15-CHRI_217021-244555_Wharf_Slab_Investigation_Memo_HDR_10-25-18.pdf

Wharf Slab Investigation Summary Memo Date: Friday, October 05, 2018

Project: Christiansted National Historic Site, Wharf Repair (CHRI 244555)

To: Doug Denk, Project Manager NPS

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

Subject: HIM.0 Hurricane Component — Repair Wharf — Investigation and Schematic Approvals

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 bulkhead includes a continuous concrete apron, located landward of the bulkhead, intended for pedestrian and light vehicle use (Figure 1).

Figure 1 – Overview of Bulkhead and Wharf Apron

Due to the poor condition of the bulkhead, including the presence of holes through the steel sheet pile, the NPS currently limits the wharf apron to foot traffic only. The purpose of the investigation summarized in this memorandum, was to evaluate the structural integrity of the existing wharf apron and the backfill below and adjacent to the wharf apron. The intent of the bulkhead replacement project is to restore the visual aesthetic of the bulkhead and to restore vehicular access (1-ton pickup truck, ambulance, or similar class of vehicle) to the wharf apron.

Field Investigation HDR and its subconsultant, GPRS conducted a field investigation of the wharf apron from September 10 — 12, 2018. The investigation consisted of detailed visual observations of the wharf apron and incorporated ground penetrating radar (GPR) scanning along the concrete apron and the surface below the concrete apron. Detailed visual observations included field measurements of the concrete apron dimensions, verification of miscellaneous site features displayed in the project survey, and observations of concrete cracks/patterns. GPR scanning relied on two types of equipment; a walk-behind scanner for overall broad sweeping radar detection, and a hand-held scanner for detailed concrete reinforcing detection (Figure 2).

Figure 2 - GPR Scanning for Concrete Reinforcing Detection

Wharf Apron Dimensions Based on observations of bulkhead sheet pile section and the dimensions of the bulkhead cap, it is apparent that the 700-foot long bulkhead and wharf apron consists of two distinct segments separated by a section for storm water drainage of the lawn associated with the fort. Referencing the stationing convention from the Schematic Drawings submittal, Table 1 summarizes the wharf apron plan dimensions. Apron width measurements reference the distance from the landward side of the concrete cap to the landward edge of the apron, or curb.

Table 1 – Wharf Apron Plan Dimensions

Station Range Apron Width Concrete Cap Width

Concrete Cap Depth

Concrete Cap Chamfer Curb Present

0+00 to 3+00 10’ – 1” 1’ – 6” 8” 1” No

3+00 to 3+15 Drainage swale – irregular shape without concrete cap No

3+15 to 7+00 15’ – 7” To 1’ – 4” 9” 3” Yes

23’ – 1”

Neither construction nor as-built drawings have been provided for the segment of the bulkhead from Station 0+00 to 3+20. Available construction (U.S. DOI, 1982) and as-built drawings (U.S.

DOI, 1985) for the segment of the bulkhead from Station 3+20 to 7+00 indicate that the wharf apron was designed as 8-inches thick with a concrete toe that extends 10 inches below the bottom of the apron. Figure 3 provides a detailed cross section from the construction drawings.

Figure 3 – Section of Wharf Apron Station 3+20 – 7+00

Based on GPR scanning of the apron, the thickness varies from 8 to 8 ¼ inches, confirming the provided drawings. The GPR was able to confirm the presence of the concrete toe in the area of Station 3+20 to 7+00. Figure 3 indicates the apron reinforcing consists of welded wire fabric (WWF) spaced on a 6-inch x 6-inch grid pattern (Figure 4), and located 3 inches from the bottom of the slab. GPR equipment confirmed the presence, spacing and depth of the WWF matched the cross section. Based on GPR scanning, the apron thickness, WWF spacing and depth match the dimensions observed from Station 3+20 to 7+00.

Figure 4 - Mapped WWF Pattern

In addition to the WWF, the apron (from Station 3+20 to 7+00) includes a single #5 steel reinforcing spaced at 4 to 6 feet on-center (red marked line in Figure 4). These #5 bars are located above the WWF approximately at the mid-height of the apron. These bars provide a connection from the bulkhead sheet pile through the apron to the concrete toe. These bars were not observed in the apron from Station 0+00 to 3+00, indicating a difference in design.

Wharf Apron Joints Several joints of varying condition and thickness exist along the wharf apron. Except for a construction joint at Station 0+33, the remaining joints appear to be ½ to 1-inch wide expansion joints. The majority of the upper joint sealant material is cracked or missing (Figure 5). As a result most joints are filled with miscellaneous debris or have vegetation growing from them (Figure 6).

Figure 5 - Joint with Missing Sealant

Figure 6 - Joint with Vegetation

Table 2 provides a list of the observed joints and a brief description of their condition. Based on field observations, it is apparent that all expansion joints should receive repairs during the wharf rehabilitation project.

Table 2 – Wharf Apron Joints Locations and Conditions

Station Joint Type Extent of Joint Condition

0+33 Construction Separates concrete pours No defects noted

0+66 1” Expansion Through slab and cap Open; vegetation filled

1+17 1” Expansion Through slab only Open; vegetation filled

1+70 1” Expansion Through slab and cap Open; vegetation filled, appears spalled

2+08 1” Expansion Through slab only Open; vegetation filled

2+46 1” Expansion Through slab and cap Open; vegetation filled

3+15 1” Expansion Through slab only Open; vegetation filled

3+36 ¾” Expansion Through slab and cap Open; vegetation filled

3+63 ¾” Expansion Through slab only Open; vegetation filled

3+73 ¾” Expansion Through slab only Open; vegetation filled

3+94 ¾” Expansion Through slab and cap Open; vegetation filled

4+34 ¾” Expansion Through slab and cap Open; vegetation filled

4+73 ¾” Expansion Through slab and cap Open; vegetation filled

5+06 ¾” Expansion Through slab and cap Open; vegetation filled

5+38 ½” Expansion Through slab only Open; partially vegetation filled, otherwise missing joint sealant

5+79 ½” Expansion Through slab only Open; partially vegetation filled, otherwise missing joint sealant

6+19 ½” Expansion Through slab and cap Partially open; where present sealant is cracked

Wharf Apron Slopes and Drainage Patterns Three distinct drainage patterns were recognized during the field investigation. Thirteen box-shaped scupper drains (14-inches long x 4-inches high) exist through the concrete cap from Station 0+00 to 2+46. Storm water drainage occurs across the width of the apron towards the water in this area of the bulkhead.

At Station 2+46 the bulkhead alignment turns to the southeast and gradually decreases in elevation by approximately 1.2 feet in 54 feet of bulkhead (-2.2% grade). Storm water from Station 2+46 to 3+00 drains along the bulkhead alignment toward the drainage swale (Figure 7). At Station 3+15, the bulkhead alignment turns to the north and gradually increases in elevation by approximately 1.1 feet in 63 feet of bulkhead (+1.7% grade). Similarly, storm water from 3+15 to 3+35 drains along the bulkhead alignment toward the drainage swale.

Figure 7 - Looking Towards Drainage Swale

From Station 3+35 to 7+00 circular PVC scupper drains (2-inch diameter) exist through the concrete cap at approximately 6-feet on-center. Storm water drainage occurs across the width of the apron towards the water in this area of the bulkhead.

Wharf Utilities The field investigation observed limited utilities along the wharf apron. Electrical conduits associated with overhead lighting were visible above grade near Station 0+07 and Station 0+46.

Figure 8 - Electrical Conduit Visible Above Grade near Station 0+46

An 18-inch diameter concrete drainage pipe was observed to cross beneath the wharf apron on a diagonal alignment starting at Station 5+15 on the landward side of the apron and terminating at Station 5+01 on the waterward side of the apron. The ground surrounding the pipe has subsided and the pipe appears disjointed landward of the apron. However, no visible signs of apron settlement were observed, and no anomalies were observed in the GPR at this area. As seen in Figure 9, the concrete apron is relatively free of cracks and other distress that would indicate ground settlement or void areas beneath the wharf.

Figure 9 – Ground Subsidence near Pipe

A subaqueous conduit was observed to exit the wharf on an alignment towards Protestant Cay at Station 6+19. A utility vault (10’ – 6” long x 2’ – 10” wide), lies approximately 5 to 15 feet landward of the apron curb (Figure 10).

Figure 10 - Utility Vault near Subaqueous Cable

Wharf Apron Defects and Soil Void Check The investigation of defects consisted of detailed visual observations of the wharf apron and was validated by results of the GPR scan the concrete apron and the surface below the concrete apron. The most prevalent defect observed was cracks in the surface of the concrete wharf apron.

The orientation, width, and nature of the visible cracks are discussed by bulkhead segment.

Except as noted, none of the cracks demonstrated observable vertical separation across the crack that would indicate a settlement induced stress.

CONCRETE DEFECTS – STATION 0+00 TO 3+00

From Station 0+00 to Station 0+33 occasional fine cracks on the order of 1/8 inch wide were observed perpendicular to the station alignment. Beginning near Station 0+60 and continuing to near Station 3+00, two nearly continuous longitudinal cracks exist along the bulkhead alignment (i.e. shore parallel). The width of these cracks are generally 1/8 inch or less. One crack lies approximately 6 – 12 inches landward of the cap, while the other crack lies approximately 5 feet landward of the cap (mid-apron).

The longitudinal cracks may have occurred from a combination of waterward lateral rotation of the bulkhead and incomplete WWF installation. Based on field observations, the bulkhead appears visibly out of plumb in this segment. Based on GPR, the WWF appeared to be missing in some portions of the apron, particularly near the concrete cap.

Transverse cracks, on the order of 1/16-inch wide or less were observed at a spacing of approximately 10 feet on-center throughout this segment. Figure 11 demonstrates the typical cracking in this section of the wharf apron.

Figure 11 - Typical Concrete Cracks Station 0+00 to 3+00

As an exception to the typical concrete crack pattern, two diagonal cracks occurs near Station 0+40 to 0+45 (Figure 12) and near Station 0+55 to 0+63.

Figure 12 - Diagonal Crack near Station 0+40 to 0+45

From the waterward edge of the diagonal crack at Station 0+63 to Station 0+72, a separation of ¼ to ¾ inch occurs at the concrete cap and wharf apron interface (Figure 13). A detailed GPR check of this area revealed that the WWF was missing near the waterward edge of the apron.

The GPR did not detect any soil voids beneath the apron.

Figure 13 - Vertical Separation at Station 0+63

In addition to observations on crack patterns, observations and measurements were recorded for miscellaneous asphalt patches in the wharf apron. Three such patches exist near the middle of the apron at Station 1+50, 2+33, and 2+50. Patches are on the order of 20 to 28 inches wide and are generally in good condition (Figure 14).

Figure 14 - Asphalt Patch near Station 1+50

CONCRETE DEFECTS – STATION 3+00 TO 7+00

Narrow longitudinal cracks are present; however, not continuous, or as frequently occurring compared with Station 0+00 to 3+00. From Station 3+00 to Station 7+00 most of the pronounced cracking occurs in the transverse direction (i.e. shore perpendicular). In general, crack widths are often wider in this bulkhead section compared to Station 0+00 to 3+00. Some transverse cracks have spalled to as wide as two inches (Figure 15).

Figure 15 - Spalled Crack near Station 5+60

Between Station 5+80 and 6+10, a slight variation of the vertical surface (< 1/16 inch) was observed along a longitudinal crack (Figure 16). This crack is approximately three feet or less from the landward edge of the concrete cap. After observing the variation in surface, a detailed GPR check of this area revealed that the WWF was missing near the waterward edge of the apron in the vicinity of Station 5+80. The GPR did not detect any soil voids beneath the apron.

Figure 16 - Longitudinal Crack near Station 5+80 to 6+10

CONCRETE CAP AND CURB

The Schematic Drawings submittal require the existing concrete cap to be removed from the bulkhead to allow construction of the new concrete fascia and cap. Therefore, a detailed inspection of the existing cap was not performed.

A 4-inch tall x 4-inch wide concrete curb exists along the landward edge of the apron from Station 3+00 to 7+00 (Figure 17). The curb will remain in place through the construction of the bulkhead repairs. The curb is generally in good condition. As an exception, two segments of curb will require repairs. The top of the curb at the edge of the lawn at Station 4+80 is broken off and should be repaired to match the general curb dimensions. A diagonal separation through the curb exists near Station 6+20 and should be cut and reconstructed from approximately Station 6+15 to 6+25.

Figure 17 - Typical Condition of Concrete Curb

Figure 18 - Damaged Curb near Station 4+80

SOIL VOID CHECK

The field investigation included a detailed scan of the entire concrete wharf apron (Figure 19) and the area of the NPS property immediately landward of the apron. GPR scan was limited to approximately 5-feet beneath the apron surface and approximately 8-feet beneath the adjacent lawn surface. No soil voids were detected by the GPR scan immediately below the wharf apron, or in the adjacent lawn.

Figure 19 - GRP Scanning Along Wharf Apron

Conclusions and Recommended Repairs

WHARF APRON DIMENSIONS

The wharf apron dimensions will remain unchanged other than the increase in width due to the outboard expansion of the proposed bulkhead.

WHARF APRON JOINTS

The wharf apron joints were observed to be defective. Most of the joints are missing sealant and, as a result, are either filled with miscellaneous debris and soil, or have vegetation growing from soil that has filled the open joint. The following repair is recommended for each expansion joint:

• Remove all vegetation and remaining joint sealant

• Remove all loose debris and soil with compressed air to expose expansion joint material

• Install backer rod within joint opening

• Install joint sealant flush with top of apron

WHARF APRON SLOPES AND DRAINAGE

The wharf apron drainage pattern is expected to remain unchanged. Detailed design drawings will attempt to mimic the existing drainage pattern as summarized below:

• Station 0+00 to 2+46 will drain across the apron, towards the proposed bulkhead

• Station 2+46 to 3+00 will drain along the apron, towards the drainage swale

• Station 3+00 to 3+15 will feature a cap design, similar to the existing, that allows drainage to occur across the proposed bulkhead

• Station 3+15 to 3+35 will drain along the apron, towards the drainage swale

• Station 3+15 to 7+00 will drain across the apron, towards the proposed bulkhead

WHARF UTILITIES

Wharf utilities will remain unplaced and will generally be left unaltered. The following improvements are recommended:

• Provide earth fill and sod/grass seed to cover the exposed electrical conduit near Station

0+00 to 0+50

• Remove concrete block from above 18-inch concrete drainage pipe

• Excavate surrounding earth to exposed pipe joint

• Install rubber seal around disconnected joint

• Backfill excavated area and replace vegetation

WHARF APRON DEFECTS

The results of the GPR scan indicate the underlying backfill is present and without observable voids throughout the wharf apron and adjacent lawn area. This matches the expected conditions based on visual observations and consideration that the concrete wharf apron is a lightly reinforced member. If widespread soil voids were present beneath the wharf apron, the concrete would show distinct vertical separation across the cracks indicating settlement induced stress. It should be understood that the conclusions regarding the backfill are specific to the point in time that the field investigation was conducted. The presence of holes through the existing bulkhead could allow backfill material to migrate though the bulkhead before construction of the proposed bulkhead begins.

As previously discussed, only two of the observed concrete cracks (Station 0+63 to 0+72 and

Station 5+80 to 6+10) demonstrated any observable vertical separation across the crack. The largest vertical separation noted was on the order of ¾ inch and covered an area less than 10-feet long. Based on the crack observations further, detailed GPR scans were conducted in these areas and indicated the underlying backfill was in fact present and without voids.

Longitudinal cracking (generally continuous from Station 0+14 and 3+00) may have resulted from previous rotation of the bulkhead. Installation of the new bulkhead will provide stability to the wharf apron and should cease any rotationally-induced cracking.

Transverse cracking generally appears non-structural and is likely induced by stresses associated with volume change. As previously discussed, the expansion joints were generally observed to be defective; therefore, the concrete behaves as if it were one continuous section. Stress relief that the expansion joints were designed to allow has largely disappeared.

Overall, based on the visual observation and the GPR scan, the wharf apron appears well suited to receive an overlay repair. The following repair program is recommended:

• Repair cracks of 1/16-inch wider or greater shall be repaired by epoxy injection or routing and sealing in accordance with ACI 224.1R

• Remove surface contaminants such as oil, grease, etc. by shot-blast or similar method

• Blow off surface debris by compressed air

• Flood coat the surface with a thin (< 0.5-inch thick) polymer coating in accordance with

ACI 548.5R

• Distribute polymer coating across walkway with squeegee

• Broadcast dry silica sand over polymer coating to act as an anti-slip surface

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

Construction of the Christiansted Harbor Bulkhead (Construction Drawings).

U.S. Department of the Interior, National Park Service, Southeast Regional Office (1985).

Construction of the Christiansted Harbor Bulkhead (As-Built Drawings).

Appendix A – GPRS Report

Appendix B –

RESERVED

References