No abstract

The interpretation of in-situ geotechnical test data needs a unified approach so that soil parameters are evaluated in a consistent and complementary manner with laboratory results. A common thread in assessing in-situ tests is the focus on the geologic stress history, often expressed by the overconsolidation ratio (OCR). For clays, the OCR can be measured by consolidation tests on undisturbed samples, yet for sands is rather problematic to address. For 6 clays, a hybrid cavity expansion -critical state model is used to match responses measured CPT, CPTu, and DMT. Specifically, tip stress, sleeve friction, penetration porewater pressures, and flat dilatometer readings are fitted by parametric input of OCR, void ratio, friction angle, rigidity index, and compressibility parameters. The undrained shear strength (s u ) of clays is best handled via critical-state concepts. Discussions are included for pressuremeter, vane, and T-bar tests. For sands, select empirical methods derived from laboratory chamber testing on reconstituted clean quartz and siliceous sands are reviewed, specifically for effective friction angle φ , OCR, and K 0 . In a novel look, a special set of undisturbed (frozen) sand samples from 15 locations in Japan, Canada, Italy, Norway, and China is used to check interrelationships for the following in-situ penetration tests: SPT, CPT, and V s . Stiffness of all soils begins with the small-strain shear modulus (G 0 = G max = ρ T V 2 s ) that can be used together with strength (s u or φ ) to evaluate stiffness over a range of strains. Supplementary testing by PMT and/or DMT can provide intermediate stiffnesses for tuning of modulus reduction schemes, as well as independent assessments of K 0 and OCR.

No abstract

Data are compiled from 31 clay sites where in-situ measurements of initial tangent shear modulus (Gmax) and cone tip resistance (qc) were available. Values of Gmax were obtained from either seismic cone penetration (SCPT), crosshole (CHT), downhole (DHT), or spectral analysis of surface wave (SASW) tests, and readings of qc were taken either by regular cone penetration (CPT) or piezocone (CPTU) tests. Multiple regression analyses indicate that in-situ values of Gmax depend on void ratio (eo), overburden stress (σ′vo), and stress history (OCR), as previously established from laboratory resonant column tests. Since qc also depends on σ′vo and OCR, a moderate association between Gmax and qc is possible, despite their incompatible strain levels. For preliminary correlative purposes, a power function relates in-situ Gmax, qc, and eo in clay deposits having a wide range in plasticity, sensitivity, stress history, and consistency.

The results of isotropic and anisotropic consolidated-undrained shear tests (CIU, CK0, U) are used to determine the critical-state pore pressure parameter (Λo). The relative advantages of using the critical-state parameter (Λo) over Skempton's pore pressure parameter (A) and Henkel's parameter (a) are discussed. The effects of over-consolidation ratio (OCR) and initial stress state (K0) on both Henkel's and Skempton's pore pressure parameters can significantly alter effective stress predictions of undrained strength. The critical-state parameter is independent of OCR, K0, and level of shear to failure, thus requiring only two basic soil constants in order to predict undrained strength: (1) the effective stress friction angle (φ'), and (2) the critical-state pore pressure parameter (Λo). An “extended” critical-state model is developed using the equivalent pressure concept for overconsolidated states. The method then provides a simple analytical representation of undrained stress-strain behavior and pore pressure response for clays with different values of OCR. One additional soil constant (Cc: the virgin compression index) is required in order to model stress-strain behavior. The validity of the critical-state theory is substantiated by data from over ninety different clay and silt soils reported in the geotechnical literature. Furthermore, the critical-state concepts are shown to encompass both total stress and effective stress methods under one unified theory.