The majority of trenchless casings or host pipes that require filling with grout are circular — that is the easy part. It is usually the other details of the project where all of the problems arise. Here are some points to consider when you need to select grout for a trenchless casing.
Trenchless host pipes can be any size, from less than 1 ft to many feet in diameter. Grouting projects may range in lengths of up to several hundred feet or more in some applications. While many trenchless grouting projects are relatively flat, some projects may involve significant vertical gradients. Host pipe conditions can also vary significantly, from dry pipe in excellent condition, to leaky CSP culverts in poor condition, to open jointed casing pipes with significant water inflows.
Trenchless grouting projects may involve sliplining with electrical or communications conduits, plastic, fiberglass, steel or concrete pipe. Various types of installed conduit or pipe have different external pressure limitations, temperature limitations and buoyancy considerations. Clients don’t seem to appreciate it when their service pipes are collapsed, melted or floated out of alignment.
Specifications, defining the required compressive strength and other performance criterion, vary widely, depending upon the knowledge and experience of the design engineers. Problems usually arise when an engineering specification that may work well in one type of grouting application does not accommodate site conditions on a different project.
Grouting materials that work well in Florida in the summer may not perform as well in Minnesota in the winter. Projects located in remote areas may encounter logistical problems or material supply issues that do not arise in metropolitan areas.
Grouting operations vary in scale and cost from small projects that involve 10 to 20 cu yds, up to 100-plus cu yds and up to 1,000-plus cu yds for very large projects. Another consideration involves undertaking grouting work in stages or as a continuous operation.
Project requirements for grouting a trenchless casing beneath uninhabited wetlands may not be as stringent as for pipe installations beneath a railway corridor or congested urban environment. In some trenchless applications, grouting may be accomplished at multiple locations through a series of vertical holes that intersect the host pipe. Other trenchless grouting operations may require that all work be accomplished through bulkheads installed at each end of the casing.
Grouting of short casings may be relatively straight forward and utilize ready-mix sanded grout from a local batch plant and concrete pumping equipment from a local rental agency. Grouting of longer casings with any combination of other complicating factors will usually involve experienced crews, specialized grouting equipment and high-performance grouting materials.
Ready-Mix Sanded Grouts
Sanded grouts are economical and practical for continuous filling of short length, large diameter casings where the internal pipe has been securely blocked in place to prevent movement and where the pipe can withstand the dynamic grouting pressures involved.
Sanded grouts are not suitable for filling small diameter casings with narrow annular spaces or on larger diameter applications where PVC conduits or HDPE slipline pipes with inherent grouting pressure limitations are involved. Sanded grouts are not a practical consideration when grouting operations are interrupted for any reason, as it may be difficult to resume grout flow through partially filled host pipes.
When contractors have suitable equipment available to prepare cement-bentonite slurries, these inexpensive types of grout may be used for long distance grouting applications where low strength grout is suitable for the intended application. Cement-bentonite slurries may be formulated to deliver stable grout mixes with a range of density, cohesion and other dynamic properties, to produce various compressive strength values.
Cement grouts that include ground blast furnace slag or flyash (depending on local availability) are typically used for trenchless grouting applications where higher compressive strength grouts are required. The proportions of cement, water, slag/flyash may be adjusted to provide desired dynamic fluid properties for grouting purposes as well as to meet ultimate compressive strength requirements.
Various cement types (Portland, high-early, sulfate-resisting, low-heat) may be used to satisfy various technical requirements for specific projects. Admixtures may also be employed to enhance performance of these higher strength grout mixes.
Cement-slag/flyash grouts may be supplied by a batch plant using ready-mix trucks, by using a pre-packaged proprietary product in packages or bulk bags or by batch mixing of separate ingredients at the jobsite.
Cellular grouts are used when low-density grouts are required to avoid flotation problems with sliplined pipe. Cellular grouts are suitable for small-scale and large-scale trenchless applications.
Cellular grouts require the combination of preformed foam into cement grouts or cement-slag/flyash grouts. The proportion of preformed foam may be adjusted to achieve a range of cellular densities and ultimate compressive strength properties.
Cement grouts may be prepared onsite for small cellular grouting applications using bagged materials. For larger projects, it is usually practical for cement grout to be prepared at a local batch plant for delivery by ready-mix trucks. For very large projects, powdered cement may be delivered by bulk tanker trucks and mixed with water using on-site equipment prior to the blending with foam to produce the desired cellular grout density.
Cellular grouts may be prepared at a job site by batch mixing of cement grout with preformed foam prior to pumping. Contractors with specialized equipment for cellular grouting work may combine cement grout and foam ingredients directly through a static mixer to provide a continuous operation.
Although grouting for trenchless applications is a widespread undertaking, the relevant “knowledge base” pertaining to various grouting materials and different site conditions is not readily accessible by design engineers, contractors and owners.
In many situations, it is a challenge for people to know what questions to ask about a specific site, as there are many important parameters other than nominal diameter and length. The majority of people who call our office for assistance with trenchless grouting applications need help collecting the relevant information before we can provide solutions for their consideration.
While various product and equipment suppliers will have some technical information on the specific products or equipment sold by their companies, few suppliers will have broad application experience or knowledge covering various combinations of materials and equipment to accommodate a range of trenchless grouting applications.
The best sources for relevant technical knowledge will be specialty contractors or similar organizations that routinely undertake a wide range of trenchless grouting applications.
Peter White, P.E., is senior engineer and grouting specialist with Multiurethanes Inc. with offices in Buffalo, N.Y. and Mississauga, Ontario, Canada.