Design Basis and Control Requirements for Vacuum Preloading in Soft‑Ground Improvement

There are no universal fixed parameters for vacuum preloading design of soft‑ground improvement. Design shall be formulated on a project‑specific basis according to site geological conditions, design drawings, load grades, construction schedule and site conditions. Construction parameters and control criteria shall be adjusted targeting actual project conditions, so as to ensure ground improvement quality, settlement indicators and service safety comply with project design and specification requirements. General design requirements are specified as follows:

1、Site Levelling Design Requirements

（1）Prior to formal ground improvement construction, perform overall site levelling within the soft‑ground treatment zone. Remove uneven top‑soil and re‑grade terrain to provide a neat and flat working surface for subsequent PVD installation, sealing membrane laying and vacuum preloading operations.

（2）No fixed standard applies to site levelling. Adjust flexibly in accordance with special design documents and actual site conditions to adapt to construction requirements of different projects.

2、Ground Improvement Technology Design Requirements

（1）Select the optimal soft‑ground improvement technology by comprehensive consideration of project geology, settlement control criteria and construction schedule. Prefabricated Vertical Drains (PVDs) combined with vacuum preloading consolidation is the mainstream general‑purpose technology.

（2）No fixed values are specified for technical parameters, improvement scope and construction control criteria. Conduct targeted optimisation and adjustment according to project design documents, soil‑layer distribution and soil properties to satisfy improvement demands of various projects.

3、 Preloading Period and Unloading Confirmation Requirements

（1）The consolidation period of vacuum preloading shall be determined specially based on soft‑soil thickness, soil properties, consolidation difficulty and overall construction schedule, without a unified fixed duration.

（2）After the site reaches the designed vacuum degree, full‑load preloading shall be carried out. For conventional projects, the full‑load preloading duration shall be controlled at 90 days, which may be flexibly adjusted according to actual consolidation requirements.

（3）Before unloading, compile complete full‑process settlement‑monitoring data and consolidate acceptance documents. Unloading operations can only be implemented after joint verification and approval by the owner, designer, supervisor and constructor.

4. Site Pre‑treatment and Elevation Control Requirements

（1）Before construction, thoroughly remove boulders, plant roots, sundries and weak interlayers within newly‑filled soil on site, so as to prevent adverse impacts of poor top‑soil on PVD installation quality and ground consolidation performance.

（2）After debris clearance, level the site to the designed elevation. Elevation deviation shall comply with project‑specific design and specification requirements.

（3）For conventional projects, the elevation tolerance shall be within 10 cm. Precision requirements may be flexibly adjusted per design standards of different projects.

5. PVD Design and Inspection Requirements

（1）Specifications, models and layout patterns of prefabricated vertical drains shall comply with special project drawings. SPB‑B or SPB‑C PVDs are commonly adopted, arranged in square or staggered (plum‑blossom) patterns. Spacing and penetration depth shall strictly follow profile and detail drawings, and may be optimised according to site geological conditions.

（2） PVD quality shall satisfy the requirements of *Quality Inspection Standard for Prefabricated Vertical Drains* (JTS/T 206‑1‑2023). Complete documents including factory certificates and manufacturer self‑inspection reports shall be provided upon material delivery.

（3） Delivered materials shall be sampled and tested in accordance with specifications and project requirements. Materials can be put into site construction only after passing inspection by qualified third‑party testing institutions.

6、 Site Drainage System Design Requirements

（1）Reasonably arrange peripheral open drainage ditches combining site topography, catchment conditions and construction layout plans.

（2）The dimension, alignment and density of open ditches shall be specially designed according to site scale. Water outlets shall be located at low‑lying areas of the site to guarantee smooth drainage.

（3）Improve the surface drainage system to avoid soft‑ground soaking caused by ponding, damage to vacuum sealing structure and deterioration of preloading‑consolidation performance. Drainage schemes may be flexibly optimised for actual site conditions.

7、 Vacuum Degree and Consolidation Degree Control Requirements

（1）During vacuum preloading construction, vacuum under the membrane shall be distributed stably and uniformly. For conventional projects, vacuum degree shall be maintained above 650 mmHg (80 kPa); specific values shall follow special project‑design indicators.

（2）Before unloading, the average consolidation degree of soil layers within the PVD improvement depth shall meet design requirements, which shall exceed 90 % for conventional projects.

（3）Indicators for vacuum degree and consolidation degree are not fixed, and shall be adjusted differentially according to settlement criteria and functional requirements of different projects.

8. Pump‑off and Unloading Control Requirements

（2） Pump‑off and unloading conditions for vacuum preloading shall comply with special project design. Unloading can be performed only after consolidation‑degree or settlement‑stability indicators are satisfied and confirmed by all participating parties.

（2） Conventional settlement‑stability criterion: when the average measured settlement rate is ≤ 2 mm/day for 7 consecutive days, the site is regarded as basically settlement‑stable; pump‑off and unloading operations can be conducted combined with consolidation‑degree results.

（3）Threshold values for settlement rate control may be flexibly adjusted according to foundation‑design grades and settlement‑limit requirements of different projects.

Source: Guangzhou Sunzo Foundation Engineering Co., Ltd.

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