Discussion on Tunis soft soil sensitivity

What is it about?

In Tunis City, the sensitivity of the marine deposits at shallow depth (z = 0–20 m) varies significantly.Theinfluenceoftheprocessoftheleachingout of Tunis soft soil on its geotechnical parameters is a focalpointinthisresearch.Thisprocessleadsfinallyto moderate levels of sensitivity for Tunis clays since it appears to happen in a two steps with increasing sensitivity. The ‘‘hard water’’ leaching out and the dispersive action of organic matter (humus) lead unexpectedly to higher but still moderate level of sensitivity as measured on many Tunis sites. These sensitivity variations result from the combination of leaching out with hard ground water and high content of organic matter. This sensitivity attracted our attention and remains of high interest for the study of the behaviour of the Tunis soft clay. It is an experimental word conducted on site and in laboratory including both in situ and laboratory tests in view correlations proposal.

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Why is it important?

The marine environment deposition of Tunis soft soil during the quaternary age and the abundance of non swelling minerals (quartz) led to a low electro-kinetic potential resulting in flocculated structure. Tunis soft soil leached out, however, afterwards with ‘‘hard’’ ground water (cations Ca2?) leading finally to pore water with a high concentration of polyvalent ions and therefore with limited possibilities of highly sensitive clay development. At locations where Tunis soft soil was in contact with active organic matter (acting in dispersive, bonding cations at clay particle surface thus increasing negative charges at the particle surface), the double layers around the particles were expanding. The electrokinetic potential became higher, implying larger repulsive forces between particles. After remoulding of such type of soft soil, these forces will prevent flocculation of the clay particles. This reduces remarkably the remoulded shear strength and therefore greatly increases the sensitivity of the clay leading to high sensitivity of many of Tunis soft soils. The levels and the origin of high sensitivity in Tunis soft soils were pointed out. These variations result from a combination of leaching out with hard ground water and high content of organic matter. It is recommended to focus more on the latter aspect especially when discussing the stiffness behaviour of these clays. The relationship between the undrained cohesion of Tunis Soft Clay (TSC) and identification parameters is discussed.

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