Environmental Science & Engineering - www.esemag.com - May 2003
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Comparing Horizontal Directional Drilling versus Open Cut for the installation of a sanitary sewer

by By Peter Langan, P. Eng.

Exposing existing utilities

Pipe being pulled back

We have all heard the term “comparing apples to apples” which infers the need to compare alternatives fairly before making a decision. This also applies when comparing costs for different construction methods. It is often difficult to estimate construction costs, as the cost is not only based on materials, labour and contractor profit; it is also driven by market conditions such as the availability of experienced contractors at the time of tendering and construction.

Currently there are many highly qualified contractors that perform Open Cut installation of sewers; however, there are fewer contractors capable of Horizontal Directional Drilling (HDD) of sewers. Site constraints often dictate which method of sewer installation is specified – for example, the size or slope or geotechnical conditions may dictate one method over the other. But what happens in cases where both construction methods are appropriate? Does the designer specify what they believe is the most appropriate method? Is it appropriate to limit a tender to one method of construction or should both methods be tendered? Can both construction methods be tendered and evaluated fairly?

A recent tender for the construction of a 300 m long 250 mm diameter sanitary sewer on Leslie Street, north of Eglinton Avenue, in the City of Toronto helped tackle these questions. The sewer was required for the Carrington Condominium, being developed by Monarch Construction Limited.

Design

The design of the sewer was initially based on an open cut approach with PVC pipe. All trench backfill in the Exposing existing utilities. Pipe being pulled back. roadway was required by the City to be of non-shrinkable fill.

In order to consider the feasibility of HDD, the following items needed to be confirmed:

Could the required minimum sewer slope be maintained with HDD?
Would the municipality allow this technique?
Were the geotechnical conditions appropriate?
Were there any existing utilities or municipal services that might be a problem?

The open cut design of the gravity sewer had slopes ranging from 1.5% to 4%. The minimum slope that was needed to achieve the required sewer capacity was 1%. We believed it was technically feasible to maintain this minimum slope of 1% or greater throughout the bore.

As with most municipal infrastructure, the Public Works Department of the municipality sets the standards on what is permitted. Early on in this project the City of Toronto was approached to determine if they would be receptive to the use of HDD and high density polyethylene pipe for the installation of the sewer. HDD was considered to have merits on this project, as the sewer was located at the centre of a busy five-lane arterial road. The City was receptive to the use of this technique at this site, provided appropriate measures were incorporated.

Appropriate measures were developed and agreed upon with the City to ensure adequate control of the horizontal and vertical alignment. The following controls were developed for approval by the City:

Horizontal Control – standard location in road with a curvilinear alignment and a horizontal deviation of up to one pipe diameter.

Vertical Control – maximum depth of cover of 4.0 m with positive grade of 1% minimum (design grade) throughout. The widespread use of CCTV for sewer inspection played a role in the City agreeing to these conditions.

A geotechnical investigation was undertaken and comments specific to both open cut and directional drilling were provided. The prevailing geotechnical conditions did not eliminate HDD as an option; however, the geotechnical report cautioned that the possible presence of stones and boulders might hamper progress. Utilities and municipal services along the sewer alignment were located in the plan and their depths were approximated to confirm that there were appropriate alignments available for HDD.

After addressing these issues, it was determined that this sewer was a candidate for the HDD construction technique.

Tendering

A tender was developed to allow contractors to bid on open cut, HDD or both methods of construction. The comparison of the bids for the two methods of construction is usually straightforward; however, the City of Toronto requires contractors working on their roads to backfill all trenches with non-shrinkable backfill and do temporary restoration. A contractor retained by the City does the permanent restoration at a later date. Special conditions had to be included in the tender to factor in the permanent restoration costs, to arrive at a total project construction cost.

Both the conventional open cut and the HDD portion of the tender contained the typical tender items. An additional item was included that required the bidders to indicate the total asphalt removal area needed for their method of construction. The unit cost for permanent asphalt restoration, as provided by the City, was indicated in the contract. The permanent asphalt restoration cost was indicated in the tender as being a factor that would be considered in the evaluation of the bids. The lower the total asphalt removal area indicated by the bidder, the lower the permanent restoration cost would be. This was to the advantage of the bidders using HDD and narrow trenches. The tender also indicated that exceeding the total asphalt removal area would result in a backcharge to the contractor. This approach leveled the playing field by considering restoration costs as part of the tender evaluation, to arrive at a total project construction cost.

To the advantage of the open cut bidders, the HDD portion of the tender included two additional items for a detailed HDD work plan, and for exposing all utilities to confirm their elevations. HDD contractors often expose utilities and municipal services to help plan the bore and confirm clearances. To the advantage of the HDD bidders, one manhole was eliminated due to the City permitting a curvilinear alignment when using HDD.

All of these factors were known by the contractors during the tendering process, allowing the contractors the opportunity to make strategic decisions prior to submitting their bids.

Set-up near busy intersection

Tender Evaluation The tender evaluation was carefully undertaken. By knowing the prices for the two alternatives, and the total asphalt removal area committed to by each bidder, the final restoration cost could be accounted for, to determine the total project construction cost for each bid. The bid with the lowest total project construction cost would be awarded the project (provided all of the references were favourable).

After comparing apples to apples, the lowest total project construction cost was realized by the HDD method and the contract was awarded to A.Van Egmond Construction Limited.

For this project the percentage cost savings was determined by comparing the average price of the three low bids for each construction method. A savings of 40% without considering restoration, and a savings of 46% when considering restoration, was achieved with HDD. We believe a significant portion of this savings was through the reduction in the quantity of nonshrinkable fill required with the HDD method. These savings should not be considered as typical, as the successful method and the cost savings will vary with site-specific conditions and market conditions. However, this demonstrates that there are savings that can be achieved when both construction methods are tendered.

Construction Issues

Municipal standards are generally biased to open cut construction. Some deviations to the municipal standards were allowed to permit the HDD method. For example, on open cut projects, the manholes are typically installed during the pipe laying operation. With HDD, long lengths of pipe may be installed in the ground, requiring the manholes to be installed afterwards. The technique used to install the manholes on this project was similar to the method used for installing a manhole on a live sewer. A dog house manhole segment (i.e. inverted Us at sewer pipe entry/exit points) was used to install the bottom precast section over the sewer pipe. The base slab was poured after the dog house section was in place and leveled. The sewer pipe, which is continuous through the manhole, was cut longitudinally to the spring line (half way up the pipe) to allow access to the sewer for maintenance. Concrete benching was then poured into the manhole.

The continuous pipe through the manhole may have some hydraulic advantages. It is also possible to remove the pipe through the manhole; however, there does not appear to be any advantage in doing this.

Drop structures are typically done with the same material as the mainline pipe. For example, an open cut sanitary sewer constructed of PVC would typically use pre-manufactured PVC fittings for the drop structure. With HDPE pipe the same selection of fittings is not readily available. On this project a PVC drop structure was used, requiring a coupler fitting to adapt between the HDPE and the PVC pipe, prior to the drop structure.

These deviations from the municipal standards were rather minor; however, as HDD increases in availability and popularity, we expect that the required standards will be developed.

Conclusion

HDD is a useful tool that is available for the installation of sewers and watermains, given the correct conditions. For this project, HDD proved to be the most economical. Comparing the average of the low three tenders, a 46% savings was achieved.

The selection of the appropriate method of construction is often determined by site conditions; however, where site conditions permit both open cut and HDD, consideration should be given to tendering both alternatives. It is prudent to outline the tender evaluation criteria to ensure there will be a fair comparison of the two methods.

Peter Langan, P. Eng. is a Senior Associate with R.V. Anderson Associates Limited in Toronto, Ontario, plangan@RVAnderson.com.

Horizontal Directional Drilling (HDD) A HDD rig is used to advance a pilot hole from one location to another. The position of the drill head can be accurately controlled to advance in all directions. Upon completion of the initial pilot hole, a reamer is pulled back while rotating to enlarge the hole to a size that will permit the installation of the pipe. Drilling mud is used to stabilize the hole. High density polyethylene pipe is fused together into long lengths and is pulled back through the hole. The surplus drilling mud is displaced by the pipe and is removed. The length of sewer that can be installed will vary with the size of the equipment, the geotechnical conditions, and the diameter and wall thickness of the sewer.

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