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V.A.O.T. Historic Bridge Committee Proceedings

Historic Covered Bridge Committee Notes of December 2, 2004 for:

Present at Meeting: Committee members J.B. McCarthy, Pamela Thurber, Warren Tripp, Eric Gilbertson, Nancy Boone and Sue Scribner. Also in attendance were Mr. Laurence Melen, Mr. Westley Hazeltine and Mr. Neil Daniels from the Town of Weathersfield, Joseph Nelson representing the Vermont Covered Bridge Society, Michael Canavan from The Federal Highway Administration, Chris Benda, VTrans Soils and Foundation Engineer, John Weaver from the Structures Section, Wayne Symonds from the Construction Section, VTrans project manager Craig Keller and VTrans project engineers Todd Sumner and Tim Filbach as well as John E. Lens of GeoDesign Incorporated.

Project Manager Craig Keller provided an introduction and explanation that this meeting is a follow-up meeting to the one held in September, 2003. At that time, there were concerns raised by the town and members of the committee relative to the proposal to completely replace the south laid up stone abutment with a new "MSE" abutment. The town has wanted to see the abutment saved but intimated it would consider a concrete wall constructed in front of the stone. Accordingly VTrans had hired GeoDesign Incorporated to work with the Soils and Foundation Unit to conduct further investigation of the south abutment.

John Lens from GeoDesign presented a PowerPoint presentation. A copy of GeoDesign's Final Geotechnical Report of September 28, 2004 had previously been distributed to those in attendance. The following notes draw material from both of these sources. Two additional test borings were performed by the VTrans drill crews under the observation of GeoDesign. Both of the borings were inclined in an attempt to explore materials under the abutment. It doesn't appear that past repairs have been made of the circa 1840 abutment with the exception of a concrete cap. There is evidence of movement of stones and possible loss of small stones as well as cracking particularly in the lower courses. The north face of the abutment bulges toward the river near its base. There is more pronounced damage and loss of stones at the north corners of the abutment which is surmised to be as a result of floating debris and ice jams during flood events. There is also evidence of "running joints", or lines of joints between a series of courses not interrupted by interlocking binding courses, at several locations in the face of the abutment which lead to planes of weakness in the abutment mass. A monitoring program has been established.

Two different actions are proposed by GeoDesign. First it is recommended that some basic repairs be made including clearing any brush, trees or other vegetation growing in the stonework, repair the cap if necessary, rechinking the stonework, elongating the bearing blocks if feasible and continued monitoring. Second it is recommended that a reinforced concrete buttress be constructed against the lower portion of the east, north and west sides of the abutment/ wing walls to protect and contain the stone masonry in the base of the abutment. However, there is concern that the abutment could be undermined during the construction process. It is recommended that sheeting first be installed around the abutment construction area. Then it is recommended that the area be dewatered and excavated to the bottom of the existing abutment. Next, reinforcing steel would be set and buttress forms installed for top portion of the abutment. The concrete would next be cast and allowed to cure and then it is recommended that drilling occur for tie rods through the abutment side walls. Next, the tie rods would be installed in sleeves and tensioned. Drilling for anchors through the front face of the abutment is recommended next followed by installation, tensioning and testing of the anchors. Next, excavation for dewatering to top of rock is recommended and then excavating for bottom portion of buttress in segments up to 12-feet long. Finally, the bottom portion of the buttress would be backfilled with riprap and/ or granular fill for scour resistance on the same day.

The above recommended buttress was represented pictorially in the PowerPoint presentation and appeared to extend more than half way up the portion of the abutment that is visible. However, GeoDesign feels that the pictorial representation was not quite accurate in the presentation and indicated that 3'-6' on average would be exposed depending on the water level. The buttress is recommended to be 12-18" wide. Much discussion was generated.

It was indicated that the view of the "bulge" looks very uniform and it was speculated by Neil Daniels that it was purposely built that way with some of the lower portion having been naturally eroded away by water. Warren Tripp noted that some gaps are pretty evident on one wing. John Lens speculated that it might have been built that way and some of the smaller stones have fallen out over time. From the boring findings, there was speculation that a timber mat might be in place. Neil Daniels expressed concern that the abutment could be destroyed if timber cribbing is there and sheet piling hits it. Timber mats can easily extend 5' beyond the limits of the abutment. Warren Tripp expressed concern that the concrete scour skirt could also hit timber mats if there. As only one boring appeared to hit timber, the conversation shifted to questioning whether a timber mat actually exists. Neil Daniels doesn't feel that there actually is a timber mat based on his experience.

Eric Gilbertson stated that as there is no evidence of movement, why spend money and potentially damage the abutment. He would like to see monitoring continued for an extended period and possibly have some stone added to the front of the abutment for scour protection. It was questioned whether just adding stone would be effective and concerns were raised as to who would monitor the situation and how quickly the funding and labor could be mobilized if problems did become evident. J.B. McCarthy asked John if he had an opinion of why stones have cracked. John felt it was due to natural weathering and didn't know how long they could be expected to last without some work. John Weaver asked if any thought had been given to using geo foam. The reply was no as there haven't been any signs of lateral spread or distortion. Chris Benda cautioned the group that the monitoring period has been relatively short and may not yield the full picture. Eric feels that the work on the abutments and the work on the superstructure can be done independently and should be kept that way. However, there could be cost savings having the work done at the same time. Todd Sumner expressed that he feels there needs to be protection against ice damage. J. B. concerned about liability if no protection is added during the project. Warren Tripp and Neil Daniels inquired if the concrete buttress could be shorter. Mike Canavan expressed concern about constructability/ safety under this scenario as the abutment could be destabilized. Eric again expressed his concern that the proposed drilling could destabilize the abutment and suggested that some scour protection only be added. Neil Daniels expressed his belief that some form of concrete buttress is needed but not to the height recommended by GeoDesign. John Lens indicated that the recommended rods are for long term protection. John Weaver asked about adding more vertical rods and extending down to bedrock but John Lens was concerned that new forces would be introduced. Todd Sumner suggested having the buttress supported on drilled piles rather than ties in to the abutment. Could this still offer the support needed and how high would they need to extend. J.B. suggested adding temporary upper support, perhaps a steel wrap, and removing it after the bottom work was completed. Eric concurred that it might be prudent to have something done in the lower 3' or so but wants to know if this can be done without damaging the abutment. Nancy Boone indicated that Todd's idea is appealing as it could be reversible in the future. Warren indicated we should investigate it there is a timber log, perhaps conducting additional borings. The idea of facing the buttress was discussed and it was expressed by Eric and Joe Nelson that this idea is not appealing. Joe would like to see the historic appearance of the abutment retained with as little work occurring as possible.

The meeting concluded with the consensus that it needs to be determined if a timber mat exists beneath the abutment. Also, it needs to be further investigated if a 3-4' concrete wall can be constructed in front of the south abutment in a safe manner with a degree of confidence that the abutment will not be damaged during the construction process. The hydraulics should be checked and the Stream Alteration Engineer consulted.

Respectfully submitted,

Susan Scribner
Chair, Historic Covered Bridge Committee

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Joe Nelson, P.O Box 267, Jericho, VT 05465-0267
This file posted December 2, 2004