Auckpath Sawangsuriya

Modulus-suction-moisture relationship for compacted soils

The ultimate parameter of interest in engineering design of compacted subgrades and support fills for highways, railroads, airfields, parking lots, and mat foundations is often the soil modulus. Modulus of compacted soils depends not only on dry unit weight and moisture but also on matric suction and soil structure (or fabric) resulting from the compaction process. However, these relationships in the as-compacted state (i.e., immediately after compaction) have not yet been extensively explored. This paper presents an experimental laboratory study of the shear modulus – matric suction – moisture content-dry unit weight relationship using three compacted subgrade soils. Compacted subgrade specimens were prepared over a range of molding water contents from dry to wet of optimum using enhanced, standard, and reduced Proctor efforts. A nondestructive elastic wave propagation technique, known as bender elements, was used to assess the shear wave velocity and corresponding small-strain shear modulus (Go) of the ...

Modulus-Suction-Moisture Relationship for Compacted Soils in Postcompaction State

Despite clear evidence, changes in mechanical properties (i.e., stiffness or modulus) of compacted subgrades in response to subgrade moisture regime changes after construction have rarely been investigated in the geotechnical profession. In particular, when in-service assessment of pavement subgrade is made, the modulus-moisture variation should be addressed on the basis of unsaturated soil mechanics. This study presents the unsaturated small-strain modulus behavior of five predominately fine-grained compacted subgrade soils. The small-strain shear modulus ( Go ) of saturated compacted specimens subjected to a desorption soil-water characteristic curve (SWCC) was evaluated using bender elements. A test apparatus was designed to apply two stress state variables, the net confining pressure and matric suction, during the Go measurements. The relationship between Go and the SWCC under a constant mean net stress was developed. Additionally, the effect of compaction moisture content, compaction energy, and soil...

Effect of Suction on Resilient Modulus of Compacted Fine-Grained Subgrade Soils

Resilient modulus of compacted subgrade soils is the primary mechanical property required in the mechanistic–empirical design of pavement structures. In most cases, the resilient modulus is determined of a specimen prepared at a single compaction condition (i.e., near optimum water content and at a specified percentage of maximum dry unit weight). However, in the field, the resilient modulus changes in response to changes in moisture content and corresponding changes in matric suction. The relationship between resilient modulus and suction is described for four fine-grained compacted subgrade soils. Resilient modulus tests were conducted in accordance with NCHRP 1-28A on test specimens prepared from each soil and conditioned to different matric suctions. The summary resilient modulus increases with increasing matric suction. This relationship is quantified empirically by using a modulus ratio, defined as the ratio of the summary resilient modulus at a particular suction to a reference summary resilient modulus. Two reference summary moduli were considered: at optimum compaction conditions and at saturation. The modulus ratio is linearly related to the logarithm of matric suction for all soils.

RELATIONSHIP BETWEEN SOIL STIFFNESS GAUGE MODULUS AND OTHER TEST MODULI FOR GRANULAR SOILS

Recently, there has been a concerted effort to develop methods for direct measurement of soil stiffness, modulus, or both. A new field test device called the soil stiffness gauge (SSG), which is currently marketed as GeoGauge, shows potential to assess near-surface stiffness. A comparison is presented of moduli obtained from the SSG with moduli obtained from other tests on granular soils. The maximum singleamplitude dynamic force produced during the SSG measurement is determined to be 17.3 N. On this basis, an estimate of the shear strain amplitude produced from the SSG is made by finite element analysis. A plot of shear modulus versus shear strain amplitude on a medium sand obtained from different laboratory tests, including the SSG, is presented. The comparison of the SSG modulus with the moduli from other laboratory tests indicates that the SSG outputs a dynamic modulus corresponding to a strain amplitude approximately 20 times higher than the expected range and with a magnitude lower than it should be on the basis of the induced strain. Nevertheless, the SSG modulus is still higher than that from the resilient modulus test typically used for pavement design.

LABORATORY EVALUATION OF THE SOIL STIFFNESS GAUGE

A new alternative geotechnical field testing device called the soil stiffness gauge (SSG), also known as GeoGauge™, exhibits particular promise for monitoring in situ soil stiffness during construction quality control. However, there has been only limited research on this device regarding its characteristics and limitations. The results of laboratory testing and a finite-element analysis (FEA) of the SSG are presented. Based on the FEA and the SSG measurement in the test box, the radius of measurement influence extends to 300 mm. For two-layer materials with different stiffness, the SSG starts to register the stiffness of an upper-layer material of 125 mm or thicker. The effect of the lower layer, however, may continue to be present even at an upper-layer material thickness of 275 mm, depending on the relative stiffness of the layer materials. Caution needs to be exercised in interpreting the results from the SSG when it is used on multilayer systems, especially those with geosynthetic separators. The presence of a geosynthetic separator between the layers may cause a stiffness decoupling of the layers.

Dimensionless Limits for the Collection and Interpretation of Wave Propagation Data in Soils

The use of bender elements to generate and receive shear waves in soils has become a very popular technique in geotechnical engineering studies. However as with any other wave propagation technique, the interpretation of bender element-collected data is controlled by wave characteristics, boundary conditions, and properties of the medium. This paper presents experimental data and simple closed-form solutions in order to investigate and to evaluate the effects due to the near field and boundary conditions in different types of specimen geometries and boundary conditions. Results yield dimensionless limits that must be taken into account to properly monitor soil parameters and to avoid misleading results in the interpretation of wave propagation data from the bender elements.

Application of Soil Stiffness Gauge in Assessing Small-Strain Stiffness of Sand with Different Fabrics and Densities

The soil stiffness gauge (SSG) exhibits particular promise for determining the in situ soil stiffness at small strains. Because the SSG is new, its application in laboratory stiffness measurements is limited. The use of the SSG in assessing small-strain stiffness of sand with different fabrics, densities, and specimen sizes is presented herein. Two types of test containers were utilized and the results indicate that the SSG stiffness obtained from both containers show a similar trend but are offset by a constant value, which might be due to the effect of specimen dimensions and boundary configurations. A comparison with other small-strain stiffness tests indicates that the Youngs moduli obtained from the seismic tests are consistently higher than those from the SSG tests. The plots of shear modulus versus shear strain amplitude suggest that the SSG modulus appears to be corresponding to a strain amplitude level higher than the strain amplitude of the seismic test, even though the SSG induces a strain amplitude comparable to that of seismic tests. Nonetheless, the SSG is found to be a potential and useful device for assessing the stiffness of sand with different fabrics and densities.

Alternative Testing Techniques for Modulus of Pavement Bases and Subgrades

The importance of stiffness measurements has gained increased recognition in geotechnical applications in pavements. Two alternative testing techniques: bender elements and soil stiffness gauge (SSG) have been recently adopted as they show some potential and promising means of monitoring the stiffness and/or modulus of pavement materials. Since each technique has its own range of stress and strain levels, the relationship between the elastic moduli and nonlinear behavior exhibited by soils at large strains is required so that the measured modulus can be adjusted or corrected to a modulus corresponding to the desired strain levels. This paper presents the implications of these testing techniques in stiffness and/or modulus assessment of pavement bases and subgrades. To adjust the modulus measured in these materials, the desired strain amplitudes must be known. The strains incurred in the pavement base and subgrade layers that are subjected to the typical traffic loadings are summarized from a number of studies including finite-element analyses, large-scale model experiments, and in-situ test sections. The typical range of strain amplitudes imposed by the bender elements and the SSG is compared with those incurred in the pavement base and subgrade layers to evaluate their suitability in the assessment of pavement layer stiffness and/or modulus. Finally, some comments on the practical implications of these techniques to monitor the pavement layer stiffness and/or modulus are provided.

Estimating van Genuchten Parameters α and n for Clean Sands from Particle Size Distribution Data

The effect of median particle size and breadth of particle sizes in clean sands on the SWCC and the van Genuchten parameters α and n is described. For uniform sands, the air entry suction increases (α decreases) and the steepness diminishes slightly (n decreases) for the drying SWCC as the particles become finer. Increasing the breadth of particle sizes also increases the air entry suction (lower α) and reduces the steepness (lower n) of the SWCC. Breadth of the particle size distribution has a larger effect on steepness and n of the SWCC, and median particle size has a larger effect on the air entry suction and α. Similar effects occur for the wetting SWCC, although the impact on the water entry suction is less than the impact on the air entry suction. The systematic effects of median particle size and breadth of particle sizes on the SWCC are captured in a pedotransfer function (PTF) to estimate α and n for clean sand based on d 60 and C u . Comparison with other SWCC data shows that α estimated with the PTF typically is within a factor of 2 of the actual α and n estimated with the PTF is within ± 2 of the actual n. These findings are applicable to α for sands with nonplastic fines provided the fines content is less than 20%, but apply to n only for sands with negligible fines content. A procedure to convert between van Genuchten and Brooks-Corey parameters for sands is also presented.

Stiffness and Strength Based In-Place Evaluation of Compacted Unbound Materials

Several non-nuclear and portable tools for structural properties assessment of pavement materials have been currently introduced to the market. They are capable of directly measure in-place stiffness and strength of compacted pavement materials, which are the fundamental properties for mechanistic design and performance evaluation of the pavement system. This paper presents stiffness and strength based methods for rapid in-place monitoring compaction quality control during highway construction in Thailand. Results indicated that these methods exhibited good potential for construction quality control as well as the development of performance-based specifications in Thailand.