بررسی عملکرد پوشش بتنی کانالهای آبیاری با توجه به خصوصیات بستر محل

Stability of concrete linings of irrigation canals network is major challenges facing the
national irrigation industry. A vertical rise phenomenon of expansive soils due to swelling
pressure occur instability and cracks in the linings. Subgrade materials that exhibit volume
change from change in soil moisture are referred to as expansive or swelling clay soils.
Characteristic expansive or swelling materials are highly plastic clays and clay shales that often
contain colloidal clay minerals such as the montmorillonites. The differential movement caused
by swell or shrinkage of expansive soils can increase the probability of damage to the foundation
and superstructure. Differential rather than total movements of the subgrade soils are generally
responsible for the major structural damage of concrete linings of irrigation canals. Differential
movements redistribute the structural loads causing concentration of loads on portions of the
subgrade and large changes in moments and shear forces in the structure not previously
accounted for in standard design practice. Damages can occur within a few months following
construction, may develop slowly over a period of about ۵ years, or may not appear for many
years until some activity occurs to disturb the soil moisture. The probability of damages
increases for structures on swelling foundation soils if the climate and other field environment,
effects of construction, and effects of occupancy tend to promote moisture changes in the soil.
The magnitude of damages to structures can be extensive, impair the usefulness of the
structure, and detract aesthetically from the environment. Maintenance and repair requirements
can be extensive, and the expenses can grossly exceed the original cost of the subgrade.
Among many factors in placing a quality concrete lining in irrigation canals, the subgrade is
an important one often overlooked. The subgrade has a substantial impact on base and
subsurface drainage requirements, as well as long-term linings in irrigation canals ride quality
and overall performance. However, subgrade stability and strength are often misunderstood as
part of design and construction requirements. Concrete linings, unlike asphalt pavements,
distribute loads through slab action. In other words, the load is spread over a large area. This
means that a concrete slab is able to bridge an isolated weak area in the subgrade. However, the
bridging effect varies depending on the location of a weak spot. Lack of uniform support at slab
corners and edges is much more likely to result in deterioration of the linings than if the weak
area is under the slab center. Several options are available to produce a uniform subgrade
including soil modifying or stabilizing, removing and replacing, and others. The most desirable
option is one that minimizes cost while providing improved constructability and performance.
In this research project, physical and mechanical behaviors of dispersive and expansive soils,
their side effects over irrigation canals were studied. Also, stabilization and treatment methods
of these problematic soils were investigated in laboratory. Besides, laboratory test method has
been developed to simulate vertical movement of an expansive soil. The test method involves
producing expansive volume change in a Target Soil to monitor vertical movements and its
effects against concrete linings.
Effectiveness of geometry parameters on the design and stability of linings were studied and
tested over ۱۳ types of concrete linings using an Accelerated Swelling Simulator System
(ASSS). This system was developed to accelerate the swelling phases in different situations
under concrete linings. Accelerated swelling simulator system (ASSS) both reduces testing time
to less than a week and results in cracking patterns in the surface of the soil subgrade and
concrete linings that are similar to those observed in the field.
Good asset management practices can significantly increase the life of an irrigation canal and
reduce its life cycle costs. Improved knowledge of irrigation canals management and the
refurbishment of ageing canals are major challenges facing the national irrigation industry. The most significant component of the irrigation canals, both in magnitude and in replacement cost,
is stability of concrete canals. The objective of this study is to provide a practical image for the
behavior of concrete linings due to swelling pressure of expansive soils. This behavior will be
effective in construction and refurbishment of long life concrete irrigation canal over subgrades
with expansive soils. To provide this practice and image in a real scale, concrete linings with
۱۰۰x۲۰۰ cm sizes in ۵ cm thickness were cast over soil subgrade of ASSS. Results of
comprehensive tests conclude that non-uniform distribution of stresses under concrete lining and
weak points in it and also, restrains in edges of linings causes tensile and shear stresses. These
stresses almost can not be fully supported by concrete lining. Therefore, cracks occur in concrete
lining and spread over it. In other word, observation and understanding how expansive soil can
be swell and swelling pressure how can damage a concrete lining in irrigation canals will be so
effective in studying, designing and construction of these stiffened concrete linings on expansive
soils.
Test results showed that lateral stiffness under plain concrete linings and valve type linings
with small conical valves have high performance to resist high swelling pressures due to the
displacements of expansive soils. Others types of plain and reinforced concrete linings could
reduce cracks with high capacity of deformation. The measured vertical movements for concrete
linings in ۱۰۰x۲۰۰ cm sizes due to simulation of swelling of any expansive soils recorded to ۱۳۰
mm for lateral stiffness and valve type linings without any crack phenomena. This desirable
results all obtained in concrete linings just ۳ days after casting in laboratory condition. It means
deformability pattern in designing concrete linings over expansive soils is an important point for
having stable linings to eliminate or mitigate swelling pressures.