SWELLING OF ROCK AND ITS INFLUENCE ON UNDERGROUND STRUCTURE
Main researcher
: VRKLJAN, IVAN (93393) Assistants
Type of research: applied Duration from: 06/30/92. to 12/31/95. Papers on project (total): 7
Institution name: Institut građevinarstva Hrvatske, Zagreb (110) Department/Institute: GEOTECHNICAL DEPARTEMENT Address: Janka Rakuše 1,p.p.283 City: 10000 - Zagreb, Croatia
Communication
Phone: 385 (0)1 6136-444
Fax: 385 (0)1 534-737
Summary: Abstract: The mechanical behavior of swelling materials may
be strongly influenced by the physicochemical interaction between the pore
fluid and mineral phase. The behavior of swelling clay minerals can not be
discussed without the thorough understanding of physicochemical forces
acting among the mineral phase particles. The action of including
physicochemical effects significantly complicates attempts to define
behavior of swelling materials. In case of swelling clay minerals, two
basic principles of the traditional soil mechanics, i.e. the concept of
effective stresses and Darcy's law, are called into question. The
influence of some chemical-load variables on the swelling, compressibility
and strength is analyzed through laboratory testing of the bentonite clay
"Gornja Jelenska". The swelling of rocks is a serious problem during
realization and subsequent use of structures built either in or on
swelling-rock formations. A significant amount of stress due to rock
swelling must be added to the stress notation known in traditional
geotechnical practice. A survey of numerous procedures used for analyzing
stress around underground structures is presented. Practical results
obtained through application of some of these procedures are evaluated.
Research goals: OBJECTIVES AND RESULTS The research objectives are:
(a) contribute to all efforts aimed at fully defining very complex rock and
soil swelling mechanisms; (b) critically analyze the existing procedures
for modeling behavior of swelling rocks surrounding underground structures.
The swelling is an atypical geotechnical problem requiring an
interdisciplinary approach. The swelling phenomenon may be defined as a
time-dependent increase in volume caused by physicochemical interaction
between the solid (mineral) phase and the pore fluid. Swelling materials
are usually tested in two stages: (a) swelling potential testing; (b)
swelling amount testing. The swelling potential describes the relative
capacity of a material to swell under particular conditions, while the
swelling amount shows the level to which the swelling potential will rise.
Various procedures are used in practice for defining the swelling amount
and potential. All of them are burdened by several basic problems: (a) the
problem of preparing undisturbed samples, due to extremely high sensitivity
of these rocks to the change in humidity; (b) mechanical behavior of
swelling materials does not depend only on mechanical, but also on chemical
influences and temperature. In this way, the number of variables in the
behavior pattern increases considerably. Taking into account the influence
of physicochemical forces on the behavior of swelling clay minerals, the
acceptability of two basic soil mechanics principles (the effective stress
concept and Darcy's law) may be questioned; (c) the swelling process lasts
a long time due to the very low permeability. The influence of various
elements of external load (environmental loading) on the mechanical
behavior will be analyzed through various types of laboratory testing on
natural swelling clays (bentonite clays from Gornja Jelenska). In order to
accomplish this task, it will be necessary to adapt the existing laboratory
equipment to new requirements and to fabricate equipment presently
unavailable in the laboratory. According to current plans, the following
equipment will be fabricated: equipment for measuring the axial swelling
stress as related to the axial swelling strain, the equipment for measuring
the swelling pressure and the equipment for free-swelling testing. The
following results are expected: (a) A survey of information on various
swelling-rock mechanisms. A comprehensive overview of laboratory
procedures most commonly used for testing swelling amount and potential of
rocks and soil; (b) Recommendations for treating undisturbed bentonite-clay
samples during their preparation for laboratory testing; (c) Defining
mechanical behavior of treated bentonite clays. The untreated clay with
original variable cations (Ca-bentonites), as well as the treated clay in
which Ca-cations are replaced by Na-cations, will be tested. The process
of testing these clays under various environmental loading conditions will
facilitate evaluating the contribution of physicochemical forces to basic
geotechnical properties (swelling, compressibility, strength); (d) Critical
analysis of the existing procedures for modeling behavior of swelling rocks
around underground structures. Other information about the project.