Changes In Soil Strength Research Articles

Estimation of breakout force of spudcan footing in structured clay

For a jack-up rig installed in cohesive soil, assessing the extraction force is critical to ensure the safe and successful retrieval of each spudcan footing. The extraction force depends on the preceding stages, including installation and operation of the rig, due to soil strength changes around the spudcan. To investigate the entire ‘penetration - operation - extraction’ process of spudcan, an effective stress large deformation finite element analysis is conducted, with a Structured Cam-Clay model being incorporated to describe the behaviour of structured soil. The large deformation approach is validated against existing centrifuge tests. Parametric studies are then carried out to explore the influential factors on breakout force, including operation period, vertical load ratio, penetration depth, destructuring rate and soil sensitivity. A unique curve is formulated to predict the evolution of the recovery degree of breakout force over the operation period in structured cohesive soil. The soil sensitivity is identified as the dominant factor that affects the ultimate net resistance ratio, which is the key parameter required in applying the prediction curve. A simple method is proposed to estimate the ultimate net resistance ratio and a procedure to predict the breakout force is suggested.

Effects of long-term monocropping, rotation cropping, and fertilization on energy and fuel requirements for fall moldboard plowing in a clay-loam soil

With recent increases in energy costs, information on energy inputs is becoming a more important aspect in management decisions on selection of crop production systems. The effects of long-term (45–55 years) monocropping, rotation cropping, and fertilization on tillage energy (implement draft and tractor fuel consumption) were determined for a Brookston clay-loam soil in southwestern Ontario, Canada. Treatments included fertilized and unfertilized monocrop (continuous) corn (Zea mays L.), and a four year fertilized and unfertilized corn – oat (Avena sativa L.) – alfalfa (Medicago sativa L.) – alfalfa rotation with all phases of the rotation present in each year. The corn and second year alfalfa plots were moldboard plowed each fall after crop harvest, and then spring-tilled (disc, harrow) prior to planting; oat and first year alfalfa plots were not fall plowed or spring-tilled. Moldboard plow draft and tractor fuel consumption were measured annually from 2004 to 2014 using an instrumented tractor and the same plow and settings.Monocrop unfertilized corn consistently exhibited the greatest plow draft and tractor fuel consumption. Draft averaged over ten years was 13.0% higher for unfertilized than fertilized rotation corn, and 15.5% higher for fertilized than unfertilized second year alfalfa; fuel consumption was 10.4% higher for unfertilized than fertilized rotation corn and 5.1% higher for fertilized than unfertilized second year alfalfa. These differences were attributed to treatment-induced changes in soil strength. Both plow draft and fuel consumption were lower in rotation corn relative to monocrop corn, while plow draft was greater in fertilized alfalfa relative to unfertilized alfalfa due to greater root growth. This study demonstrated that long-term cropping systems can have substantial impacts on the energy required for tilling a clay loam soil.

Downslope Weakening of Soil Revealed by a Rapid Robotic Rheometer

AbstractMoving down a hillslope from ridge to valley, soil develops and becomes increasingly weathered. Downslope variation in clay content, organic matter, and porosity should produce concomitant changes in soil strength that influence slope stability and erosion. This has yet to be demonstrated, however, because in situ measurements of soil rheology are challenging and rare. Here we employ a robotic leg as a mechanically sensitive and time‐efficient penetrometer to map soil strength along a canonical temperate hillslope profile. We observe a systematic downslope weakening, and increasing heterogeneity, of soil strength associated with a transition from sand‐rich ridge materials to cohesive valley bottom soil aggregates. Weathering‐induced changes in soil composition lead to physically distinct mechanical behaviors in cohesive soils that depart from the behavior observed for sand. We also demonstrate the promise that legged robots may use their limbs to sense and improve mobility in complex environments, with implications for planetary exploration.

Reversible strengthening behaviour of subsurface layers in sandy soils – understanding variable response to strategic deep tillage

Deep sandy soils cover more than 10 M ha of Australia's southern dryland cropping lands. Farming these soils is inherently challenging, and yield gaps are often wide due to multiple cropping constraints. Physical constraints that limit rooting depth are commonplace and often attributed to traffic-induced soil compaction. However, recent observations draw attention to a reversible hardsetting behaviour in response to soil water content. This hardsetting phenomenon is poorly understood and it may explain inconsistent crop responses to deep ripping in sandy soils. We examined the dynamic behaviour of soil strength under different soil water contents in deep ripped and unripped sands from four South Australian sites. We analyzed the elemental, mineralogical, chemical and physical compositions to differentiate the nature of hardened subsurface layers. Soil compaction and cementation were ruled out due to the soil's bulk density (<1.6 g cm−3) encountered and the rapid slaking that occurred when a hardened pan was submerged in water. The hardsetting behaviour was confirmed by the abrupt change in soil strength in response to the soil water content, measured using intact samples in a laboratory-based penetrometer. This significant increase in soil strength was also observed for soils collected from the ripped plots and for repacked soil samples, with penetration resistance ranging from 0.4 to 5.7 MPa depending on soil content. In contrast to our hypotheses, the hardsetting layer could not be distinguished from the layers above or below it by any analytical method. Given soil strength is typically measured on field moist soil, and standard chemical analysis did not differentiate hardsetting behaviour, alternative methods of diagnosis are needed to understand where deep tillage is likely to have the desired long-term effect. Identifying the nature of potential binding agents responsible for increased strength upon drying would be valuable. Our results confirmed reversible hardsetting in deep sandy soils is driven by small changes in soil water content, from 0.08 g g−1 to 0.05 g g−1 upon drying. Given that rapid re-consolidation of the soil matrix after expensive deep ripping interventions are reported, there is a strong imperative to improve diagnosis, and to identify if soil ameliorants could improve longevity by disrupting hardsetting behaviour.

Comparison of Stabilized Shear Strength of Blangjerango Clay and Some Quarry Soils With Tripe Jaya Gayo Lues Limestone Ash

Soil stabilization is a way that is most considered to be cheaper than replacing old, less good soil with better ones with the aim of improving the properties of the original soil which has low bearing capacity, high plasticity index, high swelling and poor gradation, especially for areas that requires soil improvement. One of the stabilizing materials that can be used is lime as an additive which is useful for increasing soil stiffness and strength. The lime used in this research came from Tripe Jaya District, Gayo Lues Regency. This research was carried out by taking soil samples in a quarry which was then added with lime as a stabilizing agent. This soil was tested for physical properties, compaction, and a shear strength test (direct shear). Research was conducted to see the results of soil stabilization with lime on changes in soil strength when receiving the effects of both natural and artificial (remolded) factors. The results of the research can be used to mitigate cases of road or building collapse on the subgrade or infrastructure built on the land. The primary data required in this research was taken from the results of examinations and tests in the laboratory. The tests and inspections carried out are in the form of measuring the physical properties of the soil such as water content, specific gravity, plastic limit, liquid limit, and PSA (Particle Size Analyzer), as well as measuring mechanical properties in the form of soil compaction parameters with a standard proctor on the original soil and soil shear strength using direct shear test to obtain friction angle (ϕ) and cohesion (c). In this research, a comparison of the stabilized shear strength of clay soil with Gayo Lues limestone has been carried out with several other soil sources for landslide analysis. The analysis results show that Gayo Lues limestone has a good ability to increase the strength and stability of clay soil. It is hoped that this research will provide useful information in selecting the right soil stabilization material.

Effect of different vegetation roots on mechanical properties of soil stabilization on slope

Soil bioengineering is concerned with the soil stabilisation with the reinforcing agent such as plant roots. This approach is extensively popular in developing countries. Most of the study conducted on soil bioengineering is carried out by ecological researchers, whereas there have been few geotechnical research studies in India that focus on using plant roots for reinforcing purposes. This research aims to investigate the changes in soil strength caused by landslides. The soil will be stabilised using plant roots from regionally common plants in the study region. The lemon roots were collected and planted in the soil, and the alterations in geotechnical properties were investigated. The reinforcing process can result in an increase in the values of MDD, UCS, SS, and OMC due to the improved compaction of soil particles. It was found that as the percentage of plant root added to the soil increases, the MDD, UCS, SS, and OMC also increases until 1% of plant root was added by weight. After that point, these properties decreases. Hence, the most favourable proportion for soil stabilisation is 1% of plant root by weight to the soil. Thus the presence of plant roots in the soil matrix enhanced the soil's stability. Therefore, the plant roots that were examined can serve as cost-effective materials for enhancing slope stability,” particularly in places that are susceptible to landslides.

A two-dimensional effective stress framework for modelling whole-life soil strength changes due to pore pressure generation and dissipation, Part 1: Formulation

The undrained shear strength of contractive fine-grained soils changes with time, reducing due to pore pressure generation and increasing during consolidation. There is an increasing appetite to recognise these temporal soil strength changes in offshore geotechnical design, as it provides a basis for potentially less conservative designs. Contributions to this endeavour are reported across two companion papers. This first paper extends an existing effective stress framework that relates the generation of pore pressure to accumulated plastic shear strain, allowing undrained shear strength to be calculated within the context of critical-state soil mechanics. The main development is the extension of the computational domain to two dimensions, allowing calculations to be made for boundary value problems that cannot be satisfactorily simplified to one-dimensional conditions. The magnitude and distribution of accumulated shear strain surrounding objects buried in soil are quantified through a series of large deformation finite element analyses. These spatial distributions are described using a strain influence function in the new 2D framework to calculate the extent and magnitude of excess pore pressure, and in turn the mobilised soil strength around the buried object. The performance of the 2D framework is examined in the companion paper through retrospective simulations of experimental and numerical data.

A two-dimensional effective stress framework for modelling whole-life soil strength changes due to pore pressure generation and dissipation, Part 2: Applications

The accurate quantification of the temporal changes in seabed strength allows for more reliable and less conservative geotechnical design. A recently developed effective stress framework, established within a one-dimensional computational domain to quantify changes in soil strength due to pore pressure generation and dissipation, has been extended to a two-dimensional (2D) computational domain to allow for consideration of boundary value problems that are too complex to be simplified to one-dimensional conditions. The work to implement the 2D framework is reported across two companion papers. The first of the two papers utilises large deformation finite element analyses to quantify the spatial distribution of accumulated plastic shear strain. These distributions are encapsulated within a strain influence function that is used within the new 2D framework in this paper to calculate the extent and magnitude of excess pore pressure, and in turn the mobilised soil strength for a number of boundary value problems that represent typical offshore geotechnical processes. The merit of the new 2D framework is explored via retrospective simulations of existing experimental and numerical data. The resulting comparisons demonstrate the potential of the new framework, which is in quantifying the reliability of a range of geotechnical structures under complex loading conditions.

Strength Assessment of Water-Glass Sand Mixtures.

For years, the chemical injection process has aided construction works by increasing the strength and water-sealing efficiency of sandy soil. Despite its growing popularity in projects, such as seismic strengthening and liquefaction mitigation, a unified understanding of how chemically treated soil develops its strength, especially under static conditions, remains elusive. Some studies have proposed that strength is derived from the tensile effects of dilatancy, where shearing of the sandy soil causes expansion, creating tension in the interstitial hydrogel and resulting in negative pressure that consolidates the soil particles. Other studies, however, attribute this strength development to the volumetric shrinkage of the hydrogel, which the authors argue confines and compresses the sandy soil particles. Challenges are encountered with this theory, particularly with respect to the consistency of the volumetric shrinkage measurements and the timing of these measurements in relation to changes in soil strength. The aim of the current research is to shed light on this mechanism by using consolidation drainage triaxial compression (CD) tests to measure the cohesive strength and internal friction angle of chemically enhanced soil. By eliminating the dilatancy-induced negative pressure effects and coupling this with an analysis of the molecular structure of the hydrogel, the present study provides an in-depth look at the strength development mechanism and its durability. This holistic approach not only fills in the existing gaps in the understanding of this mechanism, but also paves the way for optimized construction techniques.

Recurring Rolling/Crimping Effects on Termination Effectiveness of Iron Clay Pea and Pearl Millet Warm-Season Cover Crops

Summer cover crop utilization by no-till vegetable farms is essential for continuous soil protection, especially in the southern United States where intense storms are likely to occur in hot and humid summer months. A field experiment was conducted at the National Soil Dynamics Laboratory in Auburn, AL, USA, between the summers of 2015 and 2017 to determine the effectiveness of an experimental roller/crimper in mechanically terminating summer cover crops. Iron clay peas (Vigna unguiculata, L.) planted on a sandy loam and pearl millet (Penninsetum glaucum, L.) planted on clay soil were selected to determine termination rate effectiveness in single, double, and triple rolling/crimping over the same area. Overall, termination rates for both cover crops were higher for rolling three times (71%) compared to rolling once (55%) or twice (63%). However, cover crop termination was inhibited due to rainfalls on the experimental area during the three-week evaluation period. In 2016, drought conditions and high temperatures (32.6 °C) caused biomass reduction, especially for pearl millet, of over 31% to 39% compared to 2017 and 2015. Rolling provided higher soil-water conservation compared with the non-rolled control due the cover crop mulch layer blocking sunlight, keeping the soil surface cooler and preventing water evaporation. Recurrent rolling did not cause soil compaction above the 2.0 MPa level that inhibits root growth, but changes in soil strength were dependent on the soil moisture content.

Whole-life modelling of anchor capacity for floating systems: The RSN[sbnd]CSI approach

Successive episodes of cyclic loading cause the strength of soft soils to reduce, through pore pressure build-up, and then recover, through consolidation, as shown by model testing, field studies and theoretical considerations. These ‘whole-life’ changes in soil strength affect the capacity of anchoring systems for offshore infrastructure such as floating turbines or platforms. This paper introduces a new macro-model for assessing the through-life changes in seabed strength and anchor capacity as a result of variable cyclic loading and concurrent consolidation. The model combines SN curves for damage accumulation with a critical state soil mechanics framework for changes in soil strength and anchor capacity. These methods allow the full operational lifetime of an anchoring system to be rapidly analysed, encompassing timescales from individual wave-induced load cycles, through to annual seasons and soil consolidation processes. The approach provides a new basis for whole-life modelling of anchoring systems that is sufficiently fast to allow reliability-based assessments via a Monte Carlo method. In soft soils that exhibit beneficial gains in capacity, this method provides a basis for more efficient design through reductions in anchor size.

Pore evolution and shear characteristics of a soil-rock mixture upon freeze-thaw cycling

The changes in pore structure within soil-rock mixtures under freeze-thaw cycles in cold regions result in strength deterioration, leading to instability and slope failure. However, the existing studies mainly provided qualitative analysis of the changes in pore or strength of soil-rock mixture under freeze-thaw cycles. In contrast, few studies focused on the quantitative evaluation of pore change and the relationship between the freeze-thaw strength deterioration and pore change of soil-rock mixture. This study aims to explore the correlation between the micro-pore evolution characteristics and macro-mechanics of a soil-rock mixture after frequent freeze-thaw cycles during the construction and subsequent operation in a permafrost region. The pore characteristics of remolded soil samples with different rock contents (i.e., 25%, 35%, 45%, and 55%) subjected to various freeze-thaw cycles (i.e., 0, 1, 3, 6, and 10) were quantitatively analyzed using nuclear magnetic resonance (NMR). Shear tests of soil-rock samples under different normal pressures were carried out simultaneously to explore the correlation between the soil strength changes and pore characteristics. The results indicate that with an increase in the number of freeze-thaw cycles, the cohesion of the soil-rock mixture generally decreases first, then increases, and finally decreases; however, the internal friction angle shows no apparent change. With the increase in rock content, the peak shear strength of the soil-rock mixture rises first and then decreases and peaks when the rock content is at 45%. When the rock content remains constant, as the number of freeze-thaw cycles rises, the shear strength of the sample reaches its peak after three freeze-thaw cycles. Studies have shown that with an increase in freeze-thaw cycles, the medium and large pores develop rapidly, especially for pores with a size of 0.2–20 μm. Freeze-thaw cycling affects the internal pores of the soil-rock mixture by altering its skeleton and, therefore, impacts its macro-mechanical characteristics.

Numerical Assessment of Pipe Pile Response under Seismic Excitation

The axial capacity and pile transference of loads under static loading have both been well reported, but further research is needed to understand the dynamic lateral responses. The pile load imposed during an earthquake may increase, but the soil’s ability to support it may fall as a side effect of the vibration leading to more settlement. The key objective of this work is to identify what led to the substantial lateral destruction of the piles during the seismic event due to the kinematic effects. These failures were related to discontinuities in the subsoil as a result of sudden changes in soil strength due to shaking. The kinematic stresses exerted in a single pipe pile constructed in two sand layers under two different situations (dry and saturated states) are investigated in this study using numerical modeling. The bending moments were higher in the saturated sand soil than in the dry one which may be attributed to liquefaction. Generally, the acceleration increased through the loose layer (from bottom to top), and then significantly settled within the dense layer. It could be shown that using this modeling, one can estimate how a pile foundation will behave under "kinematic" loading driven by earthquakes. Therefore, the design and installation of drilled aluminum or steel piles in sand soil could make use of these present observations.

Numerical modelling of plate anchors under sustained load: The enhancement of capacity from consolidation

Embedded plate anchors can be used to moor floating offshore facilities. In taut mooring systems, a sustained tension carried by the anchors affects their long-term capacity due to consolidation effects in the soil surrounding the anchor. The resulting gain in capacity provides a potential basis for more efficient anchoring system design. To quantify this effect, and to understand the underlying mechanism, a systematic numerical study has been performed, to capture the effect of sustained loading and consolidation on soil failure mechanisms around an embedded plate anchor. The modified Cam clay model is used, to capture the effect of consolidation on soil strength, and also to faithfully model the process by which a water-filled gap forms under the plate, due to a loss of contact between the plate and the soil, with seepage into the gap zone. A critical observation is the load transfer process when such a gap forms beneath the plate after the applied tension causes the effective stress to fall to zero. The formation of this gap is shown to depend, in a systematic way, on the sustained tension, or preload, the soil strength ratio, the relative sharing of the anchor load between compression above the plate and tension below, as well as the embedded depth of the anchor. Simple relationships that capture these mechanisms are provided, linking the net change in soil strength above and below the anchor to the change in undrained capacity. As an example, it is shown that in soft normally-consolidated clays, a sustained preload of 60 % of the initial capacity, can create a 25 % gain in capacity, at which point the factor of safety is raised from 1/0.6 = 1.67 to 2. This gain is only available for anchors embedded by more than twice their diameter. At shallower embedment, the loss of strength in the unloaded soil beneath the anchor outweighs the gain in strength in the smaller zone of soil above the anchor, and the consolidation process causes a net weakening.

Evaluating the Effect of Environment Acidity on Stabilized Expansive Clay

Abstract In this article, the effects of environmental acidity on the mechanical and volumetric properties of cement-stabilized clay soils have been investigated through various tests on experimental scale. In this study, a problematic clay was chemically stabilized by cement under three treatment conditions including short term, medium term, and long term with different conditions varying from acid to alkaline environments, which were tested by different methods to evaluate their mechanical and volumetric behavior and properties. Mechanical characteristics assessment tests in this study include compaction tests, and unconfined compressive strength, which was conducted on samples under different conditions in terms of acidity and treatment time. The results of the study indicated that soil improvement by cement increases the mechanical strength and decreases the rate of soil swelling over time and treatment duration. However, the degree of acidity of the environment affects the chemical reactions of soil and cement, especially cement hydration, which causes changes in soil strength and volume variation due to swelling.

The Strength Behavior and Desiccation Crack Development of Silty Clay Subjected to Wetting–Drying Cycles

It is commonly accepted that wetting–drying cycles have an effect on the soil strength behavior. Crack development in soil is observed by many engineers during wetting–drying cycles, which may give a good explanation for the change in soil strength. A series of laboratory tests were conducted in this study to investigate the desiccation crack development and the strength change law for silty clay subjected to different numbers of wetting–drying cycles. The results show that the desiccation cracks at the end of drying process developed in two stages: the stage of rapid growth and the stage of steady state. The change law of soil strength is similar to the cracking that decreases quickly in the former stage and slowly in the latter stage, which indicates that the cracking in the soil is the main reason for strength reduction. Based on the assumption of an isotropic and linear elastic soil mass at rest earth pressure conditions, an equation for the depth of desiccation cracking after different numbers of wetting–drying cycles was obtained with soil mechanics for unsaturated soils. Finally, the applicability of the equation was verified compared with the experiment results.

A cyclic p-y model for the whole-life response of piles in soft clay

It is evident from model testing, field studies and theoretical considerations that the strength of a soft clay can reduce and then recover – potentially to above the initial value – as a result of cyclic loading followed by consolidation. For piled foundations and well conductors, these changes in soil strength and the resulting lateral resistance affect their stiffness, capacity and fatigue. This paper introduces a new model for the cyclic lateral ‘p-y’ response of a pile in soft clay, using concepts from critical state soil mechanics, combined with a parallel Iwan model to capture the hysteric response. Example analyses show that the model can capture the general forms of behaviour observed in model tests, and is rapid and simple to implement. The model provides a new basis for whole life modelling of piles and well conductors, allowing changes in stiffness and capacity to be simulated, as well as improved modelling of fatigue accumulation. This approach allows more reliable design, quantifying the benefits and risks associated with evolving soil strength.

Influence of stress history on undrained cyclic shear strength evolution

Offshore infrastructure often interacts cyclically with the seabed over the operational life of a project. Previous research on the evolution of soil’s undrained strength under long-term, large-amplitude cyclic loading has focused on contractile clays and demonstrated that this cyclic interaction can lead to the initial generation and later dissipation of positive excess pore pressure in the soil. This process generally leads to an initial strength reduction, with subsequent densification and soil strength gains that can have consequences on the performance of seabed infrastructure during its design life. In this paper, new experimental data from T-bar penetrometer testing in kaolin and reconstituted Gulf of Mexico clays is presented. The data illustrate how the stress history, quantified via the overconsolidation ratio, affects soil strength changes during large-amplitude cyclic loading. The experiments explore both long-term continuous loading cycles and episodic loading with packets of undrained cycles followed by quiescent consolidation periods. A critical state-based framework is used to interpret the experimental data and provide predictions of the long-term steady-state strength of both soils as a function of the initial in situ state of the soil.

Consequences of gas pipeline hauling on changes in soil properties over 3 years

The manifold requirements of arable soils e.g. as construction sites for gas pipes or electricity cable hauling can result in extensive stress and deformation processes. It is therefore hypothesized, that external impacts as the load applications by the used construction machinery, the excavation and following refilling activities cause a loss of important soil functions and changes in soil strength. The recovery of soil functions requires efficient amelioration methods in order to re-achieve the site-specific crop yield potential.In course of the gas pipe installation in Eutric Cambisols in the Weichselian glacial till region of Mecklenburg West Pommerania (North Eastern Germany), undisturbed soil samples were taken below the plowed A-horizon in 50 and 80 cm depth in the traffic lanes and refilled pipeline trenches in the construction area as well as in the adjacent arable soils (which were used as references) directly and 3 years after the construction process.Analyses of soil porosity characteristics, air and saturated hydraulic conductivity as well as of soil strength (precompression stress value) indicate a severe loss of soil functions (especially air capacity and hydraulic conductivity) directly after the construction process (refilling of the material) compared to the reference soil. A harmful soil degradation could be verified down to 80 cm depth in the traffic lanes as a consequence of much too heavy machines used for the transportation and hauling of the pipes. After the refilling and leveling process together with the amelioration measures like the application of in total 5 Mg CaO ha−1 and the following cultivation of alfalfa seeded at the surface after the refilling and leveling, the air capacity and air conductivity was improved, but the precompression stress and the saturated hydraulic conductivity only slightly increased during the following 3 years. We conclude that such intense impacts require a longer time period to regain more rigid soil functions in spite of all the supportive amelioration approaches.

Experimental and numerical investigation on cavity formation from large rectangular spudcan penetration in clay

ABSTRACT At present, the research on the cavity formed by spudcan penetration is based on two-dimensional axisymmetric analysis of circular spudcans, and it is doubtful whether the results could be applicable to rectangular spudcans. In this paper, centrifuge experiments and finite element analysis are combined to study the penetration process of large scale rectangular spudcans in the clay with undrained shear strength varying with depth. Soil back-flow is found to occur preferentially from the middle of the edge and gradually extends to the entire side. The back-flow sequence of the soil near the long and short sides of the rectangular spudcan is different and is affected by the geometric characteristics, such as cone angle and aspect ratio. The influence of changes in soil strength and spudcan shape on the depth of the stable cavity is investigated through extensive parameter studies, and a new equation is proposed to provide the predictive value of stable cavity depth for engineering applications. The conclusion of the study can provide theoretical support for the application of rectangular spudcans.