Technology Advances Provide New Opportunities

By Arthur J. Miller PE, FFES, FCSI

There are several new methods for achieving sub-surface installation of pipe and conduit that are revolutionizing the old "cut and cover" mentality. They have been around for a while, yet not many engineers are cognizant of the possibilities or familiar with the potentials. These systems, environmentally protective and economically feasible, provide excellent alternatives. Trenchless construction has many attributes that often have been unrecognized. The old adage that "we have always done it this way" has no place with the advent of trenchless construction.

An example of the former would be a recent microtunnel operation at Miami International Airport in which we installed nearly 2500 feet of 30" o.d. welded steel casing beneath utilities, a main runway, three active taxiways, cargo ramps and even an owl sanctuary without impact on any. With unimpaired aircraft operations at 1.5 minute intervals the advantage is obvious. The casing was installed at a depth six feet below the water table and by boring through intermediate previously placed precast manhole structures. Ultimately the casing received a series of PVC conduits for the installation of fiber optics for control tower communication.

Another Example of a Demanding Application

An example of the latter, although on a very small scale, was our installation of 8" PVC conduits under the main divided entrance/exit roadway for the immense Baptist Hospital Complex, through which irrigation system pipes were later fed. The drilling entered the ground well outside the roadway at a 20 degree vertical angle and with a parabolic track exited in a like manner on the opposite side. Interruption or rerouting of traffic to this facility could not be tolerated.

As more and more contractors see the advantages, the utilization has increased immensely. More engineers need to adapt these procedures in their design. Steel, concrete, clay, fiberglass and PVC pipes are now used extensively. Microtunnel bores well over one thousand feet and in excess of 144" diameter have been very successful. Directional drilling systems have extended for several thousand feet and at significant depths below waterways. The number of specialty contractors and the availability of all types and sizes of equipment has reduced costs considerably. Some Public Works agencies have invited contractors to submit alternate proposals to "cut and cover" designs. It is recognized that in "cut and cover" pipe installation only about 5% of the cost is in the pipe and pipe laying. 20% is in excavation and backfill, and up to 75% in the rebuilding of pavement and other surface entities.

There is another area of economics that seldom receives adequate attention on construction projects. The total cost of Public Works projects is borne by the public through assessments and taxes. The real cost to the public includes the added cost of traffic congestion due to construction. In addition to the real costs of fuel and oil consumption, there are the less tangible costs of poorer air quality and lost time. A normal city street would carry a DHV of 200 vehicles in each lane. If the resulting delay to motorists was only 15 minutes more than normal travel time, that translates into a cost of at least $40,000 per day. Add this to the cost of "cut and cover" utility installation and the real economics are apparent. Only nominal delays produce enormous costs to the public, and an educated public is recognizing this.

Environmental Considerations Are Important

The environmental attributes of trenchless construction are phenomenal. Wetland areas, habitats of endangered species, airfield operations, canals and rivers all can now be traversed without disturbing surface habitat or operational efficiency. Major commercial areas, parks, recreational facilities, and even historic facilities can be avoided. Existing utilities, known and unknown, can be avoided by choosing a safe depth for the bore, eliminating the cost and discomfort of service interruptions. The depth of the water table is not a major factor in the installations.

Although we think mostly in regard to new service or carrier pipe installation, there is another entirely different application that is gaining acceptance. This is the rehabilitation of existing deficient pipe lines. In some cases pipe bursting or splitting has been developed to exploit the old lines. The bursting device, either pneumatic or hydraulic is driven through the old pipe line, shatters it and draws the new pipe along behind it. Slip lining and other techniques are becoming prevalent with the advent of trenchless construction.

The Construction Sciences Research Foundation, recognizing the need, has prepared and made available new master guide specifications as an aid to engineers who wish to utilize these technologies. I would recommend that all engineers become aware of the many advantages of trenchless construction technology and explain these benefits to their clients as well as to the public.

About the Author: Arthur J. Miller, PE, FFES, FCSI, Immediate Past President of CSRF and Senior Vice President of AVART, Inc., a Miami, FL based consulting engineering firm. Mr. Miller can be reached at artava@bellsouth.net

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©  Copyright 2007, The Construction Sciences Research Foundation, Inc.  Updated January 12, 2007.