What are the main causes of insufficient bearing capacity of retaining wall foundations?1. Poor foundation soil quality: The strength and stability of the foundation soil are important factors affecting the bearing capacity of the retaining wall. If the foundation soil quality is poor, it can easily lead to insufficient bearing capacity of the retaining wall. For example, soft clay (natural moisture content > 50%), mucky soil (void ratio > 1.5), loose sand (relative density < 0.3), etc., have low natural shear strength (the cohesion of soft clay is often < 10 kPa) and high compressibility, which directly leads to insufficient bearing capacity.2. Foundation settlement:Foundation settlement refers to the deformation and subsidence of the foundation soil under load. If the foundation settlement is too large, it will lead to insufficient bearing capacity of the retaining wall. (In engineering practice, if settlement of the retaining wall is observed (especially differential settlement), it can usually be regarded as one of the core bases for judging insufficient foundation bearing capacity.)
3. Excessive moisture content in the foundation:Excessive moisture content in the foundation will reduce the strength of the foundation soil, thereby affecting the bearing capacity of the retaining wall.4. Unreasonable foundation design:Unreasonable foundation design is also one of the causes of insufficient bearing capacity of the retaining wall.- For deep soft soil foundations (thickness > 5 m), shallow foundations (such as strip footings) are still used, and deep foundations such as pile foundations or caissons are not adopted, so the load cannot be transferred to the hard soil layer.
- In seasonally frozen soil areas, frost heave force is not considered, and the foundation embedment depth is less than the frost depth. Frost heave in winter disturbs the foundation soil, and after thawing and settlement in spring, the bearing capacity drops sharply.
- For expansive soil foundations, no anti-seepage measures or flexible foundations are provided. The repeated action of soil swelling upon water absorption and shrinkage upon water loss damages the integrity of the foundation and reduces the bearing capacity.
5.Dynamic load disturbance: Vibration generated by construction in adjacent sites (such as pile driving and blasting) leads to sand liquefaction (a sudden increase in pore water pressure and zero effective stress) or thixotropy of soft clay (instant loss of strength), causing a sharp decline in foundation bearing capacity.6. Construction quality defects:Uncontrolled foundation treatment or foundation construction.Disturbance of the foundation soil, inadequate treatment, or poor foundation construction quality during construction will directly weaken the actual bearing capacity of the foundation.Measures to solve insufficient foundation bearing capacity
Briefly describe 4 measures:
1. Reinforce the foundation bodyReinforcing the foundation soil is one of the effective measures to solve insufficient foundation bearing capacity. There are many methods for reinforcing the foundation soil, such as soil reinforcement, foundation reinforcement, and foundation base reinforcement.The core logic of reinforcing the foundation body is to improve the properties of the foundation soil (strength, permeability, compressibility, etc.) through physical, chemical, or mechanical means, or to adjust the load transfer path, thereby directly increasing the foundation bearing capacity and fundamentally solving problems such as settlement and sliding of the retaining wall caused by insufficient foundation capacity. However, it should be noted that the reinforcement scheme must be comprehensively designed based on the foundation soil type, the load-bearing characteristics of the retaining wall, and site conditions, and when necessary, it must be combined with structural reinforcement of the retaining wall (such as adding anchor rods or counterforts) to ensure overall stability.
2. Use reinforcing materialsUsing reinforcing materials is also one of the effective measures to solve insufficient foundation bearing capacity. Reinforcing materials can increase the strength and stability of the foundation soil, thereby improving the bearing capacity of the retaining wall. Commonly used reinforcing materials include steel bars, steel plates, steel wire mesh, etc. Note: material selection must match the properties of the foundation soil (such as soft soil, sand, expansive soil), the load magnitude of the retaining wall, and construction conditions. For example, for soft soil foundations, curing agents + deep mixing technology should be preferred, and for loose sand, reinforced materials + compaction technology should be preferred, so as to achieve the optimal reinforcement effect. At the same time, material reinforcement usuallyneeds to be combined with measures such as drainage and anti-seepage to avoid reinforcement failure later due to water damage.3. Change the retaining wall structureChanging the retaining wall structure is also one of the effective measures to solve insufficient foundation bearing capacity. Changing the retaining wall structure can reduce the load on the retaining wall, thereby lowering the requirements for foundation bearing capacity. Commonly used methods for changing the retaining wall structure include increasing the width of the retaining wall and increasing the height of the retaining wall.- When changing the retaining wall structure, avoid "blindly increasing dimensions": for example, "increasing height" will lead to more backfill soil behind the wall and greater earth pressure, which instead increases the burden on the foundation and is an unreasonable adjustment; while "increasing width" must be combined with the scientific rationality of foundation extension (for example, whether the extension exceeds the stable slope angle and whether it causes new soil sliding).
- Structural adjustment must be coordinated with foundation reinforcement measures: for example, although reinforced earth retaining walls are light in self-weight, shallow compaction or a geogrid cushion layer still needs to be provided for the foundation at the wall base to ensure that the foundation can bear the distributed stress.
4. Strengthen foundation designStrengthening foundation design is also one of the effective measures to solve insufficient foundation bearing capacity. Strengthening foundation design can improve the requirements for foundation bearing capacity, thereby ensuring the stability and safety of the retaining wall. Commonly used methods for strengthening foundation design include increasing the area of the foundation and increasing the depth of the foundation. The core of strengthening foundation design is "precise matching": clarifying foundation characteristics through detailed investigation -> selecting an appropriate foundation type according to the load (shallow foundation / deep foundation / composite foundation) -> optimizing dimensions and embedment depth to distribute the load -> adopting special measures for special foundations (soft soil, frozen soil, expansive soil) -> performing refined calculations to ensure safety.
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