Underground Construction - August 2020 - 24

CURRENTLY @ CUIRE
Use of Spray Applied Pipe Linings as Structural Renewal
for Gravity Stormwater Conveyance Conduits
Since December 2017, a group
of researchers from CUIRE
at the University of Texas at
Arlington has been developing
design methodologies for
structural applications of spray
applied pipe linings (SAPLs)
in the renewal of culverts and
drainage structures. The project
includes several partner companies - Structurepoint Inc., LEO
Consulting and Rehabilitation
Resource Solutions - and is
funded through the National
Cooperative Highway Research
Program (NCHRP).

Introduction
The key issue in current design
guidelines is whether the existing pipe is structurally sound
enough to continue to carry the
earth, and live and hydrostatic
loads imposed on it. It is wellknown that existing flexible
pipes gain structural strength
through the soil-structural
interaction, thus making them
a composite system.
There are many documented
instances of existing corrugated
metal pipes, with significant

Field Inspections for Cementitious SAPLs. Source: CUIRE

Laboratory testing of the invert-cut CMP culverts renewed with SAPL at the
CUIRE's laboratory.

24

AUGUST 2020 | UConOnline.com

invert loss, that continue
to hold their shape, due to
the load carrying capacity of
the surrounding soil. If the
existing pipe is structurally
sound enough to continue to
maintain shape and carry the
earth and live loads imposed on
it, then several internal lining
techniques might be applicable.
These include sliplining (SL),
cured-in-place pipe (CIPP),
spray-applied pipe lining
(SAPL), and close-fit pipe (CFP).
It is important to note that
any lining technique used must
be preceded by restoration of
the surrounding soil integrity.
Voids in the embedment or
invert must be filled completely
to restore the strength provided by the soil. The thickness
and strength of the internal liner, in this case, is required to be
sufficient enough to resist only
the hydrostatic loads imposed
by the groundwater table.
Although these methods
are designed to increase the
structural load carrying capacity of the existing pipe, they
do resist continued structural
deterioration and maintain

the soil-structure interaction
composite system. The linings
are required to resist the live
loads and dead loads. However,
the dead loads are calculated
in a manner similar to what is
required in a steel tunnel liner
plate structure, according to
AASHTO LRFD Bridge Design
Specifications, Section 12.13.
Consequently, the above linings
are designed as semi-structural
renewal methods.
If a pipe is deteriorated to
the point that it cannot continue to carry the earth loads
and live loads imposed on it, it
will lose its shape and collapse.
This may be catastrophic, or
may occur over an extended
time, depending on the rate
of surrounding backfill loss.
Renewal methods applied at
this stage are designed to serve
as standalone structures.
Methods such as sliplining,
cured-in-place pipe, panel
lining or close-fit pipe might
be appropriate, but the wall
thickness of the new pipe will
be thicker than in the previously described case. As a result,
the installer must assure that
the surrounding soil strength is
restored; otherwise, subsidence
of the soil may occur after the
renewal method is completed.
The existing pipe must
maintain hydraulic capacity,
further limiting renewal at
this stage, due to the reduced
cross-sectional area. If hydraulic capacity cannot be assured,
the new pipe can, under certain
conditions, be replaced via
in-line replacement techniques,
such as pipe bursting or hand
tunneling. Another renewal
method is "invert concrete field
paving" for corroded corrugated metal pipes.
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Underground Construction - August 2020

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