A case of using the vacuum preloading method to improve the foundation soil for a road in Tianjin, China, is presented. A vacuum load of 80 kPa was applied for 90 days to consolidate a 20 m thick soft clay layer. The ground settled for about 1·5 m. The average degree of consolidation estimated using settlement data was 90%. The undrained shear strength of the soil increased and the water content decreased after vacuum preloading. The procedures used for soil improvement, the instrumentation and the field monitoring data are described. Several issues concerning the practical aspects of the vacuum preloading method are discussed.

针对排水体联合真空预压处理疏浚淤泥时常出现淤堵问题,探索性提出自反滤层—免滤膜秸秆排水体。通过开展免滤膜秸秆排水体真空预压处理疏浚淤泥模型试验,研究了该排水体处理疏浚淤泥的效果和机理。结果表明:免滤膜秸秆排水体处理疏浚淤泥的含固率及加固后的颗粒分布与塑料排水体相近,但出水量较塑料排水体提高7.8%~22.2%,加固后的土体平均含水率较塑料排水体降低6.9%~13.8%。免滤膜秸秆排水体具有优良的排水特性以及全降解的生态特性,为解决真空预压处理疏浚淤泥提供了新的思路。

[Objective] To address the engineering challenge of drainage efficiency reduction caused by filter membrane clogging in traditional plastic drainage bodies during vacuum preloading of dredged sludge, this paper innovatively proposes a fully biodegradable, non-filter membrane straw drainage body (NSD) technology. Model tests were conducted to verify the engineering applicability of the NSD, reveal its drainage consolidation mechanism, and provide sustainable solutions for the green treatment of dredged sludge. [Methods] A PVC cylinder with a height of 50 cm and a diameter of 30 cm was used as the test tank, filled with dredged sludge at a water content of 147.5% (approximately 2.5 ω_L). The sludge was collected from a disposal site in Wuhe County, Anhui Province, with a liquid limit of 59% and clay content of 21.7%. The control group used traditional plastic drainage bodies, consisting of rigid tubes wrapped with filter gauze and fabric. Two setups were tested: one with constant vacuum loading and another with staged vacuum loading. The experimental group employed NSDs, made of rigid tubes wrapped with straw ropes. Four setups were tested: (1) constant vacuum loading, (2) staged vacuum loading, (3) constant vacuum loading after installing a 2-5 mm self-filtering soil layer, and (4) staged vacuum loading with the pre-installed 2-5 mm self-filtering layer. During the testing period, the effluent discharge volume was recorded every 24 hours, and the solids content of the extracted tailwater was measured during each cycle. Upon completion of the vacuum preloading, the soil’s moisture content and particle gradation were determined. [Results] Drainage efficiency significantly improved, with the experimental group’s cumulative effluent volume 7.9%-22.1% higher than the control group, indicating that the three-dimensional pore structure of straw effectively alleviates the impact of clogging on drainage. Soil reinforcement was enhanced, with the experimental group’s average water content after vacuum preloading reduced by 6.8%-15.3% compared to the control group. Particle size distribution analysis revealed that when the self-filtering soil layer was pre-installed, the clay content (d<0.005 mm) increased by 5%-11.1%, confirming that the NSD, when combined with the pre-installed self-filtering layer, not only achieved effective soil filtration but also enhanced soil stabilization performance. [Conclusion] Technical innovation: The NSD achieves membrane-free drainage through its crisscrossing internal channels, overcoming the clogging bottlenecks of traditional plastic drains while increasing drainage efficiency by more than 15%. Mechanism breakthrough: A synergistic mechanism combining the NSD with the self-filtering soil layer has been proposed, demonstrating significantly enhanced drainage performance without compromising consolidation effectiveness. Application value: An efficient and eco-friendly dredged sludge treatment technology has been developed, providing a novel approach to vacuum preloading treatment of dredged sludge.