Taeseo Ku

Evaluating the In Situ Lateral Stress Coefficient (K0) of Soils via Paired Shear Wave Velocity Modes

AbstractThe utilization of shear wave velocities toward the evaluation of the in situ geostatic horizontal stress state in soils is validated, specifically the lateral stress coefficient K0. Field shear wave velocities from paired sets of different directional and polarization modes are compiled from 16 well-documented test sites involving a variety of geomaterials. Focus is particularly placed on shear wave velocities measured by downhole tests (VsVH), crosshole tests (VsHV), and special rotary-type crosshole tests (VsHH). At these sites, field K0 stress states have been quantified using one or more direct assessment techniques, including self-boring pressuremeter, total stress cells, and hydrofracture in field testing, as well as suction measurements, special consolidometers, and/or triaxial arrangements on undisturbed samples in the laboratory. Although the specific delineation of stress-induced versus inherent or fabric anisotropy may be difficult, it is shown that the ratio of horizontally polarized ...

Stress Dependency of Shear-Wave Velocity Measurements in Soils

AbstractBased on an extensive in situ shear-wave velocity (Vs) database, this study aims to not only examine the stress-dependent characteristics of in situ Vs from a generalized Vs-stress model [Vs=α×(σc′)β], but also provide valuable reference information on site-specific stress-dependency of Vs for global geotechnical sites. For in situ Vs, the site-specific material constants (α and β) at each site had wider ranges when compared to typical values reported from laboratory tests. This is mainly attributed to different test conditions and inherent in situ site characteristics. The in situ-based site-specific material constants in this study are compared with laboratory-based material constants suggested in previous studies. Evidently, each α–β trend varies on laboratory tests and in situ tests and a unified relationship is appropriate only when β is less than 0.5. Thus, separate relationships between material constants are proposed for laboratory-based and in situ-based measurements. The results imply th...

In Situ Lateral Stress Coefficient (K0) from Shear Wave Velocity Measurements in Soils

AbstractThe at-rest lateral stress coefficient (K0) is an important soil parameter in geotechnical design problems and yet it is quite elusive in our ability to assess its value, either by laboratory or in situ tests. In this study, an innovative geophysics approach toward the evaluation of the in situ K0 profile with depth is obtained by using small-strain stiffness anisotropy ratio (G0,HH/G0,VH) in soils. The newly proposed K0 equation is derived from a database compiled from 12 test sites which have direct K0 measurements, as procured from field tests, either self-boring pressuremeters (SBP) or total stress cells (TSC), and/or laboratory triaxial tests, instrumented consolidometers, and/or suction measurements. The data show a strong relationship between the overconsolidation difference (OCD=σp′−σvo′) and stiffness anisotropy ratio, thus enabling a K0 assessment in clays, silts, and sands.

Estimating spatial variations in bedrock depth and weathering degree in decomposed granite from surface waves

AbstractIn Singapore, sudden changes of bedrock depth and highly variable weathering degrees in residual soils are commonly observed. This study employed the dispersive characteristics of Rayleigh-...

Directional properties of small strain shear stiffness in soils

A compiled database of shear wave velocity measurements in a variety of clays, silts and sands shows directional hierarchies between downhole (VsVH), standard crosshole (VsHV), and rotary crosshole (VsHH) tests. The special in situ database has been collected from 33 well-documented geotechnical test sites. Expressions relating the small-strain shear modulus in terms of effective confining stress level, stress history and void ratio are explored for each of these three modes of directional shear wave velocity. The relationships are separated initially into soil groups (intact clays, fissured clays, sands and silts), and then generalised to consider all soil types together.

Numerical Investigation of Drilled Shafts near an Embankment Slope under Combined Torque-Lateral-Load Scenario

Generally, cantilevered structure-foundation systems supporting highway signs, signals, and luminaires in the areas exposed to severe wind loadings (e.g., hurricane) have been designed under coupled torsion and lateral load scenario. Especially, mast arm cantilevered structures constructed near or on an embankment slope may have more concerns on the torsional and lateral resistance of the foundation. However, most research works have merely considered drilled shaft foundations under torsion-lateral load case with an embankment in proximity. In this study, a numerical study is performed with different soil layers to: (1) understand the combined torque-lateral load behavior of drilled shafts near an embankment slope; (2) examine the effect of both the torsion and lateral resistances in the proximity of an embankment slope. It was found that torsional stiffness decreases with increase in slope angles. Finally, design criteria (e.g., minimum allowable distance from the embankment, maximum allowable point load near the embankment) of the mast arm assembly and loads are provided.

Effect of Fine Particles on Cement Treated Sand

Cement treated sand improves mechanical properties through the cementitious bonding between cohesionless particles, thus allowing several geotechnical applications such as soil stabilization against slope failure and liquefaction. Since pure sand without any fine particles is seldom available in nature, this study aims to investigate the effect of fine particles (kaolin) in a very small proportion (<5%) on cement treated sand. Two types of cements are used: (i) Ordinary Portland cement (OPC) and (ii) Calcium sulfoaluminate cement (CSA). OPC is the widely used cementitious binder whereas CSA is a rapid hardening cement that is becoming popular due to its low carbon foot print. Three different cement contents (3%, 5% and 7%) and four different fine contents (0%, 1%, 3% and 5%) for each cement content are chosen. The stiffness and strength of the cement treated sands are measured through shear wave velocity and unconfined compressive strength respectively, after 1-day and 7-day curing periods. The results show that the influence of fine particles is visible even with fine content as low as 1%. However, the effect is different between the two types of cements used and between low and high cement contents. As the fine content increases, the increase in strength and stiffness is more for OPC than CSA and more significant at low cement content (3%) than high cement content (5% and 7%).

Undrained Shear Strength in Cohesive Soils Estimated by Directional Modes of In-Situ Shear Wave Velocity

The estimated undrained shear strength (su) is often not a unique value because it can be evaluated by various test types and/or procedures, such as different failure modes, shear strain rates, and boundary conditions. This study explores (1) the relationship between reference undrained shear strength and in situ shear wave velocity in terms of the effective overburden stress, and (2) the independent relationships to evaluate the undrained shear strength with special consideration of different directional and polarization modes (VH, HV, HH shear waves), which has not been reported. This evaluation is done via a worldwide database compiled from 43 well-documented geotechnical test sites associated with soft ground. Finally, new correlation models are proposed to estimate the undrained shear strength based on the in situ shear wave velocity as well as the plasticity index or the overconsolidation ratio. The application of the shear wave velocity–undrained shear strength relation is illustrated through two independent case studies. The proposed relationships are expected to contribute to reasonable estimates of undrained shear strength as well as offer practical guidance on even extrapolation beyond the data that is available to geotechnical engineers.

Small Strain Stiffness of Unsaturated Sands Containing a Polyacrylamide Solution

Sand improvements using organic agents have shown promising results. Polyacrylamide is one possible organic agent, which has been shown to influence the shear strength, stiffness, soil remediation, and erosion resistance of geomaterials. In this study, we explored the shear wave velocity (S-wave) and water retention curves of unsaturated sands containing polyacrylamide solutions. The shear wave velocity was measured during the water retention curve measurement tests according to the variation of the degree of saturation. The experimental setup was verified through comparison of the measured water retention curves with the published data. The results show that (1) the S-wave velocity of saturated sands increases with polyacrylamide concentration; (2) as the degree of saturation decreases, the S-wave velocity increases; (3) near the residual water (or polyacrylamide solution) saturation, the S-wave velocity increases dramatically; (4) as the degree of saturation decreases, the S-wave velocity at unsaturated conditions increases with any given water (or polyacrylamide solution) saturation, like the water retention curves; (5) the S-wave velocity increases with the increase in capillary pressure; and (6) the predicted S-wave velocity at a given degree of saturation is slightly overestimated, and the modification of the equation is required.

Post-Processing Continuous Shear Wave Signals Taken During Cone Penetrometer Testing

Continuous seismic velocity measurements use a special automated wave source and advanced post-processing analyses to provide fast, detailed, and reliable profiles of shear wave velocity (Vs) with depth. Conventional geophysical techniques such as crosshole tests (CHTs) and downhole tests (DHTs) in boreholes are slow because they have several required steps: (1) rotary drilling, (2) installation of casing and grouting, (3) inclinometer measurements (for CHTs), and (4) deployment of geophones for seismic readings. Direct-push technologies include the use of seismic cones and seismic dilatometers that offer DHT-type Vs data at intervals of 1 m or less without the need for drilling, casing, grouting, or separate field events. The recent development of a new portable autoseis source allows the generation of reliable and consistent shear waves either intermittently or as frequently as every 1 to 10 s. Continuous shear wave measurements can provide improved detailing of the small-strain stiffness (G0) at frequent depth intervals and fast field production times. Appreciable sensitivity errors in Vs calculations can be experienced because of the extremely small time shifts between adjacent shear wave records, as well as significant signal noise due to vibration, external sources, and refracted waves. This paper details continuous-interval seismic piezocone testing and explains how to handle signal post-processing in both the time domain and the spectral frequency domain in order to obtain a reliable in situ Vs profile.