Project Description

This project involved the replacement of the existing Tifft Street Bridge, in Buffalo N.Y., over railroads located between the intersection of the Tifft Street with NY Route 5 ramps and Hopkins Street. Replacement of the existing bridge required construction of a new structure and embankments, incorporating the newest design standards, including greater vertical clearances over railroads and increases in lane widths.

The new structure would be significantly shorter on the west end, meaning the new west approach embankment would extend 90 meters beyond the old abutment. The total alignment of the new embankment center line and bridge would be approximately 21 meters north of the existing center line. In addition, the existing bridge had to remain open until the new bridge could accommodate at least two lanes of traffic.

Surface Conditions
The generalized subsurface soil profile indicated a 2-to-5 meter fill layer of cinders and slag over 0.75 meters of peat (under the west embankment only), 10 to 14 meters of a soft silty clay, and finally, a 3-meter layer of sand overlaying limestone bedrock. The ground water table was within 0.5 to 1.0 meters of the existing surface.

Design Concerns
Construction of the new approach embankments, varying in height from 5 to 11 meters, would result in considerable settlements. The west approach embankment would result in settlements ranging from 600 to 900 mm, without treatment. The settlement was expected to occur over a 6-year period after the full fill height was achieved.

On the east embankment, the proposed fill would incorporate much of the existing fills. Therefore, settlements of only 300 to 600 mm were expected, but the time of the settlement would be similar to the west approach. The designers used wick drains to accelerate the settlements to within a 6-month time frame.

On the west approach, a significant portion of the embankment south side slope would be under the existing bridge. It was felt that wick drains would be necessary in this area to prevent significant differential settlement of the side slope in future years. Therefore, some of the wick drains would have to be installed to maximum depths of 17 meters with a headroom clearance of only 8 meters.

On the east embankment there was no need for wick drains under the existing bridge, since the embankment did not extend past the old abutment. However, there was still need for wick drains adjacent to the existing embankment. Borings indicated that some wick drains in the new east embankment would have to penetrate slag under the existing embankment. The depth of the slag was 4.5 meters, and it would be difficult to penetrate, as indicated by very high "N" values in excess of 100 in some locations. The slag material was very abrasive and would be difficult to drill.

Analyses indicated that side slopes of 2H:1V could be safely constructed to a height of only 8.5 meters without foundation treatment., However, after the strength gain as a result of wick drain accelerated consolidation, embankments could be constructed to heights of 11 meters, as required.

Design
The west embankment was expected to settle up to a maximum of 900 mm under maximum fill height, and the east embankment a 600-mm maximum. A surcharge height of 900 mm above final grade elevation accelerated settlement. Where settlement was maximum and stability was of concern, the spacing was 1.2 meters in a triangular pattern. In areas of lesser settlement, where stability was not a concern, the pattern spacing was 2.1 meters.

The total quantity of the original estimate was 70,000 linear meters of wick drains. Added to this quantity were the cost of 5,000 m3 of a granular material for the drainage blanket and 16,000 m2 of a geotextile. New York State DOT estimated the total cost of the wick drain solution, including auxiliary items, to be in an excess of $800,000.

Project Construction Procedure
Wick drain construction began on the east side and it became readily apparent that special drilling would be necessary to penetrate the slag fill on the area near the existing embankment. Special air rotary drills were used to predrill and the wick drain installation unit had to follow close behind. In areas farther from the existing embankment, a combination unit of vibro and static force was used to install the wick drains without predrilling. Approximately 25,000 m of wick drains were completed on the east side within 4 weeks.

Wick drains underneath the bridge on the west side were started simultaneously and completed in segments. A special sectional mandrel was developed such that it could be pinned together in a fairly rapid manner. The initial section was 6 meters long with subsequent sections of 5 m. At these locations, the holes had to be predrilled through a miscellaneous fill surface either with an auger or a special air track machine. After several days of trial procedures, a maximum production rate of 25 drains per day was achieved. This compared to a maximum production rate of over 300 drains per day on the locations not under the bridge.

The wick drains on the west side that were not under the bridge were completed in 4 weeks. It was also necessary to angle some drains under high tension wires in order to achieve full coverage of the area. The total quantity of wick drains installed on the west approach was approximately 33,500 m.

To install the drains in sections underneath the bridge there had to be more than one splice per drain. This requirement conflicted with normal specifications, which require only one splice per drain.

Project Results
Total maximum settlement of the west approach was 700 m, while on the east approach the maximum settlement was 600 mm. The settlement was achieved in both locations in approximately 4 to 5 months and agreed fairly well with predictions. While slope stability was not believed to be a concern, piezometers, slope indicators, and settlement plates were used to determine the time rate and amount of consolidation. The purpose for the slope indicators was two-fold. While stability was not thought to be a concern, the factor of safety was marginal. Secondarily, it was believed that lateral movements could be used to determine the immediate settlement component.

Project Cost
This project was somewhat unique in that the use of several different installation units had to be bid under one item. The drains were installed using standard static machines along with predrilling in some locations, static vibro machines in other locations, and a special installation machine was used underneath the bridge.

The general contractor's bid prices were for (1) wick drains at $6.90 per m, (2) granular drainage blanket at $22.00 per cubic meter, and (3) geotextile at $1.70 per square meter. Using the original cost estimate, the actual prices resulted in a wick drain solution less than $600,000, which compared quite favorably to projected costs.

Subsequent to this project, New York State DOT has added a new bid item to account for wick drain mobilization costs. This bid item attempts to account for significant under runs in quantity and/or the need for special equipment for the projects.