Casagrande Method Research Articles

Finite element simulation of plate sinkage, confined and semi-confined compression tests: A comparison of the response to yield stress

Pre-compression stress (σpc) has been widely used as a criterion for assessing the susceptibility of soil to compaction. Confined compression (CCT), semi-confined compression (SCCT) and plate sinkage (PST) tests are the most practical procedures to estimate σpc. Discrepant estimates of σpc may be obtained from the above tests mainly due to differences in boundary conditions. The aim of this study was to simulate these compaction tests using the finite element method (FEM) to compare σpc estimated from stress-strain curves with the simulated yield stress. It was hypothesized that for a given simulated yield stress, σpc estimated either at the point of maximum curvature or the point of Casagrande's method is not the same as simulated yield stress for different tests. FEM models were developed in ABAQUS with elastic-perfectly plastic law and Drucker-Prager yield criterion for the soil material. Yield stresses of 20, 50, 75, 100, 125, 150, 200 and 300 kPa were examined. The results showed that the stress at the point of maximum curvature was close to the simulated yield stress in CCT but smaller than that in SCCT and PST. Casagrande's method overestimated σpc (averagely by 40%) for all the tests. A more precise estimate of σpc was obtained at the point of maximum curvature than the Casagrande’s method in experimental CCT and PST on remolded soils. Cyclic loading-unloading CCT and PST to different stress levels showed that a safe threshold for preventing a severe plastic strain is significantly smaller (with a factor of 0.5 and 0.8 for CCT and PST, respectively) than σpc in either of the tests.

Use of Fall Cone Test for the determination of undrained shear strength of cohesive soils

CT156, Geotechnics and civil engineering, has been developing efforts to create standards with the generic designation EN ISO 17892, Geotechnical investigation and testing - Laboratory testing of soil. The recent publication of EN ISO 17892-part 6, Fall Cone Test which describes in detail the use of this test method to estimate the undrained shear strength of cohesive soils leads to the necessity for the laboratories to get familiar with this test method and to acquire the required sensibility to analyse the results obtained. Originally designed to determine the liquid limit of fine soils, for which purpose it is considered as an accurate substitute of the Casagrande method, the method may constitute an alternative to the direct shear test, which takes certainly longer time to be carried out and is more complex. The present work aims to compare the values for liquid limit obtained with the Casagrande's method and the cone method and as well as the use of this test to estimate undrained shear strength, correlating with results from other laboratory tests such as the Direct Shear Test and the Laboratory Vane Test on remoulded samples. The results obtained show that there is a good correlation between the applied test methods for determination of the liquid limit and that, for some water contents, the results obtained by the three test methods are comparable.

Effect of Atrazine Contamination on Compressibility and Permeability Characteristics of Clay

Abstract Atrazine (ATZ) is one of the most heavily used herbicides in the world, particularly in the United States and Australia. This study evaluated the compressibility and permeability characteristics of two Western Australia natural clays contaminated with varying ATZ contents. A series of one-dimensional consolidation tests using fixed-ring oedometer device were conducted, and the final hydraulic conductivity was calculated at the end of each loading stage. At the first stage, the strain–time graphs at 1,280 kPa effective overburden pressure (σ′n) were evaluated, and the results indicated that the specimens with a greater ATZ content have higher values of final vertical strain (ϵv). Then, the compression index (Cc) values were computed from the void ratio–effective overburden pressure curves (e−σ′n), and the results showed that the compression index (Cc) increased as the ATZ content of the tested clays increased. At the next stage, the coefficient of consolidation (cv) was calculated using the Casagrande method and the Taylor method. The results showed that the computed coefficient of consolidation (cv) using the Taylor method was slightly greater than those calculated using the Casagrande method in both tested clays. Also, the hydraulic conductivity (k) values of the tested clays increased as the contamination content increased (i.e., 3.1 × 10-7 ≤ k ≤ 129 × 10-7 cm/s and 4.49 × 10-9 ≤ k ≤ 95.3 × 10-9 cm/s). This behavior, which was attributed to promotion of the agglomeration and aggregation between clay particles, promoted formation of the tiny voids in the soil. Finally, the secondary compression ratio (Rα) and secondary compression index (Cα) were computed, and the results showed that the specimens containing higher ATZ content had greater values of Rα and Cα.

Consistency measurement of cement-treated marine clay using fall cone test and Casagrande liquid limit test

The main objective of this study was to investigate the instant consistency of cement-treated marine clay using the fall cone test and Casagrande (percussion cup test) method. As a result, the penetration depth of the fall cone, which determines the liquid limit close to the value obtained by the percussion cup, was changed by the presence or absence of cement. It was shown that the same sand content with a much higher cement content led to smaller liquid limit values. The liquid limit value with a small amount of cement was the highest; it then decreased with the increase in cement content. The liquid limit was correlated with the additive ratio, which is defined as the ratio of the volumes of added sand and cement particles to the volumes of whole solid particles containing soil, sand, and cement. The relationship between the liquid limit and the additive ratio for all the samples can be divided into three parts: (1) no cement content, (2) cement content below 10% (transition zone), and (3) cement content above 10%. Finally, an equation for a one-point method in the fall cone test, used to determine the liquid limit of cement-treated marine clay, was proposed based on the undrained shear strength and initial water content.

LIQUID LIMIT VALUES OBTAINED BY DIFFERENT TESTING METHODS

Specifications in European countries include a variety of methods for determining the liquid limit based on Casagrande type devices and on the fall cone penetrometer. The results of a comparative study of the liquid limits determined using these two fall-cone methods are presented for lime stabilized soil. Soil material sampled in the area of Evros Regional Unit has been stabilized with lime in order to enhance its engineering characteristics. The soil and the soil-lime mixtures subjected in Atterberg limit testing. The liquid limit values were correlated through a linear regression analysis with the rest of the consistency limits of both the natural soil and its mixtures with various lime contents. The correlation coefficients in all cases were high, with those referring to results obtained by the Casagrande method to be dominant. The comparison of liquid limit values determined by either method showed that there is a systematically good correlation between them, with the decrease rate in function of the lime content in the mixture to be more intense in the case of the drop-cone procedure. There is a need for a universal specification for the determination of the consistency limits.

EXCEL ADD-IN TO MODEL THE SOIL COMPRESSION CURVE

ABSTRACT Soil compression curve (CC) provides parameters to identify soil load-bearing capacity and susceptibility to compaction. An Excel add-in (ACC) incorporating graphical procedures for mathematical models for soil CC description and calculation of parameters was developed. By using the ACC, soil CC can be described by means of the Casagrande method, mathematically operationalized with the van Genuchten equation, with or without restrictions on its parameters, and by Dias Junior and Pierce method in its original form and also modified using the void ratio rather than soil bulk density. The ACC uses a single Excel spreadsheet for input and output data, in addition to a graphical interface and a tool for exporting editable charts. Compared to SAS statistical software, the ACC minimized the sum of squared residuals and estimated parameters of mathematical models with the same efficiency for 347 compression curves. The ACC programming script is available and can be modified or used as a framework for other programming projects.

A Study on Correlation between Liquid Limit by Cone Penetrometer and Casagrande Method

A Study on Correlation between Liquid Limit by Cone Penetrometer and Casagrande Method

Determination of erosion thickness by numerical back analysis: The case study of Badenian clays in the Carpathian Foredeep, Czech Republic

The paper describes an application of the geotechnical numerical back analysis in estimating the thickness of eroded sedimentary overburden in shallow basinal sediments. The approach is based on the back-analysis of the coefficient of earth pressure at rest K0 and on estimating the unloading from the obtained K0 value. This approach is compared with the conventional methods represented by Baldwin–Butler's “compaction curves” and Casagrande's concept of “preconsolidation stress”. The results of these two commonly used methods are incorrect if the sedimentary profile is affected by “ageing” effects, such as cementation, secondary compression etc. The method is demonstrated on the Lower Miocene marine clay, often called “Tegl” which was deposited in the Carpathian Foredeep in the vicinity of Brno, Czech Republic. The numerical back analysis was applied to galleries and adits opened during site investigation of the Královo Pole Tunnels in Brno. The application of Baldwin–Butler's equation suggested the erosion thickness of 180–270m and Casagrande's method of 100–800m, while the numerical back analysis of 0–40m.

Experimental study of primary consolidation time for structured and destructured clays

Incremental load one-dimensional consolidation tests were performed on undisturbed and reconstituted specimens of seven natural clays with a predominant clay mineral of illite. Both the settlement and the base pore pressure under step load increments were measured during the dissipation of pore pressure. The change law in the primary consolidation time determined by the pore pressure dissipation with increasing stress level is found to be contrary to that determined by the Taylor method and the Casagrande method which are often used in laboratory tests and engineering practice. The primary consolidation time determined based on the time-dependent settlement observations is smaller than that determined by the pore pressure dissipation for the natural clays investigated. Pore pressure is generally not completely dissipated at the primary consolidation time determined by the Taylor method and the Casagrande method. Such remaining pore pressure may reach a quite high percentage of the step load increment for the investigated specimens. The settlement determined by the Taylor method and the Casagrande method may be significantly underestimated due to the dissipation of remaining pore pressure. Attention should be paid to the possible distortion on consolidation behavior in laboratory tests and engineering practice due to the remaining pore pressure and the associated settlement.

The Overconsolidation Ratio of the Poznan Clays from the Area of SW Poland

The main topic of the study was to characterize the over-consolidation state of stress in the Poznan Clay and estimate the average over-consolidation ratio and pre-consolidation pressure values for different depth of deposits. The Poznan Clay occurs on large area in the region of SW Poland. The shallow occurrence of analysed soils makes them very often the subsoil of engineering structures.The over-consolidated characteristic was done on the basis of the heavy oedometric laboratory tests conducted on the intact samples taken from different depth. The yield stress ratio YSR and yield stress σ’y values were estimated using the Casagrande method. Those values are usually regarded as equal the OCR and σ’y if the pre-consolidation is due to mechanical loading.The study showed that the yield stress ratio YSR is changing with depth. The comparison of the obtained YSR values with the estimated values of OCR (according geological history) showed that they do not correspond to each other. The comparison with other values estimated for soil from other European countries might suggest that the over-consolidation of Poznan Clays could also be related to the age of the deposition.

Consolidation Analysis by the Modified Slope Method

Abstract The slope method (Al-Zoubi, 2008, Geotech. Test. J., Vol. 31, No. 6, pp. 526–530) in which the coefficient of consolidation cv was expressed as function of the EOP settlement δp and slope m of the initial linear portion of the observed δt − √t curve is modified for improving the procedure used for estimating the EOP δp. The slope method EOP δp values were estimated using the point at which δt − √t curve starts to deviate from the initial linear portion. In this note, the EOP δp is determined based on a unique relationship between the ratio of the secant slope of δt − √t curve at 50 % consolidation to the secant slope at any time arbitrarily selected beyond the initial linear portion of the δt − √t curve (theoretically, at U ≥ 52.6%) and average degree of consolidation U. The modified slope method requires a minimum of four compression-time data points for better cv estimates. Two types of EOP settlement are identified; local EOP settlement δpi is obtained at a specific Ui value as is the case in the Taylor method and global EOP settlement δp is determined independently of any U value. The modified slope method yields δp and cv values quite similar to those of the Casagrande method.

A Method to Determine Pre-Consolidation Pressure of Soft Soil in Dongting Lake Area

The advantages and disadvantages of the existing mathematical models to determine soft soil pre-consolidation pressure were compared. Based on the practical shape ofe-logPcurve, it is discovered that the Harris model is more fit to thee-lgPcurve of soft soil in Dongting Lake area than the traditional cubic or quartic polynomial models. Besides, it avoids the impact of human subjectivity of Casagrande method effectively. Consequently, the Harris model was finally selected to calculate the pre-consolidation pressure of soft soil in Dongting Lake area by numerical method.

Determination of Plastic Limit of Some Selected Soils Using Rolling Device

An attempt was made to fabricate a rolling device for determining the plastic limit of different loamy and clay soils. Five different types of soil collected from different location were used for the present investigation. Clay was taken as the yardstick of converting the soil as a continuous particle and measuring scale of this continuity was expressed in terms of clay ratio. Thus plastic limit of the soils were determined by following rolling device, Casagrande method and cone penetrometer method. It was observed that plastic limit determined by different methods have similar values for different soils and clay ratio has played important role in varying the values. Plastic limit varied from almost 20 to 25% for loamy soil having clay ratio 0.30 to 0.40 and almost 25 to 30% for clay soils having clay ratio 0.75 to 0.78. The results revealed that values of plastic limit by rolling device method were on an average 5% less than that of Casagrande method and almost same to that of the cone penetrometer method at different levels of clay ratio. However, the construction of rolling device has been found easy and economical. It provides perfect terminal point for accurate result. Hence it is a reliable technique for determining soil plastic limit for cohesive soil. But it is not a workable means for non-cohesive soil.DOI: http://dx.doi.org/10.3329/pa.v21i1-2.16767 Progress. Agric. 21(1 & 2): 187 - 194, 2010

Determining Preconsolidation Pressure of the Lakeside New District Soft Clay of Hefei Based on Casagrande Method by Matlab

The preconsolidation pressure is an important index to determine the stress history of soil and also a major calculation parameter for the analysis of soil stratum’s deformation in the different stress history. Casagrade method is worldwide applied to determine the preconsolidation pressure. On the basis of research on consolidation test about the lakeside new district soft clay of Hefei by high pressure consolidation apparatus, and conformed to Casagrande method, the paper adopts quartic polynomial and least square method to fit the compression curve, and the preconsolidation pressure of the lakeside new distract soft clay of Hefei is determined by matlab software. The result provides reference for the calculation of foundation settlement considering stress history.

An In-Depth Comparison of cv Values Determined Using Common Curve-Fitting Techniques

Abstract The coefficient of consolidation (cv) is traditionally determined by fitting observed settlement–time data to the theoretical average degree of consolidation versus time factor relationship developed by Terzaghi. Although it is widely accepted that different curve-fitting methods can produce different values of cv, very few comparisons have been conducted to assess the validity of these methods. In this study, the settlement–time data gathered from conventional oedometer tests conducted on three different clays were analysed using three common curve-fitting techniques: the Casagrande log-time method, Taylor’s root-time method, and the Cour inflection point method. A new method proposed by the authors for calculating cv, which abandons the traditional curve-fitting approach in favour of a computational-based approach, was also used to compare these results. To assess the validity of each cv value, the experimental results were compared with the theoretical average degree of consolidation curve and quantified using the root-mean-square (rms) error. The efficacy of the designated curve-fitting method was found to significantly depend upon the “shape” of the settlement–time curve generated during testing. For example, in this study, clay containing a significant fraction of fine sand often resulted in settlement–time curves that exhibited no clear inflection point, which made analysis using the Cour method very difficult. In general, the Taylor method predicted larger values of cv than the Casagrande method, and correspondingly smaller rms errors. The variance method proposed by the authors resulted in values of cv that more closely matched those generated using the Casagrande method. However, smaller rms errors were achieved using the variance method, which suggests that this technique may produce a more realistic estimate of cv than the Casagrande method.

Geotechnical characterization of geomaterial blends with zeolitic tuffs for use as landfill liners

Mixtures of volcanic and zeolitic tuffs from Jordan with other natural materials such as marl, sand, kaolinite and bentonite were prepared and tested for Proctor compaction in order to determine the best blend to achieve the highest dry density for use in a landfill liner. Compaction tests indicate that volcanic tuffs have higher dry densities than the zeolitic tuffs. The presence of zeolitic tuff in a mix decreases the dry density and increases the optimum moisture content. The study indicated that two mixtures of zeolitic tuff, marl, bentonite, kaolinite and sand gave the highest dry densities which, combined with the high cation exchange capacity of zeolitic tuff and bentonite, provide mixtures suitable for potential low cost landfill liners, if the shrinkage tests confirm that they are appropriate. The liquid limits measured using the cone penetration method of the volcanic tuffs were higher than those obtained using the Casagrande method except for the Rmah Tuff (RT) which showed the reverse. This needs further investigation.

Variance of mechanical precompression stress in graphic estimations using the Casagrande method and derived mathematical models

Mechanical precompression stress is a yardstick for the strength and compressibility of soils. The default method for the estimation of precompression stress is the graphic method according to Casagrande. It involves a subjective perception by the engineer who not only determines the point of the highest curvature visually, but decides also which points are to be used for generating the virgin compression line. In order to avoid such subjective approaches, mathematical models for the determination of precompression stress have been developed emanating from the Casagrande method. These models estimate the smallest radius of the curvature based on the minimum of the second numerical derivative. The paper has the aim to quantify the variance of subjectivity implied by the person executing the graphic method, the variance of different model approaches and the accuracy of the latter in handling the graphic values. Additionally we wanted to investigate the effect of different parameters on the ordinate of the diagram and the effect of the first load step on the precompression stress. To understand these relationships, stress/bulk density functions and stress/void ratio functions measured on 13 sites were analysed by five experienced but independent engineers and by use of three mathematical models. The mean errors of precompression stress estimations by the different testers were 0.01–0.12 and by the models 0.10–0.87 on a logarithmic scale. Expressed in kPa, increasing mean errors were observed with rising precompression stress, due to delogarithmization. For the graphical determination, they reached approx. 10–20 kPa at precompression stress levels of 60–150 kPa in typical subsoils; this means 15% on average. The handling of graphically obtained values by help of mathematical models disclosed considerable deviations between them. In the logarithmic variant, the mean absolute errors varied from 0.09 (9 kPa) to 0.40 (30 kPa) and the determination coefficients from 0.71 to 0.96. Another influence on the level of precompression stress has been observed when different variables were plotted on the ordinate of the graph. The graphically obtained values of precompression stress and those shown in the dry bulk density graph exceed the values calculated on the basis of the void ratio by the factor 1.2–1.5. Furthermore, it can be stated that in soil-compression tests with an initial load of 25 kPa higher precompression stress values were obtained than with lower initial loads (5 kPa), if the precompression values were low.

Coefficient of Consolidation by the Slope Method

Abstract The slope method explicitly relates the coefficient of consolidation cv to the slope of the initial linear segment of the observed δt-t curve and to the end of primary settlement δp. The slope method utilizes the same graphical construction as the Taylor method but eliminates the use of the factor 1.15 that usually leads to lower δp and higher cv as compared to the Casagrande method and pore water pressure measurements. Instead, the point at which the δt-t curve deviates from the linear portion (at U of about 50 %) is considered for estimating δp. Oedometer test results on four soils show that the cv values of the slope and Casagrande methods are comparable but lower than those of the Taylor method.

Determination of precompression stress from uniaxial compression tests

Soil compaction has been studied for many years due to its implications for crop yield. In addition, precompression stress is important tool in evaluating soil load capacity and its implications for soil structure. This study evaluated whether precompression stress fitted by different methods gave rise to different results when a wide range of soils was tested. Soil cores were collected from different depths at 13 sites in Sweden, Denmark and Brazil and subjected to uniaxial compression tests. Soil precompression stress was determined using five different fitting methods: (1) Casagrande by polynomial fit; (2) Casagrande by van Genuchten equation fit; (3) intercept of the virgin compression line (VCL, straight portion of stress–strain curve) and regression with the first three points of the curve; (4) intercept of VCL and regression with the first two points of the curve; (5) intersection of VCL with the x-axis at zero strain. Method 1 produced the highest precompression stress values, while method 5 differed from other methods for all sites. Methods 1 and 3 did not significantly differ for most sites. Mean precompression stress was 50–130 kPa for the different methods. Polynomial and sigmoidal equations showed a good fit for the compaction curve in our dataset ( R 2 = 0.99), but Casagrande method parameters, e.g. point of maximum curvature, its tangent and slope of VCL, were different, resulting in different precompression stress values. Choice of method proved to have a significant influence on precompression stress values. Although methods 1 and 2 are both Casagrande-based, the results frequently differed. For the stress range tested (up to 1000 kPa), the van Genuchten equation was less suitable than polynomial fitting, since the compaction curve was usually not sigmoidal and the inflection point was sometimes outside observed values. Thus, the method used to fit precompression stress must be carefully chosen to avoid overestimates and underestimates. This restricts use of the absolute precompression stress value as a limit for soil stress in order to avoid soil compaction.

Relationship Between the Atterberg Limits and Clay Content

This study investigates the liquid limit (Casagrande's method) and plastic limit (rolling and thread method) of six inorganic soils and their respective mixtures with fine silica sand. It was observed that the liquid limit and plastic limit values of the mixtures tested, except those with a low clay percentage, are linked to the respective clay size contents by a linear relationship. The Atterberg limits were subsequently recalculated using the equations of the regression lines of the mixtures governed by linear law with the clay percentages. The plotting of the plastic limit as a function of the liquid limit of these data made it possible to determine the relationship among the liquid limit, the plastic limit and clay fraction valid for inorganic soils that contain platey clay minerals and for clay size contents that are not too low. Hence, on the basis of the interdependence among the parameters considered (WL, Wp, Ip, CF, A), for a given inorganic soil, knowing only two of three parameters (WL, Wp, CF) that are measurable using standard tests, the values of other three parameters can be obtained.