BOREHOLE PRESSURE SHEAR TESTER (BPST) FOR IN-SITU EVALUATION OF WEATHERED GEOMATERIALS

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Accurate characterization of weathered geomaterials—comprising residual soils and weathered rocks—is essential for ensuring the stability and safety of civil engineering structures. These materials exhibit transitional behavior between soil and rock, making their mechanical properties difficult to assess using conventional techniques. Laboratory testing is often impractical because obtaining undisturbed samples from weathered layers is extremely challenging. Consequently, reliable in-situ testing methods are crucial for capturing true field conditions and improving geotechnical design accuracy.

Limitations of Conventional Field Testing Methods

Traditional field tests are typically developed either for soils or for intact rock masses, leading to significant shortcomings when applied to intermediate geomaterials. Weathered layers possess heterogeneous structures, variable stiffness, and complex failure mechanisms that standard tests cannot fully capture. As a result, existing techniques may produce unreliable estimates of deformation and shear strength, potentially compromising engineering decisions for foundations, slopes, and underground structures.

Development of the Borehole Pressure Shear Tester (BPST)

To address these limitations, this study introduces the Borehole Pressure Shear Tester (BPST), an innovative device that combines the principles of pressuremeter testing and borehole shear testing. The BPST applies controlled horizontal pressure and shear forces directly within a borehole, enabling simultaneous assessment of deformation characteristics and shear strength parameters. This integrated approach provides a more comprehensive understanding of the mechanical behavior of weathered geomaterials compared to single-mode testing methods.

Testing Procedure and Measurement Capabilities

The BPST operates by expanding against the borehole wall while applying tangential shear displacement, replicating realistic stress conditions encountered in situ. This allows direct measurement of deformation modulus and shear resistance under controlled loading paths. By capturing both normal and shear responses in a single test, the device reduces uncertainty associated with extrapolating parameters from separate tests and improves efficiency in field investigations.

Experimental Validation on Residual Soils

Empirical tests conducted on residual soils at simulated high relative densities and varying overburden stresses demonstrated the effectiveness of the BPST. The measured deformation moduli and shear strength parameters showed strong agreement with results obtained from conventional triaxial compression and direct shear tests. This correlation confirms the accuracy and reliability of the device for characterizing intermediate geomaterials under realistic field conditions.

Implications for Geotechnical Design and Infrastructure Safety

The introduction of the BPST represents a significant advancement in geotechnical site investigation. By enabling accurate in-situ evaluation of weathered layers, the device enhances predictive modeling, supports safer foundation design, and reduces uncertainty in stability assessments. Its ability to characterize materials that fall between soil and rock categories makes it particularly valuable for projects involving slopes, tunnels, deep foundations, and infrastructure built on weathered terrain.

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#ShearStrength
#DeformationModulus
#SiteCharacterization
#EngineeringGeology
#InfrastructureSafety
#FieldTesting
#SoilMechanics
#GeotechnicalInnovation
#SlopeStability
#SubsurfaceEngineering

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