Natural Soil Slope Research Articles

Identification of Complex Slope Subsurface Strata Using Ground-Penetrating Radar

Identification of slope subsurface strata for natural soil slopes is essential to assess the stability of potential landslides. The highly variable strata in a slope are hard to characterize by traditional boreholes at limited locations. Ground-penetrating radar (GPR) is a non-destructive method that is capable of capturing continuous subsurface information. However, the accuracy of subsurface identification using GPRs is still an open issue. This work systematically investigates the capability of the GPR technique to identify different strata via both laboratory experiments and on-site examination. Six large-scale models were constructed with various stratigraphic interfaces (i.e., sand–rock, clay–rock, clay–sand, interbedded clay, water table, and V–shaped sand–rock). The continuous interfaces of the strata in these models were obtained using a GPR, and the depths at different points of the interfaces were interpreted. The interpreted depths along the interface were compared with the measured values to quantify the interpretation accuracy. Results show that the depths of interfaces should be interpreted with the relative permittivity, back-calculated using on-site borehole information instead of empirical values. The relative errors of the depth of horizontal interfaces of different strata range within ±5%. The relative and absolute errors of the V–shaped sand–rock interface depths are in the ranges of [−9.9%, 10.5%] and [−107, 119] mm, respectively. Finally, the GPR technique was used in the field to identify the strata of a slope from Tanglang Mountain in China. The continuous profile of the subsurface strata was successfully identified with a relative error within ±5%.

Large-deformation modelling of earthquake-triggered landslides considering non-uniform soils with a stratigraphic dip

Natural soil slopes often exhibit both spatial variability and stratigraphic dips due to complex depositional processes of soil, exerting a remarkable influence on landslide behaviors. Existing research mostly focused on the influence of soil spatial variability on slope stability or landslide runout behaviors, and the investigation of the effect of stratigraphic dip on large-deformation landslide consequences are relatively limited and have not been fully understood. This study therefore proposed a random large-deformation finite-element (LDFE) computation framework, combining the coupled Eulerian-Lagrangian technique and a three-dimensional (3D) rotational random field to insight the earthquake-triggered landslide consequences considering non-uniform soils with a stratigraphic dip. The whole runout process of earthquake-triggered landslides from activation to termination is realistically reproduced. By implementing both two-dimension (2D) and 3D random LDFE analyses coupled with Monte-Carlo simulations, both effects of spatial non-uniformity and stratigraphic dip of soil on the evolution process and consequences of landslides are analyzed in detail. The results reveal that the stratigraphic dip greatly influences landslide behaviors. In addition, the 2D analyses probably underestimate the landslide consequences when contrasting with 3D analyses. These findings highlight the importance of considering stratigraphic dip of soil slope and the advantage of 3D random LDFE analysis on landslide behaviors.

Some Approaches to the Prediction of Permeability Parameters in a Finite Element Program for Early Warning

Recently, landslides often occurred in natural soil slopes in the tropical region, which correlate with the rainy season. Rainfall infiltration leads to groundwater level fluctuations. The increased positive pore-water pressures due to rainfall influence have affected the properties and behavior of the unsaturated soil slope. In this research, the Finite Element Method of SEEP/W and SLOPE/W analyzes the factor safety of the slope affected by pore water pressure change due to rainfall. The Soil Water Characteristic Curve (SWCC) and Hydraulic Conductivity function were obtained from sieve analysis and Atterberg's limit. In addition, unsaturated soil properties from the UNSODA code are estimated based on grain-size distribution using the SWRC program. The study area is in Khanom District, southern Thailand. The results show that the soil slope at the site became unstable on November 18, 2021, with F.S. = 1.0, which agrees well with the date of the disaster. In conclusion, the slope stability analysis without the parameters from the unsaturated soil hydraulic database (UNSODA) leads to the F.S. value being higher than the actual value, and the alarm estimation would be inaccurate. Doi: 10.28991/CEJ-2022-08-12-014 Full Text: PDF

Time Motion Study on Soil Protection Works

Soil protection works have become popular due to their immense contribution to protecting the building from land sliding. A building structure requires a stable foundation to ensure best construction and durability. There are various engineering techniques in soil protection works, this paper consists of the Soil nailing procedure adopted in the Northwest Commercial multi-use (NWCM) Project of Karle Infra Pvt. Ltd Company, currently executed by the vendor Geoengineering Pvt. Ltd. Soil nailing technique is used to stabilize the existing slope and construct retaining walls from top down. Steel nails are used as a reinforcement of soil which are drilled and grouted into the soil to create a compound mass similar to a gravity wall. The Soil nailing procedure is an alleviative measure to handle unstable natural soil slopes or unstable filled slopes. Soil protection works include multiple activities that occur in a systematic and sequential manner. The critical activities influencing the duration, schedule and progress of soil protection works are identified and a time-motion study is used as a tool to identify the difference in productivity to improve the progress of the soil protection work. Each activity of grouted nailing for one lift of excavation is carefully analyzed to record the time spent for that activity and to identify areas to reduce time and increase efficiency. The study can improve planning work based on the actual duration of activities identified and help to minimize delays at the construction site.

Investigation of the role of crown crack in cohesive soil slope and its effect on slope stability based on the extended finite element method

Tensile cracks in soil slopes, especially developing at the crown, have been increasingly recognized as the signal of slope metastability. In this paper, the role of crown cracks in natural soil slopes was investigated and its effect on stability was studied. A numerical modeling of slope and simulation of tensile behavior of soil, based on the Extended Finite Element Method (XFEM), was used. A numerical soil tensile test was applied to validate the use of XFEM on tensile behavior of soil before the simulation. Slope failure was simulated by using the Strength Reduction Method, which determines the potential slip surface of slope. The simulation results indicate forming of crown crack in natural soil slopes when the plastic zone starts penetrating. Therefore, it is reasonable to consider the crown crack as the signal of slope metastability. A sensitivity analysis shows the effect of cracking on slope stability if cracks are at the position of the tension zone. The stress variation analysis, from the surface slip deformation, reveals that the slope is at a state of compressive stress. When plastic zone starts penetrating, the upper part of slope generates tension zone, but the extent of tension zone is restricted by the slope failure. This suggests why tensile cracks are difficult to form and stretch in the deep part of the slope. The implementation of XFEM on slope stability analysis can be used for assessing the tensile strength of soil and forecasting the time of slope failure related disaster.

The Effects of Cement Kiln Dust on the Soil Electrical Resistivity

Electrical resistivity is a non-destructive, cost-effective and sensitive method to evaluate soil's physical and chemical properties. Electrical resistivity has been used widely in surface and subsurface exploration. The electrical resistivity is directly related to the subsurface geotechnical and geothermal properties like porosity, temperature, salinity and water content. Recently uses of waste material as an additive to improve the soil engineering properties are growing because of their cost-effectiveness. Cement Kiln Dust (CKD) is a waste material of the cement manufacturing process. CKD is widely used as an additive material in ground improvement to improve soil's geotechnical properties. This study is mainly focused on the effect of CKD on the electrical resistivity properties of the soil. In this study, the electrical resistivity of a natural soil slope treated with CKD and a test model in the laboratory was investigated. Besides, the effects of CKD on soil pH and electrical resistivity were studied by performing a series of tests to predict the soil's corrosivity potential. The soil was treated with 0, 5, 10 and 15% of CKD and the electrical resistivity of the soil was measured at different water contents, porosities and curing times. The results indicate that the soil's electrical resistivity increases by increasing the CKD content and curing time. Additionally, an increase in water content or porosity decreases the electrical resistivity of CKD treated soil. Furthermore, the electrical resistivity measurement is a practical method to determine the stabilized soil's geotechnical and geomechanical properties.

Performance of geosynthetic cementitious composite mat and vetiver on soil erosion control

An understanding of how different land covers affect soil erosion caused by rainfall is necessary in mountainous areas. The land cover usually plays an important role in controlling landslide hazards associated with these terrains. This paper presents the results of a field experiment where several types of land covers were placed on a full-scale embankment as erosion control. An 8 m wide, 21 m long, and 3 m high embankment with a 45° side-slope was built with lateric soil. The soil was compacted under a relative compaction of 70% to simulate a natural soil slope. Two sides of the embankment were divided into six land cover areas, with three different areas of bare soil, and one each of a geosynthetic cementitious composite mat (GCCM), vetiver grass, and a combination of GCCM and vegetation. Soil erosion and moisture levels were monitored for each land cover area during six natural rainfall events encountered over the experimental period. Field results were compared with a numerical simulation and empirical soil loss equation. The results revealed that the GCCM gave the best erosion control immediately after installation, but vetiver grass also exhibited good erosion control six months post-construction.

Experimental Studies on Effect of Nailing in Cohesion-Less Soil

The anchorage in the form of soil nailing can be used for stabilizing weak natural soil slopes. It can also be used forsteepening of new or naturally existing soil slopes. The nail anchorage in the soil can be in the form ofreinforcement and in the different materials with different cross section area. The most of the failure of nail in soilhave been reported as pullout failure due to tension on the nail, particularly in sandy soil. Because of increased costand safety concerned the field test to determine pullout force is difficult. Therefore the laboratory studies werecarried out in simulated conditions. In this research work, the laboratory investigation were carried out to observethe effect of tensile (pullout force) force on the soil nail in sandy soil. The pullout force on the nail was graduallyincreased and the corresponding displacement was recorded till failure of the nail due to continues sliding. The testresults were compared to Standard values in earlier studies. It has been shown that a simple loading frame withtesting tank model can be used to simulate the pullout load–displacement relationship and to determine peakpullout force of different type of nails. The various parameters studied were embedded length of the nail, the type ofsurface, the material and cross-section of nail.

Stability and characterisation of spoil heaps in European surface lignite mines: a state-of-the-art review in light of new data

The large amount of spoil material produced during the mining process imposes a significant economic and environmental liability on lignite producers. In this context, the present paper provides an overview of the geotechnical characteristics of European lignite mine spoil heaps and discusses their significance to the stability of the heaps. In order to achieve this, samples collected from spoil heaps of Polish, Czech and Greek mines are analysed and the results are compiled with data from the literature. A major conclusion drawn is that both physical and engineering properties of spoil heaps indicate a noteworthy variability, which is larger than typical in situ ground material. This is because of the additional factors affecting spoil heap deposition, such as the transportation and dumping method. Furthermore, failure mechanisms and case histories of large instabilities in lignite spoil heaps are critically discussed in order to better understand triggering failure mechanisms. It is concluded that classical assumptions made for natural soil slopes and relevant limit equilibrium models should be cautiously applied to spoil heaps. The challenges associated with numerical and probabilistic modelling of spoil heap stability, such as the inherent spatial variability of spoils and the time-dependent changes in their geotechnical properties, are also critically discussed. Finally, important research gaps in design and analysis of spoil heap stability, such as the absence of appropriate constitutive models developed specifically for spoil materials, are summarised.

Assessment of bearing capacity of interfering strip footings located near sloping surface considering artificial neural network technique

The bearing capacity of interfering footings located near the slope face suffers from reduced bearing capacity due to the formation of the curtailed passive zone. Depending upon the position of the footing, their spacing and steepness of the slope, different extents of bearing capacity reduction can be exhibited. A series of finite element investigation has been done with the aid of Plaxis 3D vAE.01 to elucidate the influence of various geotechnical and geometrical parameters on the ultimate bearing capacity of interfering surface strip footings located at the crest of the natural soil slope. Based on the large database obtained from the numerical simulation, a 6-8-1 Artificial Neural Network architecture has been considered for the assessment of the ultimate bearing capacity of interfering strip footings placed on the crest of natural soil slope. Sensitivity analyses have been conducted to establish the relative significance of the contributory parameters, which exhibited that for the stated problem, apart from shear strength parameters, the setback ratio and spacing of footing are the prime contributory parameters.

The Rise of Groundwater Due to Rainfall and the Control of Landslide by Zero-Energy Groundwater Withdrawal System

Slope failure is a common issue in tropical countries. The rise of groundwater table due to rainfall is one of the main triggering factors. There are several methods for slope stabilization such as soil nailing, retaining walls, cut and fill, vegetation and so on. Most of those methods are costly and we are in need for stabilizing methods that are more economical and easier to construct. This article introduces a new method for slope stability. This method is examined numerically and experimentally. It is represented in an automatic zero-energy groundwater withdrawal system to enhance slope stability. The system is validated in a pre-fabricated model to ensure that it works on natural soil slope. The numerical simulation is performed in Soilworks software with coupled seepage-slope stability analysis using finite element methods to check the safety factor with and without the system. The effectiveness of this method is investigated with various rainfall intensities and soil permeabilities. The results for slopes with the application of groundwater withdrawal system are compared with the results without the system. The results demonstrate the effectiveness of the proposed method in reducing groundwater table and enhancing slope stability. The factor of safety for the slope with high soil permeability drops from 1.312 before the rainfall to 1.292 and 0.93 after the third rainfall event for the slope with and without pumping groundwater respectively. For soil slope with moderate soil permeability, the factor of safety deteriorates from 1.314 to 1.157 at the end of the third day, while it remains stable with pumping groundwater. Matric suction is highly increased at the crest of the slope due to pumping.

Seismic analysis of nailed soil slope considering interface effects

Natural soil slopes are often reinforced with nails to stabilize them against earthquake loading. Although pseudo-static method is widely used in designing such slopes; it fails to provide important information such as deformation of slope, effect of soil-nail interface etc. A 2-D finite element model of typical nailed slope has been prepared during this study using OpenSees to perform seismic analysis with due consideration to soil nonlinearity, pressure dependency of soil and separation-sliding at soil-nail interface. It is found that the soil-nail interface modelling can significantly influence the permanent deformation of slope after seismic event. The overburden pressure on the nail varies significantly during the earthquake loading and the variation is more when sliding and separation is allowed at the soil-nail interface. It is also found that the model with fixed interface leads to a perception of reinforced soil acting as a relatively rigid block, which results into an unconservative design from overall deformation perspective.

Rainfall-induced landslides in Quaternary soils in Norway

Increased intensity of rainfall in later years seems to result in increased frequency of rainfall-induced landslides in natural slopes with glacial Quaternary deposits, which cover large parts of Norway. Many slides hit railway and road infrastructure, and sometimes dwellings. Typically, the soil matrix has a high content of intermediate soils (sand and silt). Natural soil slopes may have inclinations above the effective friction angle of the soils. Slope stability hence must rely on some cohesion, which often is “apparent”, i.e. caused by negative pore-water pressure (suction). Dissipation of suction during short- or long-term rainfall hence results in reduced shear strength of the slope, and may lead to slope failure. There is a scarcity of data for Norwegian soils for thorough analysis of landslide triggering based on unsaturated geomechanics. More data may result in improved landslide warning. In this paper, results from unsaturated shear box testing of samples of a silty sand taken from a landslide site in Eastern Norway are presented and discussed.

Geological and engineering analysis of residualsoil for forewarning landslide from highland areain northern Thailand

Abstract One devastating landslide event in northernThailand occurred in 2006 at Ban Nong Pla village, ChiangKlang highland of Nan province after, a massive amountof residual soil moved from upstream to downstream, viacreek tributaries, into a main stream after five days of unusualheavy rainfall. In this paper, the geological and engineeringproperties of residual soil derived fromsedimentaryrocks were analyzed and integrated. Geological mapping,electrical resistivity survey and test pits were carriedout along three transect lines together with systematiccollection of undisturbed and disturbed residual soilsamples. As a result, the average moisture content in soilis 24.83% with average specific gravity of 2.68,whereas theliquid limit is 44.93%, plastic limit is 29.35% and plastic indexis 15.58%. The cohesion of soil ranges between 0.096–1.196 ksc and the angle of internal friction is between 11.51and 35.78 degrees. This suggests that the toughness propertiesof soil change when moisture content increases. Resultsfrom electrical resistivity survey reveal that soil thicknessesabove the bedrock along three transects range from2 to 9 m. The soil shear strength reach the rate of high decreasesin the range of 72 to 95.6% for residual soil fromshale, siltstone and sandstone, respectively. Strength ofsoil decreaseswhen the moisture content in soil increases.Shear strength also decreases when the moisture contentchanges. Therefore, the natural soil slope in the study areawill be stable when the moisture content in soil level isequal to one, but when the moisture content between soilparticle increases, strength of soil will decrease resultingin soil strength decreasing.

장기간 강우특성 변화에 따른 국내 사면의 안정성

Shallow landslides and debris flows are a common form of soil slope instability in South Korea. These events may be generally initiated as a result of intense rainfall or lengthening rainfall duration because of the effects of climate change. This paper presents the evaluation of rainfall-induced natural soil slope stability and reinforced soil slope instability under vertical load (railway or highway load) throughout South Korea based on quantitative analysis obtained from 58 sites rainfall observatories for 38 years. The slope stability was performed for infinite and geogrid-reinforced soil slopes by taking an average of maximum rainfall every ten years from 1973 to 2010. Seepage analysis is carried out on unsaturated soil slope using the maximum rainfall at each site, and then the factor of safety was calculated by coupled analysis using saturated and unsaturated strength parameters. The contour map of South Korea shows four stages in 10-year-time for the degree of landslide hazard. The safety factor map based on long term observational data will help prevent rainfall-induced soil slope instability for appropriate design of geotechnical structures regarding disaster protection.

IMPROVING THE STRENGTH OF SANDY SILT SOILS BY MIXING WITH CEMENT KILN DUST

Egyptian irrigation and drainage system consists of the River Nile, its branches, canals and drains. It is very important to keep them working in good condition to assure efficient water management and distribution process. Water channel embankments made of sandy silt soils are exposed to slope stability problems. These problems are considered one of the common canal embankments problems in the delta of Nile River. Soil improvement is one of the most economic engineering solutions to overcome soil problems. This research studying the improving of sandy silt soils by mixing with cement kiln dust. Laboratory tests were carried out to evaluate the improved soil properties. Three numerical models were built for an embankment of a canal to examine the introduced method to overcome slope stability problems. The natural soil slope stability safety factor was calculated from the first numerical model. The second model used to calculate safety factor after applying the improved soil properties that were determined from the laboratory works. The third numerical model was built to simulate improving natural soil by using the traditional soil reinforcement technique. The results for the two improvement methods were compared. The results showed that the proposed method caused significant improvement for slope stability of the sandy silt soil. The proposed method has the advantage of using with existing or new structures.

Travel distance angle for "rapid" landslides in constructed and natural soil slopes

"Rapid" landslides in constructed and natural soil slopes have the potential to cause loss of life, destruction of property, and damage to the natural environment because the velocity of the slide mass is such that persons in the travel path do not have time to evacuate and the kinetic energy is such that even small landslides can severely damage buildings and other structures. This paper presents methods for prediction of the post-failure travel distance for "rapid" landslides from constructed cut and fill slopes and natural soil slopes derived from a database of some 350 "rapid" landslides in predominantly soil slopes. The characteristics of soils, both dilative and contractive on initial shearing, and the slope conditions for which landslides are susceptible to development into slides of "rapid" post-failure velocity are discussed. Methods for estimating the travel distance angle of the post-failure travel of the slide mass have been developed and are based on consideration of the failure mechanics of the initial slide (whether contractive or dilative on initial shearing), the type of slope, the slide volume, the geometry of the slope at and below the slide source area, and the degree of confinement of the travel path of the landslide.Key words: "rapid" landslide, flow slide, debris flow, debris slide, travel distance angle, failure mechanism.

Mechanistic approach to the shear strength behaviour of allophanic soils

The presence of allophane minerals imparts special engineering features to the volcanic ash soils. This study examines the reasons for the allophanic soils exhibiting unusual shear strength properties in comparison to sedimentary clays. The theories of residual shear strength developed for natural soils and artificial soil mixtures and the unusual surface charge properties of the allophane particle are invoked to explain the high shear strength values of these residual soils. The lack of any reasonable correlation between ∅′ (effective stress-strength parameter) and plasticity index values for allophanic soils is explained on the basis of the unusual structure of the allophane particle. The reasons as to why natural soil slopes in allophanic soil areas (example, Dominica, West Indies) are stable at much steeper angles than natural slopes in sedimentary clay deposits (London clay areas) are explained in light of the hypothesis developed in this study.