Construction Technology of Prefabricated Vertical Drain (PVD) with Surcharge Preloading -

In soft soil foundation treatment, prefabricated vertical drain (PVD) with surcharge preloading is one of the most commonly used and cost-effective construction technologies. It is widely applied to foundation reinforcement of roads, factory buildings, ports and other projects with simple operation and remarkable treatment effect. This paper elaborates on its core key points in plain language.

I.Core Principle

The fundamental defects of soft soil foundation are high water content, soft soil texture and insufficient bearing capacity, which are prone to settlement and deformation after direct construction. The PVD surcharge preloading process adopts the combined method of “drainage + pressurization” to rapidly discharge pore water from soft soil, compact soil particles, improve the bearing capacity of the foundation, and reduce post-construction settlement.

Analogy: The soft soil is equivalent to a sponge, the plastic drainage board serves as a vertical drainage pipeline, and the surcharge material provides external pressure. Squeezing out the internal water of the sponge can make the sponge compact and solid.

II. Core Construction Process

1. Pre-construction Preparation

Clean up the construction site, remove surface sundries, level the site ground, and set up temporary drainage facilities to avoid ponding on the construction area. Meanwhile, complete the inspection and commissioning of construction materials and mechanical equipment. Check the quality of plastic drainage boards to ensure they meet design and specification requirements; inspect and debug the board inserting machine, surcharge filling materials (crushed stone, earthwork, etc.) to ensure normal operation of all equipment.

2. Sand Cushion Laying

Lay a sand cushion with a thickness of about 50cm on the leveled foundation surface. The sand cushion can disperse upper load pressure, collect seepage water, and form a horizontal drainage channel matching the vertical drainage boards. It acts as a permeable base layer for the soft soil foundation to ensure the smooth discharge of pore water in the soil.

3. PVD Installation

Adopt a professional board inserting machine to vertically drive plastic drainage boards into the soft soil foundation. The inserting depth shall comply with the design requirements, which generally needs to penetrate the entire soft soil layer and reach the dense bearing stratum. After installation, the top of the drainage board shall be exposed above the sand cushion by more than 20cm to facilitate water drainage. During construction, twisting, folding and breaking of drainage boards shall be strictly avoided.

4. Graded Surcharge Preloading

After the completion of drainage board installation, carry out layered surcharge construction in accordance with the principle of “light load first, heavy load later and graded loading”. One-time overloading is forbidden to prevent foundation instability and shear failure. Continuously monitor foundation settlement and horizontal displacement during surcharging. The next-stage loading can be carried out only after the settlement and displacement data of the current load level tend to be stable.

The preloading maintenance period is determined by the design standard, usually lasting several months. When the foundation settlement meets the design control index and the soil consolidation is completed, the surcharge can be unloaded, and the foundation reinforcement construction is finished.

III. Key Construction Notes

1. Conduct strict incoming inspection for PVD materials. Damaged, unqualified and expired products are prohibited from use. The drainage boards shall be stored away from sunlight exposure and pollution to prevent material aging and damage.

2. Control the verticality of PVD installation strictly. Excessive vertical deviation will block the drainage channel and affect the foundation consolidation effect.

3. Strictly control the surcharge loading rate. Excessively fast loading will induce foundation sliding and instability. Stop loading immediately once abnormal settlement or displacement is monitored.

4. Arrange special personnel for whole-process monitoring during preloading. Unloading can only be carried out after all monitoring data meet the design standards to ensure the quality of foundation reinforcement.

Ⅳ. Summary

The core construction logic of PVD surcharge preloading is “graded drainage and staged pressurization”. With simple technological process, convenient construction and low comprehensive cost, it is a high-cost-performance technology for soft soil foundation treatment. It is widely used in various infrastructure and industrial construction projects, and can effectively solve the problems of insufficient bearing capacity and excessive settlement of soft soil foundations.