Chapter 13 — Problem Soils¶
13.1 What makes a soil "problematic"¶
Some soils behave in ways that surprise engineers who treat them as ordinary material. They are problem soils because their behaviour is governed by properties — volume change, collapse, organic content, or liquefaction potential — that standard classification does not capture.
13.2 Expansive (swelling) clays¶
Expansive clays swell when wetted and shrink when dried, causing heave and cracking of foundations, pavements, and structures. They are identified by:
- high plasticity index and shrinkage limit;
- free-swell and swell-pressure tests; and
- mineralogy (montmorillonite).
Mitigation: moisture control, foundation depth below the active zone, replacement with non-expansive fill, chemical stabilization (lime), and membranes to limit moisture change.
13.3 Collapsible soils¶
Collapsible soils — loose silts, loess, and some fills — collapse suddenly when wetted under load, because their open structure is held by weak bonds. They are identified by double oedometer tests (comparing natural and saturated compression) and by in-situ collapse potential tests.
Mitigation: pre-wetting, dynamic compaction, grouting, deep foundations to bypass the collapsible layer, and moisture control.
13.4 Organic soils and peat¶
Organic soils and peat are highly compressible, weak, and prone to large primary and secondary settlement. They have:
- high water content and void ratio;
- high compressibility and secondary compression;
- low strength and stiffness.
Mitigation: excavation and replacement, preloading with or without vertical drains, deep foundations, and lightweight fill.
13.5 Liquefiable soils¶
Liquefaction is the loss of strength in loose, saturated sands under cyclic loading (earthquake or vibration), when pore pressure rises to equal total stress. It is assessed by comparing the cyclic stress ratio with the cyclic resistance ratio from SPT/CPT.
Mitigation: densification (vibro, dynamic compaction), drainage (stone columns, drains), cementation (grouting), and reinforcement (piles, micropiles).
13.6 Other problem soils¶
- Sensitive clays — lose strength dramatically when remoulded.
- Dispersive clays — erode readily in water; a piping risk.
- Fills — highly variable; must be characterised, not assumed.
- Frozen ground — heave and thaw settlement.
13.7 Key takeaways¶
- Problem soils behave outside standard classification.
- Expansive clays heave/shrink — control moisture and depth.
- Collapsible soils collapse on wetting — pre-wet, densify, or bypass.
- Organic soils settle greatly — excavate, preload, or use deep foundations.
- Liquefiable sands lose strength under cyclic loading — densify or drain.