3091
Technical Committee 301 /
Comité technique 301
S211
Tension crack
Duomo
street
Slip surface
Root piles group
Root piles group
RCB
RCB Reinforced concrete beam
(*) Elevation m a.m.s.l.
GWT
Buttress
(*)
Figure 9. The Agrigento Cathedral, with an indication of subsoil
stratigraphy and the supposed slope sliding mechanism (dotted line)
(Valore and Ziccarelli 2013)
.
started to suffer differential settlements and structural problems
which led to a number of modifications from the 14
th
to the 17
th
century. In a completely unsuccessful attempt to solve the
problems, in the period 1976-1980 a large underpinning
intervention with root piles was carried out.
The paper is a nice example of the correct geotechnical
approach to the preservation of monuments, and is paradigmatic
in the sense that shows how useless – or even detrimental – an
intervention without a clear understanding of the mechanisms
do be faced can be. Underpinning had no positive effects
because it was conceived assuming that the settlements were
due to the high deformability of the upper soils.
By simply monitoring with inclinometers the slope,
Valore
and Ziccarelli (2013)
demonstrate that an active and extremely
slowly evolving shear surface exists, mostly developing in a
clayey soil stratum (AGG in Figure 9). This slope movement is
consistent with the observed pattern of fissures, and can be
considered responsible of the observed displacements. Back
calculations of the slope safety factor indicate that the safety
margins are actually low and reduce as the displacements
increase and the shear strength tends to its residual value, for
which global equilibrium would not be granted.
Monitoring, careful characterization of the subsoil and of the
superstructure and a sound mechanical interpretation of the
observed mechanisms are indeed the only tools geotechnical
engineers have in their hands to tackle problems as the one of
the Agrigento Cathedral.
It is also worth pointing out that in this case a timely correct
interpretation of the observed settlements would have led to
interventions aimed to stabilize the slope more than to underpin
the structure, eventually contributing to preserve the overall
integrity of the structure.
3 CASE STUDIES
3.1
Characterization of problematic soils
Soil characterization is an essential part of the activity of
geotechnical engineers, and the success in design or in the
interpretation of mechanisms often depends more on it than on
the calculation methods adopted. Therefore, the topic never
ends to be of great interest. This subsession includes three
papers dealing with soil characterization, with reference to some
peculiar cases.
In their paper,
Hawkins and John (2013)
report on the
behaviour of the very well known London Clay, looking at it
from an unusual point of view as they investigate the chemical
properties of the unsaturated/seasonally aerated zone of the
formation. The London Clay Formation is a silty clay deposit
which, in its upper part, is weathered and mostly aerated, thus
getting a typical brown colour. The deeper part of the deposit,
saturated, is grey. In the transition (mottled) zone among the
brown and the grey London Clay, peculiar chemical conditions
exist, often with the presence of enriched acid soluble sulphate
with a corresponding low pH. As well known, this is a rather
aggressive environment for buried structures and foundations,
and may lead to their deterioration, as demonstrated for instance
by the case history of the St. Helier Hospital in Surrey.
Hawkins
and John (2013)
have monitored the sulphate content in the soil
during the construction of an underground car park at different
depths, confirming that the upper brown London Clay has a
certain amount of sulphate, whose largest values correspond to
the brown-grey mottled transition zone. The authors argue that
the heat generated by concrete hydration may enhance the
formation and mobilization of sulphates, and appropriate
countermeasures should be taken to protect concrete from its
attack in underground construction activities.
In their paper,
Puppala et al. (2013)
focus on the heave
mechanisms occurring in high sulphate soils after the addition
with calcium based stabilisers (lime and/or cement), as a
consequence of the formation of two expansive compounds
(namely ettringite and thaumasite). The authors show several
experimental results on two high sulphate soils treated by
hydrated lime, highlighting the advantages of the mellowing
technique - firstly proposed by Harris
et al.
(2004) - in reducing
the swelling behaviour due to the expansive compounds
formation. The role played by factors such as soil mineralogy
and treatment parameters are clearly discussed in the paper.
A large scale geotechnical and geological soil
characterization project for the city of Punta Arenas, in the
Chilean Patagonia, is described by
Vasquez et al (2013)
. The
urban expansion of Punta Arena has posed a number of
geotechnical problems, as soft and complex soils are spread out
in the area.
Vasquez et al (2013)
propose a classification of the
city area in different zones having homogeneous properties and
characteristics. Even though specific investigations are always
necessary and large scale classifications cannot give design
parameters at the scale of the single structure, an overall picture
of the subsoil characteristics and the problems they pose may be
of great help in planning urban development.
3.2
Geotechnical design in problematic soils
The papers belonging to this subsection report design
considerations or experimental results mostly with reference to
new geotechnical structures. Some of the papers refer to
peculiar regional problems, some to specific projects, and some
to specific structures. In most of them, reference is made to
codes, either existing or missing, as they represent some of the
design constraints.
A paper analysing a specific geotechnical structure is the one
by
Hoffmann et al. (2013)
, in which the authors deal with the
very interesting case of embankments conceived as rock-fall
protection elements. Such structures may be more convenient
than rock-fall protection nets, because they may absorb larger
impact energies and usually present advantages in terms of
longevity and construction cost. The paper briefly describes the
results of a large number of 1g small scale experimental tests
(Figure 10) carried out on model embankments having different
characteristics (unreinforced, reinforced with geotextiles, or
unreinforced with a rip-rap up-hill facing).
The tests allowed to find interesting correlations between the
geometry of the embankment, its characteristic and the energy
and position of the impact. Based on the experimental
observations, the authors give geometric indications to avoid the
