(This article is reprinted from the National Cooperative Highway Research Program's (NCHRP) Synthesis of Highway Practice 242: Trenchless Installation of Conduits Beneath Roadways. Reproduced here is the summary from that publication. It is 82 pages in length and was published in 1997 by National Academy Press.)

Summary

More than 90 percent of the roadways in the United States were constructed before 1950. Since then, vehicle ownership and total travel mileage have increased, resulting in higher traffic density. Societal impact resulting from conventional open-cutting to install utility conduits beneath roadways has increased significantly because of this higher traffic density.

At the same time, public demand for access to various utilities has increased considerably. The need to replace much of the deteriorated underground utility infrastructure and to expand utility services will increase the need for conduits to intersect roadways. This intersection of roadways and underground conduits is critical and often requires special design, construction, and maintenance considerations. The need to minimize traffic disruption has increased the need for cost-effective alternatives that do not require roadway excavation.

Trenchless technology is a relatively new term that describes the installation of conduits beneath roadways without open-cutting. The term has been used on a global basis since the mid-1980s. However, some of the methods referred to as trenchless methods are not new. For example, auger boring and slurry boring have been used since the 1940s, and pipe jacking has been used since the early 1900s. These methods are referred to as road boring techniques or horizontal earth boring techniques. Nevertheless, many new trenchless techniques have been introduced and many advancements have taken place with the more traditional techniques. Although most of these methods require excavation for shafts, shaft locations usually can be selected to avoid or minimize traffic disruption.

It is anticipated that the use of trenchless technology will continue to increase because of its inherent advantages of minimizing disruption to society and reducing environmental impact. Another driving force behind this increase is the benefit of avoiding or minimizing the handling, volume, treatment and/or disposal of contaminated soil. In many situations, these techniques have become cost-effective alternatives to traditional open-cutting methods.

State departments of transportation (DOTs) are being asked by utility owners and contractors to evaluated the feasibility and compatibility of trenchless methods for a wide range of utility installations. In some cases, the DOT is directly involved in the design and construction of a trenchless project, whereas, in other cases, the DOT is responsible for issuing a permit to a utility owner for the installation of conduits beneath its roadways. Unfortunately, the use of a trenchless technique that is incompatible with the parameters of a particular roadway crossing can result in failure. The most common type of failure resulting from trenchless construction of conduits beneath roadways is subsidence or heave of the pavement surface. An additional area of concern is damage to nearby facilities and utilities, which can be catastrophic.

This synthesis describes eight trenchless construction techniques: (1) auger boring, (2) slurry boring, (3) pipe jacking, (4) microtunneling, (5) horizontal directional drilling, (6) pipe ramming, (7) soil compaction methods, and (8) utility tunneling. The main features and range of applications, productivity issues and special concerns, current DOT practice, and emerging technologies of these methods are described. Each description includes a range of typical unit costs and typical capital equipment costs, the accuracy that can be achieved, space requirements, compatible pipe materials, and compatible soil conditions. In addition, a case study is presented for each method that illustrates the principles and practices associated with the trenchless technique.

The most recent methods being used to install conduits under roadways, pipe ramming, microtunneling, and horizontal directional drilling, are not always addressed in DOT specifications. A survey of state DOTs indicated that states' experience with trenchless technology varies considerably. Interviews with state transportation officials indicated that trenchless technology experience of districts within a state also varies. The survey indicated that the DOTs are less familiar with the characteristics associated with these newer methods.

The survey pointed out that no DOT specification addresses all techniques. In fact, most DOT specifications only address one or two of them. The survey demonstrated a trend in the increased use of trenchless technology and that almost all respondents have no, or very little, training in the application of trenchless techniques. DOTs expressed concern for ground movement, accuracy, safety, and traffic disruption.

Installing conduits beneath roadways with trenchless technology requires not only different equipment but also different personnel skills than are needed for open-cutting. For example, the operator of trenchless equipment is attempting to install something without being able to see the excavation, spoil removal, and conduit installation process. Thus, not only is it critical to ensure that the proper equipment and method are selected for a particular application, but also that the operator and crew have adequate skills and experience.

The synthesis describes the trenchless technologies that are available to help DOTs install new utility conduits beneath their roadways; it does not address the trenchless techniques available to rehabilitate existing underground conduits. An overview of the development of the trenchless technology industry and sources of other information are included. The synthesis provides a classification system that includes major factors that affect the selection and use of trenchless alternatives. There is no one method that is compatible for all types of conduits under all possible conditions. The DOT, utility owner, and contractor should be aware of the capabilities, limitations, and risks associated with each technique.