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Determination Method for Unloading-Reloading Modulus of Coarse-GrainedSoil Based on Dynamic Cone Penetration Tests
TAO Lian-jin, DENG Li-jia, LI Shu-ya, LU Yi-wei, WANG Tian-cheng
Journal of Changjiang River Scientific Research Institute ›› 2025, Vol. 42 ›› Issue (12) : 101-107.
PDF(5697 KB)
PDF(5697 KB)
Determination Method for Unloading-Reloading Modulus of Coarse-GrainedSoil Based on Dynamic Cone Penetration Tests
[Objective] This study aims to establish a field-based method that uses heavy dynamic cone penetration test (DCPT) energy index Pindex to quantify relative density (Dr) of sandy-cobble soil and to link Dr to deformation moduli E50 and Eur, thereby overcoming the long-standing difficulties of retrieving undisturbed samples and calibrating parameters for this material. [Methods] (1) A Φ600 mm × 600 mm calibration chamber was fabricated to enable precise reconstitution of specimens at Dr=0.40, 0.55, 0.70, and 0.85. (2) A series of 63.5 kg DCPTs were performed, and penetration resistance-depth curves were analyzed to extract Pindex. (3) Triaxial consolidated-drained and unloading-reloading tests were conducted on the same Dr specimens to obtain E50 and Eur. (4) A three-tier Bayesian-Bootstrap model “Pindex-Dr-modulus” was developed and coded into PLAXIS-Hardening-Soil model. (5) The method was validated using field monitoring data of an adjacent excavation. [Results] (1) Pindex decreased with Dr following a power law (R2≥0.93). When Dr increased from 0.40 to 0.85, Pindex dropped by 62%, outliers decreased by 47%, and repeatability error remained <3%. (2) E50=112.4 Dr^1.87 MPa and Eur=318.6 Dr ^1.64 MPa. Eur/E50 decreased exponentially from 2.8 to 2.1. (3) Ten-fold cross-validation yielded a mean absolute error of Dr =0.028. The relative errors of the predicted E50 and Eur were <8% and <7%, respectively. (4) FE simulations using the predicted moduli yielded an average relative displacement error of 6.1% compared to 18.4% (Mohr-Coulomb) and 12.7% (Modified Mohr-Coulomb), and the maximum vertical displacement deviation of the station reduced from 5.2 mm to 1.7 mm. (5) The proposed method was applicable to sandy-cobble layers in the upper and middle reaches of the Yongding River alluvial fan, western Beijing (Dr=0.35-0.90, cobble content ≤70%). [Conclusion] The study presents the first continuous field method linking DCPT impact energy, relative density, and deformation moduli for sandy-cobble soil without undisturbed sampling. The compact power-exponential model can be directly implemented in commercial software, providing in-situ parameters for deformation analysis of excavations and tunnels in such formations. The significant improvement in deformation prediction accuracy provides immediate advantages for risk control and support optimization during tunnelling or excavation near existing metro structures.
coarse-grained soil / triaxial test / heavy dynamic cone penetration test / relative density / unloading-reloading modulus
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