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Chemical Grouting Is a Key Component of Sewer System Maintenance
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.
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.
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.
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.
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.
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.
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