BCD: A Soil Modulus Device for Compaction Control

There is a trend toward using the modulus as an alternative to dry density in compaction control because of the undesirable nuclear source in the current field density gage and because a modulus is often used in the design of roadway bases and compacted fills. The Briaud compaction device (BCD) is a new instrument used to obtain a soil modulus in only a few seconds; it consists of leaning on a rod equipped with a thin circular metal plate at the bottom end and recording the bending of that plate under a standard load. This article describes the theory and the experiments that have been performed to validate the new instrument. This validation is based on a comparison to a simple plate test and on a numerical simulation of the BCD test. A recommended procedure is outlined in the conclusions. The BCD is used in the lab on top of the soil in the Proctor mold to obtain the lab modulus versus water content curve and select a target value. Then the BCD is used in the field to verify that the target modulus has been achieved.     

Ohmic Conductivity of a Compacted Silty Clay

In its natural state, loess can be considered as an unstable soil, which develops large deformations when moistened. In Argentina, loess is used in most Geotechnical constructions, including embankments and liners. The interest of this work to evaluate the potential application of electrical conductivity measurements for monitoring the effects introduced by remolding and compaction in the soil. Samples of loess were compacted at varied densities and mixed with electrolytes of different concentrations. Electrical conductivity was measured with a two electrode cell. The effects introduced on the measured conductivity by frequency, degree of saturation, soil density, temperature, and electrolyte type and concentration are addressed. Additionally, hydraulic permeability tests were performed on compacted specimens of loess and the relationship between electrical and hydraulic conductivity was determined. It is concluded here that the ohmic conductivity of compacted specimens depends mainly on the salt concentration in the pore fluid, and volumetric water content. The effect of compaction density was observed to be less significant. The whole behavior of electric conductivity of loess is well described by the Archie’s law.     

Influence of Clod-Size and Structure on Wetting-Induced Volume Change of Compacted Soil

Volume changes due to wetting may occur in naturally deposited soils as well as earthen construction (e.g., compacted fills or embankments). Depending on the stress level, some soils exhibit increase in volume upon wetting (swell) while others may exhibit decrease in volume upon wetting (collapse). The work described in this paper focused on wetting-induced volume changes in compacted soils. Motivation for this work stemmed from observations of earthen structures that exhibit problematic behavior under wetting conditions, even though soils were compacted to engineering specifications (i.e., at or above minimum density and within moisture content ranges). Not only is this problematic behavior a concern but also the laboratory tests used to predict settlement of constructed facilities may not properly model the actual behavior of soil compacted under field conditions. For example, settlements experienced by compacted fills may be different from settlement predictions based on one-dimensional oedometer tests. These differences are partly related to the variations in the soil structure in tested specimens that arise because soil clods compacted in the laboratory are smaller than soil clods compacted in the field. The term “soil structure” includes the combined effects of soil fabric and interparticle forces. “Fabric” generally refers to the geometric arrangement of particles, whereas interparticle forces include physical and physicochemical interactions between particles. The soil structure in this case is associated with specimen preparation methods and is influenced by several factors including soil composition (including pore water chemistry), compaction method, clod sizes, initial moisture condition of clods, dry density or void ratio, and compaction moisture content. A laboratory research study was conducted to investigate the influence of variations in clod-size and structure on one-dimensional volume change, with emphasis on wetting-induced volume change, for nine different fine-grained soils. The results of the study suggest that the influence of structure in one-dimensional oedometer tests depends on soil type and nature of the clods in the compacted soil. Clayey soils appear to be influenced more by differences in structure, whereas silts or clayey sands of low plasticity (PI<10) do not appear to suffer as much from structure effects in one-dimensional oedometer tests. This is attributed to more extensive clod development in clayey soils. Furthermore, the moisture condition of clods appears to have an important influence on volume change behavior.     

Laboratory Investigation of Plough Sole Reformation in a Simulated Paddy Field

The study aimed to determine the number of cultivation cycles required to reform the plough sole once it has been destroyed or removed from the paddy. An ad hoc infiltration column experiment, which is a cylinder of 50?cm in diameter and 140?cm long, was setup to simulate the processes of ploughing and compaction, the two major forces exerted by a tractor, for developing a plough sole in rice paddy. Three experimental conditions were investigated, namely developing the plough sole by ploughing, compaction, and a combination of ploughing and compaction. The results of the change in the infiltration rate, soil dry bulk density, and weight percentage of clay in an experimental soil column were measured and evaluated. The experimental results show that, the infiltration rate decreases to 63, 29, and 2% of its initial value with ploughing eight times, compaction 30 times, and combination of ploughing and compaction 14 times, respectively. Moreover, the soil bulk density increases from 1.51 to 1.53, 1.61, and 1.71?g?cm?3, respectively. Finally, the weight percentage of clay in the plough sole as a result of clay particles moving down from above and increases from 6.2 to 8.2, 6.2, and 14.8%, respectively, over the experimental period. The ploughing rearranged clay content distribution and the compaction increased soil bulk density. Applying these two practices in sequence effectively increased the soil bulk density and reduced the infiltration rate. The results also indicate that after 14 ploughing and compaction the infiltration rate did not further decrease, suggesting that the soil structure could no longer be changed and the plough sole had successfully been reformed. The quantitative result of this work provides valuable information on how to rehabilitate a plough sole once it has been destroyed and needs to be reformed from the paddy field.     

In Field Soil Characterization: Approach Based on Texture Image Analysis

Building on the development of a tool for in place soil investigation based on the use of endoscopy, this paper presents a method for soil characterization using the images recorded by this tool. Various techniques have been explored including texture analysis which is very attractive because it is based on global image analysis. The use of a third order moment resulting from spectral analysis and its value for soil characterization is presented. The influence of various parameters (particle size distribution, mineralogy, water content, and compaction) on the moment evolution is studied.     

Relationships between In Situ and Roller-Integrated Compaction Measurements for Granular Soils

To evaluate compaction meter value and machine drive power roller-integrated compaction technologies, a field study was conducted with 30-m test strips using five granular materials. The test strips were compacted using a prototype CS-533E vibratory smooth drum roller and tested for various compaction parameters using in situ test methods (e.g., nuclear moisture density, dynamic cone penetrometer, plate load tests, etc.). To characterize the roller machine-ground interaction, soil testing focused on measuring soil compaction parameters of the compaction layer, to a depth not exceeding 300?mm. The experimental testing of five test strips provided roller data and in situ measurements for several stages of compaction that were used in performing statistical regression analyses. The relationships between data from the roller-integrated compaction technologies were investigated with special consideration for the relative variation that was observed for each measurement system. Statistical averaging mitigated measurement variability and revealed statistically significant (R2>0.9) relationships between in situ and roller-integrated compaction measurements. This research demonstrates statistical analysis techniques for which calibration procedures using roller-integrated compaction technologies may be developed.     

Design of Compacted Lateritic Soil Liners and Covers

Laboratory tests were conducted on three lateritic soil samples to illustrate some pertinent considerations in the design of compacted lateritic soil liners and covers. The three design parameters investigated are hydraulic conductivity, desiccation-induced volumetric shrinkage, and unconfined compressive strength. Test specimens were compacted at various molding water contents using four compactive efforts. The compaction conditions were shown to have some relationship with soil compaction using either the plasticity modulus or the plasticity product (i.e., clay index). For construction quality assurance purposes, the traditional approach was compared with the modern criterion. Deficiencies associated with the traditional approach for soil liners found in literature also apply to lateritic soils. Overall acceptable zones were constructed on the compaction plane to meet design objectives for hydraulic conductivity, volumetric shrinkage strains, and unconfined compressive strength. The line of optimums was identified as a suitable lower bound for overall acceptable zones of lateritic soils. The volumetric shrinkage strain was also identified as the second most important design parameter for lateritic soils. The shapes of the acceptable zones were affected by the fines contents of the soils.     

Impact of Soil Type and Compaction Conditions on Soil Water Characteristic

Tests were conducted to determine the variation of water content and pore water suction for compacted clayey soils. The soils had varying amounts of clay fraction with plasticities ranging from low to high plasticity. The unsaturated soil behavior was investigated for six conditions, covering a range of compactive efforts and water contents. The experimental data were fit to four commonly used models for the water content-pore water suction relationship. Each model provided a satisfactory fit to the experimental data. However, the individual parameters obtained from the curve fits varied significantly between models. The soil water characteristic curves (SWCCs) were more sensitive to changes in compaction effort than changes in compaction water content. At similar water contents, the pore water suction increased with increasing compaction effort for each compaction condition and soil type. For all compaction conditions, the lowest plasticity soils retained the smallest water content and the highest plasticity soils retained the highest water content at a specified suction. In addition, SWCCs for soils compacted in the laboratory and in the field were similar.     

Case Study of a Full-Scale Evapotranspiration Cover

The design, construction, and performance analyses of a 6.1?ha evapotranspiration (ET) landfill cover at the semiarid U.S. Army Fort Carson site, near Colorado Springs, Colo. are presented. Initial water-balance model simulations, using literature reported soil hydraulic data, aided selection of borrow-source soil type(s) that resulted in predictions of negligible annual drainage ( ? 1?mm/year). Final construction design was based on refined water-balance simulations using laboratory determined soil hydraulic values from borrow area natural soil horizons that were described with USDA soil classification methods. Cover design components included a 122?cm thick clay loam (USDA), compaction ? 80% of the standard Proctor maximum dry density (dry bulk density ～ 1.3?Mg/m3), erosion control measures, top soil amended with biosolids, and seeding with native grasses. Favorable hydrologic performance for a 5?year period was documented by lysimeter-measured and Richards’-based calculations of annual drainage that were all <0.4?mm/year. Water potential data suggest that ET removed water that infiltrated the cover and contributed to a persistent driving force for upward flow and removal of water from below the base of the cover.     

Estimation of Soil–Water Characteristic Curve of Clayey Till Using Measured Pore-Size Distributions

The soil–water characteristic curve (SWCC) of fine-grained soils is usually determined experimentally. In the design of mine waste covers and landfill liners, the unsaturated hydraulic conductivity function, k(h), is often derived theoretically from the measured SWCC. Implicit in these derivations is the transformation of the SWCC to a pore-size distribution (PSD), typically assumed to be constant and monomodal. However, PSD measurements of a clayey till compacted at various water contents after compaction, after flexible-wall permeability testing and before and after SWCC tests show that the PSD of the same material varies significantly under the stated physical conditions. Predictions of the SWCCs using PSDs measured both before and after the SWCC tests significantly underpredicted the values measured. By applying a simple transformation to the PSD to account for the scaling effect from the porosimetry samples (approximately 1 g dry weight) to the SWCC test samples (approximately 200 g dry weight), the predicted SWCCs were found to envelop the measured values. A simple model that simulates the change in PSD during the SWCC test predicted water contents close (1% root mean square error) to the measured SWCCs.     

Soil Water Content Monitoring Using Electromagnetic Induction

The use of electromagnetic (EM) induction measurements was evaluated to predict water content in the upper 1.50 m of a prototype engineered barrier soil profile designed for waste containment. Water content was monitored with a neutron probe, and bulk soil electrical conductivity was monitored with a Geonics EM38 ground conductivity meter at ten locations at approximately monthly intervals over a three-year period. A simple linear regression model accurately predicted average volumetric water content of the profile at any location at any time (R2 = 0.80,σ = 0.009) and spatially averaged volumetric water content over the entire area at any time (R2 = 0.99,σ = 0.003). Although some temporal drift was present in the model residual values, the impact on predicted water content was negligible. Therefore, once the model is calibrated with the neutron probe over a sufficient range of water contents, further neutron probe measurements may not be necessary. EM induction has several advantages over traditional water content monitoring techniques, including nonradioactivity, speed and ease of use over larger areas, and noninvasive character.     

Expansive Soil Test Site Near Newcastle

A field site was established in 1993 near Newcastle, Australia, as part of a long-term study into expansive soil behavior. The primary objectives in establishing the site were to collect high quality data with which to check current design methods for lightly loaded building foundations and to develop improved understanding of the physical processes that drive unsaturated expansive soil behavior. The site was instrumented to allow measurement of soil water content, soil moisture suction, and ground movement to depths of 3 m. The site was provided with two ground covers to simulate moisture boundary conditions due to the presence of typical structures. This paper presents some of the more important findings from the 7 years of data acquired so far. These include a qualitative assessment of the overall site behavior, and quantitative observations of the range of total suction and water content changes with depth, the depth to which moisture changes occur, and the contributions to surface movement from ground movement at various depths. The shape of a mound developed beneath a flexible cover on an initially dry site is examined, and the effects of a large tree on moisture changes are reported.     

Microporosity Structure of Coarse Granular Soils

To date the microporosity structures of coarse soils with various coarse/fines contents are still not fully understood. In this study, the pore-size distributions (PSDs) of five types of soil varying from gravel to clay were characterized using mercury intrusion porosimetry. The soil with a coarse content below 70% (i.e., fines content above 30%) is found to have a fines-controlled microstructure, which is sensitive to water content changes. Such soil forms a dual-porosity structure due to compaction, in which both intraaggregate pores and interaggregate pores are dominant. After saturation, the dual-porosity structure evolves into a unimodal porosity structure dominated by the intraaggregate pores. During drying, such soil exhibits a significant reduction of total volume. The soil with a coarse content above 70% instead has a coarse-controlled microstructure, which is stable upon water content changes. Such soil maintains dual-porosity structures no matter if the soil is compacted, saturated, or dried. As an example of application, the measured PSDs are used to predict the soil water characteristic curves (SWCCs) for the test soils and the predictions are consistent with the SWCCs measured in the laboratory.     

Effect of Fly Ash on Engineering Properties of Expansive Soils

This note presents a study of the efficacy of fly ash as an additive in improving the engineering characteristics of expansive soils. An experimental program has evaluated the effect of the fly ash content on the free swell index, swell potential, swelling pressure, plasticity, compaction, strength, and hydraulic conductivity characteristics of expansive soil. The plasticity, hydraulic conductivity and swelling properties of the blends decreased and the dry unit weight and strength increased with an increase in fly ash content. The resistance to penetration of the blends increased significantly with an increase in fly ash content for a given water content. Excellent correlation was obtained between the measured and predicted undrained shear strengths.     

Organoclay with Soil-Bentonite Admixture as Waste Containment Barriers

Organoclays, clays modified by cationic surfactants, for engineering applications have recently drawn great attention because of their high organic removal capacity. In this study, the potential use of organoclays with soil-bentonite admixtures as waste containment barriers is investigated by experimental tests such as batch equilibrium sorption studies, compaction tests, and hydraulic conductivity tests. Sorption isotherms of total organic carbon (TOC), a gross organic term, by five different types of soil admixtures are nonlinear. The soil specimen with more organoclays exhibits higher organic sorption capacity and a larger retardation factor. The specimens with 20% by dry weight of bentonite have higher optimum water content and plasticity. With the addition of bentonite in the soil material consisting of completely decomposed volcanic rock (CDV) (natural soils) and organoclays, the hydraulic conductivity to leachate decreases from about 10?7 to 10?8 cm∕s. This indicates that the presence of bentonite in the admixtures is important in reducing hydraulic conductivity.     

Foundry Green Sands as Hydraulic Barriers: Laboratory Study

A laboratory testing program was conducted to assess the use of foundry sands from gray iron foundries, typically called green sands, as hydraulic barrier materials. Foundry green sands are mixtures of fine uniform sand, bentonite, and other additives. Specimens of foundry sand were compacted in the laboratory at a variety of water contents and compactive efforts and then permeated in rigid-wall and flexible-wall permeameters to define relationships between hydraulic conductivity, compaction water content, and dry unit weight. Additional tests were conducted to assess how hydraulic conductivity of compacted foundry sand is affected by environmental stresses such as desiccation, freeze-thaw, and chemical permeation. Results of the tests show that the hydraulic conductivity of foundry sand is sensitive to the same variables that affect hydraulic conductivity of compacted clays (i.e., compaction water content, and compactive effort). However, hydraulic conductivities <10?7 cm∕s can be obtained for many foundry sands using a broad range of water contents and compactive efforts, including water contents dry of optimum and at lower compactive effort. The hydraulic conductivity of foundry sand was generally unaffected by freeze-thaw, desiccation, or permeation with 0.1 N salt solution or municipal solid waste leachate, but was incompatible with acetic acid (pH = 3.5). Hydraulic conductivity of foundry sands correlates well with bentonite content and liquid limit, with hydraulic conductivity less than 10?7 cm∕s being achieved for bentonite content ≥6% and∕or liquid limit >20.     

Laboratory Evaluation of the Briaud Compaction Device

Soil compaction quality control plays an important role in earthwork construction. Compacted dry density is only loosely related to the actual deformation of the compacted soil. Rather than using dry density as the controlling factor for compacted fills, it would be better to measure properties more closely related to soil compressibility. The Briaud compaction device (BCD) is a simple, small-strain, nondestructive testing apparatus that can be used to evaluate the modulus of compacted soils. The use of the BCD as a field testing device for compacted soil quality control may be more beneficial than the current practice of measuring in situ dry density. In this study, the laboratory procedures of the BCD were evaluated for compacted silt. The modulus determined by the BCD was compared to the dynamic elastic moduli (Young’s and shear moduli) determined from ultrasonic pulse velocity testing on the same compacted silt samples. The BCD modulus correlated well with the ultrasonic pulse velocity results with R2 value of 0.8 or better. Finally, a repeatability and reproducibility study conducted on the BCD showed a variation of 4% from the mean when only the soil properties were altered.     

Best-Fit Models to Estimate Modified Proctor Properties of Compacted Soil

Regression models were developed to estimate the optimum moisture content and maximum dry density of clayey and fine-grained soils using physical and index properties from 30 soil samples collected in Central Italy and 41 soils described in the literature. The liquid limit of the soils analyzed ranged between 18 and 82%, the plasticity index between 1 and 51%, and specific gravity between 2.47 and 3.09. The most significant regression variables were the specific gravity and the Atterberg limits. The developed models are accurate and can be used as a simple tool to approximate the maximum dry density and optimum water content of clayey and fine-grained soils.     

Effective Stress Behavior of Quasi-Saturated Compacted Cohesive Soils

Important geotechnical structures constructed on compacted cohesive soils often involve compaction either around or on the wet side of optimum water content. In general, at these water content values, water voids are continuous and air voids are occluded, and the soil may be assumed to be in a state termed as “quasi-saturated.” This paper evaluates the effective stress behavior of such quasi-saturated compacted specimens of Gangetic silt and Canyon dam clay in the broad framework of the conventional modified Cam-clay model. The initial state of quasi-saturated compacted specimens is shown to lie on the recompression line in w versus ln(p′) space. The actual recompression line on which the specimen state would lie, and the corresponding equivalent past maximum pressure, are found to depend only on the amount of compaction energy and the soil structure, and are independent of the molding water content or initial dry density. It is observed that, at low effective confining stresses, quasi-saturated compacted soils behave like overconsolidated soils and the effective stress paths during undrained shear lie on the Hvorslev surface. However, at confining stresses greater than the past maximum pressure, these soils behave like normally consolidated soils and the effective stress paths move practically along the Roscoe surface toward the critical state line.     

Predicting the Erosion Rate of Chemically Treated Soil Using a Process Simulation Apparatus for Internal Crack Erosion

Chemical stabilization is an effective ground improvement technique for controlling erosion. Two stabilizers, lignosulfonate and cement, were used to study how effectively they could stabilize erodible silty sand collected from Wombeyan Caves, NSW, Australia. To conduct this research, four dosages of cement (0.5, 1, 1.5, and 2%) and four dosages of lignosulfonate (0.1, 0.2, 0.4, and 0.6%) by dry weight of soil were selected. All treated and untreated soil specimens were compacted to 90 and 95% of their maximum dry density to study the effect of compaction level on erodibility. The erosion characteristics of treated and untreated soil samples were investigated using a process simulation apparatus for internal crack erosion designed and built at the University of Wollongong. The findings of this study indicated that both chemical stabilizers increased the resistance to erosion because of their cementing properties. It was also found that the critical shear stress increased linearly with the amount of stabilizer, and the coefficient of soil erosion decreased as a power function of the critical shear stress.