foundation system of burj al arab

Behavior of Expansive Soil under Raft Foundation

Expansive soil shows volume changes upon change in moisture content in soil and movement is found to dominate in vertical direction. Plastic cohesive soils like CH are most susceptible to suffer volume change which is enriched with montmorillonite.

Lean clays like CL can also result structure damage when moisture content in soil is sufficiently large. These types of soils swell during wet season and shrink during dry seasons. 


These soils have an inherent fissure system that influences extent of volume changes within a season or a time frame depending on construction period or part of it. Numerous fissure permits run-off water to penetrate deep into soil strata. Desiccated zones increase depth of active volume change of soil, Za whereas fewer fissures prohibits flow of water which limits depth of penetration; thus depth of volume change within a season is also reduced. To analyze foundation performance due to soil movement, it is characterized depending on deformation mode

• Center lift

• Edge lift


Center lift:


In this type of movement mat is deformed upward at center relative to edge. This is due to increase in water content of soil and heave at center relative to foundation perimeter or reduction in moisture content and subsequent shrinkage near perimeter as compared to center. 


When foundation is constructed on ground surface, it prevents evaporation from ground surface; thus eliminate moisture loss due to transpiration from existing vegetation. The moisture content of soil, therefore, is increased, especially at center of mat foundation.

At the central portion of soil under mat, environmental variations have least effect. Soil outside or near perimeter have tendency to dry out during dry season which results settlement at edge relative to center.

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