Page 239 - Analysis and Design of Energy Geostructures

214 Analysis and Design of Energy Geostructures

(Di Donna and Laloui, 2013). When soils are subjected to a stress that is lower than
the preconsolidation stress, they are characterised by a small and reversible deforma-
tion. In contrast, when soils are subjected to a stress that becomes the preconsolidation
stress, they are characterised by a significant and partially irreversible deformation.
The preconsolidation stress corresponds to the maximum experienced density or the
lowest void ratio (Di Donna and Laloui, 2013). It also represents the limit between a
reversible and a partly irreversible mechanical behaviour of soils.

5.3.4 Overconsolidation ratio
The overconsolidation ratio (OCR) allows characterising the stress history of soils.
0
This parameter can be computed from the preconsolidation stress, σ , and the over-
p
0
burden vertical effective stress for a given soil, σ ,as
z
σ 0
OCR 5 p ð5:2Þ
σ 0
z
In principle, values of OCR , 1 are associated with a underconsolidated state for
the soil. A value of OCR 5 1 is associated with a normally consolidated (NC) state
for the soil. Values of OCR . 1 are associated with an overconsolidated (OC) state for
the soil.
In practice, soils are considered NC if the OCR is within the range of 1 2, that is
if the current stress state is close to the maximum that the soil has ever supported,
while they are considered OC if the OCR is greater than 2, that is if the current stress
state is lower than the maximum that the soil has ever supported.
Underconsolidation can result from processes including, for example deposition at
a faster rate than consolidation, rapid drop in the groundwater table, insufficient time
since the placement of a fill or other loading for consolidation to be completed, and
disturbance that causes a material structure breakdown and decrease in effective stress
(Mitchell and Soga, 2005). Normal consolidation can result from significant loading
processes leading to effective stress equilibrium with the overburden effective stress.
Overconsolidation can result from unloading phenomena caused, for example by the
movement of the groundwater table, mechanical unloading, ageing, desiccation or
physicochemical interactions (Lancellotta, 1995).

5.3.5 Relative density

The relative density can be considered as a proxy to characterise the stress history of
coarse-grained soils. The considered parameter crucially characterises the mechanical
behaviour of coarse-grained materials together with the effective stress state. It is a