Global Journal of Engineering Sciences (GJES)
Stress
Reduction on Buried Conduits Using EPS Geofoam
Authored by Ahmad Abdul Haleem Mohammed
Abstract
In the
recent years, the Kingdom of Saudi Arabia started some huge projects in the
field of infrastructure development through all the Kingdom regions. Reinforced
concrete pipes are considered to be a good and economic choice for sewage pipes
because they are less expensive, locally produced, and environmentally safe.
The use of reinforced concrete pipes is not limited to the sewage systems, but
they are also used in water and oil transfer as well as in storm water drainage
systems and utility tunnels throughout the Kingdom cities.
The
current paper aims to identify the trouble zone above buried conduits to be careful
during installation pipes or construct buildings on these zones and to
investigate how to improve the performance of rigid conduits under heavy
overburden soil loads using EPS-Polystyrene around it and compared the geometry
of trouble zone above pipe with different diameters of pipes and modulus of
elasticity of inclusion. The results of the study show the improvement in the
performance of reinforced concrete pipes under high overburden pressure by
reducing the vertical stress above buried pipes around 95% when using a thin
layer of EPS-geofoam around pipes due to the development of arch action above
the pipe. Furthermore, the trouble zone determined for different diameters of
pipes to protect buildings that will constructed from settlement.
Keywords: Buried pipes; Stress reduction; EPS
geofoam; Disturbance zone; FLAC; Reinforced concrete
Introduction
In the
recent years, engineers started to use new materials for the sewage pipes such
as plastic, reinforced concrete, and fiberglass. Reinforced concrete (RC) pipes
are considered an economic and environmentally safe alternative when engineers
think of installing utility pipelines for sewage and water projects. Compared
wither other types such as steel, composite, or plastic pipes, concrete pipes
are built using locally produced, less expensive, and less harmful to
environment materials. The serious problem that threatens the increase the
usage of concrete pipes is their ability to resist high overburden pressure
when burying them at high embedment depth underground surface and their effects
on constructed buildings.
There
are two types of pipes can be used for transfer fluids under the ground
surface, rigid and flexible pipe. Rigid pipes are generally limited by thrust
in the pipe wall and cracking in the pipe like reinforced concrete, plain
concrete and clay pipes. Flexible pipes are generally limited by deflection,
buckling, and yielding in the pipe wall like metal and plastic pipes [1].
EPS is
the abbreviation for Expanded Poly Styrene. Expanded polystyrene, EPS-Geofoam
is a lightweight material that has been used in engineering applications around
the world since at least the 1950s [2]. (EPS) geofoam is an ultra-light
material that used in transportation infrastructure projects to reduce vertical
and horizontal stresses subjected to buried pipeline and culvert systems [3].
It is a lightweight cellular plastic material consisting of fine spherical
shaped particles which are comprised of 98% air [4]. EPS geofoam is
manufactured by pre-expanding polystyrene beads which are molded and fused in
block-molds using dry saturated steam [5].
During
the period from 1988 to 1992 researchers measure the deformation and the
vertical and horizontal earth pressure on buried concrete pipes using hydraulic
pressure cells. Hydraulic earth pressure measuring cells can installed next to
pipe in both sides and above pipe below and above compressible layer with
measured distance [6]. For installations with granular backfill material, the
long-term measured vertical pressure above the pipe ranged from 23% to 25% of
the overburden pressure and about 45% for the one with cohesive soil backfill.
Also, the type of soil used in the embankment construction affect the
performance of induced arching because the field with granular fill reduced the
vertical pressure over the culvert more than the one with silty- clay
embankment. The results also show that the deformation of EPS compressible
layer is greater in cohesive fill than in granular fill.
The
final compression of the EPS geofoam compressible layer at the end of
embankment construction ranged from 27 % to 32 % for concrete pipes with
granular fill and 50 % for cast-in-situ box culvert with cohesive fill. So, the
induced trench installation meth- od is successful in reducing the vertical
loads on the buried pipes and culverts and it depends on the selection of
backfill material with higher stiffness like granular fill material [6]. Table
1 shows a list of physical model studies on induced trench for different re-
searchers from 1979 to 2008 [7].
Objectives
This
research aims to achieve the following objectives:
•
Identifying the geometry of disturbance zone above buried pipes with different
diameters of pipes surrounded with inclusion that graded from flexible to rigid
materials.
•
Investigating the reduction of the vertical stress on buried pipes and
improving the performance of it by installing a compressible inclusion around
pipe.
Methodology
In
this research the study of a problem will made by a numerical model using FLAC
8.0 (finite difference analysis software). FLAC is a two-dimensional explicit
finite difference program used for engineering mechanics computation [8]. This
program used for engineering problems to simulates the behavior of structures
built of soil, rock, concrete or other materials [8]. FLAC also contains many
special features like interface elements, plane stress, plane strain,
groundwater and consolidation and structural element models. It also contains a
database for materials and its properties that will use in a model. Analytical
verification for numerical results done using equations to calculate a vertical
stress above buried pipe.
Numerical model
The
numerical analysis done by using FLAC finite difference analysis program.
Models and simulation are done for a concrete pipe under the ground surface 20m
deep that surrounded with 20cm thickness of cover material (Figure1) that
tested with different modulus of elasticity from flexible to rigid material.
This
test show how the stress on pipe will reduce by distribute the loads around
pipe with arch action method because there is an incompressible layer around
it. Also, from strain border above pipe the disturbance zone geometry can be
determined. Dimensions used in this simulation are 40m X 40m and its divided
into grids with 200 X 200 elements which that each element is 20cm X 20cm.
Tables 2&3 shows the properties of materials from database of FLAC that used
to prepare a model
Loading and boundary
conditions: This model ran with a dead load of backfill soil without adding any
external loads with the following boundary conditions (Table 3):
a. Fix X axis at i = 1 and i =
201.
b. Fix Y axis at j = 1.
Results and Discussion
Figure 2 from FLAC software
shows the shear strain increment that indicates the disturbance zone above
buried pipe and show the top and bottom width of this zone by colouring the
strain line and its geometry near the shape cone. Shear strain: it is defined
as the ratio of the change in deformation to its original length perpendicular
to the axes of the member due to shear stress of soil. It is dependent on shear
stress and shear modulus (Figures 2&3).
Simulation done 12 times with
a model showed in Figure 1 for different diameters of concrete pipes with
ranged modulus of elasticity of cover material from 25KPa to 25GPa. Figure 3
shows the curve between diameter of pipe and the bottom width of disturbance
zone directly above pipe for each modulus of elasticity of inclusion material.
We can notice that the bottom width directly proportional to the diameter of
pipe in the case of flexible pipe and inversely proportional in the case of
rigid pipe. Figure 4 shows the curve between diameter of pipe and the top width
of disturbance zone above pipe near the surface of ground for each modulus of
elasticity of inclusion material. We can notice that the top width directly
proportional to the diameter of pipe in the case of flexible and rigid pipe.
Figures 5&6 from FLAC
simulation show the difference in values of vertical stress between flexible
and rigid inclusion around pipe and how the flexible inclusion reduces the
stress around 95%. Figure 7 shows the curve between diameter of pipe and the
vertical stress (Syy) above pipe near the surface of inclusion material for
each modulus of elasticity. We can notice that the vertical stress above
flexible material reduced about 95% from rigid material. Figures 8&9 show
the vertical displacement of simulation from FLAC and the curve between
diameter of pipe and the vertical displacement (Y-disp) above pipe near the
surface of inclusion material for each modulus of elasticity. We can notice
from curve that the flexible material causes more vertical displacement than
rigid material. Figure 10 shows a vertical section for a geometry of
disturbance zone above concrete buried pipe surrounded with inclusion
Conclusion
Model studies of circular
rigid buried pipe 20m deep underground surface with the use of inclusion material
surrounded it with different modulus of elasticity have been performed and
modeled by using a finite difference program (FLAC 8.0), the following
conclusions are made.
• A bottom width of
disturbance zone that is directly above buried pipe is directly proportional to
the diameter of pipe in the case of flexible pipe and inversely proportional in
the case of rigid pipe.
• A top width of disturbance
zone that is near to the ground surface is directly proportional to the
diameter of pipe in both cases flexible and rigid pipe but the case of flexible
has higher values compared to rigid one.
• When a vertical stress (Syy)
above surface of pipe compared between flexible and rigid material around pipe,
it is reduced by 95% with the case of flexible pipe because the compressible
inclusion distributes loads around pipe with arch action method.
• The flexible material causes
more vertical displacement (Y-displacement) than rigid material above buried
pipe (Figure 10).
Recommendation
Determining the disturbance zone,
vertical stress and vertical displacement above buried pipes by changing the
properties of fill material and inclusion.
Notation
The following symbols are used
in this paper:
Do = Outer diameter of pipe (m);
H = Height of fill up to pipe
(m);
PL = Prism Load;
VAF = Vertical Arching Factor;
w = Soil unit weight (N/m3)
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