Miniature Penetrometer Research Articles

Characterization of Lime and Polymer Treated Ultra-Soft Clay Soils Using the Modified Vane Shear and Correlating the Shear Strengths to the Electrical Resistivity and CIGMAT Miniature Penetrometer for Nondestructive Field Tests

With the growth in construction along the coastal regions, ports and offshore deepwater sea beds, ultra soft clay soils are encountered and are affecting the performance of the pipelines and other anchor systems placed in it. In this study, untreated and treated ultra-soft clay soils shear strengths were quantified using the modified vane shear device and correlated the shear strength to the CIGMAT miniature penetrometer penetration and electrical resisitivity. The ultra-soft clay soils were prepared by varying the bentonite content from 2 to 10%. The soil with 10% bentonite was treated with 2–10% lime and with 1–10% polymer separately to enhance the shear strength. The shear strength, pH, CIGMAT miniature penetrometer and electrical resistivity were used to characterize the untreated and treated ultra-soft clay soils. Untreated soft soil strength varied from 0.01 to 0.17 kPa when the bentonite content in the ultra-soft clay soil was increased from 2 to 10%. With 2% lime treatment the shear strength of 10% bentonite soil was increased to 0.27 kPa. Adding 10% lime to the 10% bentonite ultra-soft soil reduced the shear strength to 0.15 kPa. With 10% polymer treatment, the shear strength was increased by 45 times to 6.8 kPa for the 10% bentonite ultra-soft soil. The CIGMAT miniature penetrometer penetration varied linearly with the shear strength of the untreated ultra-soft clay soils and treated ultra-soft clay soils with 10% bentonite. pH was almost constant for untreated ultra-soft clay soils while it increased by 22% and 35% for the 10% lime and 10% polymer treated ultra-soft clay soilss respectively. Relative electrical resistivity decreased by 246% when the bentonite content was increased from 2 to 10% in the ultra-soft clay soils. Addition of 10% of lime to the ultra-soft clay soil with 10% of bentonite content decreased the relative electrical resistivity by 171%. Addition of 10% of polymer to the ultra-soft clay soil with 10% of bentonite content reduced the relative electrical resistivity by 545%. Power law, linear and Vipulanandan correlation models were used to predict the shear strength-resistivity relationship for the untreated, lime-treated and polymer-treated ultra-soft clay soils respectively. In addition, linear model was used to predict the water content relationship with electrical resistivity for the untreated and treated ultra-soft clay soils. The CIGMAT miniature penetrometer was modeled using 3-D axisymmetric finite element method, which predicted the penetration of CIGMAT penetrometer that agreed well with the experimental results of the untreated and treated ultra-soft clay soils. The CIGMAT miniature penetrometer penetration and the electrical resisitivity monitoring are two non-destructive methods that can be easily adopted in the field and the property correlations can be used to determine the shear strength of the ultra-soft caly soils in the field.

Empirical Factors for Miniature Cone and T-bar Penetrometers for Kaolin Clay

Undrained shear strength is a key parameter of clay and is popularly determined by penetrometers. Miniature penetrometers are preferred to estimate the undrained shear strength of clay for small-size model tests. In order to estimate undrained shear strength, an empirical factor should be established to correlate the tip resistance to the undrained shear strength. However, published data on miniature penetrometers are limited. This paper aims to evaluate empirical factors on miniature cone and T-bar penetrometers using a cylindrical chamber. Kaolin clay specimens were prepared in the chamber under five pre-consolidation pressures to control their undrained shear strength. Two miniature cones with diameters of 10 mm and 16 mm and a T-bar penetrometer with 10-mm diameter and 40-mm length were used to investigate penetration rate, boundary, and empirical factor. Unconsolidated-undrained triaxial tests and vane shear tests were carried out to measure undrained shear strength. Finally, empirical factors for the penetrometers were constructed to correlate tip resistance to undrained shear strength. The empirical factors established at 20 mm/s of a penetration rate for 10-mm-diameter cone, 16-mm-diameter cone and T-bar penetrometer are 18.33, 13.09 and 12.5, respectively.

Characterization and finite element analysis of lime and polymer treated ultra-soft clay soils using the electrical resistivity and miniature penetrometer methods

Characterization and finite element analysis of lime and polymer treated ultra-soft clay soils using the electrical resistivity and miniature penetrometer methods

CPT-induced change of lateral earth pressure in modeling test

The main objective of this study is to investigate systematically the lateral earth pressure induced by the cone penetration test (CPT) conducted at various distances from the rigid container wall. A series of centrifuge CPTs were performed in dry Ottawa sand using two miniature penetrometers with diameters of 4 and 12 mm. The CPT locations varied from 2 to 42 times the cone diameter. The change of lateral earth pressure was measured using tactile pressure sensors. Results from this study have shown that the lateral earth pressure change was mainly associated with the initial relative density of soil, the distance away from the testing location and the diameter of the cone penetrometer. The change of lateral pressure can be as large as 150–200 kPa at a distance of 2–4 times the diameter of the CPT. The effect of initial soil relative density on the change of lateral earth pressure was greater when using the 4 mm CPT than the 12 mm CPT. Some residual lateral pressure was observed after completion of the CPT in dense sand tests.

Use of epoxy in developing miniature ball penetrometers

Ball penetrometers used in geotechnical centrifuge facilities for characterising the soil strength profile are typically around 12 mm in diameter. The minimum size is limited by the traditional design approach, with ball and shaft machined from steel or aluminium alloys, and axial strain gauges placed on the shaft to measure the penetration resistance. This paper describes a new fabrication method that uses epoxy to cast and cure the ball penetrometer and shaft as one piece. This allows the strain gauges to be set inside the penetrometer shaft, reducing the size of the ball penetrometers. The manufacture of three miniature ball penetrometers of 4 mm, 5 mm and 6 mm diameter is described. Example results using the penetrometers in a drum centrifuge highlight the versatility and consistency of the new devices. A comparison with the more established T-bar in characterising the undrained shear strength profile of soft clays is also provided.

Numerical study of miniature penetrometer in granular material by discrete element method

The resistance of driving a slender penetrometer into soil is often used for material characterization in geotechnical engineering. The penetrometer diameter is usually much larger than the mean grain size of soil and the resisting pressure is independent of the penetrometer size. For miniature penetrometers, however, the resisting pressure is known to show size dependence. In this paper, a numerical study of driving miniature penetrometer into polydisperse granulates is presented. The three dimensional numerical analyses are carried out based on the discrete element method. The relationships between resisting pressure and penetrometer diameter are obtained. The numerical results are compared with tests in the literature. Moreover, the effect of penetrometer geometry is also considered.

Strength Measurement for Near-Seabed Surface Soft Soil Using Manually Operated Miniature Full-Flow Penetrometer

A manually operated penetrometer (DMS) fitted with cylindrical (T-bar) and ball penetrometer tips was developed for measuring the profiles of undisturbed and remolded undrained shear strength within box-core samples. This paper summarizes the findings of a series of miniature penetrometer tests and vane shear tests that were carried out on reconstituted clay from a local site in Western Australia. The aim of the tests was to evaluate the potential of the DMS in characterizing the shear strength of seabed surficial sediments. It was found that the DMS gave essentially identical T-bar and ball penetration resistances but these were up to 17% lower than the net cone resistance. From the comparison between the T-bar and ball penetration resistance and the shear strengths measured from vane shear tests, average N factors of 11 and 14 were obtained for intact and fully remolded conditions, respectively. The test results suggest that the DMS is a reliable and efficient means of obtaining intact and remolded shear strength profiles.

Pore-pressure generation in cyclic T-bar tests on clayey soil

A model miniature T-bar penetrometer, designed for in-flight applications at the COPPE geotechnical centrifuge in Rio de Janeiro, Brazil, comprises a cylindrical crossbar of 15·2 mm diameter and 112·5 mm long, attached at right angles to the end of a vertical shaft. During penetration, the resistance is measured by a sensitive load cell situated at the vertical shaft immediately above the crossbar. Additionally, a set of two Kulite miniature pore-pressure transducers have been inserted inside the cylindrical crossbar, one at each side of the vertical shaft, facing the bottom part of it. This paper presents a set of cyclic T-bar penetration tests carried out on an artificial clayey soil made of 20% bentonite and 80% kaolin. The generated excess pore pressures are measured at both sides of the bar during the penetration cycles, and compared with values found in the literature. The maximum excess pore pressure, as well as the peak soil resistance, show a significant decrease during soil cyclic penetration. The variations in the excess pore pressure due to the cyclic penetration of the T-bar were also monitored using additional pore-pressure transducers, which were fixed at the bottom of the soil layer. The results were compared with the measurements obtained from the pore-pressure transducers located on the cylindrical crossbar.

A Simple Technique for Measuring Wetting Front Depths for Selected Soils

The depth of the wetting front within a soil sample in infiltration measurements, especially in soils that develop cracks on drying, is difficult to ascertain simply and nondestructively. A technique was developed to determine wetting front locations on prepared soil beds, with a miniature penetrometer probe of the needle type, immediately following a simulated rainfall event. The method involves placing a 0.5-kg weight atop a miniature penetrometer probe and measuring the penetration depth of the probe relative to a known datum. Five texturally different soils were tested under similar laboratory conditions to evaluate this method. The penetrometer-based method provided accurate estimates of the wetting front position in laboratory simulated rainfall infiltration studies for clay, silty clay, and sandy clay soils that differed from visually observed depths by <1 mm. For the silt loam soils, however, this method underestimated mean wetting front depths by as much as 4 mm, with a standard deviation of 1.6 mm and 95% confidence limits of ±2.5 mm. The penetrometer method was especially useful for detailed characterization of wetting front depths in soils where wetting was highly variable or irregular (e.g., cracking clay soils).

SW—Soil and Water: State Parameter Interpretation of Cone Penetration Tests in Agricultural Soils

Cone penetrometer resistance in saturated sands has been shown by earlier workers to be an exponential function of a state parameter in critical state space. The present investigation demonstrates that this concept of a state parameter-penetrometer function (SPP-function) also holds good in a partly saturated agricultural soil. The two coefficients which characterize the SPP-function have been evaluated from extensive tests carried out in a miniature penetrometer calibration chamber. These coefficients were found to vary in a systematic manner with the moisture content of the sandy loam soil used in the experiments, as do the three basic parameters necessary to characterize the critical state boundaries of this soil. The paper presents a method of using this experimentally obtained information to interpret the pore space of the soil from measured penetrometer resistance in that soil. The performance of the proposed method was checked against penetrometer readings made under carefully controlled laboratory conditions in an indoor soil tank. The prediction accuracy was poor, but it is felt that this could be improved by using an iterative solution in place of the single step method used in the validation. The tedious and time consuming experimental work described in the paper is confined to a single sandy loam soil, typical of a light agricultural soil. In order to generalize the solution, it is necessary to test the procedure over a wide range of soils, including clay. If the SPP-functions can be established for these conditions then the state parameter concept could prove to be a powerful tool in the interpretation of cone penetrometer readings.

A combined penetrometer and surface relief meter for studying drill coulter performance

SUMMARYA miniature penetrometer which measures soil strength in a vertical section across a drill line is described. It has a cone diameter of 3.6 mm and a range and resolution of 0–5000 and 5.4 kPa respectively. Data are recorded directly on to a portable digital tape logger which interfaces directly with a microcomputer for data analysis. The use of a horizontal reference height, rather than the soil, enables measurement of the soil surface relief. Results are presented showing the mean soil strength in a vertical section (200 mm wide by 130 mm deep) in drill lines in autumn and spring cultivated soil, with a horizontal and vertical resolution of 10 and 3 mm respectively.