Analysis of Surface Water Anti-Floating Issues in Underground Structures

With the rapid development of urban construction, the utilization of underground space has gained increasing attention. Consequently, the anti-floating issues in underground engineering have become more prominent. When building structures have relatively light self-weight, are deeply embedded, and are located in areas with high groundwater levels, insufficient anti-floating measures can lead to buoyancy exceeding the sum of self-weight and anti-floating bearing capacity. This results in overall anti-floating stability failure, and cases of damage caused by basement uplift are not uncommon. To ensure structural stability and normal functionality, the anti-floating problem in basements has drawn significant attention from engineers.

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01 Surface Water Anti-Floating Issues Are Often Overlooked

The rise in groundwater levels can be attributed to two main factors:

1. Seepage of groundwater from surrounding areas.
2. Inflow of surface water.

Designers typically focus on determining the anti-floating design water level in geotechnical reports while overlooking the impact of surface water inflow. Why is this the case?

• Geotechnical Engineers: After excavation, monitoring groundwater levels affected by surface water inflow becomes difficult, so construction-phase anti-floating water levels are rarely mentioned.
• Designers & Reviewers: They assume construction-phase anti-floating measures are temporary and should be handled by contractors, focusing only on normal-use anti-floating design.
• Construction Teams: They may lack awareness of surface water effects. If the basement foundation is impermeable rock with tight surrounding support, they might rely solely on design specifications without additional anti-floating measures.

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02 What Is a “Fat Trench”?

Before discussing construction-phase anti-floating issues, we must first understand a common construction term—fat trench.

A fat trench refers to the space between the foundation pit wall and the underground exterior wall or pile caps, essentially an extra excavation area for construction work. Typically, fat trenches are 0.5–1.2m wide. Since they are exposed, they are prone to water accumulation. Therefore, drainage ditches and sump pits are often installed at the bottom to collect and discharge water.

After completing underground structural construction, the fat trench is backfilled to the natural ground elevation.

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03 What Is the “Water Basin Effect”?

How does surface water inflow into the fault trench affect underground structures? To explain this, we must understand the water basin effect.

As previously mentioned, before backfilling the fat trench, heavy rainfall can cause surface water to flow into the foundation pit, accumulating beneath the basement slab. The pit acts as a large water basin, while the basement becomes a smaller basin submerged within it. If the resulting buoyancy exceeds the combined resistance of the basement’s self-weight and anti-floating measures, structural damage occurs—this is the water basin effect.

Key Consideration:
When using rented materials like SMW (Soil Mixing Wall) methods for pit support, developers often rush to backfill the fat trench and remove steel beams. However, improper backfill material selection or inadequate compaction can still allow water infiltration, leading to the water basin effect even after backfilling!

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04 How to Prevent the Water Basin Effect?

To avoid the water basin effect, strict control in the following aspects is crucial:

1. Determine Construction-Phase Anti-Floating Water Level

• Consider both original hydrogeological conditions and surface water inflow post-excavation.
• Verify anti-floating stability using actual construction loads (excluding backfill soil).
• Refer to JGJ 476-2019 Technical Standard for Anti-Floating of Building Structures (Section 5.3.2):
  

  “During construction, factors such as possible water accumulation, changes in groundwater recharge/discharge, and incomplete structural loading must be considered.”

2. Implement Anti-Floating Measures When Necessary

• Use drainage and pressure-limiting methods (e.g., sump wells, well-point dewatering, deep well dewatering).
• Follow JGJ 476-2019 requirements:
  

  “Groundwater levels must remain ≥1.0m below the basement slab, with fluctuations ≤0.5m.”

• Critical: Avoid stopping dewatering before backfill completion—many anti-floating failures occur due to premature dewatering cessation.

3. Optimize Fat Trench Backfill Materials

• Avoid permeable materials (e.g., gravel). Instead, use:
  • Compacted clay, lime soil.
  • Flowable fill, plain concrete, or rubble concrete for impermeability.
• Standards for reference:
  • JGJ 3-2010 (High-Rise Building Concrete Structures): Recommends graded backfill for lateral support.
  • GB 50007-2011 (Foundation Design): Requires compaction (≥0.94 density).
  • JGJ 476-2019: Emphasizes impermeable backfill (clay, flowable fill, concrete).

Cost Consideration:

Material	Relative Cost
Plain Soil	Low
Lime Soil	Moderate
Flowable Fill	High
Plain Concrete	Very High

Given large backfill volumes, material choice significantly impacts project costs (differences can exceed millions).

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05 Key Recommendations

Engineering practice shows that surface water-induced anti-floating issues require proactive measures beyond theoretical design. We recommend:

1. Surface Sealing: Install low-permeability concrete strips (≥1m beyond fat trench edges).
2. Drainage Systems: Use seepage wells, blind drains, and discharge ditches for organized drainage.
3. Base Preparation: Avoid permeable subgrade materials; backfill over-excavations with concrete/flowable fill.
4. Utility Protection: Waterproof pipe joints, trenches, and culverts to prevent leaks.

Design managers must bridge design-construction gaps, eliminate blind spots, and prioritize safety to ensure cost-effective and reliable project outcomes.