RFQ No. 02-26-03 Attachment B.PDF

City of Keene City Hall Rear Wall Repair Project

Final Design Report July 26, 2022 *Final appendices updated January 2023 i

Table of Contents

Executive Summary ................................................................................................................ ii

I. Introduction and Purpose:

II. Background:

III. Investigations and Observations:

IV. Potential Cause of Movement:

V. Alternative Analysis:

VI. Conceptual-Level Construction Costs:

VII. Summary and Recommendations:

Appendix A: Prior Reports List Appendix B: Town Brook Culvert Exhibits Appendix C: Repair Exhibits Appendix D: Geotechnical Report Appendix E: Conceptual-Level Construction Cost Estimates ii

City of Keene, NH City Hall Rear Wall Repair Project

Executive Summary

The Preliminary Design phase of this project evaluates potential causes of movement and associated cracking at several areas of Keene City Hall identified by the City and the preparation of design alternatives to remediate and repair the areas, which include:

1) Secondary Front Entrance (Front Entrance, Vestibule area), the secondary front entrance area located on the western side of the building, which abuts Washington Street on City Hall, and;

2) Rear Wall, the two (2) story, bump-out area located on the eastern side of the City Hall building (rear wall), immediately adjacent to the entrance into the City Hall basement.

The primary objective of this project is to identify and implement a preferred alternative(s) to stabilize and repair the identified cracked areas of the building. The following is a summary of the work and implementation recommendations for consideration by the City.

Summary:

1. Project investigations suggest movement has continued since the cracks were recorded in 1982-1983, evidenced in part by enlarged brick cracks and movement, cracking around the front entrance vestibule and reports by City staff of interior changes such as jamming of doors. A survey scan conducted in November 2021 indicates the brick on the rear wall is out of plumb by over 3 inches.

2. A visual inspection of the timber floor joists that support the second floor in the rest room area of the bump-out indicate that outward movement of the wall is causing a separation with the joists. The visual review in March 2022 indicates an inch or perhaps more of apparent reduction / loss of bearing of the joists has occurred. This area should be repaired at this time to avoid a complete separation and loss of bearing, which could result in a collapse of the floor.

3. A structural engineering investigation commissioned by the City (2017 Report identified herein) stated that the brick cracking in the Rear Wall area “...constitutes an unsafe condition which should be corrected” and this current 2021 work does not dispute this statement.

4. Review of prior studies, records and analysis of soils as part of this project conclude there was likely negligible to no influence on the brick cracking in the building’s rear wall at the bump-out as a result of the 1982-1983 construction of building renovations and the adjacent City parking deck. The cracks may well have been present in a hairline condition prior to that work and made worse from construction activities.

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5. The continuing building movement is likely a result of changing soil conditions associated with the nearly 200-year-old Town Brook culvert that is located under the City Hall basement floor.

6. The 2021 investigation indicates foundation soil is likely eroding into and around the culvert via groundwater discharge and stormwater surges, causing a loosening / weakening of soils both under the basement slab and in the vicinity of the foundation.

The loosened/weakened soil needs to be stabilized and the areas repaired.

7. Left unattended, the soil erosion is expected to continue, decreasing the stability of the building foundation soils in the vicinity of the culvert. This will increase the potential for damages, injury and loss of life.

8. The Town Brook Culvert should be sealed and abandoned in-place to prevent future eroding of soil into it and the associated weakening of foundation soil.

Recommendations: The following recommendations are intended to be constructed in three

(3) phases. Phase I (items 1-3 below) includes immediate stabilization of the rear wall and movement monitoring. Phase II (items 4-9 below) includes movement monitoring, culvert abandonment, soil stabilization and repairs to the front and rear areas. Phase III, Foundation Reinforcement (item 10 below), would occur only if continued movement is detected through the monitoring efforts following implementation of Phases I and II.

Phase I: Immediate Action

1. Implement Rear Wall Repair Alternative #3, Repoint and Attach as soon as possible in the summer of 2022. This alternative, discussed in detail in Section V. below, provides for a stabilization of the floor joists at the rear bump-out wall. Design drawings suitable for competitive bidding and construction have been developed and were provided to the City in May 2022.

2. Visually monitor the floor joists at their bearing locations on the wall for ongoing movement on a regular (weekly) basis and be prepared to close access to the area, both interior and exterior, until the stabilization repairs have been installed and are judged to be working effectively.

3. Temporary bracing of the rear wall is an option until remedial work is conducted if the interior space use is to remain occupied.

Phase II: Near-Term Action (2-3 years)

4. Install movement monitoring equipment at the rear wall bump-out to monitor further movement of the brick. The City has been provided with a proposal to install monitoring devices, which consists of a series of crack gages (passive monitoring) and crack monitors (active monitoring). If movement is observed and the rate increases, the iv city should restrict public access to the interior spaces in the rear wall area (i.e., second floor restroom) and exterior areas within the fall zone of brick and other debris until repairs are implemented. Further movement of the brick on the rear wall bump-out may result in collapse without remedial work.

5. Implement Town Culvert Alternative #4, Abandon-in-Place with Polymer Foam Injection. Fill, seal and abandon the existing culvert and remove it from service. This alternative will eliminate the condition where surrounding soils could be eroding into and washing away from it, causing loss of foundation soil support. Roof and floor drains and other utilities that currently drain into the culvert will need to be relocated and rerouted as part of this work. Abandonment may cause potential impacts to the buildings located further downstream and thus will require coordination with the affected property owners to avoid or mitigate impacts.

6. Stabilize the foundation soils surrounding the culvert and the building foundation (both rear wall and front entrance areas) in the vicinity of the culvert using a polymer foam injection program. Polymer foam is designed specifically to strengthen loose soils in difficult to reach places and largely avoids significant excavation and soils work.

7. Rebuild the front entrance vestibule and the associated deteriorated structural steel and foundation bearing and support areas under the Front Entrance vestibule area.

Stabilization of the underlying soil would be treated with the polymer foam injection program.

8. Continue a Long-Term Movement Monitoring Plan following construction of Alt #3 for future movement of the rear wall. The objective is to determine the extent the polymer foam is able to stabilize the soils and arrest the wall movement.

9. Prepare Construction Documents (plans and technical specifications) for Items 2-6 at this time, and move forward with the implementation of Items 1-6 above.

Phase III: Long-Term Action (if needed, should movement continue)

10. Budget for design and construction funding to install Foundation Reinforcement support systems if long-term monitoring indicates building movement is not arrested or reoccurs after the polymer foam injection program has been implemented.

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Project Cost Summary: An itemized construction cost estimate was prepared for the alternatives identified above. The results are included in Appendix E and summarized below.

Recommendation Number and Description Recommendation Alternative Description

Conceptual Cost

(rounded) Phase I: Immediate Action

1 Bump-Out Stabilization Repair: $50,000 Phase II: Near-Term Action (2-3 years)

2 Wall Movement Monitoring Equipment $50,000 3 Town Brook Culvert: Abandon-in-Place w/Polymer Foam Injection $375,000 4 Stabilize Foundation Soils with Polymer Foam Injection $100,000 5 Front Vestibule / Entrance Repair $300,000 6 Rear Wall: Repoint & Attach $150,000 Phase II subtotal = $975,000 Final Design & Quality Assurance = $300,000 Total Phase II= $1,275,000

Phase III: Long-Term Action (if needed, should movement continue) 9 Foundation Reinforcement Helical Piles and Concrete $500,000 -

$750,000 Final Design & Quality Assurance = $150,000

Total Phase III = $650,000 - $900,000

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I. Introduction and Purpose:

DuBois & King, Inc., (D&K) and our project partners Sanborn, Head & Associates, Inc. (Sanborn Head) and PC Construction (PCC) were retained in February 2021 by the City of Keene (City) to investigate several specific locations of movement (and associated cracking) of the City Hall building’s exterior fascia. The building is located at 3 Washington St. in Keene. The City desires to develop plans and specifications for construction of repairs to these areas.

The City has recognized there are multiple areas of cracking and related deterioration throughout the City Hall building and adjacent structures. However, the City’s scope of work specific for this project is limited to two (2) areas identified below.

1) Secondary Front Entrance (Front Entrance), the secondary front entrance area located on the western side of the building, which abuts Washington Street on City Hall,

2) Rear Wall, the two (2) story, bump-out area located on the eastern side of the City Hall building (rear wall), immediately adjacent to the entrance into the City Hall basement;

The purpose of this project (to the extent practical and within the scope of this project) is to identify the primary cause(s) of the movement, evaluate several repair alternatives and to prepare preliminary repair designs and associated project costs. Final plans and construction documents would be prepared in a subsequent contract to be negotiated in the future.

A key issue associated with this project is the poor underlying subsurface soils under the building. As such, a geotechnical investigation was included as part of the project scope of work. The conclusions of this investigation are summarized below, with details presented in Sanborn Head’s geotechnical report, which is appended to this main report.

II. Background:

Based on reports from City staff and prior investigations, the cracks in the rear wall were first documented following construction of the City Hall parking deck in 1983. Since then, several staff noticed cracking, and investigations into the cracking have been conducted and summarized in technical reports.

1) Prior Reports: A number of prior reports and investigation summaries were compiled by the City and provided to the D&K team as reference material for this project. A listing of the reports is included in Appendix A.

a. The most recent prior structural, visual condition assessment was performed on behalf of the City by Engineering Ventures, P.C. (EV) dated August 8, 2017 (2017 Report). The assessment encompassed the entire building, therefore much of the report covers topics outside of D&K’s focus/area of work as described above.

Relative to the Front Entrance, the 2017 report noted the cracking in the pavers and ceiling below, severe corrosion of steel beams supporting the entry slab, and wall damage in the area. Foundation movement (no further description) was stated as

Page 2 of 20 26 July 2022 the cause of cracks observed in the pavers and ceiling below, steel corrosion was presumed to be caused by compromised slab joint sealant, and the installation of pipe penetrations was thought to contribute to the wall cracking. The report recommended reconstruction of the entry floor structure with steel beams, concrete slab, and waterproofing. It further recommended repairing damage from pipe penetration installation and repairing the crack foundation walls with grout injection or concrete facing.

Relative to the Rear Wall, the 2017 Report noted the severe cracking in the brick walls but did not offer an opinion as to what may have caused the damage in this specific area. A suggested potential repair would consist of construction of wood or cold-formed steel stud walls and new foundations inboard of the cracked, brick walls. The new walls would act as vertical support for floor and roof framing, act as lateral support/shear walls, and act as back-up structure that the unreinforced brick walls can be tied to.

The 2017 Report also referenced the 1983 Soils Engineering, Inc., geotechnical report for the design of adjacent two-story parking structure and, based on information in this geotechnical report along with the 2017 visual observations, offered a generic opinion that foundation cracking was likely caused by differential settlement of the underlying soft clay layer. The 2017 Report further opines that future settlement will likely be minimal due to the decades-long time passage since construction/renovation. The 2017 report also recommends that, although the building movement is likely unrelated to the Town Brook, the City continue to investigate the waterway.

Other, general observations, deficiencies and recommendations were included in the 2017 Report concerning large portions or the entirety of City Hall. It was recommended that an analysis be conducted regarding these deficiencies and that repairs and upgrades be made as required.

2) Town Brook Culvert Location: Multiple prior reports describe the history of the Town

Brook culvert, and as such its background is not reiterated in this report.

a. The upstream limit of the Town Brook culvert is located outside the building at manhole labeled as AMH MH-1 on the attached sketch provided by the City DPW.

This manhole is approximately 25 ft off the northwest corner of the City Hall building, just off the curb line adjacent to the drive entrance to the backside of the building.

b. The downstream limit of the Town Brook culvert is on Roxbury St, where the culvert discharges into the recently (2021) reconstructed City storm drain system.

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c. Significant portions of Town Brook upstream of manhole AMH MH-1 had been previously intercepted and diverted into the City’s storm drain system. The culvert itself does not exist upstream or downstream of these manholes.

III. Investigations and Observations:

1) Visual Review of two (2) Cracking Areas: D&K visited the site on several occasions to visually observe the structure and the cracking at the Front Entrance and the Rear Wall crack areas.

a. The first visit was conducted on March 17, 2021. The focus of the visit was to observe cracking at both the front entrance and rear wall, but it also included observing other areas and the building as a whole to collect any information that might aid the project. Observations from D&K’s March 17, 2021, site visit are summarized below:

b. Front Entrance (west side of building):

ii. The cracking and related damage within the front entrance observed on March 17, 2021, is consistent with that described in the 2017 report. The extent of cracking extends along the entire periphery of the door, along the archway over the door along the edge line with the ceiling, vertical cracks along each corner where the doorframe intersects with the brick walls, and along the base of the door.

iii. Concrete deterioration was observed on the exposed face of the entrance stair and ramp retaining wall parallel with Washington Street. This deterioration appeared to be freeze-thaw deterioration and spalling at embedded guardrail post location and not attributable to building movement. City made repairs to this area during 2021 based on observations during subsequent site visits.

iv. Step cracking was noted in the interior masonry basement walls that are oriented perpendicular to the exterior basement wall along Washington Street. Step cracking is diagonal cracking that generally follows the mortar joints of masonry, this type of cracking is typically an indication that masonry below the crack has settled. In this case, the step cracking suggests differential settlement towards the interior of the building.

v. Cracking observed in masonry walls below the front entrance where plumbing lines penetrate the walls, reference Photograph 1. Cracking appears to have resulted in some movement of the floor framing above.

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vi. Subsequent to the March 17, 2021, site visit, the 1982 city hall renovation drawings were reviewed to see if any modifications were made that may have increased loading and thus potential settlement on the interior of the building. While some modifications were made in the area of the entrance (to build a ramp and infill a stair opening, brick pavers were also added), no appreciable load increase or load path modification were noted.

c. Rear Wall (east side of building) and City

Hall basement:

i. The cracking observed in the brick masonry exterior wall on the north, east and south faces of the rear wall bump-out appear consistent with those described and photographed in the 2017 report. Stepped cracking along the north elevation is indicative of settlement to the east and step cracking on the east elevation indicative of settlement to the south. The east elevation wall is out-of-plumb from the foundation to the second floor with rotations towards the parking structure to the east. The cracking pattern observed is indicative of the bump-out settling towards the adjacent parking structure footings.

ii. East elevation wall rotations are more pronounced at north end than the south.

Information taken from a Nov. 2021, 3D scan of the building indicates the east wall is 3.28-inchs out of plumb at the north end and 0.69-inchs at the south end.

iii. Building structural systems at the rear bump-out are understood to consist of board sheathing supported by wood floor joists and rafters. Floor joists and rafters are supported by the multi-wythe exterior brick masonry walls. No mechanical attachment between the floor or roof framing and brick masonry wall was observed. The east brick masonry bearing wall has rotated to the east towards the parking garage resulting in the framing pulling out of the bearing pockets.

Photograph 1: Steel framing under vestibule

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iv. The City is not aware of any official crack width measurements or monitoring for comparison to the current conditions. Crack width measurements appear to be written on the north wall at the northeast corner of the rear bump-out, see Photograph 2. This writing seems to indicate a crack width of 3/4-inch at an unknown date and 1 1/8-inch on 5/19/2010.

This information indicates the wall has moved since 2010.

v. As part of the 1982 renovation, an addition containing a stair and elevator shaft was added to the northern four-story portion of the building. Existing drawings indicate this addition is supported on pile foundations. Cracking of the existing structure is located north of installed piles for the building addition. These piles were also installed just west of the Town Brook Culvert. We know through our subsurface evaluation program that the soils at and around the Town Brook culvert are loose, and it is possible that this pile driving activity may have contributed to the existing building cracks. The observed cracking appears that the rear wall bump-out is settled away from the building towards the parking garage.

vi. Subsequent to the March 17, 2021, site visit, the 1982 City Hall renovation drawings were reviewed to see how the modifications may have affected this area. Based on this review, the 1982 renovation work may have contributed to the building movements observed. These drawings indicate original windows to be removed and infilled at the rear bump-out. As wall cracking is within the window infills on the north elevation, this provides a point in time when cracking was not present at the rear wall bump-out.

d. A second visit was conducted on October 7, 2021, by D&K. PC Construction (PCC) staff was on-site to perform selective demolition in the basement under the front entrance area to observe the structure and its condition. Observations from this site visit are summarized below.

Photograph 2: Crack Width Field Measurements at North-east Corner

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Figure 1 Partial Basement Floor plan – Excerpt from 1982 Drawings

i. The City Hall basement area is divided into two spaces below the front entrance that are not interconnected, reference Figure 1 for a depiction of these spaces.

Based on the 1982 renovation drawings, the dividing wall is centered under the front entry. Starting in the north basement space, PCC removed ceiling finishes to expose the underside of the front entrance floor structure.

ii. PCC also removed ceiling finishes in the south basement space to confirm that the construction is the same on both sides of the divider wall. As the ceiling finish was difficult to remove, selective demolition was limited to understanding the structural layout and framing condition. Cracking in the ceiling generally coincided with the steel beams, and the cracked finishes were removed to uncover the beams.

iii. The front entry floor structural system consists of steel angles spanning between interior masonry bearing walls. The steel angles support a reinforced cast-in-place concrete slab constructed on a steel mesh stay-in-place-form. A cementitious or plaster ceiling finish was applied to the underside of the steel mesh.

iv. In the north basement space, steel to the north appeared to be in relatively good condition, but to the south near the divider wall, steel corrosion was prevalent, especially to the vertical legs of the angles. Corrosion observed is attributed to water infiltration through front entry cracks.

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v. In the south basement space, all visible steel appeared to have significant corrosion and section loss. Where corrosion was present, the toe of horizontal angle leg was rotated down and away from the slab. This rotation appears to be associated with the volumetric increase of deteriorated steel pushing the angle leg away from the underside of the slab. Corrosion observed is attributed to water infiltration through front entry cracks. Some of the angle bearing seats appeared to be compromised and in need of reinforcement/repair.

vi. The steel mesh formwork was corroding, more so in the south basement space.

Concrete in contact with the corroded steel mesh was typically loose, but upon removing the layer of loose material, sound concrete was encountered.

Deterioration observed is attributable to water infiltration through front entry cracks.

vii. While section loss and corrosion of framing has taken place, excessive deformation and movement was not observed. Movements at the front entrance do not appear attributable to the steel framing deterioration.

viii. The interior face of the basement wall along Washington Street was not plumb.

Using a two-foot-long level, it was estimated that the top end was pitched approximately 1/2-inch out towards the exterior. It is unknown if this is related to building movement or original construction.

ix. While on site, City-employee James Kirby stated that he hung new doors on the occupied floors in November 2020 and that several of the doors have since been trimmed/cut to keep them functional due to movement of the building.

x. While on site, the City opened an access hatch to the Town Brook within the basement to see if perhaps it was drier than normal. It was observed to have standing water and is appeared to be consistent with prior observations.

2) Town Brook Culvert: The culvert’s current location, configuration and material type has been generally documented through prior reports and assessments and outlined below.

a. In November 2021, the City’s Engineering Department compiled the most current map depicting the culverts’ location and sections of various materials that constitute the culvert. Several exhibits, which were prepared by the City and are located in Appendix B, depict the culvert location, material sections, roof drains and associated utilities.

b. One objective of this project was to clean out the accumulated sediment from the culvert under the basement floor and conduct a video inspection of it to the extent possible. The video inspection was to facilitate an opinion of the culvert’s condition and the ability to line the culvert and maintain its current service or if it requires abandonment.

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c. The process began with the City installing an access port manhole (AMH MH1) on the upstream end of the culvert on the exterior of the building. The manhole installation was completed on August 17, 2021, along the edge of Washington St.

near the northwest exterior corner of the building.

d. Eastern Pipe Services (EPS) was retained to remove the accumulated culvert sediment using this access port. EPS initiated this work on August 19, 2021. The video camera was able to transverse approximately 26 ft along the culvert until it met a tree root ball obstruction, which corresponds with a tree located in the greenway adjacent to the sidewalk in front of the building. This 26-ft point in the culvert was estimated to be only several feet upstream of the building wall and foundation. The remote camera was not able to maneuver beyond the root ball obstruction, and the video was terminated at that point.

e. Additional access ports were reviewed, such as the Roxbury St culvert and the existing port located in the basement, but none were considered achievable by EPS due to limiting factors such as inaccessibility for the EPS vacuum truck in the back of City Hall and too many obstructions in the culvert along the Roxbury access manhole.

3) Specialty Contractor Site Review: Sanborn Head arranged for and coordinated the site review of Town Brook Culvert and the foundation on January 19, 2022, with a summary outlined below.

a. Richard Porter, a representative of Engineering Foundation Technologies (Nashua, NH) specializes in the application of structural polymer for soil and foundation stabilization. Mr. Porter opined that the filling of the culvert and strengthening the loose soils in the immediate vicinity of the culvert is a suitable application of polymer foam.

i. Polymer foam is a two-component, high-density, expanding thermoset polyurethane system. In general, structural polymers are injected into the base and subgrade soils at targeted depths and the material expands and hardens to fill soil voids. As it cures, polymer foam stabilizes and stiffens loose and/or weak soils.

It is able to strengthen weak soils under foundations, thus increasing bearing capacity. It is also able to encapsulate and seal buried infrastructure such as the culvert.

ii. The application of polymer foam injection technology has been incorporated into the alternatives discussed below because the engineering team concludes it will achieve the project objectives while avoiding significant impacts to the City Hall basement for a competitive cost.

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b. Jeffrey Costa, P.E., a representative of Helical Drilling (Braintree, MA), also reviewed the site with Sanborn Head on Jan 19th. Mr. Costa’s expertise focuses on the use of helical piles to provide strength to foundation systems that experience movement for a variety of reasons, including poor underlying soils. He was able to provide our team with valuable technical and cost information relative to installation of helical piles to reinforce the City Hall foundation along the rear wall area. Mr. Costa also provided cost information to seal the culvert with cement grout as an alternative to polymer foam injections. Mr. Costa’s provided information has been incorporated into several of the repair alternatives identified below.

4) Geophysical (GPR): Hager-Richter Geoscience (HRG) (Salem, NH), under subcontract with Sanborn Head, conducted Ground Penetrating Radar (GPR) scans in the sidewalk along Washington Street on April 12, 2021, and on the basement concrete slab on October 4, 2021. The objective was to identify and map the location of the Town Brook culvert, to identify and evaluate potential any voids below the slab in the vicinity of the culvert near any exterior footing. HRG’s report is included in the Geotechnical Report in Appendix D and is summarized below.

a. There were three (3) priority areas to scan in the basement based on areas that were accessible with GPR equipment, including:

i. Area 1 is within the Storage Area of the basement,

ii. Area 2 is in the Boiler Room and,

iii. Area 3 is in the front part of the building, adjacent to Washington St. The sidewalk along Washington street was also scanned but not included in the GPR report as this work was used to locate a suitable location for AMH MH1.

b. The Town Brook culvert was sensed in each area. Areas 1 and 3 had indications of disturbed soils or possible voids. Area 2 had indications of dense gravel soils. Both of these observations were confirmed by slab cores and hand auger probes (see Section 4.3 of the HRG report).

c. Prior to Sanborn Head’s project involvement, Catamount Subsurface Investigation performed a ground penetrating radar survey on the inside of the City Hall building with the intent of locating the brick arch culvert and any voids under the slab. This report is appended to the Geotechnical Report.

5) Soil Borings: Sanborn Head conducted a subsurface exploration program between

August 17 and August 19, 2021. The subsurface exploration program consisted of two

(2) test borings (SH-1, SH-2) as shown on the Geotechnical Report’s Figure 2. Two monitoring wells were also installed. MW- 1 was installed adjacent to SH-1 and MW-2 was installed in boring SH-2. Utilities near the test borings were cleared by HRG’s GPR work on August 13, 2021, before soil boring work was performed.

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6) Slab Cores and Hand Auger Probes: Sanborn Head conducted an additional subsurface exploration program on October 26, 2021, in the basement of City Hall.

a. The subsurface exploration consisted of drilling 13 basement floor concrete slab core locations within the Storage Room (located on the east side of the building) with two (2) of the location being hand auger probes (TH-1 and TH-2) and 12 slab core locations within the boiler room as shown on Figure 3. The purpose of the slab cores was to observe any voiding below the floor slab (as sensed by the GPR work), as well as to evaluate the location of the Town Brook culvert.

b. PC Construction performed the slab cores using hammer drill with a 2-inch rotating bit. Once the concrete slab was penetrated (slab was 1 to 2 inches thick in the Storage Room, and varied between 3 to 8.5 inches in the Boiler Room—see table on Figure 3 of the Geotechnical Report). An extension was placed on the drill to extend the probe to at least 5 feet below grade. TH-1 and TH-2 were hand auger probes using a 1-inch diameter sampler. Samples were obtained continuously from below the concrete slab to 2.6 ft below grade (TH-1) and 5.3 ft below grade (TH-2).

c. Within the Storage Room, the crown of the brick culvert was encountered approximately 27 inches below the slab, as evidenced by core bit refusal. Fill soils were observed on top and sides of the culvert, which consisted of brown loose silty sand that was moist to wet. At about 5 ft below the slab, a gray denser silty sand was observed. Groundwater was observed about 3 to 4 ft below the slab.

d. Within the boiler room, a vapor barrier with 1 to 2 inches of crushed stone was located below the slab. Below the crushed stone was a dense sand layer which could only be penetrated by 1 to 2 inches using a T-bar hand probe. No soil samples could be obtained with the hand auger probe.

7) Three-Dimensional Survey Scan: A high definition (HD) survey scan of the City Hall building exterior and areas of the basement within the project area was conducted on December 6, 2021. Survey scan targets were installed at several locations on the exterior brick rear wall in the vicinity of the cracking.

a. The objective of the survey scan was to 1) create an accurate record of the current extent of cracking in the two areas and 2) provide a basis for long-term measurement of these two areas for movement. The survey scans are a digital product and the deliverable are provided to the City in cloud-based, hyperlink format. The scan is downloadable for long-term recordkeeping and use.

b. The HD scan provided important information regarding the current condition of the rear wall. Measurements from the scan indicates the wall in the vicinity of the cracks is out of plumb by upwards of 3 inches.

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c. A plan view of the building and basement floor is prepared from this scan and intended for use in subsequent design and construction use.

IV. Potential Cause of Movement:

Section 7.0 of the above-referenced geotechnical report provides detailed discussion as to the probable causes of the settlement of the foundation and the associated cracking that this project is addressing. In summary, the geotechnical report concludes:

1) Clay Consolidation: Consolidation of the underlying clay layer likely occurred during the original construction of the City Hall building. This clay consolidation would have been largely complete by time the construction activities in the early 1980s City Hall and parking garage occurred. It is possible that some of the 1980s construction may have been the initial cause of settlement and associated brick cracking. However, the geotechnical investigations for this project indicate this clay layer has largely stabilized, and since there has been no known increased loading from the building (and actually there was a reduction of load to the main building based on the historic fire that eliminated the upper floor of the City Hall), ongoing consolidation of the clay is not likely occurring. Therefore, while clay consolidation may have contributed to past movement, it is not anticipated to contribute to future movement, as consolidation settlement is believed to be largely complete.

2) Town Brook Culvert Leaking causing loss of Soil Strength: As stated above, a majority of the culvert’s interior could not be video inspected due to inaccessibility, so the full extent of culvert deterioration is unknown. However, prior photographs and videos do indicate some level of damage and deterioration of the brick.

a. Given the culvert’s 200-year-old age and presence of silts and sediment in the culvert’s interior, it is probable the surrounding water table is leaking into the culvert, eroding the fines from the soil matrix, loosening the soil.

b. It is noted the culvert has likely been damaged during past construction and soil boring activities. For example, the 1980 geotechnical boring report noted that brick, voids and wood were encountered. Because the soil boring was taken in close proximity to the culvert, and because the culvert consists of brick, voids and wood, there is a high likelihood the soil boring auger and hollow stem casing penetrated into the culvert causing damage and providing an entrance opening for soil to erode into.

c. The City has also stated the culvert has experienced pressurized conditions and surging of stormwater within the culvert during storm events. The condition of repeated (over time) pressurized water followed by a negative pressure surge during receding conditions is also a likely contributor to soil fines being eroded into the culvert and washed away.

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d. The results of the hand soil probes below the basement floor slab (discussed above) identified loose soils adjacent the culvert pipe in the basement of the building bump-out at the rear wall. The probed loose soils support the belief that groundwater is likely leaking into the culvert causing soil fines to erode from the soil matrix and loosening the soil in the vicinity of the rear wall foundation. This loosened soil in turn reduces the soil bearing capacity and induces settlement of the footings.

e. This loosened soil could be a contributor to the foundation movement (as stated above) which would cause and/or perpetuate a wall crack condition.

V. Alternative Analysis:

The team has identified and evaluated several alternatives to address the underlying issue of continued loss of soil and associated reduced soil bearing capacity. In addition, we have identified and evaluated several alternatives to repair the cracked areas in the Rear Wall and Front Entrance areas, as outlined below.

Further, a phased approach for the implementation of these alternatives is recommended. The first step will be to eliminate the erosion of soils into the culvert and to strengthen the surrounding soils. Once the soils are stabilized and unable to further erode into the culvert, the rear wall and front entrance vestibule areas would be repaired. From there, a monitoring program of the wall areas is recommended to assess if movement has been arrested. The foundation reinforcement work identified below would be deferred until or if movement reestablishes.

A No-Action (or Do-Nothing) alternative was briefly discussed during the identification and consideration of alternatives. The engineering team did not further develop this alternative, and we recommend against its consideration, because no action is expected to continue to allow the erosion of foundation soils, further decreasing the foundation bearing. Left unchecked, the movement of the building is expected to continue along with the ever-increasing probability of a structure collapse. The foundation soils need to be stabilized in order to reduce the potential for a structure collapse.

1) Town Brook Culvert

The total length of the culvert to be modified is estimate to be 175 ft, described as follows: The length of the culvert under the City Hall portion of the building itself was field measured by DPW to be 100 ft. The culvert length from the edge of City Hall on Washington St to the upstream access manhole is 40 ft. Based on discussions with the City, the assumed downstream (easterly) end of the culvert abandonment will be at the City’s property line, near the walkthrough heading to Roxbury St. This equates to an overall culvert abandonment length of approximately 230 ft. The remaining length of the culvert (easterly) to the new manhole on Roxbury St will be addressed at a later time by others.

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2) Town Brook Culvert Alternative #1, Slip-line the Existing Culvert with a Cured-in-Place Pipe line (CIPP), and leave the culvert in service: This alternative is based on leaving the culvert in place and in service. The basement floor slab would be cut at predefined intervals to expose the culvert and provide direct access to it for cleaning and for installing and curing the CIPP. Unsuitable and loose soil surrounding the brick culvert would be removed to the extent it is accessible and replaced with a structural granular borrow and compacted in place with a small plate compacter. Underlying and/or inaccessible loose soils could be strengthened with polymer injection foam. This alternative would require additional cleaning effort to create a suitable culvert interior to install and cure the CIPP. Manual work around the roof and floor drain pipe ports (plus other drain penetrations) would be required to create the proper seal. The drains themselves would not be lined with CIPP due to accessibility and because they are located above gradient. Several access portals for the slip-line alternative would be installed.

3) Town Brook Culvert Abandonment Alternatives: The preferred method to reduce the potential for future foundation movement is to eliminate the existing condition in which the underlying soils are able to erode into the culvert or be washed away from the exterior of the culvert and then to stabilize the soils to the extent possible. Three (3) alternatives have been considered to achieve this preferred method, as outlined below.

a. Alternative #2, Remove the Existing Culvert: remove the culvert from under the basement slab and from service: The actual brickwork of the culvert, to the extent it is accessible, would be removed, unsuitable and loose soil removed and backfilled with compacted structural granular borrow and roof drains relocated. The practicality of this alternative is considered to be poor, because the presence of the basement interior walls (i.e., several load bearing masonry walls) would restrict the cutting of the floor slab and access to the culvert.

b. Alternative #3, Abandon the Existing Culvert In-Place: fill the culvert to prevent water from leaking into it and remove it from service. As with Alt #2, the roof and floor drains would need to be relocated to a different outlet. Accumulated sediment would need to be removed, the culvert cleaned and filled with flowable fill and/or cement grout. Much of the brick would remain in place but would be sealed off, thus eliminating the potential for the movement and erosion of subsurface soils.

c. Alternative #4, Abandon the Existing Culvert with Polymer Foam Injection: based on field input from a specialty contractor (Richard Porter, Engineered Foundation Technologies, LLC), the culvert could be filled with a polymer foam. The polymer foam technology is able to compact and solidify the existing accumulated loose silts and sediments located in the culvert, thus avoiding the need to clean the culvert.

The foam would also be injected on the outside perimeter of the culvert pipe to strengthen the loose soils around the pipe providing a soil improvement to the

Page 14 of 20 26 July 2022 bearing soils for the slab and building foundations. Refer to Section III. 3) above for discussion on polymer foam.

d. The culvert abandon-in-place alternatives would generally include:

i. Relocate the roof, floor, and yard drains that currently discharge into the culvert from City Hall. Prior investigations by the City DPW indicate several roof and floor drains tie into the culvert. The drains would be relocated to the nearest accessible drain system in the vicinity of City Hall.

ii. Alternatives 1, 2 and 3 will require cleaning the culvert and removing the accumulated silts and sediments. Because attempts using a vacuum truck in 2021 were unsuccessful, we have assumed cutting of the basement floor slab at defined intervals to expose and access the culvert’s interior.

iii. To access and clean the culvert, D&K has assumed that five (5) cut locations would be required. Each cut would be 5 ft in length along the culvert and 4 ft in width.

Remove the top section of brick off the culvert at each cut location. Use a portable vacuum and water pressure system to mobilize and remove accumulated sediment. Where the culvert was exposed, the project would include the removal of loose / unsuitable soil from the exterior of the soil where needed. Backfill with plate-compacted structural granular backfill (sands/gravels).

iv. Each alternative includes the strengthening of loose subsurface soils around the culvert, particularly in the vicinity of the building’s foundation walls. Polymer foam injection is an effective technology that is expected to be included with these improvements due to the limited access to the culvert in the basement.

e. Alternatives 2 and 3, Abandon-in-Place alternative, would require adequate levels of accumulated sediment removal to install a flowable fill/concrete grout to sufficiently fill and seal the culvert interior.

f. Alternative 4, Polymer Foam Injection, avoids the need to cut the basement floor slab. The polymer foam would be injected into the culvert and surrounding soil by drilling ¾-inch injection ports through the concrete slab and pumping the polymer foam through the ports. Based on the site visit discussed in Section III. 3) above, three (3) rows of injection ports would be installed at a 5-ft interval along the treated length of culvert. One row would be located over the center of the culvert and the injection port would extend to the invert of the culvert. The remaining two rows would be placed along the exterior of the culvert to strengthen the loose soils.

Additional injection ports would be installed along the foundation wall to strength the soils in those areas. Access into the constrained basement is possible because small, portable equipment is suitable to drill and install the injection ports and to pump the polymer foam into the ports.

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g. Town Brook Culvert Recommendation: As stated above, the preferred long-term solution is to eliminate the potential for water to leak into the culvert and the associated migration and erosion of subsurface and foundation soils into it.

Therefore, removing the culvert from service is the preferred alternative and is recommended for this project.

i. The most cost effective and practical way to accomplish this is Alternative 4, Abandon the Culvert in Place with Polymer Foam Injection.

ii. The culvert does not require cleaning, as the accumulated silts and sediments can be intermixed and compacted with the polymer foam and becomes hardened within the cured polymer matrix. In addition, the team understands that organics, such as leaves and root wads, will also mix into the matrix.

iii. The inclusion of polymer foam injection for soil improvement is part of this alternative in the vicinity of the building foundation walls. As indicated above, the investigations identified the presence of loose soils in the vicinity of the foundations, which is believed to be contributing to the foundation movement.

The polymer foam injection is expected to strengthen the soils and reduce the potential for future settlement and associated movement.

iv. Because the upstream limit of the culvert begins just upstream of the City Hall building and several known roof and floor drains tie into it, there are no significant adverse impacts from removing it from service. The key risk to this alternative is any unknown roof or lateral drains that discharge into the culvert could cause water damage elsewhere.

v. Final design will include the use of dye testing of all known and visible roof and floor drains to confirm and rule out connection with the culvert.

4) Building Crack Repairs: Three (3) alternatives have been considered to stabilize and restore the integrity of the Rear Wall area. Based on observed condition, only one option was deemed feasible for the front entrance. All of the building repair options assume that no additional foundation movement is anticipated.

a. Rear Wall Repair:

i. Alternative 1: Remove and Replace. This alternative consists of deconstructing the existing rear wall in the vicinity of the bump-out. The wall would be reconstructed using modern construction materials and techniques. Modifications could be made to the building to permanently remove the existing rear wall, although this was not considered as an alternative.

ii. Alternative 2: Add Stud Bearing Wall. This alternative aligns with recommendations contained in the 2017 Report and consists of modifying the

Page 16 of 20 26 July 2022 foundation and adding a new light framed stud wall on the inside of the existing brick masonry wall. The new stud wall would support the existing floor framing and the existing brick wall attached with post-installed renovation ties. Cracking at exterior brick surfaces would be repointed. This alternative requires demolition and reconfiguration of interior spaces to allow for the completion of work. The City is advised the mortar bed thickness at the crack locations will be visibly wider than adjacent areas.

iii. Alternative 3: Repoint and Attach. This alternative consists of repointing cracks and resetting loose brick in the existing masonry walls and installing stainless steel dowels across the joints (i.e., crack stitching). A preservative treated wood nailer would be fastened to the inside face of the brick wall and existing floor framing at each floor and roof elevation. The City is advised the mortar bed thickness at the crack locations will be visibly wider than adjacent areas.

b. Rear Wall Repair Recommendation: The recommendation is to implement Alternative 3: Repoint and Attach to structurally stabilize the rear wall at the bump-out area and restore the associated building envelope. This alternative was selected as the least cost of the three alternatives in lieu of more significant renovations associated with the other alternatives.

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c. Front Entrance Repair Recommendation: As previously indicated, only one remedial option was developed for the front entrance based on existing conditions. The goal of these repair recommendation is to structurally stabilize the existing floor structure, make upgrades to the building thermal and building envelope to minimize future corrosion and maintain ADA access into the building. The proposed work at the front entrance consists of the following:

i. Removal of existing storefront, brick pavers, concrete slab and supporting steel framing. Extent of removal to be from the inside wall of the entry vestibule to the outside face of the existing building.

ii. Installation of new steel lintels over masonry openings in interior masonry basement walls for building systems.

iii. Repointing and crack stitching of interior masonry basement walls at locations to support the proposed repairs.

iv. Installation of…