Spruce_Tunnel-Portal_Structure_Technical_Memorandum_3-30-16.pdf

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Technical Memorandum

Technical Memorandum

To: John Maloney (DEA) Project: Spruce Railroad Tunnel Evaluation

From: Thomas Pallua cc: Carol Ravano

Date: 03.29.2016 Job No.: 5419.0

Subject: Pricing Comparison for Long Tunnel Portal Options

1.0 Tunnel Portal Options for Long Tunnel

After the January 26, 2016 site visit with representatives from FHWA, NPS, DEA, and Clallam County, we agreed on final portal rehabilitation for the Long Tunnel by using portal structures. Portal lengths were estimated at 35 feet for the south portal and 30 feet for the north portal. We recommend extending the portal structure approximately 5 feet into the tunnel; therefore, final portal structure lengths result in 40 and 35 feet for the south and north portals, respectively. Outside of the tunnel, the structure will be backfilled with soil to minimize the impact energy from falling rocks. The client requires the front part of the portal structure be covered with a timber façade, to mimic the original look of the railroad tunnels (see picture below).

Figure 1. Original Spruce Railroad tunnel portal with timber façade.

Spruce Railroad Tunnel Evaluation Pricing for Portal Options

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For the portal rehabilitation, different options are available and are outlined below, along with engineer’s estimates for each tunnel portal option.

1.1 Timber Portal Structure

A timber portal structure which mimics the original structures for the tunnels could be constructed.

However, preliminary structural analysis indicates that a timber portal consisting of 10x18 inch timbers with less than 7.5 feet of soil on top will not withstand the energy deriving from the worst case scenario rockfall impact, which is modeled after site conditions at the south portal of the Long Tunnel, i.e.

potential rockfall heights of 80 feet and kinematic impact energies of approximately 12,000 ft-lbs. This type of structure could provide protection from small falling rocks, but would be subject to failure from bigger boulders (1 cubic-foot and bigger) and landslides or rockslides. If this structure were used, it would have to be in addition to shotcrete or Tecco Mesh system to provide rock stability.

For this reason MJA decided to not further pursue this option and no detailed engineer’s estimate is provided.

1.2 Steel Sets with Concrete Backfill

This is a standard portal structure frequently used for tunnel portal rehabilitation purposes. It consists of steel sets, with channel lagging providing an interior form and a standard exterior form, and then backfilled with concrete. This is a free standing structure and does not need to be backfilled with soil for structural reasons. Nevertheless backfilling is recommended to protect the structure from rockfall and limit potential damage and to facilitate placement of a protective berm on top of the portal.

Table 1. Unit pricing for Long Tunnel – Steel Concrete Portal Option

Long Tunnel

Item Quantity Unit Unit Cost Item Cost Steel and Concrete South Portal 40 LF $12,000 $480,000

Steel and Concrete North Portal 35 LF $12,000 $420,000

TOTAL $900,000

1.3 CONTECH Multi-Plate Structure

We talked to CONTECH representatives Michael Blank and Doug Maxwell. Michael Blank met with Carol Ravano and Thomas Pallua on Thursday, March 10, 2016 to discuss and recommend the options that would best fit the Spruce Tunnel Rehabilitation Project. Their Multi-Plate arch pipe was recommended because of the existing estimated tunnel dimensions.

Multi-Plate structures are made from sturdy, heavy gage, corrugated steel plates that are pre-formed to various shapes and sizes, then galvanized for long-term protection and performance. The plates are delivered to the job site and bolted together to form a Multi-Plate structure optimally suited for the

March 2016 3 McMillen Jacobs Associates project. The dimensions of the Multi-Plate system needed for this project will not be known until the perimeter of the portal can be seen after blasted material is removed from the site. Also, this structure must be backfilled around its entire perimeter to perform properly.

Since the Multi-Plate structure doesn’t have the same dimensions as the tunnel, the part of the structure that is extended into the tunnel needs to be backfilled with concrete to form a bulkhead wall. The end result for the bulkhead will look similar to the pictures below. A footing as shown in Figure 3 is also needed.

Figure 2. Example for a bulkhead wall/backfill between portal structure and tunnel.

Figure 3. Example for a bulkhead wall/backfilled Multiplate structure (source CONTECH Structures Reference Guide).

Bulkhead wall

Footing

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Table 2. Unit pricing for Long Tunnel – Contech Multi-plate Portal Option

Long Tunnel

Item Quantity Unit Unit Cost Item Cost Contech Multi-Plate 5 gage steel South Portal (material + labor)

40 LF $2,000 $80,000

Contech Multi-Plate 5 gage steel North Portal (material + labor)

35 LF $2,000 $70,000

Footing Rock Excavation 17 CY $220 $3,740

Footing Concrete 11 CY $925 $10,175

Bulkhead Wall (concrete backfill) 10 CY $925 $9,250

Design cost $20,000

TOTAL $193,165

2.0 Backfill of the Portal Structure and Wood Facade

Backfill material will be necessary on top of both portal structures (corrugated steel and steel-concrete) to absorb the energy of falling rocks; backfill material is necessary on the sides and on top of the corrugated steel structure for proper structural performance. It is recommended to backfill the concrete and steel portal for protection against rockfall. To contain the portal structure’s backfill material, head- and wing-walls are needed (see pictures below). MJA believes that either mechanically stabilized earth (MSE) or gabion basket walls would be ideal for this purpose due to the site location. The MSE wall is constructed with soil and artificial reinforcing elements, which can vary but include steel and geosynthetics. A gabion basket is a cage, cylinder, or box filled with rocks, concrete, or sometimes sand and soil for use in civil engineering.

Figure 4. Example for a MSE-type wing-wall being constructed to cover a corrugated steel structure (source Contech).

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Figure 5. Example of a finished and backfilled corrugated steel structure using MSE retaining structures on the sides and on top.

Figure 6. Example for a gabion basket-type wing-wall that covers a corrugated steel structure (source Contech).

Some of the many options for mechanically stabilized earth (MSE) walls include:

1. Type of reinforcing mats (these are layered between the soil): Steel reinforcing mats (Hilfiker) or geogrid (Tensar, etc.)

2. Slope on the face of the wall: this can be battered (sloped back), vertical or a stepped face. The steps allow plants to grow; however, this could present a safety hazard if accessible.

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3. Facing material: Options include soil held with a geotextile that allows vegetation to grow, gabion facing (decorative stone within wire baskets), and pre-cast or cast-in-place panels. The panels are typically the most expensive option.

Various options would be provided based on the wall type/manufacturer selected. The prices of MSE-type retaining structures are difficult to estimate because of unknown square footage and the suitability of the on-site material for backfill. Based on DEA’s information 3,300 cubic yards of material would be available to backfill the portal structures. Most likely, the material on site will not be suitable for this purpose (not enough fines content and too coarse grained, containing boulders and cobbles). We recommend assuming the worst case scenario, where all backfill material is imported.

Table 3. Unit pricing for Long Tunnel – Retaining Structures and Facade

Long Tunnel

Item Quantity Unit Unit Cost Item Cost Gabion Basket Head- and Wing-wall including imported backfill (assume 30 ft high x 50 ft wide x 2 for both portals)

3,300 SF $60 $198,000 1

MSE Head- and Wing-wall including imported backfill (assume 30 ft high x 50 ft wide x 2 for both portals)

3,300 SF $50 $165,000 1

Wood façade (assume 5 ft long each) 10 LF 9,000 $90,000

TOTAL $255,000-288,000

Table Note: 1. For estimating purposes, either the Gabion basket or MSE type retaining structures should be chosen, not both.

3.0 Summary

Before we can proceed with 100% PS&E, we need a decision from DEA and Western Federal Lands Highway Division (WFLHD) as to which portal and backfill scheme we should design. The WFLHD should be aware that the interior of the portals will either be corrugated, galvanized steel in the case of the Multi-Plate system, or formed concrete in the case of the steel set and concrete portal structure. For purposes of the estimate, we have assumed the timber façade outside each portal is 5 feet long. We will proceed with 100% PS&E for the portal structures once we hear back from the client.