GM 07-17_Geotechnical Design & Const. Information.pdf

Memorandum

Western Federal Lands Highway Division 610 E. Fifth Street

Vancouver, WA 98661-3801

Subject: GEOTECHNICAL INFORMATION Date: January 12, 2018 Geotechnical Design & Construction Information—Wells Creek Bridge

From: Robert Kraig, Geotechnical Engineer Reply to: HFL-17, Geotechnical Services GM 07-17

To: Betty Chon, Project Manager, Wells Creek Repairs @

FDR 3300 MP 6.1 & FDR 3300 MP 5.1,

Mt. Baker National Forest, WA FS ERFO 2015(1)-20(1)

Introduction & Background

In September 2016, the Western Federal Lands Highway Division conducted site reconnaissance to investigate damage that occurred along Wells Creek Road (FDR 3300) milepost (MP) MP 6.1, and MP 5.1 as part of a larger scoping effort. This memorandum contains geotechnical and construction recommendations for the damage sustained at the Wells Creek Bridge located on FDR 3300 at MP 6.1 and the roadway embankment and stream bank failure that occurred on FDR 3300 at MP 5.1.

The two sites addressed in this memorandum are part of larger regional damage that recently qualified for assistance under the Emergency Relief of Federally Owned Roads (ERFO) program.

The sites are in the Mt. Baker-Snoqualmie National Forest (Forest) in Whatcom County, Washington, about 50 miles driving distance to the east from Bellingham, as shown in Figure 1.

Wells Creek Bridge at FDR 3300 MP 6.1

The Wells Creek Bridge repair site is located on Wells Creek Road (FDR 3300) at MP 6.1 at the northwest end of the bridge where it crosses Bar Creek as shown in Figure 2A. Reference to mapping (and not visible in the figure) shows that Bar Creek flows into Wells Creek approximately one mile to the northeast. Irrespective of that, the site is denoted as the Wells Creek Bridge in this memorandum and the project documents. The repair at this site will require reconstruction of the approach road leading to the bridge from the northwest and replacement of stream bank protection.

Following the site visit, we conducted a data search to locate previous documents relating to the local geology, bridge construction, historic geotechnical reports and memoranda, and previous repairs. We were unable to locate any prior geotechnical reports or memoranda for the site.

However, we obtained the original bridge as-builts, which show that it was constructed in 1960 (then referred to as the Barr Creek Bridge).

Figure 1: Site Location

Figure 2A: FDR 3300 MP 6.1 Site Vicinity (Image date March 2017)

FDR 3300

Wells Creek Bridge Repair

Bar Creek

The existing bridge is a single span, cast-in-place concrete, cantilevered, T-beam type with a concrete driving surface. It is supported by four concrete columns (two at each end). The construction documents show that the columns are seated on shallow foundations (spread footings) that are 5.5’ x 5.5’ (in plan) and 2’ thick, founded on “gravel and boulders,” with a maximum foundation pressure of 3 tsf. The Wells Creek Road is unpaved near the bridge. The as-builts show that the embankment was originally constructed at a 1:1.25 slope (V:H) with a toe that extended past the support columns and into the stream channel, thus restricting the width of the stream channel. It is unknown whether riprap was used as toe protection, as it is not shown on the as-built plans. Excerpts of the as-built plan set are provided as Attachment 1. Note that the elevations shown on the project plan set and the as-builts are different due to the conversion between the base coordinate systems used.

High stream flows during the winter of 2004/2005 caused loss of embankment in the stream channel at the northwest side of the bridge. Repairs were made under the ERFO program in 2005.

The repairs included placement of a riprap revetment consisting of Class 7 sized rock, and gabion baskets to retain the roadway embankment on the northeast side. Note that the Class 7 size used in the ERFO repair was likely based on the gradation size appearing in an earlier version of the Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects (FP- 03), which is known to be smaller than the same size now specified in the FP-14. The previous ERFO repair details are provided as Attachment 2.

The most recent damage to the site occurred during a storm event in winter 2015 that resulted in widespread damage in the region, thus qualifying the site for repair under the ERFO program.

Geotechnical subsurface investigations were not performed at this site.

Embankment and Streambank Scour at FDR 3300 MP 5.1)

This site is located along Wells Creek as shown in Figure 2B. The damage at this site has been progressive such that the original damage occurred during the same ERFO event that caused the damage at the FDR 3300 MP 6.1 site; however, subsequent high stream flows have since resulted in additional damage, thus resulting in the loss of the entire roadway as described in the following section.

Figure 2B: FDR 3300 MP 5.1 Site Vicinity (Image date March 2017)

Site Conditions

Figures 3 through 7 show the condition of the site in June to September of 2016. The recent damage resulted in the loss of most of the riprap revetment constructed under the 2005 ERFO repair and the loss of about 14 feet of the approach road at the northwest end of the bridge as seen in Figures 3 and 4. The gabion baskets supporting the roadway embankment to the northeast remained in place (Figures 5 and 6), but were undermined beneath the bridge (Photo 6) with the remainder lost during the high flow event (Figures 3, 4 and 6).

It is thought that the riprap present at the site was installed during the 2005 ERFO repair. It is predominantly angular to subangular as compared to what we interpret to be native stream material that is predominantly subround (Figures 3 and 5). The spread footings at the base of the bridge support columns were not visible (Figure 7) and consequently not inspected during the site visit.

Embankment Failure

FDR 3300

Wells Creek

Figure 3: View of embankment failure from upstream (June 2016)

Figure 4: View into lost approach (June 2016)

Riprap from ERFO Repair

Native to stream

Gabion baskets lost

Gabion baskets lost during damage

Figure 5: Gabion baskets supporting embankment at the downstream (northeast) side. Note interpreted native stream material and remnants of the 2005 ERFO repair (September 2016)

Figure 6: View of undermined gabion baskets below bridge superstructure (September 2016)

Gabion baskets lost during damage

Native to stream

Riprap still in place from ERFO repair

Undermined gabion baskets

Gabion baskets from ERFO repair

Gabion baskets still in place

Figure 7: View of the concrete support columns (September 2016)

Reference to the geologic map excerpt in Figure 8 shows the site positioned along the boundary of mapped Landslide Deposits (Ql) located to the west and north (Holocene), and Younger Alluvium deposits (Qyal) to the east (Holocene but may date to the Pleistocene). The Ql mapped unit is described as diamictons composed of angular clasts of bedrock and surficial deposits derived from upslope. The Qyal unit is described as moderately sorted deposits of cobble-gravel to pebbly sand along rivers and streams. We did not observe angular clasts of bedrock near the site.

What we observed at the site is interpreted to be consistent with the Ql unit, and that it likely includes the gravel and boulders (native stream bed material) shown in the vicinity of the footings in the as-builts.

Figure 8: Geologic map excerpt (Geologic Map of the Mt. Baker 30 by 60 Minute Quadrangle, Washington, R.W. Tabor, R.A. Haugerud, Wes Hildreth, and E.H. Brown, 2003) showing approximate location of site

Embankment and Streambank Scour at FDR 3300 MP 5.1)

This site is also shown in the geologic map excerpt appearing in Figure 8. Per the geologic mapping, the site is located within the Qyal mapped unit (described under the previous section) with a mapped unit of argillite and sandstone (KJna) located to the east.

The damage resulting from the original embankment scour event was described as “extended into roadbed 3-feet,” and extending for about 250 feet along the road. Damage at this site (as reported in the original Damage Site Report or DSR) is shown in Figures 9 through 16. Except for Figure 9, Figures 9 through 16 show the condition of the site in June 2017. The image in Figure 9 was taken shortly following the initial damage at the site. The additional damage incurred due to subsequent high flow events is most noted by comparing the images appearing in Figure 9 and Figures 10 and 14.

There is a steep valley wall directly to the east of the site. Seeps within the hillside near road grade and ponding water were noted in the inside ditch during the June 2017 site visit. (See Figures 14 and 15.)

A subsurface investigation was not performed at this site and no field samples were collected.

Therefore, subsurface conditions are essentially unknown. Based on the photographs, the embankment beneath the road appears to be comprised on predominantly granular soils that may contain gravel and cobble-size fragments. Note the stream-deposited material adjacent to the active stream channel comprised of cobble and boulder-size fragments appearing in Figure 13.

FDR 3300 MP 6.1

N

Qyal

FDR 3300 MP 6.1

KJna

Figure 9: View of damage at FDR 3300 MP 5.1 looking south following initial storm damage

Figure 10: View of damage at FDR 3300 MP 5.1 looking to the south (June 2017)

Figure 11: View of damage at FDR 3300 MP 5.1 looking to the north (June 2017)

Figure 12: View of damage at FDR 3300 MP 5.1 looking to the north (June 2017)

Figure 13: View of damage at FDR 3300 MP 5.1 looking to the north (June 2017)

Figure 14: View of seepage collecting in the drainage ditch (June 2017)

Figure 15: View of seepage collecting in the drainage ditch (June 2017)

Recommendations

We have designed a modified Geosynthetic Reinforced Soil Integrated Bridge System (GRS-IBS) abutment wall to repair this site, with a height of approximately 18 feet. Typically, a GRS-IBS abutment wall is comprised of three components—a GRS wall (abutment) to provide the earth retention, a Reinforced Soil Foundation (RSF) as the foundation, and an Integrated Approach to provide a smooth transition from the approach road to the bridge deck.

GRS-IBS structures are stable and are typically more economical than other conventional wall systems. They can be put into service quickly and are well suited to projects constructed in areas that are difficult to access with heavy equipment. Standard (non-specialized) earthwork equipment and labor can be utilized to construct GRS-IBS systems.

GRS-IBS abutments are typically constructed on top of a RSF, which provides sufficient support and a smooth and level foundation and helps to distribute bearing stresses. In some cases, if the bearing capacity of the foundation material is adequate, then the RSS thickness can be reduced, or eliminated, such as when competent rock is encountered above the scour depth. For this project, we do not consider the RSF to be an appropriate foundation for the GRS-IBS and an alternative is proposed, as discussed below.

We interpret that the foundation conditions at the site are sufficient to support a GRS abutment wall without constructing an RSF. This is inferred owing to the spread footings for the bridge being constructed on dense (interpreted) gravel and boulders as shown on the as-builts, and that the bridge does not appear to be exhibiting the type of distress usually associated with settlement.

The top of an RSF (if constructed) would typically coincide with the calculated depth of scour, with the bottom of the RSF extending several feet below the scour depth. When a RSF is not used, then the toe of the GRS abutment wall would need to be placed at or below the reported scour elevation, or other means to raise the toe that are resistant to scour must be considered.

Scour depth information was provided by the WFLHD Hydraulics Team at 2606 feet elevation, which corresponds with the top of the footing elevations, and the Ordinary High Water (OHW) elevation was reported at 2615 feet. The reported scour information requires the foundation excavation to extend to at least 2606 feet, which is below the OHW elevation. We anticipate that the dewatering efforts required during construction might be significant since the interpreted dense native granular materials in the streambed may be very porous, thus making dewatering efforts challenging. To overcome this, a 7.5 feet thick Foundation Block consisting of grouted Class 5 riprap has been designed to have a bottom that corresponds with the scour elevation and a top that is at (or higher than) the expected stream elevation during construction, thus allowing construction of the GRS abutment in-the-dry.

To provide a smooth and level surface to found the GRS abutment wall on (discussed below), a concrete leveling pad is proposed at the top of the foundation block. Grout coverage in the foundation block is assumed to be approximately 25 percent by volume, which will bind the riprap matrix with sufficient grout coverage. Riprap protection will be required to protect the toe of the wall face and be constructed high enough to satisfy hydraulic requirements. A conceptual illustration of the modified GRS-IBS system with a grouted riprap foundation block for the project is provided in Figure 15. Refer to the contract plan set for specific requirements for the GRS-IBS system.

Figure 15: Cross section of a conceptual GRS-IBS abutment system with a grouted riprap foundation block for the Wells Creek Bridge site at MP 6.1

The GRS abutment wall will be constructed by placing alternating layers of closely-spaced geosynthetic reinforcement fabric with high quality, free-draining granular backfill. Typically, concrete masonry units (CMUs) or segmental retaining wall units (SRWUs) are used to construct the face of this system with the reinforcement layers embedded between the CMUs or SRWUs, or a positive connection between the SRWs and the reinforcement is provided. This project has been designed with SRWUs as the facing for the GRS abutment wall. Since the blocks are not rigidly attached to each other, the wall is considered as a flexible structure. The closely-spaced reinforcement layers take up the stresses that develop behind the wall face; consequently, the facing blocks do not act as a retainer of the soil backfill, but rather as protection against erosion.

Typically, a bearing bed and beam seat are incorporated in the upper portion of a GRS abutment wall (not shown in the figure), on which the bridge superstructure is seated. The modified GRS- IBS system developed for this project does not require a bearing bed and beam seat since the bridge superstructure will not be bearing on the GRS abutment. The GRS abutment wall for this project is designed to be constructed behind the end of the existing bridge superstructure and will be constructed to a height sufficient to prevent escapement of backfill materials, as can be seen in Figure 9. A reinforced concrete approach slab will be constructed to provide a transition from the approach road to the bridge, where the bridge superstructure would typically be seated.

The WFLHD Hydraulics Section has proposed permanent streambank stabilization that include wood buffers at this site. Relevant information pertaining to the design and construction of the wood buffers proposed at the FDR 3300 MP 5.1 site will also apply to this site, as discussed in the following sections.

Embankment and Streambank Failure at FDR 3300 MP 5.1)

It is our understanding that the WFLHD Hydraulics Section has proposed permanent streambank stabilization for this site that includes embankment reconstruction, riprap revetment, and wood buffers consisting of log piles, deflector toe logs, deflector logs, flank logs, coarse woody debris, and conserved streambed material. The design was transmitted to the design team through a design memorandum with the subject title Wells Creek Repairs – WA FS ERFO 2015(1)-20(1) Bank Stabilization Report—Preliminary, dated December 6, 2017. Refer to the contract plan set and physical data for details pertaining to the proposed design for this site.

Construction Advisory

We did not have the opportunity to inspect the segment of the Wells Creek Road that leads to the site from the north. It is our understanding that the site is not accessible from that direction, and that access to the site will be from the east. The Contractor’s materials stockpiling, logistics, and equipment staging must take this limitation into account.

A Government-provided material source is not available for this project. The materials excavated at the bridge abutment are not suitable for use as GRS open-graded backfill, but may be used for embankment fill (unclassified borrow). It is the Contractor’s responsibility to provide all other construction materials from available commercial or private sources that meet the contract requirements. If required, refer to the final contract documents for wasting materials.

As discussed earlier, no subsurface investigation or soils testing was performed for the project.

However, the soils in the nearby cuts and embankments were observed to be predominantly cohesionless and fine to coarse grained and containing cobble-size fragments. Additionally, based on the information obtained from the geologic mapping, the information appearing on the as-builts, and the observations made during the site visit, we do not anticipate that bedrock will be encountered during construction. The contractor should be prepared to encounter dense gravel and cobbly material that may contain boulders larger than 12 inches.

It is recommended that construction proceed during the drier months from July into August to take advantage of lowest stream flows—particularly during construction of the grouted riprap foundation block.

The preferred design for the project is a modified GRS-IBS abutment system. Construction procedures for a GRS-IBS structure are outlined in Chapter 7 of the Geosynthetic Reinforced Soil Integrated Bridge System, Interim Implementation Guide, Publication No. FHWA-HRT-11-026 (Adams, et al). An FHWA-produced construction video is available on YouTube at the link provided below. It is recommended that the Contractor and all personnel involved in construction of the GRS-IBS watch the video. However, as discussed earlier, some components of a typical GRS-IBS system are not used in the proposed modified design for this project.

http://www.youtube.com/watch?v=w_5WFoAdoUw

The face batter of the GRS-IBS is limited to a maximum of 8%, but may be constructed vertical.

The Contractor must take into consideration the effects the batter will have at the top of the GRS wall in order to meet the requirements at, and below, the end of the bridge superstructure as specified in the plan set.

Construction will require excavation into dense granular materials that will be below OHW elevation, with dewatering efforts likely to be challenging. To minimize risk, construction should commence during seasonal low stream flows, but it is anticipated that dewatering will still be required. The GRS abutment wall for the project has been designed to be constructed on top of a grouted riprap foundation block to raise the bottom of the abutment wall at or above the anticipated stream level elevation during construction. We anticipate that the grouted riprap foundation block can be constructed in the presence of light to moderate water infiltration at the base of the excavation, but saturated conditions within the grouted mass are to be avoided until after the specified curing period. Refer to the contract documents for the required curing period.

As of the release of this memorandum, the Contractor will be required to verify the elevation of the top of the bridge footings during construction, which will correspond with the assumed scour elevation for the project. If the top of the footings are higher than what is shown in the plans, the base of the grouted riprap foundation block may be raised to correspond with the top of the footings. If the top of the footings are lower than what is shown on the plans, notify the CO before excavating below the planned elevation. The CO will notify the Project Geotechnical and Hydraulics Engineers for additional guidance, if required.

Since the conditions at the base of the foundation block will not be known until excavation reaches the designed elevation, we recommend that a qualified WFLHD geotechnical representative be present to observe the foundation prior to constructing the grouted riprap foundation block to evaluate the support capacity and potential scour susceptibility of the foundation materials, or the CO may inspect the foundation and report the conditions to the project geotechnical engineer.

http://www.youtube.com/watch?v=w_5WFoAdoUw

Adjustments may be recommended to the CO, as necessary, for boulder obstructions or loose material prior to construction of the foundation block. A rough surface at the base of the foundation is ideal and preferred for increased frictional resistance and stability at the base of the foundation block.

Embankment and Streambank Scour at FDR 3300 MP 5.1)

As discussed previously no subsurface investigation was performed at this site and subsurface conditions are essentially unknown. To construct the wood buffer at this site, the Contractor must be prepared to install approximately 13 to 15 feet long, 12 to 18-inch diameter log piles into stream deposits containing cobbles and boulders of unknown density. The embedded depth of the log piles is anticipated to be approximately 8 feet. Installation methods might include driving the piles using impact or vibratory methods, augering, or possibly excavating and backfilling around the piles.

Refer to the design memorandum described under the Recommendations Section and the final plan set for additional construction information.

Closing

This memorandum provides the results of document searches and observations made during a site visit performed in September 2016 to the Well Creek Bridge at MP 6.1 along the Wells Creek Road in Mt. Baker Snoqualmie National Forest. The observations, interpretations, recommendations and guidance provided herein are intended only for this project and should not be used for any other use without the explicit consent of the project geotechnical engineer. No subsurface investigations or soils testing were performed for the project and no previous geotechnical report or memoranda were identified. Potentially unfavorable subsurface conditions may exist which could impact the project. If adverse geological or ground conditions are encountered during construction, it is recommended that the project geotechnical engineer be consulted for further evaluation and possible revised recommendations.

The information contained in this memorandum was intended to provide support to the project design team during project development and to provide supplementary information to the Construction Contractor. Final project design and construction requirements will be contained in the final plan set.

For questions regarding this memorandum, please contact Robert Kraig at 360-619-7699 or robert.kraig@dot.gov.

~ ~ ~ ~ ~ End Memorandum ~ ~ ~ ~ ~ cc: Betty Chon Nathan Jenks

Carolyn Sourek Benn Oltmann Jennifer Chariarse Stephen Henry

Geotechnical File

Attachments

1. Original Bridge As-Built Plan Sheet Excerpt

2. 2005 ERFO Repair Plan Sheet Excerpts

(Intentionally blank)

Attachment 1

Barr Creek Bridge As-Builts

Attachment 2

Previous ERFO Repair Details