Soft foundations have excessive natural water content and low bearing capacity, and are prone to sliding or consolidation settlement under loads. How to handle them is a challenge. What should be done?
Common Issues
Stability Problems
(1) Stability of fill and slopes. In the case of upper fill, when the foundation soil strength of embankments or sea dikes built on soft clay is insufficient, circular sliding can occur, causing overall shear failure. Even if overall shear failure does not occur, excessive lateral displacement and resulting additional settlement can cause local shear failure and affect the normal use of the road (sea) embankment. Slopes formed by excavation also have stability issues.
(2) Foundation bearing capacity problems.
(3) Earth pressure problems on retaining walls, sheet piles, etc.
(4) Horizontal tension problems of piles (during earthquakes or under horizontal forces). When piles pass through soft layers and act on the bearing stratum, they are bearing piles; their vertical bearing capacity is not much related to the soft soil layer they pass through. However, when subjected to horizontal forces, due to the low strength of the soft soil between piles, the piles cannot resist the bending moment caused by horizontal forces and may break. In this case, improvement of the soil between piles should also be considered.
Settlement Problems
Excessive settlement of foundation soil can cause negative friction on piles, leading to damage to the pile foundation or superstructure. In addition to settlement caused by upper loads, lowering of groundwater level can also cause settlement. Settlement caused by shear deformation of the foundation has been mentioned in the stability problems. Special emphasis should be placed on the relationship between total settlement and differential settlement. When total settlement is large, differential settlement is inevitably large, because the building itself, site conditions, and environmental loads cannot be completely symmetrical.
Liquefaction Problems
Under dynamic loads (earthquake forces, blasting, machinery, vehicles, waves, etc.), the pore water pressure in saturated loose sand increases, and effective stress decreases. When effective stress reaches zero, the sand behaves like a liquid. At this time, light structures such as pipelines (oil, gas, water pipes) will float up, and heavy structures will sink. Highway and railway subgrades, subjected to vehicle vibration loads, experience increased pore water pressure and decreased strength, leading to mud pumping and ground subsidence.
Seepage Problems
Quicksand and piping, etc. (in water conservancy, foundation pits, manually excavated piles, etc.).
Common Soft Foundation Treatment Methods
Rolling and Compaction
Rolling and compaction methods include hammer compaction, mechanical rolling, vibratory compaction, dynamic compaction (dynamic consolidation), etc.
Principle and function: Utilizing the principle of compaction, the surface soil is compacted by mechanical rolling and tamping. Dynamic compaction uses powerful tamping energy to generate strong shock waves and dynamic stresses in the foundation, forcing the soil to consolidate and densify.
Applicable scope: Suitable for gravel, sand, silt, low-saturation cohesive soil, miscellaneous fill, etc.
Replacement and Cushion Layer
Replacement and cushion layer can be specifically divided into: sand-gravel cushion, plain soil cushion, lime-soil cushion, slag cushion, etc.
Principle and function: Using higher-strength materials such as sand-gravel, plain soil, lime-soil, and slag to replace the weak surface soil of the foundation, improve the bearing capacity of the bearing layer, spread stresses, and reduce settlement.
Applicable scope: Suitable for treating weak soil foundations such as hidden ditches and hidden ponds.
Drainage Consolidation
Drainage consolidation can be specifically divided into: natural foundation preloading, sand well preloading, plastic drainage strip preloading, vacuum preloading, dewatering preloading.
Principle and function: Install vertical drainage bodies in the foundation to accelerate consolidation and strength growth, improve foundation stability, accelerate settlement development, and complete foundation settlement in advance.
Applicable scope: Suitable for treating saturated soft soil layers; for peat soil with extremely low permeability, it must be treated with caution.
Vibro-Densification and Compaction
Vibro-densification and compaction can be specifically divided into: vibroflotation densification, lime-soil compaction piles, sand-gravel piles, lime piles, blasting densification.
Principle and function: Adopt certain technical measures, through vibration or compaction, to reduce soil porosity and increase strength. If necessary, during the vibration and compaction process, backfill sand, gravel, lime-soil, plain soil, etc., to form a composite foundation with the foundation soil, thereby improving the bearing capacity and reducing settlement.
Applicable scope: Suitable for treating loose sand, silt, miscellaneous fill, and collapsible loess.
Replacement and Mixing
Replacement and mixing can be specifically divided into: vibro-replacement, deep mixing, high-pressure jet grouting, lime piles, etc.
Principle and function: Adopt special technical measures to replace part of the weak soil in the soft foundation with sand, gravel, etc., or mix cement, lime, or mortar into part of the soft foundation to form reinforced bodies, which together with the untreated soil form a composite foundation, thereby improving the bearing capacity and reducing settlement.
Applicable scope: Cohesive soil, dredged fill, silty sand, fine sand, etc. When the drained shear strength is 20 kPa, use vibro-replacement with caution.
Reinforcement
Reinforcement can be specifically divided into: geosynthetic reinforcement, anchoring, root piles, reinforced soil.
Principle and function: Embed high-strength geosynthetics, steel strips, and other reinforcing materials in the foundation soil to enable the soil to withstand tensile forces, prevent fracture, maintain integrity, improve stiffness, change the stress and strain fields of the foundation soil, thereby improving the bearing capacity and deformation characteristics of the foundation.
Applicable scope: Soft foundations, fill and high fill, sand.
Others
Other treatment methods include grouting, freezing, underpinning technology, deviation correction technology, etc.
Principle and function: Treat soft soil foundations through unique technical measures.
Applicable scope: Determined according to actual conditions. There are many foundation treatment methods, each with its own applicable scope, limitations, advantages, and disadvantages. No method is universal.