Preliminary Geotechnical Considerations prepared by GEI Consultants June 2024.pdf

GEI Consultants, Inc. www.geiconsultants.com

Technical Memorandum GEI Consultants, Inc. 455 Winding Brook Drive, Suite 201, Glastonbury, CT 06033 860.368.5300

Via Email: twilliams@mmaltzan.com

To: Tim Williams, Managing Principal, Michael Maltzan Architecture, Inc.

From: Ma/hew Glunt, P.E.

cc: Gina Morrow, Guy Nordenson and Associates

Paul Baldinger, S/L/A/M Collabora3ve

Date: June 20, 2024

Re: Preliminary Geotechnical Considera3ons

New Federal Courthouse

Har7ord, CT

Project No.: 2405226

We present this memo to discuss preliminary founda3on and earthwork considera3ons for each of the two sites currently under considera3on for development.

Background

We understand the referenced project will involve a new 281,000-sf courthouse facility, constructed at either of these two loca3ons in downtown Har7ord:

61 Woodland Street – This property adjacent to the Park River includes a 4-story brick building currently housing State offices, with parking in the rear. (Woodland Site)

Church, High, and Allyn Streets – This assemblage would include several vacant proper3es bounded by High Street, Allyn Street, and South Church Street, which are currently in use as surface parking lots. (Allyn Site)

We also understand, based on ini3al site layouts and massing, that a Woodland Street facility would likely be on the order of 8 to 9 stories in height with 1 level of underground parking, while a

Church/High/Allyn facility would be on the order of 12 stories in height with up to 2 levels of underground parking.

Expected Subsurface Conditions

We conducted a search and review of all available subsurface informa3on proximate to both sites from public and internal (GEI) sources. In par3cular, the following sources were relied on:

• Geotechnical inves3ga3on at corner of Ann Ucello Street and Asylum Street, approximately 500 feet from the Allyn Site, conducted by GEI in 2015 for a confiden3al client in conjunc3on with a development that was never implemented.

Preliminary Geotechnical Considera3ons 2

New Federal Courthouse

June 20, 2024

GEI Consultants, Inc.

• Recent construc3on of a new parking garage at St. Francis Hospital, approximately 1,000 feet north of the Woodland Site. Though not the geotechnical engineer-of-record for the project, GEI was involved in other aspects and is familiar with the subsurface condi3ons and founda3on systems.

• Recent new construc3on at the West Middle Community School, approximately 1,800 feet east of the Woodland Site. Though not the geotechnical engineer-of-record for the project, we had geotechnical data from the project in our files obtained from public sources.

• Original founda3on plans for the Woodland Street facility, which included pile types and planned lengths.

Woodland Site

From the provided documents, we understand the current building on this property was constructed in or about 1951 on vacant agricultural land.

From our ini3al review, to be confirmed by subsurface inves3ga3ons on the site at a later stage, we expect subsurface condi3ons on the Woodland Site to be as follows, proceeding downward:

• Exis3ng fills where grade-raise was required during ini3al construc3on of the facility.

• Approximately 5 to 10 feet of fine-grained medium-s3ff silt and sandy silt.

• Approximately 100 feet of firm to very soH layered (“varved”) clays and clayey silts. These deposits, known locally as Connec3cut Valley Varved Clay (CRVC), are remnant of a former glacial lake and are prevalent to varying depths within the Har7ord area.

• Approximately 10 feet of dense to very dense gravelly Glacial Till soils.

• Expected sandstone bedrock at approximately 120 feet. In regard to any drilled founda3ons that require rock anchorage, this rock forma3on is generally moderately soH and fractured.

• Groundwater at approximately 8 to 10 feet below grade.

Allyn Site

From the provided documents, the site has been in con3nuous commercial and residen3al use over mul3ple itera3ons since least 1885. At present, all above-grade structures have been demolished and the site is in use as surface parking. The prevalence and character of any remnant below-grade elements is not known at this 3me.

From our ini3al review, to be confirmed by subsurface inves3ga3ons on the site at a later stage, we expect subsurface condi3ons on the Woodland Site to be as follows, proceeding downward:

• Exis3ng fills to depths of about 10 to 12 feet below grade, generally to the depth of previous below-grade construc3on. Given its history, high variability and the presence of debris, as well as poten3ally intact elements such as grade slabs, basement walls, etc., would be expected.

• Approximately 5 to 10 feet of sandy silt to silty sand alluvial soils.

• Approximately 10 to 15 feet of firm to very soH layered (“varved”) clays and clayey silts (CRVC).

• Approximately 5 to 10 feet of dense to very dense gravelly Glacial Till soils.

• Expected sandstone bedrock at approximately 30 to 40 feet.

Preliminary Geotechnical Considera3ons 3

• Groundwater at approximately 10 to 15 feet below grade.

Preliminary Building Foundation Considerations

Exis3ng soils at both sites are not suitable for direct support of the new facility. As discussed further below, ground improvements or deep founda3ons will be required. The primary risk items and cost drivers for this and subsequent evalua3ons will be:

• Thickness and character of exis3ng fills, par3cularly the presence of large structural elements or debris which may impede founda3on installa3on.

• Thickness of the firm to very soH clays (CRVC) underlying both sites and, thereby, depth to rock.

• Depth to groundwater, especially in regard to construc3on of below-grade parking.

At this 3me, in reference to the expected types listed below, we would expect building founda3on costs to be significantly higher for the Woodland site.

Woodland Site

Deep founda3on support with driven piles or drilled-in elements such as micropiles or drilled shaHs would be required to support a new facility of the scale planned at the Woodland Site. Ground improvement systems such as aggregate piers or rigid inclusions are not generally feasible to the depths required at this site. The overall depth and type of deep founda3on will be highly dependent on lateral capacity requirements. Drilled shaHs or, to a lesser degree, large (approx. 12-inch diameter) micropiles anchored into sandstone bedrock at a depth of about 120 feet would be expected to provide sufficient compressive and lateral resistance. We understand both types are in use suppor3ng the large St. Francis

Hospital facili3es just to the north. Steel H-piles driven to refusal on rock may also be considered if sufficient lateral resistance can be achieved. In either scenario, we recommend the use of a structural slab to form the lowest level.

Re-use of exis3ng deep founda3ons may be considered at the Woodland Site. In our experience, successful implementa3on of these programs can be extremely challenging. This would require mapping and overlay of exis3ng and new pile caps, invasive test pits during the design phase to expose and document the exis3ng piles, and load tes3ng during construc3on to confirm their capacity, which we note would carry founda3on risk forward into the construc3on phase.

Allyn Site

Based on our preliminary review, a system consis3ng of large spread foo3ngs and soil improvement by rigid inclusions is considered feasible at the Allyn site. Soil improvement by conven3onal (un-grouted) aggregate piers would likely not provide the required bearing strength, given the expected building scale.

The columns would be extended to bear in the glacial 3ll at depths of 25 to 30 feet below site grade and designed and constructed to a performance specifica3on. Pre-excava3on to remove obstruc3ons and installa3on of steel casing will likely be required through the upper fill. A crushed stone bearing pad would be installed below each spread foo3ng to transfer the load to the piers.

Preliminary Geotechnical Considera3ons 4

Ground improvement elements would be designed and installed by a specialty geotechnical contractor and held to a performance specifica3on that includes an allowable bearing pressure and maximum se/lement. Based on experience with similar projects in similar geologic condi3ons, the specialty contractor will design a system intended to make effec3ve use of reinforcing elements and meet the performance specifica3ons. An allowable bearing pressure on the order of 8,000 psf can likely be used in conjunc3on with rigid inclusion ground improvement.

Given the expected variability and depth of exis3ng fills, we expect that rigid inclusion support on a grid layout would also be recommended to support lower-level grade slabs.

Seismic Design

The current (2022) edi3on of the Connec3cut Building Code document mirrors the 2021 Interna3onal

Building Code, with excep3on of the revisions and supplemental informa3on provided by state building officials.

At this 3me, in reference to the expected design parameters listed below, we would expect seismic design to have a significantly higher impact at the Woodland site.

Woodland Site

Based on expected subsurface condi3ons, and our knowledge of nearby sites in this locale, we believe the most likely Site Class will be “E” (soH soil) due to the presence of very thick, firm to very soH clays.

There is a chance that Site Class “D” (s3ff soil) is achievable with the use of shear-wave velocity measurements at the site, which tend to be less conserva3ve.

• Site Class E would correspond to Seismic Design Category C

• Site Class D would correspond to Seismic Design Category B

The representa3on of Site Class and Design Category could have large implica3ons on the structural design, and thus cost, of this development given its scale. Therefore, if this site is chosen, the use of shear wave velocity measurements to define seismic Site Class is strongly recommended.

Defined seismic hazards such as liquefac3on and lateral spreading are not expected to influence structural design at this site.

Allyn Site

Based on expected subsurface condi3ons, and our knowledge of nearby sites in this locale, we believe the most likely Site Class will be “D” (s3ff soil). Given the rela3vely shallow depth to rock, there is a chance that Site Class “C” (very dense soil) is achievable with the use of shear-wave velocity measurements at the site, which tend to be less conserva3ve.

• Site Class D would correspond to Seismic Design Category B

• Site Class C would correspond to Seismic Design Category A

Preliminary Geotechnical Considera3ons 5

The representa3on of Site Class and Design Category could have large implica3ons on the structural design, and thus cost, of this development given its scale. Therefore, if this site is chosen, the use of shear wave velocity measurements to define seismic Site Class is strongly recommended.

Defined seismic hazards such as liquefac3on and lateral spreading are not expected to influence structural design at this site.

Excavation and Shoring

We understand that 1 to 2-level below-grade parking is expected at both sites for new facili3es. This will warrant significant soil disturbance and excava3on, con3nuous dewatering, and – in some instances -temporary excava3on support.

Given the increased expected requirements for temporary excava3on support and disposal of contaminated soils, we would expect earthwork costs to be significantly higher for the Allyn site.

Woodland Site

Excava3ons on this site would be expected to proceed through exis3ng fills where present, medium-s3ff silts and sandy silts, and into fine-grained clays (CRVC). Our experience is that these soils are slow to drain and highly susceptible to disturbance by construction equipment. A working platform of 12 to 24 inches of crushed stone over geotextile fabric, and potentially geogrid, would be required on the exposed subgrade to allow installation of deep foundations and general movement of construction equipment.

Given the minimum building setbacks required, significant requirements for temporary excavation support are not expected for this site.

Allyn Site

Excava3ons on this site would be expected to proceed through significant exis3ng fills with debris, alluvial silts to sands, and – if a second below-grade level is added - into fine-grained clays (CRVC). The considerations and cautions listed above for the Woodland site for working on these fine-grained soils would also apply here.

Given the site’s history and initial rounds of testing by others, we expect that significant quantities of polluted and/or contaminated soil would require removal from the site. Depending on the contaminants ultimately identified and the levels of such, disposal of these materials would likely come with significant cost. We also note that suitable disposal facilities for this type of material are becoming harder to come by in this region, and transportation/disposal costs could be significantly higher than currently estimated by the time this project goes into construction. Dewatering effluents would likely also require proper treatment.

A temporary excavation support system, such as drilled soldier piles and timber lagging, would be expected to be required for this site.