20_WY.PFH.4-1(4).Beartooth_Highway.URS.Ravine_Bridge.2003.pdf

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TECHNICAL MEMORANDUM

To: Matt DeMarco Geotechnical Engineer Federal Highway Administration Central Federal Lands Division 555 Zang Street #259 Lakewood, CO 80228

From: Craig B. Parent, P.E.

Project Engineer

Date: June 20, 2003

Subject: Beartooth Ravine Retaining Wall Structure Selection Recommendation

FHWA T.O. No. DTFH68-01-T-00039 URS Project No. 22234609.00300

The following memorandum identifies the wall structural system and advantages of utilizing precast construction when compared to cast-in-place construction for the Beartooth Ravine Retaining Wall. The retaining wall will be a precast post-tensioned concrete Z-wall on a cast-in-place footing founded on micropiles. The advantages of this type of structure includes a reduced construction period, elimination of formwork, significantly less material and lower foundation loads.

The retaining wall consists of five components, the micropile foundation, a cast-in-place concrete footing, ground anchors, a precast double-tee post-tensioned wall and a cast-in-place concrete cantilevered slab. The precast wall web depth will vary along the height of the wall to decrease the cost of the wall and simplify the anchorage of the post-tensioning. The post-tensioning will consist of up to four threaded high-strength rods that will be anchored into the footing and at the top of the double-tee section. Ground anchors will be used to resist the high horizontal loads and reduce the number of micropiles. The wall can accomodate roadway drainage inlets, culvert pipes and underdrains between the webs of the double-tee section.

Block-outs can be cast into the panels to accomodate outlets for pipes. Architectural features, such as a rustication with rough sawn boards, can be cast into the front face of the wall.

The construction sequence of the wall will be

1) The micropiles are driven and installed to the required depth.

2) The cast-in-place footing is then constructed which includes the placement of the high strength post-tensioning rods.

3) Ground anchors will also be cast into the footing and anchored into rock on the fill side of the wall.

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4) After the footing has attained sufficient strength to support the wall dead loads and the ground anchors are tensioned, the precast wall sections can be placed. Each of the post-tensioning rods will slide through bar sheathings or ducts that were placed in the wall units prior to casting the concrete. The wall units will have an adjustment system that will provide the verticality of the wall unit for proper alignment and fit-up.

5) The anchor plates and anchor nut will be placed and tightened in order to provide stability to the wall system. As long as the footing has achieved a high enough strength at this time, the rods may be tensioned to their design value.

6) After tensioning of the wall system has occurred, the rods will be grouted along their entire length. The wall will then be back-filled and the cast-in-place slab construction begun.

The primary advantage to using precast construction is the significant reduction in the construction time. Due to the short construction season in this area, it is necessary to build the wall as quickly as possible. If the entire wall was cast-in-place, the contractor would have to wait for the footing to cure prior to building the wall portion and then have to wait for the wall to cure prior to adding any additional loads. For the precast wall, the wall units can be cast while excavation, pile installation and footing construction/curing take place. After the footing has attained sufficient strength, the wall units can be placed immediately.

Another advantage which also reduces construction duration is the elimination of formwork.

The cast-in-place wall would require significant formwork construction and shoring in order to pour concrete and maintain wall stability until the concrete cures.

The design of the precast wall is more efficient in that it uses significantly less material than the cast-in-place concrete wall. The volume of concrete used for the precast wall section is reduced by approximately 1/3 when compared to the cast-in-place section. This reduces the amount of concrete that will be trucked to the site. In addition, the loads to the micropiles are substantially reduced which will reduce the overall cost of the wall.

The disadvantages of the precast wall include a more complicated structure, fit-up of the wall is more difficult and the construction requires heavier equipment. Due to the fact that we are proposing a more efficient structural section, the section requires post-tensioning steel. Post-tensioning requires a contractor with experience in this type of construction. The fit-up of the wall is complicated by the precise locations of the high strength rods and requires the use of templates with set rod locations when constructing the footings and each precast unit. Lastly, the units are large and in some cases will be approximately 30 feet high. This will require heavy equipment to place each precast unit in place.

It is our belief that the advantages of the precast wall overcome the disadvantages. The fact that we can eliminate as much construction time as possible while saving costs with the reduction in materials outweigh the disadvantage of using a more complex construction system.

cc: Tim Rogers, Bert McCauley, FHWA CFLHD

TECHNICAL MEMORANDUM