Soft clay and compressible soil: controlling settlement
Soft clays and silts cannot be densified by impact or vibration. Under load they settle slowly and by large amounts unless the ground is reinforced or the load is carried deeper.
How it is recognised
- Low undrained shear strength in clay or silt layers.
- High compressibility in laboratory consolidation tests.
- High groundwater and slow drainage.
Why it matters
Long-term settlement can continue for years after construction, and bearing failure is possible under tanks, embankments and stockpiles.
Methods usually considered
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VR
Vibro Replacement (Stone Columns)
Soft-to-firm soils that can support stone columns; also speeds drainage.
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Rigid Inclusions
Very soft soils, or where settlement limits are strict.
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DR
Dynamic Replacement
Soft layers near the surface over large areas.
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Bored Piles
High structural loads carried to a deeper bearing stratum.
Which method fits depends on the site investigation and the performance requirement. More than one may be combined.
How the choice is made
- Strength and thickness of the soft layer.
- Type of structure and its settlement tolerance.
- Depth to a competent layer.
- Programme: how soon the structure must be loaded.
How the result is verified
Reinforcement is verified by load tests on columns, inclusions or piles and, where specified, by settlement monitoring.
Tests typically used
Other ground problems
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