The decision on whether to require contact grouting and backfilling of pipelines, conduits and various other types of utilities installed by trenchless methods is an issue that needs evaluation and resolution during the early design phase of the project.
As used in this paper, the term “contact grouting” means the filling of voids between the casing pipe and the excavated ground in a two-pass lining system or the filling of voids between the carrier pipe or conduit and the excavated ground in a one-pass lining system. The materials used to fill the voids utilizing contact grouting is assumed to be a grout made from Portland cement, water and perhaps additives that are placed by pumping at some predetermined maximum injection pressure at several injection locations along the casing or carrier pipe.
The term “backfilling” means the filling of the annular space between the casing and the carrier pipe, conduit and other utilities in a two-pass lining system. The material used to fill the annular space is assumed to be cellular concrete, also called cellular grout, which is placed by pumping at some predetermined maximum injection pressure or placed by gravity utilizing vertical drop holes. Neat cement grouts and sanded grouts are also sometimes used. The use of drop holes is complicated in small diameter tunnels since a hole must be provided in the crown of the casing to allow the backfill to enter the tunnel via the drop hole. The backfill material can also be pneumatically placed sand. As used in this paper, trenchless methods include auger boring, pipe jacking, pipe ramming and microtunneling.
The decision to require contact grouting and/or backfilling is an important one which: may improve the long-term performance and reduce maintenance costs during the design life of the project, can minimize surface and buried utility settlement issues, will impact construction costs and schedule and can help mitigate other third-party impacts. The evaluation and decision to require the contractor to perform contact grouting and/or backfilling are based on a host of project specific conditions and requirements. Several of these can include:
- Site geology and groundwater conditions
- Depth of installation below the ground surface
- Diameter of the excavation
- The trenchless excavation method used
- Surface and buried utility settlement concerns
- Potential third-party impacts
- The casing and carrier pipe materials
- Corrosion issues
- Operating pressure of the installed system
- Function of trenchless installation for example: water supply, sanitary sewer, storm sewer, oil, gas, electrical, communication, etc.
- Long-term performance and maintenance costs considerations of the project during its design life
- The possibility of encountering hazardous materials during excavation
Contact Grouting: When contact grouting a tunnel in soil, the size and extent of the voids between the casing and the ground in a two-pass lining system or between the carrier pipe and the ground in a one-pass lining system will depend on several factors, such as:
- Designed overcut of the machine or shield
- Any over-excavation experienced during the mining process
- Geology: cohesive vs. non-cohesive soils
- Steering accuracy
- The effective use or non-use of lubrication systems during pipe jacking
Because some of these factors may change over the course of the tunnel drive, the potential void’s size (depth), as well as its radial and lateral extent can vary, which makes estimating the volume of the contact grout required to fill the void somewhat difficult. The permeability of the soil surrounding the excavation will also have an effect on the volume of the contact grout actually placed. Keeping in mind that the goal of contact grouting is to fill the void created during the excavation process and not to permeation grout the surrounding soil.
When contact grouting a rock tunnel, the void space is usually more constant, making volume calculations of the contact grout required to fill the void more accurate. The size of the void is usually limited to the design overcut. However, rock fallouts and overbreaks can, in some types of rock, add substantially to the void space and thus to the grout takes. When contact grouting a rock tunnel, care still must be also taken to avoid grouting the rock surrounding the excavation. Contact grout can travel into cracks and open joints in the rock surrounding the excavation. The problem of grouting the surrounding ground while contact grouting either soil or rock tunnels can be minimized to some degree by utilizing a maximum 1:1 water cement ratio grout and limiting the injection pressure at the grout port or delivery line to approximately 10 psi or less. In establishing the maximum grout injection pressure the forces applied to the casing or carrier pipe must be considered to avoid overstressing. Heaving of the soil above the tunnel, especially for tunnels with shallow cover, must also be addressed. The specified required 28-day unconfined compressive strength (UCS) of the contract grout also needs careful consideration.
When contact grouting a trenchless project in soil, the contact grout is simply replacing a small portion of the in situ soil that was removed by the excavation process and therefore, in theory, the grout needs only to be as strong as the soil removed. In practice, taking into consideration quality control issues during the proportioning, mixing, injecting and curing of the contract grout, a somewhat higher UCS than that of the in-situ soil is specified. A required UCS for the contact grout, injected into soil, of 500 to 1,000 psi is usually adequate.
When contact grouting a trenchless project in rock in which the carrier pipe, installed as a one-pass system, is operating at atmospheric pressure the contact grout used to fill the void need only have an UCS adequate to support the dead and operating loads. However, if the system will be operated as a pressure conduit, the contact grout may be required to transfer those loads to the surrounding ground. In these cases, it is common to see contact grouts specified with a 28-day UCS requirement between 3,000 to 5,000 lbs per sq in. (psi).
The contact grout is usually injected through grout ports installed in the casing or carrier pipe during fabrication. To conduct contact grouting utilizing grout ports, man-entry inside of the casing or carrier pipe is required to allow hook up and removal of the grout delivery line at the grout ports. Because of this man-entry requirement, it is generally agreed within the industry that a minimum of approximately 36-in. inside diameter is needed. Contact grouting of casing and carrier pipe of smaller diameters than 36 in., while possible, is simply not practical. Also, as the casing and carrier pipe diameters become smaller the need to grout, for example to mitigate surface settlement, by filling voids is likewise reduced because of the smaller excavation required.
Backfilling: When backfilling the void or annulus between the casing and the carrier pipe, conduits or other utilities installed within the casing, in a two-pass lining system, the volume of backfill materials needed to fill the void can generally can be calculated fairly accurately. The most common backfill material is cellular concrete also called cellular grout. The backfill materials can also consist of pneumatically placed sand. Neat cement grouts and sanded grout mixes are also sometimes used for backfilling but their use is less common. When utilizing cellular concrete, neat cement grout or sanded grout mixes as backfill, several factors must be considered, such as:
- Pumping distances
- Density of the mix being placed
- Buoyancy of the carrier pipe, conduit or other utilities being embedded in the backfill
Heat of hydration of any of these cement-based backfills and its effect on the carrier pipe or whatever other materials and systems that will be in contact with or be embedded in the backfill
With a properly designed mix, the correct mixing and pumping equipment and experienced grouting personnel, pumping cellular concrete long distances is achievable. Successful pumping with distances in excess of 5,000 ft have been reported for cellular concrete placements.
Cellular concrete mixes can be formulated to produce grouts with wet densities ranging from approximately 20 lb/cf to approximately 120 lb/cf. As density decreases, so does the UCS of the hardened cellular concrete. As density increases, the cellular concrete’s effect on buoyancy of the carrier pipe, conduit and any other embedded items during placement also increases. The density of the grout will also be an important factor if groundwater or construction generated water is present in the annular space. If standing (pooled), seeping and/or flowing water are present in the annulus, the wet density of the gout must be greater than the density of water.
The heat of hydration of cellular concrete or the cement grout backfill can cause problems for the materials being embedded in the backfill. While carrier pipes or conduits made from fiberglass and reinforced concrete are generally less affected, carrier pipes made from steel and PVC experience thermal expansion, and softening and loss of structural integrity, respectively. When pre-installed grout ports are not used, PVC pipes are often used as grout delivery lines. These PVC lines will deflect and may become unusable from the elevated temperatures caused by the hydration of the cellular concrete. Several ways to mitigate the problem caused by the heat of hydration include: replacing a portion of the cement in the mix with flyash, limiting the quantity and lift height of the backfill injected during a single placement, and in some cases filling the carrier pipe and conduit with water or even circulating water through the carrier pipe and conduits during backfill placement and during the curing period.
The decision to contact grout and backfill are important ones during design. These decisions should be made early in the design process. The decision to grout will cost the project construction time and money. The decision not to grout may result in surface settlement issues, third-party impacts and could affect the long-term operation and maintenance costs of the trenchless installation. The addition of contact grouting and backfill by change order after the construction contract has been awarded can prove very costly. Therefore, “The Determination: To Grout or Not to Grout” is an important design decision and should be made early during project design.
Raymond W. Henn and Tracy J. Lyman are principals with Lyman Henn Inc. in Denver.