By: Richard N. Schantz, P.E.
Sewer line joint testing and chemical grouting provide new
opportunities for sewer cleaning and televising companies to expand their
maintenance and rehabilitation services. Well-maintained and functional sewer
systems are a major asset in any community, but they require regular cleaning,
inspection and maintenance. Chemical grouting too often is viewed as a stopgap
measure to reduce groundwater infiltration. In reality, grouting does much more
to maintain sewer line integrity. It also is a soil-sealing process that
stabilizes the sewer bedding soil, preventing washout of bedding fines and
resulting pipe misalignment and joint failure.
Recent draft changes to EPA-administered permit programs,
specifically the Capacity, Management, Operation and Maintenance (CMOM)
program, requires each permittee to “properly manage, operate and
maintain at all times all parts of the collection system” that they own
or control. The CMOM program specifically requires the following.
-- Routine
preventive operation and maintenance.
-- Identification
and prioritization of structural deficiencies, and identification and
implementation of short-term and long-term rehabilitation actions to address
each deficiency.
-- Standards
for the installation of new sewers and for rehabilitation and repair projects.
-- Procedures
and specifications for inspecting and testing new sewers and rehabilitated and
repaired facilities.
This means increased opportunity for contractors in sewer
system inspection, maintenance and rehabilitation, especially in communities
that have neglected their collection systems.
Why Chemical Grouting?
Chemical grouting is a proven process that has been
successfully used for more than 40 years. The process preserves existing sewer
systems by stopping groundwater infiltration and stabilizing the bedding soil
around structures. It also stops migration of wastewater and groundwater
through sewer bedding and prevents the washing of fines from the bedding soil
into the system.
Chemical grouts immobilize water in the voids between soil
grains and particles. In this way, they stabilize the soil and prevent the flow
of groundwater through the soil and into the sewer through joints, connections
or cracks.
Chemical grout is not cement that fills a void or crack and
then hardens to form a new structure. Instead, it is a low-viscosity liquid
that, when applied, flows through the sewer joint, connection or crack into the
bedding soil around the area of injection.
Low viscosity is what makes chemical grouts so effective in
soil stabilization and sealing. Once injected, the chemicals easily flow
between the soil grains into the surrounding soil and gel or coalesce,
entrapping the water and soil to form a large “doughnut” of sealed,
stabilized soil around the point of injection. The result is a mass of solid
soil that seals off the flow of groundwater.
When jetting a sewer line, technicians often will find an
inordinate amount of soil and sand being drawn from the pipe. Experienced
operators know this can be a sign of open joints or cracks where bedding
material fines are being liquefied outside the sewer pipe wall and flushed into
the system through adjacent joints and cracks.
The jetting process is, in fact, removing this bedding. With
the loss of pipe bedding material, the pipes settle, losing their alignment and
opening joints to additional infiltration. Older sewer systems that show any
type of groundwater infiltration or misalignment should be joint-sealed using
chemical grout as a first step in maintenance and rehabilitation.
Successful Grouting Factors
Any successful project depends upon cooperation between the
contractor and the customer. The same applies to chemical grouting. The contract
must reflect the realities of grouting, and each party must understand its
contractual responsibilities.
For example, the contractor typically is responsible for
equipment, crew, application knowledge and productivity. The municipality,
meanwhile, typically is responsible for grout usage, as the quantity of grout
used depends on sewer line and bedding soil conditions that the contractor
cannot control.
While the contractor’s labor and equipment may be at a
fixed price, the chemicals are paid for on a unit-cost basis. This ensures a
full chemical injection into all areas that require sealing and restabilization
of the pipe and surrounding soil. Fixed-price contracts may limit the amount of
chemical grout applied—to the detriment of the final result.
The grouting operator is critical to the project. The
operator works at the control panel of a test-and-seal grouting truck,
observing the operation on a video inspection camera. A quality operator
understands through experience the chemical pump flow rates, discharge
pressures, injection point pressures and chemical cure times required to seal
and stabilize each area.
Test and Seal Equipment
Grouting operations are performed between manholes. Every
test-and-seal system has the same basic components.
-- A
truck to carry controls, electric generator, grouting equipment, chemicals and
a TV system to the point of operation and provide a work platform.
-- A
video system with a skid-mounted TV camera coupled to the monitor in the
control room.
-- A
winch at the remote manhole, powered through a cable and controlled by the
operator in the control room.
-- A
grout chemical injection system consisting of two separate chemical tanks,
pumps, valves and hosing. The two pumps work in unison at a preset ratio of one
chemical to the other (1:1 for acrylamide, 8:1 for urethane).
-- A
four- or five-part reel-mounted hose, 500 to 700 feet long, that delivers two
separate chemicals, packer inflation air, test air and lateral plug air
pressure or vacuum (for lateral grouting) to the test-and-seal packer at the
working point in the sewer line.
-- A
test-and-seal packer--the joint isolation, testing and chemical injection
device that is moved through the sewer line from joint to joint.
The Grouting Process
Before grouting, the sewer line must be cleaned and dirt,
grease and stones removed. To start grout operations, a power winch is located
at the remote manhole, and a towing cable is threaded through the sewer line
back to the grout truck at the local manhole. The TV camera is mounted on a
skid looking backward toward the local manhole. The grout packer then is
attached to the camera skid, and the grout hose assembly is attached to the
packer.
This train of equipment is launched through the local
manhole. The remote winch, controlled by the operator on the truck, tows the
chain of gear with rear-viewing TV camera, packer, hose and TV cable through
the sewer line from the local manhole toward the remote manhole.
The operator uses the TV camera to locate each sewer joint.
The packer is positioned over the joint and is inflated at both ends, isolating
the joint at the midpoint of the packer. This resulting space is called the
void area.
The packer is fitted with a void pressure transmitter that
provides continuous void pressure information to the operator. This pressure
data tells the operator how the test-and-seal operation is proceeding.
Either air or chemical grout is pumped into the void area to
pressurize and test the joint. If the joint fails to hold pressure, chemicals
are injected until the injection pressure reaches a predetermined level. The
chemical pressure slowly rises in the void area as material is injected until
there is a sudden increase in void pressure, signifying that chemical has been
injected to the point of refusal and that sealing should be complete.
The chemical grout then cures rapidly (in 40 to 60 seconds).
The joint is retested to confirm a complete seal. If necessary, the joint is
regrouted and retested.
Grouting Cost Factors
The major cost components in grouting are equipment, crew
and chemicals. These items must be related to expected production rates in
order to project test-and-seal cost on a per-joint or per-line basis.
A grouting crew usually consists of two people: an
experienced lead operator and a supporting member of the crew to help install
and remove equipment and mix chemicals.
Chemical usage varies greatly—from one to three
gallons or more per joint for 8-inch pipe. Higher usage is not unusual on lines
where extensive washout of backfill has occurred. That is why it is misleading
to quote firm-price chemical costs per joint or line.
Production rates also vary but can be reliably predicted
based on pipe diameter, pipe segment length, number of joints, manhole access
and set-up time. For example, an 8-inch clay pipe in 3-foot segments covering
about 300 feet, manhole to manhole, has 100 joints. At a production rate of
five minutes per joint including set-up and knockdown, a crew could test and
seal about 12 joints per hour and complete one segment, manhole to manhole, in
a typical working day.
A Practical Solution
Complete grouting of sewer
joints substantially extends sewer life. Municipal sewer agencies will benefit
from a complete job that fully saturates the soil around each leaking joint
with chemical grout. Saturation not only will stop infiltration but eliminate
bedding soil washout and thus stabilize the sewer line, maintain sewer
alignment and stop the shifting of pipes and joints caused by ground water
action. WWD
About The Author: Richard "Dick" N. Schantz, P.E., is vice president of operations and a principal with Aries Industries, Sussex, Wis. You can reach him at 800-234-7205 or e-mail [email protected].
