Evaluation of penetrometers for measuring soil strength

Cone index, as determined by a cone penetrometer, is frequently used as a measure of soil strength. The index is a compound parameter involving components of shear, compressive and tensile strength and soil metal friction. In order to assess the effect of soil type and condition on the relative contributions of these components to penetration resistance, the forces required to push blunt and sharp probes into two soils under a range of moisture contents and bulk densities were investigated. The maximum penetration force in homogeneous soil was not uniquely related to dry bulk density or cohesion, but varied with soil moisture content.At high and low moisture contents, the soil tended to interact with the shaft of the penetrometer thus increasing the resistance to penetration. At low moisture content, bodies of compressed soil formed in front of the probe, effectively changing the probe geometry.It was concluded that interpretation of cone index in typical layered field soils is difficult. Even in homogeneous soils, the proportion of shear, compressive and tensile components that the cone index reflects varies with soil condition.     

Cone penetration resistance equation as a function of the clay ratio, soil moisture content and specific weight

The cone penetrometer is a simple versatile device which is widely used to monitor the strength of a soil in terms of its resistance to the penetration of a standard cone. The soil penetration resistance is a function of soil moisture content, soil specific weight and soil type. The soil type is characterised by means of a clay ratio which is the ratio of the clay content of the soil to the content of silt and sand.Based on the classical bearing capacity theories for strip foundations, a general cone penetration resistance equation is developed to represent the variability of cohesion and friction angle by means of soil type and moisture content. The empirical relationship is shown to give an accurate prediction of the cone penetration resistance for a wide range of soils from a loamy sand to a heavy clay (clay ratios 0.10–1.60) and over a wide spectrum of soil moisture contents from 10 to 65% w/w.     

The determination of plough draught—Part I. prediction from soil and meteorological data with cone index as the soil strength parameter

Using cone index as an indication of soil strength, empirical equations are developed in accordance with soil mechanics theory to relate soil moisture content to plough draught. The plough draught equation comprises a quasi-static component dependent on cone index and a dynamic component which is a function of the soil specific weight, plough speed and mouldbard tail angle. It is further argued that the cohesive and frictional components of the cone penetration resistance can be predicted by means of a simple equation comprising a reciprocal function of the square of the soil moisture content and a linear function of the soil specific weight. The cone index equation explained 98% of the experimental data for threthree soils over a wide range of moisture contents. These empirical equations, together with a soil moisture model, provide a method of predicting plough draught directly from soil and meteorological data.     

Effect of wetting and drying on soil physical properties

Agricultural soils are subject to seasonal wetting and drying cycles. Effect of drying stress, as influenced by one cycle of wetting and drying, on physical properties of a clay–loam soil was investigated in the laboratory. The physical properties studied were soil bulk density, cone penetration resistance, shear strength, adhesion and aggregate size and stability. Three drying stress treatments were made by wetting air-dried soil of initial moisture content of 12% (on dry weight basis) to three different higher moisture contents, namely 27, 33 and 40%, and then drying each of them back to their original moisture content of 12%. Thus, the soil was subjected to three different degrees of drying stress. The results showed that the soil strength indicated by cone penetration resistance and cohesion, and soil aggregate size, increased with the degree of drying stress. However, the soil bulk density did not change significantly with the drying stress.     

Non-linear finite element analysis of cone penetration in layered sandy loam soil – Considering precompression stress state

Axisymmetric finite element (FE) method was developed to simulate cone penetration process in layered granular soil. The FE was modeled using ABAQUS/Explicit, a commercially available package. Soil was considered as a non-linear elastic plastic material which was modeled using variable elastic parameters of Young’s Modulus and Poisson’s ratio and Drucker–Prager criterion with yield stress dependent material hardening property. The material hardening parameters of the model were estimated from the USDA-ARS National Soil Dynamics Laboratory – Auburn University (NSDL-AU) soil compaction model. The stress–strain relationship in the NSDLAU compaction model was modified to account for the different soil moisture conditions and the influence of precompression stress states of the soil layers. A surface contact pair (‘slave-master’) algorithm in ABAQUS/Explicit was used to simulate the insertion of a rigid cone (RAX2 ABAQUS element) into deformable and layered soil medium (CAX4R ABAQUS element). The FE formulation was verified using cone penetration data collected on a soil chamber of Norfolk sandy loam soil which was prepared in two compaction treatments that varied in bulk density in the hardpan layer of (1) 1.64 Mg m⁻³ and (2) 1.71 Mg m⁻³. The FE model successfully simulated the trend of cone penetration in layered soils indicating the location of the sub-soil compacted (hardpan) layer and peak cone penetration resistance. Modification of the NSDL-AU model to account for the actual soil moisture content and inclusion of the influence of precompression stress into the strain behavior of the NSDL-AU model improved the performance of FE in predicting the peak cone penetration resistance. Modification of the NSDL-AU model resulted in an improvement of about 42% in the finite element-predicted soil cone penetration forces compared with the FE results that used the NSDL-AU ‘virgin’ model.     

Traffic-soil-plant (maize) relations

Twenty-five treatments consisting of three vehicle contact pressures, 62, 41 and 31 kPa (0.63, 0.42, 0.32 kg/cm²), four numbers of tractor passes (1, 5, 10, 15,) before and after seeding groups, and a control of zero traffic were used to study the effect of soil compaction on corn plant root growth and distribution in a Ste. Rodalie clay soil. The average dry bulk density values for 0–20 cm depths measured during the season varied from a minimum of 0.89 g/cm³ to a maximum of 1.12 g/cm³ depending on the severity of the treatment. Root distribution maps were obtained for all the treatments by field measurements coupled with root washing methods. An average root density of 5.7 mg/g of soil in an uncompacted control plot was reduced to less than 2 mg/g in a plot with 15 passes of 0.63 kg/cm² contact pressure. Soil penetration resistance values in various plots were compared, and a statistical model was obtained in terms of the traffic treatments, soil moisture content and depth. Yield reductions and penetration resistance were compared to root distrubution density results.     

Soil response to track and wheel tractor traffic

Experiments were conducted on a Eudora silt loam to determine the effect of tracked and wheeled tractor traffic on cone penetration resistance and soil bulk density at three different soil-water content levels. Treatment plots were ripped to a depth of 0.45 m and irrigated 5 days prior to the experiment. Significant differences in penetration resistance and bulk density were observed between the treatments within the plowing depth (0.30 m). After the tractor passes, the average penetration resistance recorded was about 7.5% higher and the soil bulk density was about 3% higher in the wheel treatment plots. However, at the soil-water content level close to Proctor optimum (15% w/w), no significant difference was observed in the average penetration resistance of the two treatments.     

Remote electronic acquisition of soil cone index measurement

The cone penetrometer is widely used in tillage and off-road mobility research as an indicator of soil strength and density characteristics. Light-weight, manually operated units are especially useful in recording cone index determinations at remote field locations. Such units permit a single operator to measure and record penetration force vs depth in graphic form. However, the interpretation and analysis of such data has remained a tedious manual operation which has limited the number of determinations which are practical for a given field experiment. The system described in this paper allows one person to determine and electronically record penetration force vs depth relationships using a standard cone penetrometer (ASAE S313.1, 1979). A CMOS (complementary metal oxide semiconductor) microprocessor is utilized to sample and digitize analog signals and to record them on a magnetic tape cassette. An identifying code can be associated with each measurement and the microprocessor is subsequently utilized in interpreting cassette-stored data and transmitting it to a remote computer terminal or minicomputer for processing and analysis. Thus, this relatively low-cost system significantly enhances manual acquisition and interpretation of cone penetrometer measurements.     

Experimental Study of the Impact and Penetration of A Cone in Frozen Sand

A reverse experiment technique is used along with the technology of measuring rods to study the impact and penetration of a steel conical body in frozen sandy soil. This paper presents the dependences of maximum values of the force of resistance of cones with base diameters of 10.0, 12.0, and 19.8 mm to penetration into sand on the impact velocity in the range of values 100–400 m/s. The numerical solution of the problem in an axisymmetric formulation with the use of the “Dinamika-2” software package is used to show the effect of waves reflected from the walls of the container on the contact force. A comparative analysis of the forces of resistance to penetration of the shocker into compacted dry, water-saturated, and frozen sandy soils is carried out.     

Correlation dimension of paddy soil strength in China

Embedding phase space R^m is reconstructed from the spatial series g(x) of cone indices measured in two paddy fields near Nanjing, China. The correlation dimension D₂^m for each field is derived from the correlation integral C^m(r) and the neighbours distance r in log–log scale. Results show D₂^m increases as m, and tends to 5.0, which expresses the estimate of correlation dimension for each soil strength profile measured.     

Cone resistance in sand by incremental method

A new method has been developed for the determination of cone resistance under drained conditions. Numerical methods are used for the solution of the differential equations of plasticity theory for soils and for the determination of the stress states in the soil produced by the penetration of the cone. It is assumed that the stresses produced by the penetration of the cone remain ‘locked in’ the soil and constitute boundary conditions for further penetration. The computation starts with the cone base at the surface and is continued by successively incrementing the depth by a small amount. Charts are given for the computation of cone resistance in sands for various friction angles. The importance of the effect of the shear stresses generated at the surface of the cone and characterized by the interface friction angle, δ, is discussed in detail.     

Determination of the soil pressure distribution around a cone penetrometer

The objective of this paper was to investigate the pressure distribution around a cone penetrometer using a pressure sensing mat under laboratory conditions. The investigation was conducted under (1) constrained conditions using cylindrical split pipe molds and (2) unconstrained conditions using a soil box. These tests were conducted in Capay clay and Yolo loam soil containing two different moisture conditions and two compaction levels.In the constrained tests, a maximum radial pressure of 111 kPa was observed in the Capay clay soil with 3.4–4.3% d.b. moisture content and three blows of compaction (cone index value of 2040 kPa) when using the 41 mm diameter split pipe mold. These pressure levels decreased to 82 and 22 kPa, respectively, when 65 and 88 mm diameter molds were used. In both the Capay clay and Yolo loam tests, the average radial pressure and average cone index values showed similar trends.In the unconstrained tests, a maximum pressure of 9.0 kPa was observed in the Capay clay with 4.5% d.b. moisture content and three blows of compaction (cone index value of 550 kPa) at a horizontal distance of 25.4 mm from the vertical axis of the cone penetrometer and minimum pressure levels in the range of 0.2–0.3 kPa when the horizontal distance of the penetrometer was in the range of 56.8–66 mm. The pressure levels are much smaller than the ones obtained in the constrained tests and may suggest that the pressure distribution under field conditions is small at a distance of 25.4 mm or higher from the tip of the cone.The experimental data were statistically analyzed to identify significant factors. The results of the analysis for the constrained test indicated that the mold diameter and number of blows significantly increased the pressure readings within the soil mass. Increasing the mold diameter led to a decrease in the average radial pressure and increasing the number of blows contributed to an increase in the average radial pressure. In the unconstrained test, the average radial pressure distribution at a given point were significantly influenced by the horizontal distance of the point from the vertical axis passing through the center of the penetrometer shaft, soil type, and soil moisture content. Higher pressure values were obtained in the Capay clay tests compared to the Yolo loam tests. In all cases, the pressure levels were greater for the drier soil than for the moist soil.     

Effect of multiple passes of tractor with varying normal load on subsoil compaction

A field experiment was conducted on alluvial soil with sandy loam texture, in a complete randomized design, to determine the compaction of sub-soil layers due to different passes of a test tractor with varying normal loads. The selected normal loads were 4.40, 6.40 and 8.40 kN and the number of passes 1, 6, 11 and 16. The bulk density and cone penetration resistance were measured to determine the compaction at 10 equal intervals of 5 cm down the surface. The observations were used to validate a simulation model on sub-soil compaction due to multiple passes of tractor in controlled conditions. The bulk density and penetration resistance in 0–15 cm depth zone continuously increased up to 16 passes of the test tractor, and more at higher normal loads. The compaction was less in different sub-soil layers at lower levels of loads. The impact of higher loads and larger number of passes on compaction was more effective in the soil depth less than 30 cm; for example the normal load of 8.40 kN caused the maximum bulk density of 1.53 Mg/m³ after 16 passes. In 30–45 cm depth layer also, the penetration resistance increased with the increase in loads and number of passes but to a lesser extent which further decreased in the subsoil layers below 45 cm. Overall, the study variables viz. normal load on tractor and number of passes influenced the bulk density and soil penetration resistance in soil depth in the range of 0–45 cm at 1% level of significance. However, beyond 45 cm soil depth, the influence was not significant. The R² calculated from observed and predicted values with respect to regression equations for bulk density and penetration resistance were 0.7038 and 0.76, respectively.     

Simulating shear behavior of a sandy soil under different soil conditions

Understanding of soil shear behavior is very important in the field of agricultural machinery and soil dynamics. In this study, a discrete element model was developed using a simulation tool, Particle Flow Code in Three Dimensions (PFC^3D). The model simulates direct shear tests of soil and predicts soil shear behavior, in terms of shear forces and displacements. To determine and calibrate model parameters (stiffness of particles, strength and stiffness of bond between particles), laboratory direct shear tests were conducted to examine effects of soil moisture content and bulk density on shear behaviors of a sandy soil. Three soil moisture levels (0.02%, 13.0%, and 21.5%) and four bulk density levels (0.99, 1.28, 1.36, and 1.50 Mg/m³) were used in the tests. The test results showed that in general drier and denser soil conditions produced higher shear forces. Based on the test results, the bond strengths of the model particles were determined from soil cohesion and internal friction angle. The model particle stiffness was calibrated based on the yield forces from the tests. The calibrated particle stiffness varied from 1.0 × 10³ to 8.2 × 10³ N/m, depending on soil moisture and density levels. The bond stiffness calibrated was 1.0 × 10⁷ Pa/m for all soil conditions.     

Compaction characteristics for the towed and driven conditions of a wheel operating in an agricultural soil

High axle loads, duration of strain as well as strain rate due to applied stresses, and field moisture condition have been found to contribute to compaction in the field. Numerous previous investigations on agricultural soil compaction were carried out with relatively dry soil. The aim of this study was to investigate the interrelationships between compaction, applied load, vehicle speed and a certain practical range of soil moisture content through a soil bin investigation of the compaction which results from the passage of a towed and a driven wheel. Soil pressure and the corresponding bulk density were analysed using a model proposed by Bailey et al. (J. agric Engng Res. 33, 257–262 (1986)) and ANOVA techniques. The results showed that compaction was higher at the higher moisture content level for both towed and driven conditions of the wheel, and that it was applied load that had the greatest contributory effect. Also, compaction was higher in the case of the driven wheel as compared to the towed wheel due to the phenomenon of slip sinkage. Bailey's model, it appears, can be utilized in the field for a practical estimation of compaction resulting from the passage of a towed wheel.     

The effect of dynamic impact and water potential on some surface soil properties

A direct shear test with a superimposed impact was used to simulate the action of a track on the soil surface and to study the effect on soil surface properties. Results showed that impact increased bulk density, reduced saturated hydraulic conductivity and decreased cone penetrometer resistance. An impact plus shear treatment reduced the residual shear strength to approximately 60 kPa compared with 85 kPa for a shear only treatment. Water tension also greatly influenced the changes measured with the order of greatest change being −5>−10>−60>t-100>−300 kPa. The results are discussed with respect to soil trafficability and soil structural change with vehicle passage.     

The inception and evolution of earthmoving soil mechanics

Soil and test conditions important to earthmoving machinery have been found to be significantly different from all other fields of endeavour with the partial exception of tillage studies. This could be the subject of a long dissertation. Broadly, however, soil conditions which produce critical mobility problems are much too soft and/or wet to be of concern to the earthmoving contractor who has to meet rigid specifications on acceptable types and moisture contents of fill soils. Occasional soft spots are considered as nuisances instead of indicators of the need for major design compromises.

Civil engineers are concerned with the same types of soil, but in a vastly different context. They must design soil structures which will never reach initial soil failure. Earthmoving processes, on the other hand, must accept soil failures in many different forms and degrees and utilize post-failure soil strength to perform their tasks efficiently.

Tillage studies display many important similarities to earthmoving studies, particularly in regard to the types of soil failures of importance. They are, in reality, merely another form of earthmoving; by definition, if nothing more. Earthmoving processes can range into much stronger soils, but this alone is insufficient to set them apart.

The term Earthmoving Soil Mechanics was introduced in 1962⁽²⁰⁾. This paper more clearly defines the implications of the new terminology and illustrates the first successful application of soil mechanics and model analysis principles in the earthmoving industry.     

An investigation of the bevameter soil physical measurements in the prediction of soil tool draft

The relationship between the parameters measured during soil testing using the bevameter system and the horizontal forces acting on a simple soil-cutting blade were investigated. Field experiments were conducted on untilled, compacted soil and on recently-tilled soil. On both soils, five sites were randomly chosen and bevameter and draft measurements were performed The parameters measured were modulus of soil deformation, wet and dry bulk density, soil moisture content, tool operating depth, tool operating velocity and horizontal draft. A statistical analysis of the data indicated that a mathematical model for predicting draft should contain the following variables: operating depth, dry bulk density and modulus of deformation. A linear regression analysis of draft versus modulus of deformation showed significance at the 95% confidence level on the untilled sites at all measured depths. A similar analysis of the tilled site indicated significance at the 70 mm depth only. The usefulness of the bevameter deformation modulus as an indicator of draft was found to be limited to shallow depths.     

静力触探锥头阻力的近似理论与实验研究进展

锥头阻力在静力触探试验中扮演着十分重要的角色.从不同角度,对触探中锥头阻力的研究进行简要阐述,对承载力理论、空洞膨胀理论、应变路径法及运动点位错法等几种理论分析方法进行了回顾.另外,对数值分析和实验研究的进展情况进行了叙述.并对各种方法的适用性进行了比较.承载力理论虽然简单,但忽略了土的压缩性和探杆周围初始应力的增加,所以不能精确地模拟锥头的深层贯入.空洞膨胀理论提供了一个分析锥头阻力的简单而较精确的方法,它考虑了土的压缩性(或膨胀性)和锥头贯入过程中锥杆周围应力增加的影响.但这种方法是将锥头贯入与空洞膨胀之间做了一个等效模拟,所以不同的模拟方法,得到的结果差别较大.应变路径法能够有效解决饱和粘土中的不排水贯入,但不适用于砂土.运动点位错法因为考虑了部分排水,所以能较好地预测固结系数,但采用了线弹性分析,故位错法在其他方面的应用还需要大量的试验验证.有限元法在处理锥头贯入这类慢侵彻问题时缺乏一种很好的处理技术,导致它在进行破坏荷载计算时有显著的误差和数值计算困难.标定槽试验将在验证和建立锥头阻力与土的性能关系方面继续起到一个重要作用,但其结果需经过校正后才可应用到现场.最后对该领域的研究趋势进行了讨论.      

Changes in some mechanical properties of a loamy soil under the influence of mechanized forest exploitation in a beech forest of central Belgium

Modification of some soil mechanical properties (penetration resistance and consolidation pressure) induced by vehicle compaction during mechanized forest exploitation was studied in an acid and loamy leached forest soil of the loessic belt of central Belgium. In situ penetration tests and laboratory Bishop–Wesley cell tests were undertaken for the two main soil horizons of a beech high-forest, i.e. the eluvial E horizon (5–30 cm depth) and the underlying clay-enriched B_t horizon (30–60 cm depth). Both undisturbed and wheel-rutted soil areas were studied (E and B_t horizons vs. E_g and B_tg horizons).

Results show that: The experimental overconsolidation pressure of the eluvial reference horizon (E) is about 50 kPa higher than the value calculated from soil overburden pressure; this probably results from suction action during dry periods. The clay-enriched reference horizon (B_t) shows the same trends. In wheel-rutted areas, seven years after logging operations, the E_g horizon memorizes only 14.5% of the wheel induced stress due to forest machinery.

In the compacted B_tg horizon, the experimental overconsolidation pressure represents 96% of the exerted theoretical stresses due to harvesting actions. The good recording of the exerted stresses, after seven years, can be explained by: (1) The B_tg depth which keeps it from seasonal variations i.e. from desiccation–moistening or freeze–thaw cycling; (2) amorphous and free iron accumulation inducing a “glue” effect of the B_tg soil matrix, which could stabilize the soil structure and prevent recovery to initial conditions. These results provide clear evidence that on loessic materials, soil compaction due to logging operations leads to modifications in both physical (bulk density, total porosity) and mechanical (penetration resistance and consolidation pressure) soil properties.