seismic behavior of inclined liquefiable soil and pile interaction system

The statistical data of earthquake hazard indicate that the liquefaction and large lateral flow deformation of ground is one of the main reasons of pile foundation failure. It is significant to focus on the seismic dynamic behavior of pile foundation in inclined liquefiable soil. Based on the Opensees finite element platform, an integrated finite numerical model of inclined liquefiable soil-pile group-structure interaction is established in this study, according to the shaking table model test of inclined liquefiable soil. The nonlinearity of pile-soil contact and the shear localization of soil layer are considered in the numerical simulation. The comparison of the numerical simulation result with the shaking table test results are verified. In order to discuss the influence of different overlying soil layer on the seismic behavior of soil and pile system, a typical inclined liquefiable soil-pile group-structure interaction finite element model is established by the proposed numerical simulation method. The results show that the overlying soil layer has an obvious influence on the seismic dynamic behavior of the pore pressure, the horizontal displacement of soil and pile. The increase of the thickness of overlying soil can reduce the liquefaction degree of sand and improve the mechanical performance of pile foundation.

Model Test Analysis of Groundwater Level Fluctuations on Karst Cover Deformation Taking the Monolithic Structure of Guilin as an Example

Engineering practice and real-life cases show that the geological conditions of the Guilin overlying karst site are complex. In particular, the groundwater, which drives the accelerated formation of soil cavities, and the thickness of the overlying soil layer, which affects the speed of the groundwater subsidence process. Therefore, this paper is based on a physical model to evaluate the effects of groundwater level changes caused by different factors on the deformation of karst cover. The model tests are simulated for different cover thicknesses (6 cm, 9 cm, 12 cm, 15 cm, 18 cm) under rainfall and other recharge, cavity supply, and drainage conditions at the same density (1.40 g/cm3) and initial water content (30%), respectively. The results show that with the increase of rainfall and other recharge time, the basic change trend of different cover thicknesses is that the infiltration curve changes faster at the beginning and slows down at the end, but the thicker the cover, the slower the overall deformation; at a certain rate of cavity recharge and drainage, the thicker the cover, the smaller the deformation caused by the fluctuation of groundwater level. The cavity recharge makes the cover displacement obvious, in the order of 0.304 cm, 0.173 cm, 0.118 cm, 0.068 cm, and 0.056 cm. After the formation of the cavity, the rainfall, other recharge, and the cavity supply and drainage accelerate the destruction and deformation of the soil body and the upward development of the cavity. The research results provide theoretical support for the subsequent prevention and control of karst collapse in covered karst areas, and have certain practical engineering significance.

Analysis of the Influence of Groundwater Level Fluctuation on Karst Soil Cave Collapse

Artificial water pumping and rainfall will induce groundwater level fluctuations, leading to large-scale karst collapse. Therefore, the combination of physical model test and FLAC3D numerical simulation is adopted to simulate the soil cave collapse caused by three different initial water level fluctuations in this paper. The influence of various initial water level heights on the variation of water-gas pressure, soil pressure of the overlying soil layer, and deformation in the existing soil cave during the fluctuation was analyzed. The relationship between initial water level height, deformation and collapse of the cave, and water-gas pressure was established, and the action law of water level fluctuations on the collapse of soil cave was introduced. The results show the following: (1) The change of water-gas pressure positively correlates with the initial water level height, directly affecting the overlying soil layer deformation. (2) The change of soil pressure of overlying soil positively correlates with the change of water-gas pressure under different initial water level fluctuations. (3) The deformation and collapse difficulty of the soil cave negatively correlates with the initial water level. The higher the initial water level, the stronger the negative pressure absorption and positive pressure gas explosion effect on the overburden on the soil cave and the collapse will occur with fewer water level fluctuations. (4) The numerical simulation results are consistent with the laboratory model test. These laws elucidate the change of water-gas pressure in different initial water level fluctuations and reveal the collapse mechanism of water level fluctuations. It will provide essential theoretical support for further research on the action law of hydrodynamic factors on karst collapse and a scientific and reasonable basis for urban groundwater pumping control, karst and roadbed collapse prevention and prediction.

Region-dependent site conditions in China: Evidence from borehole data statistics

Consideration of site effects is vital in modeling and predicting earthquake ground motions. However, site conditions can vary markedly between different localities. The regional dependency of site conditions in China has not yet been investigated systematically. In this study, profiles of 6,179 boreholes were collected from four regions in China, i.e., the Capital Metropolitan (CM), Xinjiang (XJ), Guangdong—Guangxi (GG), and Sichuan–Yunnan (SY) areas. Quantitative characteristic parameters including site category, equivalent shear wave velocity (Vse), and the thickness of the overlying soil layer (H) defined by the Chinese seismic code, and VS30 and the median shear wave profile were all analyzed to confirm the dependency on site conditions among the four regions. Investigation revealed that the majority of sites in most regions are classified as Class-II sites with no Class-IV sites, except in CM. In comparison with the other three regions, a larger number of GG sites are classified as Class-I. Sites in XJ are generally characterized by small H values (<20 m) and large Vse values (250 –450 m/s), while those in CM are characterized by large H values (average: ∼50 m) and small Vse values (150 –300 m/s). Generally, the Vse and H values are similar in GG and SY, i.e., the sites are covered by an overlying soil layer that is ∼20 m thick with an average Vse value of ∼250 m/s. A very thin (<5 m) overlying soil layer is observed at many more sites in GG than at sites in the other three regions. The VS30 values in CM, SY, and GG all approximately follow a lognormal distribution with various logarithmic means, i.e., 257.2, 299.6, and 360.9 m/s, respectively. However, the VS30 values of most sites in XJ broadly follow a uniform distribution with a range of 200 –550 m/s. We summarized the characteristics of the median VS profiles of each VS30-based category (C1, C2, and C3) for each region. The median VS profiles between any two regions are mainly manifested as parallel trends for sites in category C1, whereas they present trends of intersection for sites in categories C2 and C3. The findings of this study could serve as a basis for the establishment of a regional empirical model for site-dependent seismic response in China.

Analysis of the seismic effects of the local slope site of Longtoushan market town in Ludian Ms6.5 earthquake

In the Ludian Ms6.5 earthquake (Yunnan, China), Longtoushan market town and its vicinity showed significant differences in earthquake damage. To explain this phenomenon, this paper discusses the local engineering geological conditions, local topographic effects, and seismic response of the soil layer in Longtoushan market town. The results show that complex topography and varied engineering geological conditions will lead to significant differences in ground motion, and then lead to significant differences in building earthquake damage. Slope topography has an obvious influence on ground motion. From the foot of the slope to the top of the slope, the degree of influence gradually increases, and the influence in different directions is different, as shown: the closer to the top of the slope, the more significant the difference. This indicates that the serious damage to buildings built on the slope is caused by the amplification effect of local terrain and the differential effect of ground motion. Although the site belongs to Class II site, the near-surface geotechnical properties and their combination characteristics, the physical state and thickness of the overlying soil layer, the variation characteristics of shear wave velocity, the non-linear dynamic characteristics of the soil, and other factors play a decisive role in the amplification degree of ground motion. The significant difference in ground motion on the surface of the slope site leads to a significant difference in seismic damage to buildings on the site. The covering soil layer has a significant effect on the acceleration response spectrum. The conclusions obtained in this paper can provide a scientific basis for the site selection of engineering construction and seismic design of building structures.

Experimental Analysis on Mechanical Characteristics of Foundation Soil in Rift Valley Area of Kenya Nairobi-Malaba Railway

The Rift Valley section of Kenya Nairobi-Malaba Railway locates in the Great Rift Valley of East Africa, with complex engineering geological conditions and well-developed geological structures. During the rainy season from March to May 2018, four large-scale ground fissures were formed in the first-stage project of the Nairobi-Malaba Railway of the valley floor section, accompanied by uneven surface settlement and trenches, seriously endangering the safety of the railway and its nearby projects. Through field investigation, it is preliminarily considered that the main reason induced ground fissures and surface settlement is the underlying soil layer being eroded by groundwater. The gully is further formed by surface water erosion on a base of uneven surface subsidence. The geological exploration trench at DK77 ground fissure revealed that the overlying soil layer is respectively grayish-yellow silty clay, cyan-gray volcanic ash, and brownish-yellow silty clay from top to bottom, and the underlying bedrock is volcanic tuff with wide cracks. The fluid flows out or into the bedrock through the cracks developing channels for groundwater up-down flowage. Under the erosion of groundwater to the underlying soil, this study proved the possibility of the occurrence of uneven settlement. When exposed to the groundwater, the underlying soil will exhibit special physical and mechanical properties which are conducive to the occurrence of ground cracks and subsidence. The conventional geotechnical tests are conducted for the three types of overlying soil, and the results reveal the causes of ground fissures and surface settlement from the physical and mechanical properties of the overlying soil and provide a basis for the further qualitative analysis of the mechanism of ground fissures and surface settlement.

Analysis of the Mechanism of Ground Fissures in the Nairobi–Malaba Railway Rift Valley Area in Kenya

During the rainy season from March to May 2018, four ground fissures were generated in the rift valley area along the first phase of the Nairobi–Malaba Railway in Kenya, among which the ground fissures B3 highway, DK76, and DK77 were larger in scale, which seriously endangered the safety of the railway. According to the author’s previous study, the ground fissure B3 highway belongs to a typical fault-type fissure, and the ground fissures DK76 and DK77 are qualitatively concluded to be generated under the groundwater erosion of collapsible soils. To quantitatively summarize the mechanism of ground fissures, DK76 and DK77, physical model tests on the overlying soil properties, compactness, the intersection angle between hidden cracks and the water flow direction, the height of confined water level, and the groundwater flow rate are conducted to analyze the effect of these factors on the formation and expansion of ground fissures. The test results show that the soil with strong collapsibility under the action of underground erosion produces fissures, collapses, and holes similar to those in the rift valley area along the first phase of the Nairobi–Malaba Railway in Kenya; the loose overburden is conducive to the occurrence of surface collapse; the collapse of the soil surface is the most severe when the underlaid cracks are parallel to the water flow direction; the higher confined water level and faster inflow rate will aggravate the collapse of the overlying soil layer.

Experimental investigation of local scour around cylindrical pile foundations in a double-layered sediment under current flow

Scour is a natural phenomenon that occurs when the soil around piles erodes, and it is significantly affected by the characteristics of the seabed or riverbed materials, the nearby submerged obstructions, and the water flowing past. Water–sediment interaction is a critical process that may lead to the loss of bed materials and influence the mechanical behavior of foundations. Exploring the scour mechanism is essential for predicting scour depth and designing appropriate countermeasures, especially those that consider essential concepts in geomechanics as well as complex geological conditions. This study presents a series of tests on piles embedded in single- and double-layered sediments that are composed of coarse, medium, and fine sands. The results show that the properties of the overlying soil layer govern the initiation of erosion and the final scour range, while those of the underlying layer determine the final scour depth. As the exposed sediments and their scour resistance at the water–soil interface change during the scour, predictions of scour depth that are merely based on the properties of the surface layer of the sediment may not be accurate. Based on the experimental results, an innovative predictive framework of scour depth was proposed that considers sediments with two layers.