Geological description
Soils from the Holocene period, unconsolidated, flowing muds, black in colour with a very strong organic component, sometimes in the form of pure rotting vegetable matter (peat bogs). The mud compresses by 50% of its volume under a footstep. Extreme coastal areas (Ebro Delta, Valencian Albufera, marshes of the Odiel, rías of Huelva/Cádiz).
Foundation ?
Inviable with any natural means. Drive tests of enormous length (> 25–40 m in friction) or improvement of the soil block through rigid inclusions, drain wicks, multi-year preloading, or compacted gravel columns.
Excavatability ?
The machines get swallowed up. It requires working platforms (geotextile mats and one metre of hardcore) only so that the machines do not physically sink into the mud when operating.
Settlements ?
Exceptional (metric). A metre of roadbase on the surface may subside by half a metre, dragging the subsoil down to the bottom in the first week (extremely high lateral flow and primary creep).
Water table ?
Typically, the groundwater table is at an elevation of +0.0 m or at sea level, with corrosive salinity that destroys metals.
Seismic risk ?
Organic soils and muds greatly amplify seismic waves due to their extreme softness. They are the most unfavourable soils in the event of earthquakes: high amplification, long duration of shaking, and risk of liquefaction in interbedded sandy layers. The foundation must necessarily bear on underlying firm strata by means of piling.
Construction advantages
- Plots usually located in highly attractive coastal areas (deltas, beaches).
Site limitations
- Worst possible scenario: the soil has the consistency of honey.
- Catastrophic settlement. A small layer of concrete can sink by half a metre.
- This implies friction piling of more than 20 metres of depth or a costly soil improvement.
- High chemical aggressiveness towards concrete and steel due to organic matter and decomposition salts.
Where this soil is found
Alerts
Hazard- NEGATIVE SKIN FRICTION: The piles will suffer extreme negative skin friction: the mud sinks and pulls the pile downwards towards the base; the reinforcement must be over-stressed or the concrete shaft must be sleeved with bituminous materials.
- Take care of saline corrosivity in flood-prone coastal areas; the concrete cover must obligatorily exceed 5-7 cm.
- Nauseating odour during drilling where there is organic H2S; mandatory monitoring due to the risk to workers in foundation trenches.