Abstract:

[Objective] To improve the density of dredged silty soil foundation and eliminate the liquefaction risk, we investigated the reinforcement effectiveness of pneumatic vibratory probe compaction method. The focus is on quantitatively analyzing the improvement in physical and mechanical properties of the treated soil, systematically evaluating the enhancement effects of the pneumatic vibratory probe compaction method, and establishing a scientific effectiveness assessment system, thereby providing a novel technical solution for the treatment of weak coastal foundations. [Methods] Comparative tests involving non-filler vibroflotation method and dynamic compaction with pre-drainage were conducted, supplemented by laboratory experiments and in-situ testing. First, field tests were carried out to evaluate the pneumatic vibratory probe compaction method for reinforcing dredged silty soil foundations, during which key construction parameters were determined through theoretical calculations and trial compaction. The excess pore water pressure during construction was monitored in real time using vibrating wire piezometers. Based on the analysis of the maximum pore pressure ratio, the effective horizontal reinforcement range per point was determined to be 1.15 m. Ultimately, a triangular point arrangement was adopted, with the spacing between vibro-points set at 1.8 m. Moreover, wellpoint dewatering was innovatively employed as an auxiliary measure. [Results] Pneumatic vibratory probe compaction method significantly improved the physical and mechanical properties of the soil: the moisture content decreased from the initial range of 25%-38% to 21.8%-35.5%, a reduction of 5.9%-25.7%, and the void ratio reduced from 0.7-1.07 to 0.63-1.02, a reduction of 6.1%-23.9%. Significant improvements were observed in cone tip resistance, sleeve friction, standard penetration test (SPT) blow counts, and surface wave velocities of the soil layers. Notably, SPT blow counts increased by 60%-260%, static cone penetration (CPT) cone tip resistance rose by 39%-75%, and surface wave velocities showed an increase of 18%. All these indicators met design requirements. More importantly, the treated site was completely free from liquefaction risks, demonstrating a substantial enhancement in seismic performance. Comparison between non-filler vibroflotation method and dynamic compaction with pre-drainage revealed that while the dynamic compaction with pre-drainage performed well in shallow soil reinforcement, its effectiveness was limited for deep layers below 8 m. The non-filler vibroflotation method exhibited good performance in soils with high sand and silt content but showed a significant decline in effectiveness when clay content was elevated. Economic analysis indicated that dynamic compaction had higher construction costs, whereas the pneumatic vibratory probe compaction method and non-filler vibroflotation method had similar costs, demonstrating the former’s notable economic advantage. [Conclusions] The wellpoint dewatering auxiliary measure effectively resolves construction challenges associated with high-moisture-content surface soils, creating favorable conditions for the successful implementation of the pneumatic vibratory probe compaction method. Within the treatment zone, the pneumatic vibratory probe compaction method generates greater excess pore water pressures in the middle-to-lower soil layers. This not only induces premature liquefaction but also significantly improves drainage conditions in silty soil layers, thereby expanding the influence range of single-point vibration and substantially enhancing overall reinforcement effectiveness. Furthermore, this technique offers notable advantages including simple construction procedures, no filler requirement, low cost, high work efficiency, and energy-environmental benefits. These characteristics confer both significant economic and environmental advantages, demonstrating broad application prospects for treating weak coastal foundations.

Key words: dredged silty soil, pneumatic vibratory probe compaction method, precipitation compaction method, pore pressure ratio, in-situ measurements