1067
Technical Committee 106 /
Comité technique 106
4 EFFECTS OF CLIMATE AND VEGETATION
A consistent number of papers are being published in recent
years presenting attempts to evaluate the consequences of
increasingly severe climatic conditions. This conference is not
an exception, as nearly one third of the papers are broadly
related to issues related to this theme.
Liu & Yasufuku
(Japan) present a self-watering system of
new conception to support superficial vegetation in arid climate.
The basic idea is to bury clayey inclusions in coarser soil to
exploit their retention properties to store water. The proposed
system should be able to regulate the capillary fringe, and
reduce evaporation, in turn helping in preventing from
salinization. A model test, numerical simulation and design
specifications are presented in the contribution. The
performance of the proposed system will depend mostly on
water retention properties, hydraulic conductivity of the soil,
vegetation activity and fresh water availability, but also on the
geometrical configuration of the water trap.
Mitchell
(Australia) investigates expansive soil movements
under climatic impact, for vegetated an non-vegetated areas.
The aim of the study is to evaluate the resilience of existing and
new structures, and to provide useful revision guidelines for
foundation design standards. A simple one-dimensional model
averaged with depth is adopted to calculate heave and
settlement of expansive soil, subjected to the moisture excess or
deficiency predicted for the next half century, summarised by
means of a simple moisture index.
Hemmati & Modaressi
(France) analyse the stability of
slopes under infiltration accounting for vegetation. In their
analysis, performed with the aid of a finite element model, both
infiltration and evapotranspiration are explicitly accounted for.
The latter is described by means of an empirical function giving
the evapotranspiration flux as a function of root density and
depth, based on potential evapotranspiration. The model allows
a preliminary evaluation of the effects of vegetation on slope
stability, and shows that these depend also on the retention
properties of the vegetated soil.
In all previous contributions the effect of vegetation is
accounted for by means of previous literature relationships,
coming either from agronomy or hydrology. Although these
relationships usually prove to be sufficient for a preliminary
description of soil-vegetation-atmosphere interaction, they
disregard the role played by the retention and transport
properties of the soil. The contribution by
Ng et al.
(Hong
Kong) is intended to provide an insight in time evolution of
suction resulting from vegetation activity. A laboratory set up
was designed and manufactured to compare the evolution of
suction and water content in vegetated soil horizon with the
same bare soil. Two different types of vegetation were
investigated, and their effect on energy distribution was
evaluated. The environmental conditions were carefully
controlled, and suction and water content profiles were recorded
at increasing depth. As the soil state in turn affects the root
growth, the root system was characterised after the test. The
results allowed quantifying the effects of the two different types
of vegetation on the soil suction profile, both in terms of
interception and evapo-transpiration.
5 SWELLING, SHRINKAGE AND CRACKING
In fine grained soils, multiphysics processes, starting from soil-
atmosphere interaction, are accompanied by relevant volume
changes, often ending in cracking and degradation. This aspect
of the mechanical behaviour is a common issue of various
applications in geotechnical engineering, including foundations,
liners and mine tailings, to which some of the contributions
presented to this session refer.
Adem & Vanapalli
(Canada) discuss a simple approach for
vertical displacements of expansive soils. The approach is based
on a simple suction-strain relationship, and can be used to
predict vertical displacements promoted by suction changes in a
one-dimensional scheme. The case of a residential site in the
city of Regina, located on a highly expansive clay deposit, is
described to suggest how the model can be applied. Suction
changes profile were calculated by means of a numerical
analysis in which the climatic history during one year was
imposed. As the stiffness is assumed to be a function of the
degree of saturation, hysteretic water retention behaviour will
give different stiffness along drying and wetting path. The latter
feature is confirmed among others by
Ejjaaouani et al
. (Maroc),
who present experimental data of a wetting and drying cycle on
an expansive clay, and discuss possible source of non-reversible
volume change.
Stanciu et al.
(Romania) analyse cement stabilization to
reduce the swelling and shrinkage potential, as a
countermeasure against potential structural damage of structures
founded on expansive clay. Comparing different active clays,
the Authors propose a unified swelling classification chart,
based on grain size distribution, liquid limit, plasticity index and
activity of the clay. With reference to this comprehensive
classification, the beneficial stabilising effect of different
cement types is evaluated.
As a consequence of swelling and shrinkage, cracking may
occur in active soils. The issue is of relevant interest for many
engineering systems, although cracking occurrence and crack
patterns are still difficult to be predicted and characterised.
Auvray et al.
(France) present a device to analyse cracking
evolution as a function of the hydraulic state. The soil
investigated is an active mixture of silt and bentonite. Samples
with a diameter to height ratio of about 5 were prepared by
static compaction and left evaporating in a controlled climatic
chamber. The height and the mass of the samples were
recorded, together with the crack pattern, which was tracked by
photographic imaging. Three samples were analysed during
drying. In spite of similar initial conditions and similar vertical
strain, significant differences in the crack pattern were observed
(Fig. 8). The crack area was found to be higher for the sample
that experienced lower lateral shrinkage, while it decreases at
decreasing lateral constraint. Although the result is consistent,
no reason for different behaviour is evident, as the boundary
conditions were identical for the three samples. Nonetheless, it
can be observed that cracking is a strongly localised mechanical
processes, hence it is dominated by local heterogeneity which
may be responsible for different cracking patterns.
More systematic cracking development and evolution were
observed by
Avila et al
. (Colombia), who specifically designed
the moulds in order to force repeatable crack pattern. The
Authors discuss the stress state in the sample subjected to
drying shrinkage, highlighting the role of boundary conditions
on the overall behaviour of the soil. For the simple geometrical
scheme adopted, the position and the sequence of cracks could
be predicted based on a careful simplified stress analysis.
Figure 8. Surface analysis of the crack pattern upon drying on three
theoretical identical samples (from
Auvray et al.
)
