13-CHRI_217021-244555_Concrete_Bulkhead_Investigation_Memo_HDR_10-25-18.pdf

Concrete Bulkhead Investigation Summary Memo Date: Monday, October 01, 2018

Project: Christiansted National Historic Site, Replace Wharf Bulkhead (CHRI 217021)

To: Doug Denk, Project Manager NPS

From: Josiah Berg, P.E., Project Manager HDR Robert DiRienzo, P.E., Project Engineer, HDR

Subject: LIC.0 Line Item Component — Replace Wharf Bulkhead — Investigations

Introduction

This technical memorandum is provided in support of the Replace Wharf Bulkhead project for the

National Park Service (NPS), in Christiansted, St. Croix, U.S. Virgin Islands. The NPS seeks to replace a ±700-foot long bulkhead located at the Christiansted National Historic Site on NPS property. The majority of the bulkhead consists of a cantilever steel sheet pile design; however, the easternmost ±200 feet (Figure 1) consists of a concrete gravity wall connected to bedrock.

Figure 1 – Overview of Eastern Segment of Bulkhead Looking East

The presence of surficial bedrock along the eastern segment could present serious construction challenges for installing the proposed cantilever sheet pile bulkhead. Cantilever bulkhead design requires sufficient embedment of the sheet pile to provide lateral and rotational stability against earth and water pressures. Achieving the required embedment depth in this segment of bulkhead would require extensive pre-drilling and/or trenching of the surficial rock. The success of this activity is not possible to predict until construction is underway. It is possible that the required embedment depth could not be met during construction. Therefore, the NPS has authorized HDR to provide a detailed investigation of the existing concrete gravity wall. The objective of this investigation was to confirm whether the condition of the concrete is sufficient to provide another

40 years of design life. If so, this would eliminate the necessity of installing a bulkhead into bedrock and would substantially reduce construction risk.

40 years of design life. If so, this would eliminate the necessity of installing a bulkhead into bedrock and would substantially reduce construction risk.

Field Investigation HDR and its diver-engineer subconsultant, Collins Engineers, Inc. conducted a field investigation of the concrete gravity wall on September 11, 2018. The investigation consisted a Level I investigation (Figure 2) on 100% of the gravity wall, and a Level II and Level III investigation at four locations along the gravity wall.

Figure 2 – Collins Engineering Conducting Level I Investigation

In accordance with ASCE (2015), the Level I effort consisted of a tactile examination above and below the water to check for obvious major damage and deterioration and does not include clearing of the structure or removal of marine growth. The Level II effort consisted of cleaning a 1 ft by 1ft patch at the waterline, mid-depth, and the mudline to further examine the gravity wall at four (4) distinct locations (i.e. every 50ft). The Level III effort included non-destructive [Schmidt Hammer, and ground penetrating radar (GPR)] testing and partially destructive testing (extraction of 2-inch diameter concrete cores for laboratory testing). Locations for Level II and Level III efforts were selected to avoid contact with or disturbance to marine habitat.

Level I Effort

CONCRETE GRAVITY WALL CONDITIONS

Referencing the stationing convention from the Schematic Drawings submittal, the concrete gravity wall exists from Station 5+02 to 7+00. Figure 3 provides a cross section from the available construction drawings (U.S. DOI, 1982).

Based on visual observations, the concrete gravity wall appears in sound condition with little to no spalling or delamination noted. The concrete is intact with only minor surface defects including honeycombing (<2% of the surface) and hairline cracking (on the order of 1/16-inch wide). The diver-engineer team noted some separation (ranging from 2 to 4 inches vertically x 6 inches horizontally) between the bottom of the concrete gravity wall and the channel bottom. This separation occurred from Station 5+20 to 5+80.

Figure 3 - Cross Section of Concrete Gravity Wall Station 5+02 to 7+00

DEPTH SOUNDINGS

Level I effort included collection of depth soundings along the bulkhead. Table 1 summarizes the soundings taken on September 11, 2018. Water level reference was obtained from the National Oceanic and Atmospheric Association (NOAA) website for Station ID 9751364 – Christiansted Harbor, St. Croix, VI. Elevations reference mean lower low water (MLLW).

Table 1 – Concrete Gravity Wall Soundings

Station (A)

Water Level (ft – MLLW)

(B) Mudline to Top of Cap (ft)

(C) Waterline to Top of Cap (ft)

(D) = (A) + (C) Cap Elevation

(ft – MLLW)

(E) = (B) + (D) Channel Bottom

Elevation (ft – MLLW)

5+02 1.0 8.4 3.3 4.3 -4.1

6+00 1.0 5.0 3.2 4.2 -0.8

7+00 1.0 4.1 3.2 4.2 0.1

TIMBER WALE OBSERVATIONS

Finally, the Level I efforts included observations of the timber wales which generally appeared in good condition. An upper wale measuring 6-inches wide x 12-inches deep runs continuously from Station 3+87 to 7+00. Minor defects include checks, and splits, while moderate defects include light corrosion to the stainless steel fasteners (Figure 4).

Figure 4 - Typical Timber Wale Conditions

The lower wale shown in Figure 3 was only present from Station 5+02 to Station 5+20. The wood fender panels were missing entirely.

Level II and Level II Effort The diver-engineer team cleaned a total of four sections of the concrete gravity wall to perform Level II and Level III tasks. Sections were selected at 50 to 60 foot intervals and with care to avoid contact with or disturbance to marine habitat.

NON-DESTRUCTIVE TESTING

Non-destructive testing included Schmidt Hammer readings and GPR scanning to detect steel reinforcing. Schmitt Hammer readings showed that concrete compressive strengths were generally in the 5,000-6,000psi range, as summarized in Table 2:

Table 2 – Schmidt Hammer Reading Summary

Station # of Tests Sch. Hammer Mean Rebound Sch. Hammer Compressive Strength (psi)

5+25 3 48.88 7066.7

5+71 3 43.54 5783.3

6+22 3 38.37 4666.7

6+85 3 40.08 5033.3

As shown in Figure 3, the gravity wall design required #5 steel reinforcing in the vertical and horizontal direction set on a 2-foot x 2-foot grid pattern. A #11 dowel connects the gravity wall to bedrock, and the #5 bars lie waterward of the #11 dowel. Based on the given dimension of 6 inches from the waterward edge of the gravity wall to the center of the dowel, the clear cover of the reinforcing from the waterward edge of the wall was intended to be approximately 4 inches.

The GPR scan confirmed the vertical bar spacing of 2-feet on-center. However, the clear cover was typically 8 to 9 inches deep; greater than expected based on the construction drawings.

The GPR scan was unable to detect the horizontal bars. As shown in Table 1, the distance from the waterline to the top of cap is 3.2 to 3.3 feet. Of this distance the concrete cap takes up 1.4 feet, leaving approximately 1.8 feet between the bottom of the cap and the waterline. The construction drawings indicate the upper horizontal bar should lie 4 inches below the key joint (dimensions not given – appears to be 4 inches). Using an estimate dimension of 8 inches (0.67 feet), the upper horizontal bar should have located approximately 1.1 feet above the water line.

Based on the GPR scans the upper bar was either left out of the construction or was placed lower than the design intended. GPR equipment was not able to scan below the waterline to further investigate the horizontal bars.

PARTIALLY DESTRUCTIVE TESTING

The concrete gravity wall relies on its mass to provide lateral resistance against earth and water pressures. Unlike a cantilever bulkhead, the gravity wall does not bend. Therefore, the primary function of the steel reinforcing is not to resist structural load, but to prevent cracking due to stresses associated with volume change (shrinkage and temperature).

The design life of the gravity wall is a function of the concrete’s ability to prevent chlorides in the salt water from penetrating to the steel reinforcing, and beginning the corrosion process. If present, corrosion cracking of concrete would manifests itself along a line that is parallel with the reinforcing. As corrosion cracking progresses the concrete would exhibit delamination/closed spalling. This would be detectable by striking the concrete with a hammer and listening for hollow sounding strikes, or by performing Schmidt Hammer readings.

As a more thorough examination, the diver-engineer team extracted two-inch diameter concrete core samples (Figure 5), ranging in depth from two to six inches, from the four locations shown in Table 3. The diver-engineer team filled the extraction holes with a non-shrink marine grout.

Samples were provided to an engineering laboratory for conducting chloride ion concentration tests and unconfined compressive strength tests.

Figure 5 - Concrete Core Sample for Chloride Ion Testing

Table 3 provides a summary of the chloride ion concentration tests and unconfined compressive strength tests.

Table 3 – Summary of Concrete Laboratory Testing Results

Station Compressive Strength (psi) Chloride Penetration (mV) Chloride Penetration (% Cl)

5+25 N/A -46.03 2.267

5+71 6200.0 -26.0 0.958

6+22 6420.0 N/A N/A

6+85 6900.0 -39.97 1.048

Conclusions and Recommended Repairs

CONCRETE GRAVITY WALL CONDITIONS

Based on the sound visual observations (little to no spalling or delamination, only minor surface defects), extending the design life to an additional 40 years appears viable. The noted separation between the bottom of the concrete gravity wall and the channel bottom (Station 5+20 to 5+80) would require attention if the face of the gravity wall were to remain unaltered. However, as noted below, some alterations are required to align the concrete gravity wall with the proposed sheet pile bulkhead.

DEPTH SOUNDINGS

As shown in Table 1, the depth of the channel bottom is limited to approximately four feet or less for low tidal conditions. With a tidal variation of only 0.74 feet (NOAA), this section of bulkhead is not well suited for boating activities. The upcoming Replace Wharf Bulkhead project should not provide any marine cleats along this section of bulkhead.

TIMBER WALE OBSERVATIONS

Based on visual observations of the timber wale, the upper 6-inch x 12-inche timber generally appeared new and in good condition. If desired, these timbers could be removed from the existing concrete cap and re-used in the upcoming Replace Wharf Bulkhead project.

CONCRETE TESTING RESULTS

Based on the laboratory testing results, extending the design life to an additional 40 years appears to be feasible, provided the continued exposure to weathering and salt water chlorides is eliminated and the existing concrete is allowed to age naturally.

The results of the visual and laboratory testing conclude that it is viable to extend the design life of the concrete gravity wall. However, some alterations are required to align the concrete gravity wall with the proposed sheet pile bulkhead:

Cut the existing concrete cap flush with the apron to extend the apron waterward Install steel dowel rods into the existing gravity wall face Install steel reinforcing, similar to the existing reinforcing, waterward of existing gravity wall Align the gravity wall with the proposed sheet pile bulkhead by casting new concrete for a thickness of 18 to 24 inches waterward Install a new concrete cap to match the proposed sheet pile bulkhead concrete cap o Provide isolation joint in the concrete cap at Station 5+02 to avoid differential movements of the two adjacent segments of bulkhead

References American Society of Civil Engineers (ASCE) (2015). Waterfront Facilities Inspection and Assessment. Manual of Practice (MOP) 130. Reston, Va.: American Society of Civil Engineers.

NOAA Tides & Currents. (n.d.). Retrieved September 25, 2018, from http://tidesandcurrents.noaa.gov/. Station ID 9751364

U.S. Department of the Interior, National Park Service, Southeast Regional Office (1982).

Construction of the Christiansted Harbor Bulkhead (Construction Drawings).

http://tidesandcurrents.noaa.gov/

Appendix A – Collins Engineers, Inc.

Report

Appendix B –

RESERVED