Wuhan is renowned nationwide as the 'River City,' and its groundwater resources are extremely abundant, especially in areas along both banks of the Yangtze River. To avoid structural damage caused by the overall uplift of basements due to groundwater buoyancy, which can lead to huge economic losses, the anti-floating design of basements is particularly important.

Traditional anti-floating schemes can be divided into two types:

① 'Compression,' which uses backfill materials or relies on structural self-weight to balance groundwater buoyancy; ② 'Tension,' which uses anti-uplift piles or anti-floating anchors to prevent structural uplift.
The main functions, corresponding types, and applicable conditions can be summarized as follows:

I. Introduction to Water Release and Pressure Reduction Engineering and Construction Technology

① Project Introduction: Compared with traditional anti-floating schemes, the 'water release and pressure reduction method' introduces and discharges groundwater through water release and pressure reduction devices installed on the basement side walls or bottom slabs, thereby reducing or eliminating the buoyancy effect of groundwater on the basement.

② Anti-floating Principle: An anti-floating method combining internal and external measures, allowing groundwater to enter the basement drainage system in an organized manner through water release holes set on the side walls or bottom slabs of underground structures. Combined with the existing drainage system, groundwater is pumped and discharged to lower the outdoor groundwater level and reduce the pressure causing structural uplift.

③ Construction Technology: Pre-embedded sleeves are installed synchronously with the main structure. Pre-embedding work is carried out before concrete pouring of side walls or bottom slabs, and the water release device is installed after the backfilling around the basement is completed.

On-site engineering example

II. Cost Composition of Water Release and Pressure Reduction

The main cost components of water release and pressure reduction engineering include equipment costs, installation costs, design technical service fees, and the corresponding increase in civil engineering costs for drainage ditches and collection wells. The number of devices determines the cost of the entire water release and pressure reduction project.

III. Key Points of Cost Control for Water Release and Pressure Reduction

Water release and pressure reduction is a monopolistic patented project. For conventional projects, owners generally conduct research on prices of other real estate projects and directly negotiate prices with construction units. Attention should be paid to whether the negotiated price covers later equipment maintenance and repair costs. With changes in the real estate environment, controlling development costs and accelerating development pace, the 'water release and pressure reduction method' scheme is gradually being used in some housing construction projects.

IV. Case Comparison

Case 1

(1) Project Overview

A project has a total construction area of 540,000 m², including: high-rise residential buildings with a total construction area of 360,000 m², street-facing commercial buildings with a total construction area of 41,000 m², kindergartens and supporting facilities with a total construction area of 6,000 m²; a single-story basement with a construction area of 200,000 m².

(2) Case Background

After basement excavation, the buoyancy effect of perched water on the basement should be considered. The anti-floating design water level is considered according to the outdoor ground design elevation. According to design code requirements, the pure basement part requires anti-floating treatment.

(3) Problems to be Solved

Compare the conventional treatment scheme of anti-floating anchors or anti-uplift piles with the new water release and pressure reduction anti-floating scheme in terms of construction period, cost, etc., to decide the final project execution plan.
(4) Scheme Comparison

① Scheme 1 (Original proposed scheme: traditional anti-floating scheme — anti-floating anchors)
The scheme provided in the early stage of the project was anti-floating anchors, specifically: a total of 4,929 anti-floating anchors with a diameter of 300 mm, designed effective anchor length of 6.5 meters, and characteristic value of vertical uplift resistance of 250 KN.

② Scheme 2: (New anti-floating scheme — water release and pressure reduction)

The owner invited units qualified for the design and construction of the 'water release and pressure reduction method' anti-floating scheme to conduct on-site inspection of the soil conditions after earthwork excavation, and combined with the preliminary geological detailed survey report, provided an anti-floating scheme of the 'water release and pressure reduction method' that meets design requirements. The specific scheme is (307 side wall water release devices, 439 bottom slab water release devices, corresponding to newly added civil drainage ditches)

Layout plan of water release and pressure reduction devices

Schematic diagram of side wall and bottom slab construction for water release and pressure reduction

(5) Economic Comparison:

① Scheme 1 (Original proposed scheme: traditional anti-floating scheme — anti-floating anchors)

Note: Pile length is calculated as 6.5 m according to the design scheme, and prices refer to adjacent projects. The estimated total cost including tax is 5.7858 million yuan.

② Scheme 2 (New anti-floating scheme — water release and pressure reduction)

After price inquiry of a project in Wuhan, the water release and pressure reduction devices for this scheme are calculated at 3,600 yuan per set, and the estimated total price is 3.13 million yuan. The details are as follows: (including the corresponding increase in civil engineering drainage ditches and collection wells for the proposed new water release and pressure reduction devices);

From the above, the water release and pressure reduction devices are about 2.65 million yuan more economical than traditional anti-floating anchors.
(6) Construction Period Comparison:

The construction of anti-floating anchors is on the critical path in the project schedule network diagram. The planned construction period for this project is about 60-80 calendar days, while the 'water release and pressure reduction method' scheme is on a non-critical path, mainly involving pre-embedded holes during main structure construction and later installation of anti-floating water release devices, which will not affect the total construction period. Compared with the two anti-floating schemes, the 'water release and pressure reduction method' anti-floating scheme can greatly shorten the total construction period.

Case 2

(1) Project Overview

A project consists of 1 49-story office building, 2 36-story hotels and hotel apartments, a 4-7 story commercial podium, and a 3-story connected basement. The basement depth is about 17 meters, and the basement area is about 35,000 m².

(2) Case Background

① The basement is mainly embedded in cohesive soil layers, below which are limestone and mudstone layers, both being relatively impermeable layers; ② Groundwater consists of perched water and confined water. Since the confined water depth is greater than 30 m, the basement anti-floating design only considers the effect of perched water.

(3) Scheme Comparison

① Scheme 1: (Original proposed scheme: traditional anti-floating scheme — anti-floating anchors)
The anchors use 3 25mm diameter Grade III steel bars, and the cement mortar grade is M30. The length of a single anchor is not less than 15 m, extending into gravelly clay, strongly weathered mudstone, shale, and siliceous rock layers, with a hole diameter of 200 mm. The uplift bearing capacity of a single anchor is 250 kN or 400 kN, and a total of 8,013 rock-socketed anchors are installed.

② Scheme 2: (New anti-floating scheme — water release and pressure reduction)

Water release and pressure reduction devices are arranged along the basement exterior walls and in local areas in the middle of the basement, with a spacing of 2~3 m. The side wall water release holes are arranged at a height of 600 mm above the basement bottom slab structural surface, and the bottom slab water release holes are set in drainage ditches.

(4) Economic Comparison

① Scheme 1: (Original proposed scheme: traditional anti-floating scheme — anti-floating anchors)

Note: Pile length is calculated as 15 m according to the design scheme, and prices refer to other projects. The estimated total cost including tax is 21.8879 million yuan.

② Scheme 2: (New anti-floating scheme — water release and pressure reduction)

From the above, the water release and pressure reduction devices are about 17.44 million yuan more economical than traditional anti-floating anchors.

(5) Construction Period Comparison

V. Implementation Achievements

By comparing the water release and pressure reduction anti-floating scheme with the anti-floating anchor scheme, it has significant advantages in both economy and construction period. The construction method of this technology is simple and has been applied in several projects in Wuhan and other regions. The current operation is good with no adverse hidden dangers. Under the premise of consistent geology and seepage conditions, the larger the basement volume, the greater the advantages in economy and construction period.

VI. Case Summary

(1) Compared with traditional anti-floating schemes, the 'water release and pressure reduction method' has the following advantages in basement anti-floating design:

① Simple concept, easy implementation, and convenient construction;

② Reasonable use can significantly reduce costs and shorten construction period;

③ Since water buoyancy is controlled and reduced, the structural forces on underground buildings are reduced, and certain design optimization can be carried out for basement bottom slabs and side walls;

④ Due to the lowering of groundwater level around underground buildings, the probability of water seepage in the basement is reduced, which helps the waterproofing and seepage resistance of underground buildings;

⑤ It solves the anti-floating problem during the construction period and reduces the probability of basement uplift caused by insufficient consideration of anti-floating during construction in conventional anti-floating methods.
(2) Adopting the water release and pressure reduction anti-floating scheme will incur certain construction costs and later operation and maintenance costs; through market research, the annual operation and maintenance cost is about 2,000 to 3,000 yuan (electricity for pumping), with no other additional costs.
(3) Adopting the water release and pressure reduction scheme, the side wall water release devices leave about 0.5 m long vertical pipes outside the wall, and newly added drainage ditches in some areas may have a minor impact on the appearance and use function of the basement.
(4) The new water release and pressure reduction devices also have certain applicable conditions:
① Applicable to all underground construction projects where basement uplift is caused by perched water;

② Applicable to underground construction projects where there is a confined water layer below the basement bottom slab, but there is a relatively thick impermeable layer between the confined water layer and the basement bottom slab.

③ Applicable to underground construction projects where the underground building is embedded in relatively impermeable soil layers and the base below is a relatively impermeable layer.
In summary, new construction projects can use the 'water release and pressure reduction method' anti-floating scheme while ensuring that the anti-floating performance of underground construction projects meets structural safety requirements, which will create considerable value for the construction unit while achieving the desired anti-floating effect.