GSA WNRC-Geotechnical Report-20210621.pdf

8890 McGaw Road, Suite 100

Columbia, MD 21045 T: 410-997-8900 F: 410-997-9282 www.pennoni.com

June 4, 2021 (Revised June 21, 2021)

MTFAX 20021

Ms. Meagan W. Jancy, AIA MTFA Architecture 3200 Lee Highway Arlington, VA 22207

Re: Geotechnical Engineering Services Washington National Records Center Retaining Wall Design 4205 Suitland Road Suitland, MD

Dear Ms. Jancy:

Pennoni Associates Inc. (Pennoni) is pleased to submit this report summarizing our geotechnical and geophysical engineering services performed at the existing retaining wall located on the southwest side of the Washington National Records Center at 4205 Suitland Road in Suitland, Maryland. The general purpose of our services was to perform field explorations, laboratory testing, engineering analysis, and provide a report summarizing our services and recommendations. These services were performed in general accordance with Pennoni’s revised proposal dated January 14, 2021 and subsequent MTFA Consultant Agreement dated February 25, 2021. This report has been revised to include recommendations regarding foamed glass aggregate (FGA) which is being considered as lightweight fill.

As you are aware, Pennoni is also providing survey and site civil engineering services for this project.

To assist with the performance of our services, we reviewed the following documents:

• A drawing sheet containing soil boring logs and soil boring locations performed at the Washington National Records Center titled, “Log of Soil Borings & Soil Notes,” Drawing No. “7-1-1,” prepared by Abbott Engineering, Inc., dated February 4, 1965;

• A drawing sheet containing soil boring logs from borings performed along the southern wall of Washington National Records Center titled, “Log of Soil Borings,” Drawing No. “7-1-3,” prepared by Abbott Engineering, Inc., dated February 4, 1965;

• A plan-view drawing sheet containing the Washington National Records Center building and retaining wall foundation layout titled, “Foundation & Ground Floor Plan – Part I,” Drawing No. “7-2-1,” prepared by Abbott Engineering, Inc., dated February 4, 1965;

• A section-view drawing sheet containing retaining wall foundation wall sections and details titled, “Foundation Wall Details,” Drawing No. “7-2-12,” prepared by Abbott Engineering, Inc., dated February 4, 1965;

• An 18-page drawing package titled, “Repair Retaining Walls, Sidewalks, Masonry, and Modify Storm Water System,” prepared by Gauthier, Alvarado, & Associates, Inc., dated August 17, 1992;

• A memorandum titled, “Memorandum of Record – Retaining Wall at the Washington National Records Center,” prepared by GSA, dated June 16, 2016; and

• A report titled, “Structural Assessment Report – National Archives and Records Administration Suitland Warehouse Building – Federal Records Center – Investigation of Damaged Concrete Retaining Wall,” prepared by GSA, dated July 2, 2019.

MTFAX 20021 June 4, 2021 (Revised June 21, 2021) Page 2 Ms. Meagan W. Jancy, AIA WNRC Retaining Wall – Geotechnical Engineering Services

BACKGROUND

We understand that MTFA Architecture (MTFA) is working with the United States General Services Administration (GSA) to investigate the cause of observed retaining wall distress on the southwest side of the Washington National Records Center (WNRC) building within the Suitland Federal Center campus in Suitland, Maryland. A Site Location Sketch, SL-1, is attached indicating the approximate location of the aforementioned retaining wall.

A structural assessment of the retaining wall was performed by a GSA Structural Engineer on June 21, 2019 summarized in the above referenced “Structural Assessment Report.” The report indicates observations of concrete retaining wall distress consisting of cracking, spalls, delaminated concrete, efflorescence, and exposed reinforcing steel along the face of the retaining wall. Additionally, observations of slight deflection at the top of the retaining wall were noted. The findings of the report indicate that the cause of the damage may be due to water penetration behind the retaining wall and failure of previously performed concrete repairs (described below). Recommendations within the report indicate that the wall is in need of repair and that additional field investigation and structural analysis should be performed on the retaining wall to determine repair options.

Based on the recommendations of the “Structural Assessment Report,” GSA requested a field investigation and structural study to evaluate the existing conditions of the wall and provide repair/replacement recommendations based on the results of the investigation/study. Pennoni is working as a part of the design team to develop these recommendations related to retaining wall repair/replacement design recommendations for the project.

HISTORICAL RESEARCH

Based on review of historical structural drawings referenced above titled, “Foundation & Ground Floor Plan – Part I,” Drawing No. “7-2-1,” and “Foundation Wall Details,” Drawing No. “7-2-12,” prepared by Abbott Engineering, Inc., dated February 4, 1965, and historical background information contained in the above referenced “Structural Analysis Report,” the WNRC building is a two-story structure that was constructed in 1966 and is approximately 820,000 gross square feet. The building contains office spaces, storage/warehouse areas, loading docks, and a chiller plant. A site retaining wall was constructed on the west side of the WNRC building and is located approximately 102.4 linear feet west of the building. The upper loading dock area contains seven loading docks and is supported by the retaining wall and a below-grade structure. The lower loading dock area contains five loading docks with access to the below-grade structure beneath the upper loading dock area. The access ramp and loading dock areas are primarily used for tractor trailer traffic.

Topography behind the wall within the upper loading dock and ramp ranges from elevation 270 feet to 289 feet, NAVD88. Topography in front of the wall ranges from elevation 260 feet to 271 feet, NAVD88.

The retaining wall is approximately 240 linear feet in length, approximately 22 feet in maximum exposed height (top of wall elevation of 290.08 feet, NAVD88) and tapers (i.e., slopes) to a top of wall elevation of

270.5 feet, NAVD88 on the north end. The retaining wall structure is indicated to be supported on a continuous foundation bearing at approximately elevation 265.5 feet with a minimum embedment of approximately 3.5 feet below existing grade. The retaining wall foundation width ranges from 4.66 to 7.75 feet and varies depending on the height of the retaining wall. Reinforcing sizing and spacing varies as the retaining wall varies in height. Six reinforced-concrete ties are indicated to connect the retaining wall to the existing structure and the top of the ties are located at approximately elevation 281.75 feet (NAVD88). The ties connect to the retaining wall with two diagonal legs that converge approximately 20 feet from the backside of the retaining wall to form one tie that extends perpendicular to the wall and connects to the existing building foundations. Two-inch diameter weep holes were indicated to be installed within the retaining wall at approximately 11.25 feet on-center spacing at the base of the wall. An approximately 4 feet

MTFAX 20021 June 4, 2021 (Revised June 21, 2021) Page 3 thick layer of course gravel on the backside of the retaining wall above the top of footing is indicated on the drawings with the weep holes located below the top of the course gravel layer. Figures F-1 and F-2 contain excerpts from the above referenced historical drawings with retaining wall foundation sizes, wall heights, reinforcing steel size and spacing, and concrete tie-back details.

Based on review of the historical structural drawings referenced above titled, “Repair Retaining Walls, Sidewalks, Masonry, and Modify Storm Water System,” prepared by Gauthier, Alvarado, & Associates, Inc., dated August 17, 1992, repairs to the retaining wall and site/civil improvements appear to have been performed in 1992 and included:

• Concrete repairs to the face of the wall;

• Installation of gravel drains on the backside of the retaining wall;

• Installation of additional weep holes at the base of the exposed portion of the retaining wall;

• Pavement subgrade re-compaction and restoration;

• Curb and sidewalk replacement; and

• Open-grass area re-grading to generate a swale and positive drainage away from the backside of the retaining wall.

The repair work appears to have been performed to address observed distress of the concrete retaining wall and an area of settlement (approx. 9-inch depression) within the existing pavement area adjacent to the backside (upper level) of the wall. The concrete repairs were performed in areas where cracking was observed on the face of the retaining wall. The drawings indicate a “patching concrete” was used and that chipping and removal of distressed concrete was performed, but no further details on the material used for the repair work were provided. Approximately 650 cubic feet (CF) of concrete patching and 600 linear feet (LF) of crack repair was indicated to have been performed. Vertical construction joints at approximate 20 feet on-center spacing were added during the repair work. Installation of 12-inch diameter uncased vertical gravel drains spaced approximately five feet on center at the backside of the wall extending from the top of the wall to the top of existing retaining wall footing was also indicated to be performed. The attached Figure F-3 contains excerpts from the above referenced historical repair drawings.

FIELD EXPLORATION

On August 11, 2020, representatives from Pennoni met with representatives from GSA on site to review the existing conditions and identify the proposed testing. Pennoni subsequently developed Activity Hazard Analysis plans titled, “GPR and Concrete Coring – WNRC Retaining Wall” and “GPR, SPT Borings and Test Pits

– WNRC Retaining Wall,” and submitted the work plans to GSA on April 16, 2021 and May 3, 2021 documenting our proposed field exploration program and safety measures. The work plans were subsequently approved by GSA prior to our field exploration.

Pennoni conducted field explorations in two phases consisting of retaining wall concrete coring for subsequent laboratory testing performed on Tuesday, April 20, 2021 and then a second phase of geophysical and geotechnical field exploration performed on Thursday, May 6 and Friday, May 7, 2021. The geophysical and geotechnical field exploration consisted of the following:

• Subsurface utility location;

• Ground penetrating radar (GPR) in an attempt to locate retaining wall concrete ties;

• GPR to estimate reinforcing steel spacing in accessible portions of the retaining wall face;

• Test pits to expose the retaining wall concrete ties and concrete foundation to facilitate observation by McMullan;

• Standard Penetration Test (SPT) borings within the upper loading dock area to evaluate the subsurface soils behind the retaining wall; and

• Visual observations of the existing retaining wall, weep holes, and site features.

MTFAX 20021 June 4, 2021 (Revised June 21, 2021) Page 4

The sections below have been prepared with respect to each portion of the field exploration performed for the project.

A. RETAINING WALL CONCRETE CORING

On Tuesday, April 20, 2021, Pennoni performed concrete coring consisting of six, 4-inch diameter retaining wall cores, labelled (CC-1 through CC-6) at three general locations along the base of the retaining wall as indicated on the attached Concrete Core Location Sketch LP-1 and agreed upon by Pennoni and McMullan prior to performance. The core locations were selected at readily accessible areas at the base of the retaining wall due to safety concerns with mounting the drill at higher elevations along the face of the wall. Two cores were extracted from each general location for petrographic testing and compressive strength testing, respectively. Coring was performed using an electric coring machine with a diamond tipped core barrel. Water was used to cool the drill bit and minimize dust during coring and a shop vacuum was used to collect the water during the drilling process. The concrete cores obtained were collected by Pennoni and delivered to our laboratory for measurement and photographs following the completion of the field work. Three cores (CC-1, CC-3, and CC-5) were prepared for petrographic testing and three cores (CC-2, CC-4, and CC-6) were prepared for compressive strength testing. Two of the retaining wall cores, labelled CC-3 and CC-4, were advanced to full depth of the retaining wall. Photographs of the concrete cores obtained, P-1 through P-6, are attached. The concrete core locations were patched in kind with approved, Rapid Set® CEMENT ALL®, concrete mix after the performance of the concrete coring.

B. UTILITY LOCATE SERVICES – BLOODHOUND

Blood Hound, LLC, performed utility locate services on Monday May 3, 2021 along the upper loading dock area, ramp, access road, and open-grass areas on each side of the retaining wall as a subcontractor to Pennoni. Utility mark-out was performed on the site and detected utilities consisted of electric, stormwater, water, and an unknown utility. The existing utilities detected by Bloodhound were included on an existing site plan prepared by Pennoni and are depicted on the attached Geotechnical Testing Location Sketch, LP-4.

C. GEOPHYSICAL FIELD EXPLORATION – GPR

On Thursday, May 6, 2021, GPR was used to collect indirect subsurface information within the areas of interest. The GPR method uses high frequency pulsed electromagnetic waves. An electromagnetic wave is generated from the transmitting antenna which travels through the subsurface material. As the electromagnetic wave front propagates downward and a change in the electrical property of subsurface material occurs, a portion of the wave energy is reflected back to the surface. The arrival of the reflected wave is recorded by the receiving antenna. The system records a continuous image of the subsurface by plotting the two-way travel time of the reflected wave front versus the distance traveled along the ground surface. Typically, GPR signatures characteristic of anomalies become evident where a bright inverted (black-white) reflection exists. The inversion of the normal polarity (white-black) occurs due to the velocity contrast between soils (or other obstructions) of different density, air, and/or water.

A GSSI SIR 3000 control unit and 400 megahertz (MHz) antenna were used to obtain continuous radar profiles along the profile lines shown on the attached GPR Location Sketch, LP-2. A total of seven GPR scans were performed parallel to the retaining wall on the retained soil side (upper loading dock area) and at various distances from the face of the retaining wall in an attempt to locate retaining wall concrete ties. The effective depth of penetration was on the order of 10 to 15 feet below existing ground surface elevation.

MTFAX 20021 June 4, 2021 (Revised June 21, 2021) Page 5

A StructureScan Mini XT and 2700 MHz antenna were used to obtain continuous radar profiles along the profile lines shown on the attached GPR Location Sketch, LP-3. The high-frequency antenna was utilized along the front face of the retaining wall in accessible areas in attempt to estimate reinforcing steel spacing. The effective depth of penetration was on the order of 2 feet. A total of 41 GPR profiles were collected along accessible areas of the existing front face of the retaining wall in an attempt to estimate reinforcing steel spacing. Scanned locations are presented on the attached GPR Location Plan, LP-3. Three representative GPR profile, GPR-1 through GPR-3, are also attached indicating typical reinforcing spacing along the retaining wall.

D. GEOTECHNICAL FIELD EXPLORATION – TEST PITS AND VISUAL OBSERVATIONS OF WALL

Three test pits labelled “TP-1” through “TP-3” were performed on May 7, 2021 at the project site and are indicated on the attached Geotechnical Testing Location Sketch, LP-4. Test pits labelled, “TP-1” and TP- 2,” were performed along the back side of the retaining wall in open-grass areas to expose an existing concrete tie. The test pit labelled “TP-3” was performed on the front side of the retaining wall to expose the retaining wall foundation. Test pit locations were established in the field by Pennoni. Test pits were advanced to depths ranging from approximately 4 to 6 feet below the existing ground surface (Elev. 272 feet to Elev. 282 feet, NAVD88).

Visual observations of concrete distress and exposed reinforcing steel at the existing retaining wall, exposed retaining tie-backs, as well as additional site features were performed on April 20, 2021, May 6, 2021 and May 7, 2021 by representatives of Pennoni. The structural engineer apart of the design team, McMullen (John O’Donald), was also on site on May 7, 2021, to observe test pit excavations and concrete distress. Photographs of the concrete ties, foundation, distress, and apparent settling of the bituminous concrete area behind the existing retaining wall, labelled P-7 through P-21, are attached.

At the conclusion of the documentation, the test pits were backfilled with the excavated soils and were placed in lifts and tamped with the excavator bucket, with the excess soil mounded above the excavation.

Settlement and softening of the replaced soil are common resulting in depressions at the ground surface, additional restoration may be necessary at the test pit locations. Test pits were performed by DMY using a Case CX36 Mini Excavator. Our J. Donahue, PE directed the field work and N. Rex and S. Corcoran, PE (DE) observed the test pit excavations, logged the soils encountered, and performed measurements of the exposed concrete ties. Test pit logs are attached.

E. GEOTECHNICAL FIELD EXPLORATION – SPT BORINGS

Two SPT (ASTM D1586) borings labelled, “B-1” and “B-2,” were performed on May 7, 2021 along the backside of the retaining wall in existing bituminous concrete pavement areas to a depth of approximately 30 feet below existing grades. The test borings were performed to observe in-situ soil conditions behind the retaining wall to depths below the retaining wall foundation to assist in estimation of soil parameters for use in wall design. Continuous sampling was performed to a depth of 15 feet below existing grades and then in five feet intervals thereafter to termination depth. The borings were backfilled with the excavated soil cuttings and topped with bituminous concrete “cold patch” to match the existing pavement thickness.

The test borings were performed by DMY using a CME 55 truck rig. Our J. Donahue, PE directed the field work and S. Corcoran, PE (DE) observed the test drilling. The test boring locations are depicted on the attached Geotechnical Testing Location Sketch, LP-4. Test boring logs are attached.

MTFAX 20021 June 4, 2021 (Revised June 21, 2021) Page 6

LABORATORY TESTING PROGRAM

A. CONCRETE CORE LABORATORY TESTING

The concrete cores were delivered to our laboratory for measurement and photographs following the completion of the field work. Three cores (CC-1, CC-3, and CC-5) were prepared and sent to a laboratory subconsultant, Wiss, Janney, Elstner Associates, Inc. (WSE) in Cleveland, Ohio, for petrographic testing.

Three cores (CC-2, CC-4, and CC-6) were prepared and tested for compressive strength testing in our Pennoni concrete laboratory. Results of the petrographic testing and compressive strength testing are attached herein and summarized below. The results were previously distributed to the project team in draft form via electronic mail on May 19, 2021.

B. SUBSURFACE SOILS LABORATORY TESTING

Following conclusion of the SPT drilling, the soil samples were delivered to our laboratory for visual classification and testing. The geotechnical laboratory program consisted of the following:

• Moisture Content (ASTM D 2216) 10 tests

• Sieve Analysis (w/out hydrometer, ASTM D 422) 4 tests

• Atterberg Limits (ASTM D 4318) 1 test

Samples for testing were selected by Pennoni. Results of the laboratory testing are attached herein.

SUBSURFACE CHARACTERISTICS

A. GEOLOGY

According to the Geologic Map of Maryland (1968) the project site is within the Upland Deposits (Western Shore). The Upland Deposits typically includes gravel and sand, commonly orange-brown, locally liminite-cemented, minor silt and red, white, or gray clay. The Upland Deposits Include Brandywine, Bryn Mawr, and Sunderland Formations with typical thicknesses ranging from 0 to 50 feet. The Brandywine formation includes sand and marl of two levels and of different ages. The early Brandywine consists of about 50 feet of sand and gravel lying at altitudes of 200 to 400 feet and is believed to be of Pleistocene or late Tertiary age. The Bryn Mawr typically consists of reddish-brown to yellowish-brown silty quartz sand to sandy silt with interbedded medium to coarse clayey sand with gravel. The Sunderland formation is part of the Columbia group consisting of ice-borne boulders, gravel, sand, and loam, which change their mutual distribution both vertically and horizontally, after manner of shore deposits.

B. SUBSURFACE SOIL CHARACTERSTICS

Subsurface stratigraphy encountered within the SPT borings and test pits generally consists of a surficial layer of topsoil or bituminous asphalt underlain by fill material generally appearing to be re-worked on-site material consisting of loose to medium dense fine to medium sand with varying amounts of gravel and silt to firm to very stiff clayey silt with varying amounts of gravel and sand (Stratum F). The fill material was underlain by fine to medium to coarse sand and to some gravel with some to little silt (Stratum A).

Stratum F was not fully penetrated during our field exploration at locations TP-1 through TP-3. Brick fragments were observed in Stratum F material at locations TP-1 and TP-2. The Stratum A material was not fully penetrated during our field exploration at locations B-1 and B-2.

MTFAX 20021 June 4, 2021 (Revised June 21, 2021) Page 7

Pennoni observed an apparent void directly beneath the eastern most exposed leg of an existing tie at location TP-1. The void extended approximately 10 inches below the bottom of the tie in an approximate 6-inch by 18-inch area. A photo of the apparent void beneath the existing tie is attached (Photo P-17).

Based on the test pits and SPT borings performed by Pennoni and review of the historic test borings mentioned above, we have developed the following generalized subsurface stratigraphy utilized for our subsequent analysis and recommendations. The soils encountered in the test borings and test pits were generally dry to moist and signs of moisture trapped behind the wall were not encountered.

Table 1: Generalized Subsurface Stratigraphy for Design

Stratum Approximate

Thickness (feet) Description

USCS

Classification

T 0.75 – 1.0 TOPSOIL -

P 1 5” BITUMINOUS CONCRETE (ASPHALT) underlain by 7” of GRADED AGGREGATE BASE COARSE (GABC)

F [1] 18.25 to 19

FILL: Fine to medium SAND, varying amounts of silt and gravel (loose to medium dense, dry to moist) to FILL: Sandy SILT with varying amounts of gravel (firm to very stiff, moist)

A [2] 21.5 – … Fine to medium to coarse SAND and to some gravel with some to little silt (medium dense, moist)

SM

B [3] 8.3 – 20.8 Very fine sandy SILT (firm to stiff) ML to SM

C [4] … Silty CLAY to Clayey SILT, trace of fine sand to SILT, trace of fine sand (firm to stiff)

CL-ML

to

ML

NOTES:

[1] – The FILL stratum was only encountered in the Pennoni field exploration performed on May 7th, 2021.

[2] – Stratum A was not fully penetrated during the Pennoni geotechnical field exploration performed on May 7th, 2021. The estimated thickness of the stratum was estimated based on the historical borings performed in 1964.

[3] – The estimated thickness of Stratum B was estimated based on the historical borings performed in 1964.

[4] – Stratum C was only encountered and not fully penetrated in borings 21, 23, 24 and 25 based on the historical borings performed in 1964.

The subsurface soils encountered in the historic borings referenced above generally appear similar to the soils encountered during our field exploration. The Stratum B very fine sandy silt and Stratum C clayey silt to silty clay strata were not generally observed in the Pennoni field exploration but have been included in Table 1: Generalized Subsurface Stratigraphy for Design, as the historic borings were extended deeper than the borings performed as part of our exploration.

C. GROUNDWATER

Groundwater was not observed during our field exploration. Review of the geotechnical borings performed as a part of the soil boring data presented in the above referenced structural drawing titled, “Log of Soil Borings,” Drawing No. “7-1-3,” prepared by Abbott Engineering, Inc., dated February 4, 1965, indicate groundwater level readings were recorded after drilling and after approximately 24-hrs at borings labelled Hole No. 21, 23, 24, and 25, at depths ranging from 16.3 to 26 feet (elevation 246.8 to 260 feet) from existing grade.

MTFAX 20021 June 4, 2021 (Revised June 21, 2021) Page 8

RESULTS AND ANALYSES

A. GPR FIELD EXPLORATION – EXISTING RETAINING WALL REINFORCEMENT

Pennoni collected a total of 41 GPR profiles along accessible areas of the existing retaining wall’s front face. The historical drawing details are provided in Figures F-1 and F-2 attached. Review of the collected GPR profiles performed along the face of the retaining wall generally indicated the following:

• The interpreted vertical reinforcing steel spacing within the front face of the retaining wall was generally interpreted to be on the order of 6 to 12 inches (+/-) along scanned areas;

• The interpreted horizontal reinforcing steel spacing within the front face of the retaining wall was generally interpreted to be on the order of 12 inches (+/-) along scanned areas;

• Reinforcing steel clearances from retaining wall face to front face reinforcing steel were generally interpreted to be on the order of 2 inches (+/-); and

• The retaining wall thickness generally appears to be on the order of 16 to 17 inches (+/-).

The accuracy of the above general reinforcing steel measurements are limited by the accuracy of the geophysical equipment, with has accuracy limitations of +/- 5% of the total depth of the scan or ¼ inch, whichever is greater. The back face reinforcing steel identified on the above referenced historical drawings were not able to be interpreted during our geophysical field exploration due to a combination of potential limiting factors consisting of the presence of front face reinforcing steel, moisture within the concrete, and soil backfill on the backside of the retaining wall.

B. VISUAL OBSERVATIONS OF RETAINING WALL AND SITE FEATURES

Visual observations of accessible areas of the existing retaining wall were performed at the project site during our field exploration including the retaining wall face, exposed tie-backs, exposed retaining wall foundation, and surrounding site features. Observations at the existing retaining wall face included cracks, spalls, delaminated concrete, efflorescence, and exposed portions of reinforcing steel. Typical examples of these observations are shown on Photos P-11 through P-16. Typical signs of retaining wall sliding and rotation were not generally observed based on visual observations;

however, historical survey information was not available for comparison. The previously performed repairs appeared to be in various stages of disrepair including loss of concrete at the surface and exposed reinforcing steel. The thickness of the repairs was generally on the order of 2 to 3 inches thick. During concrete sounding of the retaining wall performed by McMullan (John O’Donald), the concrete patch repair appeared hollow sounding and after several blows with a hammer would fracture.

Visual observations of the retaining wall weep holes indicate the following:

• Generally appeared dry with evidence of minor flow through the weep holes, as observed in Photo P-12, attached;

• Primarily granular soils on the backside of the weep holes appeared dry to moist based on visual observations; and

• Efflorescence in areas around the weep holes was observed in Photo P-12, attached.

Visual observations of the concrete tie-back and existing foundation indicate that the above referenced historical drawings appear to generally coincide with measured dimensions (thickness, width, etc.) of the exposed tie-back and foundation. The following noted observations were made after exposing the concrete tie-back:

• An apparent void was observed beneath one leg of the concrete tie-back near the back face of the retaining wall, as observed in Photo P-17 attached.

MTFAX 20021 June 4, 2021 (Revised June 21, 2021) Page 9

• A localized area of exposed reinforcing steel was observed at one leg of the concrete tie-back at the back face of the retaining wall, as observed in Photo P-18 attached; and

• Concrete appears to be poorly consolidated at localized area on the exposed tie-back when the two legs of the tie converge to form one tie, as observed in Photo P-19 attached.

Additional site observations were made regarding site features at the project site and the following noted observations were made:

• An apparent depression of the existing pavement area in the area immediately adjacent to the below-grade structure was observed similar to the depression noted on the above referenced Gauthier, Alvarado, & Associates, Inc. plan set, as observed in Photo P-20 attached; and

• Observations of cracking and separation in existing concrete sidewalk areas from the backside of the retaining wall, as observed in Photo P-21 attached.

C. CONCRETE LABORATORY TESTING RESULTS

The concrete cores obtained were collected by Pennoni and delivered to our laboratory for measurement and photographs following the completion of the field work. Three cores (CC-1, CC-3, and CC-5) obtained were prepared and sent to a laboratory subconsultant, Wiss, Janney, Elstner Associates, Inc. (WSE) in Cleveland, Ohio, for petrographic testing. Three cores (CC-2, CC-4, and CC-6) were prepared and tested for compressive strength testing in our Pennoni concrete laboratory. The petrographic analysis included physical and chemical composition and alkali-silica reaction (ASR) of the concrete cores. ASR occurs at the interface between cement paste and the surface of reactive aggregates generating silica gel. When water is introduced into concrete with the cement paste and reactive aggregates ASR can be induced. Silica gel within the concrete absorbs the water and expands/swell in size causing the concrete to crack. The cracking of the concrete leads to further pathways for water to enter the concrete exacerbating the ASR condition and potentially leading to reinforcing steel corrosion within the concrete further distressing the concrete.

The petrographic analysis performed on concrete cores (CC-1, CC-3, and CC-5) generally indicate the following:

• Concrete cores determined to be composed of generally the same concrete mixture with a w/c ratio of approximately 0.45 to 0.50;

• Minor evidence of ASR was observed in the extracted concrete cores (concrete cores obtained from bottom portion of retaining wall below areas of distress);

• Alkali silica gel was observed for all three cores after exposure to simulated conditions with a high relative humidity;

• The cores were determined to be marginally or not air-entrained (air content ranging from 1.5% to 4.0%);

• Evidence of moisture migration was observed in the cores, but generally no evidence of freeze thaw distress; and

• Chloride contents were measured to be negligible.

The three concrete cores (CC-2, CC-4, and CC-6) were tested for compression in general accordance with ASTM C39 and C42. Prior to compression testing, each core was photographed in the received condition and subsequently prepared for compression testing in accordance with ASTM C42. The ends of each core were ground smooth using a horizontal lapping machine to conform to the smoothness and parallel requirements of ASTM C42. The cores were then moisture conditioned in sealed, non-absorptive bags for five (5) days prior to compression strength testing. The cores were trimmed to maximize the length to diameter ratio to 2 to avoid the need for correction factors. Photographs of

MTFAX 20021 June 4, 2021 (Revised June 21, 2021) Page 10 the cores in the received, prepared, and post testing are provided for reference in the attached report.

Table 2 below summarizes the physical dimensions and compression test results.

Table 2: Summary of Concrete Core Compressive Strength (ASTM C39/C42)

Core ID Received

Length (in)

Prepared Length

(in)

Diameter (in)

Area (in2)

Ultimate Load (lbs)

Compressive Strength

(psi)

CC-2 14.28 7.90 3.98 12.44 69,631 5,597

CC-4 16.92 7.96 3.98 12.44 88,365 7,103

CC-6 11.43 7.98 3.98 12.44 90,572 7,280

Average Compressive Strength (psi) 6,660

The observed failure modes for each core were observed to be Type 2 – “Well formed cone on one end with vertical cracking”, as defined by ASTM C39. The average compressive strength of the concrete cores tested was approximately 6,660 psi.

Further specific information and analysis regarding the petrographic analysis and compressive strength testing of the extracted cores is discussed in the attached petrographic and compressive strength testing report prepared by WJE and Pennoni, respectively.

CONCLUSIONS AND RECOMMENDATIONS

Based on the results of our document review, GPR surveys, laboratory testing, and analysis, we offer the following conclusions and recommendations in support of retaining wall repair/replacement design to be performed by McMullan.

A. RETAINING WALL REINFORCEMENT

The existing retaining wall generally appears to be approximately 16 to 17 inches thick, contain front vertical and horizontal reinforcing steel with spacing ranging on the order of 6 to 12 inches on center, and reinforcing steel clearances on the order of 2 inches. The geophysical findings of the concrete retaining wall appear to be in general agreement with the data indicated in the above referenced historical structural design drawings (1965). As noted in the results, the interpreted reinforcing steel was limited to the front face of the retaining wall. The back face reinforcing steel identified on the above referenced historical drawings were only partially able to be interpreted during our geophysical field exploration due to a combination of potential limiting factors consisting of the presence of front face reinforcing steel, moisture within the concrete, and soil backfill on the backside of the retaining wall.

B. RETAINING WALL DISTRESS

Observations at the existing retaining wall face included cracks, spalls, delaminated concrete, efflorescence, and exposed portions of reinforcing steel. The concrete patch repairs performed in 1992 appeared to be failing and were observed to be fractured when sounding the patch areas. The previously repaired concrete areas were generally observed near the top of the retaining wall.

Concrete cores were extracted along the base of the retaining wall and concrete laboratory testing performed on the samples consisting of petrographic analysis and compressive strength testing. The laboratory results indicated that the retaining wall concrete was generally intact, sound concrete with an average compressive strength of 6,660 psi. Minor evidence of ASR was observed in the concrete

MTFAX 20021 June 4, 2021 (Revised June 21, 2021) Page 11 cores and was generally not interpreted to be associated with distress observed within the concrete cores. The concrete cores examined indicated that the concrete was marginally to non-air entrained with evidence of moisture migration observed in each core. No indications of freeze-thaw distress were detected in the examined cores, but moisture migration was identified indicating the cores were exposed to significant amounts of moisture throughout the service life of the wall.

As mentioned above, the concrete cores were obtained at the base of the retaining wall. Signs of distress were noted along the full height of the retaining wall but more sever signs of distress were noted at higher elevations of the wall. The tested concrete cores and the results of the petrographic testing may not be fully indicative of the severity of distress observed at higher elevations of the wall.

The observed efflorescence along the face of the retaining suggests significant moisture migration through the wall is occurring. Moisture is required for ASR and freeze-thaw damage to occur in the concrete which further leads to retaining wall distress. ASR and freeze-thaw damage may be more prevalent than represented by the laboratory testing conducted on the concrete cores obtained at the base of the wall as evident from the more significant signs of efflorescence, exposed reinforcing steel, and cracking observed at higher elevations. Further exploration consisting of concrete cores obtained from upper portions of the retaining wall can be performed to confirm or refute these indicators based on visual observations of retaining wall distress.

Moisture migrating through the retaining wall indicated in the laboratory testing and visual observations suggest that water may be infiltrating through existing cracks in the wall. Water migration into the retaining wall will lead to further distress of the concrete and reinforcing steel through ASR and freeze-thaw action. Furthermore, the installed gravel drains and weep holes at the base of the wall may not be operating as intended to drain the water on the backside of the wall. The weep holes appeared dry during our field exploration and little evidence of water flowing out of the weep holes was observed (i.e., minimal staining on the face of the retaining wall beneath the weep holes). Recommendations for mitigating water infiltration and proper drainage of water from the backside of the retaining wall are provided in repair options below.

C. RECOMMENDATIONS FOR REPAIR OPTIONS

We understand that a wall repair option and a wall replacement option have been requested by GSA.

Due to the limited corrosion/distress observed at this time, it may be possible to repair only that section of retaining wall which was observed to be distressed and provide repairs that minimize migration of surface water to the back of the wall.

Wall Repair Option Based on our conversations with McMullan, we understand that the repair option currently being considered consists of the following:

• Removal and replacement of isolated areas of the wall where significant damage was observed and patching of other distressed areas

• Removal of existing concrete sidewalk and adjacent bituminous concrete to allow for access to the back side of the wall

• Removal of subgrade soils behind wall down to the concrete tie elevation

• Placement of waterproofing on the exposed backside of the wall down to the concrete tie elevation

• Replace excavated soil behind wall

• Place an impermeable soil layer behind the wall to minimize water intrusion behind wall

• Place a horizontal drain pipe above impermeable soil layer that ties into site drainage

MTFAX 20021 June 4, 2021 (Revised June 21, 2021) Page 12

• Placement of new concrete curb directly behind the wall with new flashing installed over the top of the wall and the curb

• Replacement of bituminous concrete asphalt behind wall and potential regrading of parking lot to promote positive drainage away from the wall towards the entrance ramp

• Cleaning and maintenance of the retaining wall weep holes could be considered for optimal performance of original design and repair intent.

The repair option, as compared to complete replacement, has the benefit of requiring less disturbance, minimal support of excavation, and does not result in complete demolition of the retaining wall. This option has the advantage of being less costly than complete replacement and less time intensive to perform the repair modifications; however, this option is not anticipated to result in a wall with the same service life as a complete wall replacement (i.e., the wall is still over 55 years old and will continue to deteriorate). Future maintenance and repairs should be anticipated. Excavation for a replacement wall may also disturb adjacent utilities including water, sanitary sewer, and storm sewer, as well as requiring greater disturbance to building and loading dock operations. A partial replacement/repair of the existing wall may impact building operations on a limited basis, but a complete shutdown of trucking operations may be avoidable if this option is selected.

If a repair option is selected, Pennoni will work with the design team to provide specific geotechnical recommendation for design of the repairs as the design progresses.

Wall Replacement Option Complete replacement has the advantage of replacing all damage as well as allowing for the areas behind the retaining wall to be addressed down to the foundation elevation. This is considered a “least risk” option in terms of potential for future issues associated with the retaining wall and retained area and will result in a longer service life as compared to wall repairs. This option has the disadvantage of being the higher design and construction cost, as well as the longer construction schedule to complete the work. Additional consideration if complete replacement of the existing retaining wall is selected is the impact on building operations and potential shutdown of loading operations. The construction operations may impact the loading dock areas and should be further discussed with the project team and client if this option is selected.

If the wall replacement option is chosen, Pennoni will work with the design team to provide specific geotechnical recommendation for design of the repairs as the design progresses. For conceptual design purposes, it is our opinion that a replacement wall can be founded on a traditional shallow foundations system (e.g., strip footings). We estimate a maximum net allowable bearing capacity of approximately 3,500 pounds per square foot (psf) for foundations bearing in the natural Stratum A medium dense sand generally encountered at the existing footing depth. Due to the variable composition and consistency and the potential for detrimental differential settlement, we do not recommend the foundations bear in the Stratum F fill material. Densification of the foundation subgrade in place prior to footing/foundation construction and/or localized undercuts to reach the bearing conditions suitable to achieve the allowable bearing capacity may be necessary if loose Stratum A sand material is encountered at foundation subgrade elevations.

The subgrades of all foundations exposed to freezing temperatures during construction and/or the life of the structure should be established at least 30 inches below adjacent exposed grades or otherwise protected against frost action.

MTFAX 20021 June 4, 2021 (Revised June 21, 2021) Page 13

D. LATERAL EARTH PRESSURE PARAMETERS

The soil parameters presented in the table below can be used to estimate lateral earth pressures to design below grade structures and temporary shoring (if necessary). All excavation bracing should be designed and constructed in accordance with OSHA earthwork standards. If the top of the structure is restrained from movement, thereby preventing the mobilization of active soil pressures, the structure should be designed using the at-rest pressure coefficient, ko.

The earth pressure coefficients are based on the assumption of vertical walls, horizontal backfill, no surcharges, no wall friction, and a safety factor of 1.0. Drainage behind the wall and incorporation of drains through the wall must also be considered to reduce hydrostatic pressure.

Table 3: Lateral Earth Pressure Parameters

Parameter Stratum F Stratum A Engineered Granular Fill

Foamed Glass Aggregate

(FGA)

Unit Weight 115 pcf 120 pcf 135 19

Angle of Internal Friction, degrees

28 30 36 40

Cohesion, psf 0 0 0 0 ka 0.36 0.33 0.26 0.22 ko 0.53 0.50 0.41 0.36 kp 2.77 3.00 3.85 4.60

Below grade walls should be designed to withstand lateral earth pressures and any surcharge loads within a 45° slope from the base of the wall. Based on Table 3.11.6.4-2 – Equivalent Height of Soil for Vehicular Loading on Retaining Walls Parallel to Traffic in AASHTO LFRD Bridge Design Specifications (2012), a vehicle surcharge load of 250 psf should be considered for loads located a minimum of 1 foot from the back of the wall.

The space between the back of the retaining wall and excavations limits should be backfilled with an inorganic, free draining granular material, free of debris. Our experience indicates that the on-site, near surface soils of Strata A can be reused for the construction, provided all organics and debris larger than 3 inches in its greatest dimension be removed prior to reuse. Significant amounts of cohesive soil (silt) were encountered within the Stratum F fill material and this material should not be reused for construction behind the wall. We recommend that the foundation walls be backfilled with soils that classify as SC/SM or more granular. High plasticity soils (MH, CH) are not acceptable for use as wall backfill. Provisions for importing structural fill should be included in the contract documents. If necessary, granular fill should consist of well-graded material with a maximum particle size less than 3 inches, not more than 20 percent passing the No. 200 sieve and have a plasticity index (PI) not greater than 8 percent. We understand that a lightweight aggregate such FGA is being considered for backfill behind the wall and recommended parameters have been included in the table above.

To achieve a desirable balance between minimizing excessive pressures against walls and reducing the settlement of the wall backfill, heavy earthwork equipment should maintain a minimum of 1-foot space per foot of vertical wall height away from the wall. Lighter compaction equipment should be used close to the wall and the loose thickness of the lifts should be reduced to no more than 6-inches to achieve proper compaction. The soil should be compacted in accordance with the Earthwork section below.

MTFAX 20021 June 4, 2021 (Revised June 21, 2021) Page 14

E. SUPPORT OF EXCAVATION

Depending on the option chosen, construction phasing, continued operation of the loading dock during construction, existing utility locations/depths, and other design/construction variables, support of excavation may be required during construction. Examples of support of excavation systems include benching/sloping of the soils, soldier pile and lagging walls, and steel sheet pile walls.

Benching/sloping of soils is typically the most cost-effective option for support of excavation. All excavation bracing should be designed and constructed in accordance with OHSA Earthwork standards.

Pennoni will work with the design team to provide geotechnical specifics to assist with design of support of excavation, if necessary.

F. GROUNDWATER AND SURFACE WATER MANAGEMENT

The groundwater observations made in the borings suggests that free-standing groundwater is not anticipated in repair/ replacement of the existing wall. In the event that wet conditions and/or seepage are encountered due to perched water, sumps and pumps should be adequate to maintain stable conditions during foundation construction. Additionally, drainage should be provided during construction such that surface water cannot be impounded behind the wall. The foundation excavations should not be used as a detention basin or sump. During construction, surface runoff should be prevented from entering the excavations by creating soil berms or diversion swales along the perimeter if the excavation is expected to be open for a long period of time. Where ponding does occur, the water should be pumped immediately and grades should then be established to prevent further ponding.

LIMITATIONS

This work has been performed in accordance with generally accepted professional practice in the fields of geotechnical and geophysical engineering. This warranty is in lieu of all other warranties either expressed or implied. Our conclusions and recommendations are based on the data revealed by this exploration. We are not responsible for any conclusions or opinions drawn from the data included herein, other than those specifically stated, nor are the recommendations presented in this report intended for direct use as construction specifications. This report is intended for use with regard to the specific project described herein;

any changes in loads, structures, or locations should be brought to our attention so that we may determine how they may affect our conclusions. An attempt has been made to provide for normal contingencies, but the possibility remains that unexpected conditions may be present which we are not aware of. If additional or contradictory data are revealed in the future, we should be notified so that modifications to this report can be made, if necessary. If we do not review relevant construction documents and witness the relevant construction operations, then we cannot be responsible for any problems that may result from misinterpretation or misunderstanding of this report or failure to comply with our recommendation.

MTFAX 20021 June 4, 2021 (Revised June 21, 2021) Page 15

We trust that the information presented in this report is what you require at this time and we thank you for the opportunity to assist you with this project. If you have any questions, or if you need any further assistance with this project, please contact this office at your earliest convenience.

Sincerely, PENNONI ASSOCIATES INC.

Steven J. Corcoran, PE (DE) J. Steven Donahue, PE Staff Geotechnical Professional Project Geotechnical Professional

Attachments: Site Location Plan SL-1…