2023_06_26 Downtown Bathroom Geotech Report.pdf

Alpha Geotechnical and Testing Services, Inc. 4778-A Woodlane Circle, Tallahassee, FL 32303 (850) 514-4171 www.alpha-geotech.com

June 26, 2023 File No. 23-5626

Mr. Cam Whitlock, AIA Architects Lewis + Whitlock 206 West Virginia Street Tallahassee, FL 32301

Subject: Subsurface Exploration and Geotechnical Evaluation for Downtown Restrooms – 131 South Broad

Street, Thomasville, Georgia

Dear Mr. Whitlock:

As authorized by you on 6/7/23, Alpha Geotechnical and Testing Services, Inc. has completed a subsurface soil exploration at the subject site. The purpose of this exploration was to evaluate subsurface conditions encountered in our test borings as they relate to construction of a new restroom building. Two soil borings had been planned within the general footprint of the proposed structure but apparent buried concrete caused us to terminate the southwest bore at 5.2’ deep. Four additional bores were attempted in this area but likewise found “concrete” obstructions. The bore at the northeast on the other hand was advanced to the planned 15’ deep. A few laboratory classification tests were done to aid in our evaluation.

As a summary of our findings, after coring through 5” to 6” of concrete, all borings at the southwest end of the building area found variable sand with silt, often transitioning to mixed orange and tan clayey sand then terminating on apparent impenetrable concrete obstructions between 2’ and 7.2’ below the existing surface. On the northeast end of the building, mixed red, yellow and orange lean clay with some concrete debris was found below the cored concrete slab until about 5.3’ deep. Apparent naturally occurring lean clays were then penetrated until a clayey sand stratum was found from 12’ to termination at 15’ deep. Based on the penetration resistance readings obtained with our sampling device and with a portable “penetrometer” device, the near surface manmade fill soils are in a generally loose or soft condition; the deeper, naturally occurring lean clays are stiff to very stiff and the underlying clayey sands are loose to medium dense.

Groundwater was not encountered in the borings at the time of drilling (middle June, 2023).

Because the soils are generally loose or soft in the upper 5’, we recommend intensive compaction must be performed within footings to increase bearing capacity and reduce potential settlement. With this intensive compaction accomplished, a safe allowable bearing capacity of 2,000 pounds per square foot should be available.

The recommendations submitted in this report are based upon the data obtained from the soil borings presented on Figure 1 and the structure loading conditions outlined. This report does not reflect any variations that may occur between or away from the borings. Possible variations may not become evident until during construction or during additional investigation. Finally, we recommend a review of final design drawings and specifications by our office, to determine if recommendations made herein have been properly interpreted and implemented. This exploration only deals with the near surface soil deposits. It is not intended to include analysis of deeper soil or rock strata where cavities and caverns may exist. Sinkholes do occur in Thomas County; however, this report does not address the possibility of sinkhole occurrence at the site. This report documents our findings and recommendations and was prepared exclusively for use by our Client and their Consultants only for this project.

Sincerely, Alpha Geotechnical and Testing Services, Inc.

Stephen P. Shanley, PE

GA #22376

This item has been digitally signed and sealed by Stephen P. Shanley, PE on the date adjacent to the seal. Printed copies of this document are not considered signed and sealed and the signature must be verified on any electronic copies.

Alpha

Foundation Evaluations Environmental Studies Construction Materials Testing

Alpha Geotechnical and Testing Services, Inc.

Professional Engineer Firm PEF 004552

Subsurface Exploration and Geotechnical Evaluation – Downtown Restrooms – 131 S. Broad St., Thomasville, GA -2- June 26, 2023 (File #23-5626)

Alpha Geotechnical and Testing Services, Inc. 4778-A Woodlane Circle, Tallahassee, FL 32303 (850) 514-4171 www.alpha-geotech.com

1.0 PROJECT DESCRIPTION

We understand that an approximate 17’ x 31’ restroom facility will be constructed on the northeast portion of the property identified by the Thomas County Property Appraiser as parcel #005019015 located at 131 South Broad Street in downtown Thomasville, Georgia. Currently, the site of the improvements is a concrete covered parking lot with primary access from Remington Avenue (the property is in the northwest corner of the intersection of S. Broad Street and Remington Avenue).

You provided us a copy of a preliminary site plan showing the footprint of the proposed downtown restroom facility (see excerpt on attached Figure 1 at the end of this report). Although topography is not shown on the drawing, the site is visually very flat with minimal slope toward Remington Avenue, so we anticipate site grading will require minimal cut and fill (estimate ± 1’) to construct a level building pad. We expect the building will be a pre-engineered metal structure or masonry construction with concrete floor slab on grade and perimeter “turned down” shallow spread footings or continuous “stem-wall” footings. Wall footing loads are not expected to exceed 4 kips (a kip is 1,000 pounds) per lineal foot and column footings should be no more than 60 kips each.

The recommendations contained in this report will not necessarily apply if loading conditions are in excess of these estimates, so please advise if needed.

2.0 FIELD EXPLORATION

To evaluate subsurface conditions, two soil test borings were proposed to be advanced to 15’ below existing ground surface. It was first necessary to core through the concrete parking slab to expose the underlying soils. Our boring designated B-1 at the southwest of the site encountered an impenetrable material (likely buried concrete) at 5.2’ below the surface so we cored additional locations nearby to conduct supplemental borings yet found impenetrable “concrete” at depts varying from as little as 2’ (B-1C) up to 7.2’ deep (B-1D). Boring B-2 done at the northeast of the site found some concrete debris but we were able to advance that bore to termination at 15’. Borings B-1 and B-2 were performed with our mobile drill rig in accordance with the Standard Penetration Test (SPT) method (ASTM D 1586). B-1A through B-1D were performed with hand operated augers (ASTM D 1452) and supplemented with readings from a dynamic cone penetrometer (DCP) device to evaluate the consistency (relative hardness) of the soils (ASTM Special Technical Publication #399).

An excerpt from the preliminary site plan provided to us is included on the attached Figure 1 to display our boring locations. The locations were determined by taped measurement from existing site features. Therefore, locations should be considered accurate to the degree of the method of measurement used.

3.0 SUBSURFACE CONDITIONS

3.1 General

Subsurface conditions encountered during our field exploration are shown on the soil boring profiles presented on Figure

1. The stratification lines represent the approximate boundaries between the soil layers, but subtle changes in the soil matrix may make these changes more gradual than the boundary lines tend to illustrate.

The soil descriptions shown adjacent to the boring profiles on Figure 1 are based on a visual/manual classification procedure in accordance with the methodology presented in ASTM D 2488. We supplemented these with a few laboratory classification tests to confirm our classifications in accordance with the Unified Soil Classification System

(ASTM D 2487).

Subsurface Exploration and Geotechnical Evaluation – Downtown Restrooms – 131 S. Broad St., Thomasville, GA -3-

Alpha Geotechnical and Testing Services, Inc. 4778-A Woodlane Circle, Tallahassee, FL 32303 (850) 514-4171 www.alpha-geotech.com

3.2 Soil Conditions

The soils at the southwest of the site (B-1 through B-1D) differ somewhat from those at the northeast (B-2). Beneath the concrete, the southwest bores typically advanced into apparent manmade fill consisting of tan with orange mottles or marbling sand with silt until about 2’ to 2 ½’ deep where orange and tan marbled clayey sand that also appears to be manmade fill was found until each of the five bores attempted in the area found impenetrable materials (likely buried concrete) at depths ranging from as little as 2’ (B-1C) to as much as 7.2’ (B-1D); the average depth to impenetrable materials at this southwest end is 4.2’. On the other hand, the northeast boring (B-2) found manmade fill consisting of mixed red, yellow and orange lean clay with some concrete debris inclusions below the 6” thick concrete slab until 5.3’ below the surface. Apparent naturally occurring red, orange, yellow and white marbled lean clay underlain by brownish red, yellow, purple and dark gray marbled lean clay were next encountered in this northeast bore until 12’ below the surface where brownish red with gray marbling clayey sand was discovered until termination at 15’ deep.

Based on the penetration resistance “N-values” and “P-values”, the near surface sandy to clayey sand fill soils at the southwest of the site are in a generally loose condition and the lean clay fill at the northeast are generally soft to medium. The deeper naturally occurring lean clays are stiff to very stiff and the deeper clayey sand layer is loose to medium.

The reader should examine the individual boring profiles on Figure 1 for a more detailed description of the subsurface conditions at the locations drilled.

3.3 Groundwater Conditions

A groundwater table was not detected in any of the borings. However, “perched” groundwater could exist within the sand with silt layers atop possible clayey sands or lean clays after prolonged heavy rain periods but should dissipate within a few days.

4.0 LABORATORY TESTING PROGRAM

Laboratory testing was performed on selected samples to aid in soil classification and to further define the engineering properties of the soils. The laboratory tests included Natural Moisture Content (ASTM D 2216), Percent Finer than the U.S. No. 200 Sieve (ASTM D 1140, to assess percent silt and clay), and Atterberg Limits tests (ASTM D 4318, to evaluate plasticity characteristics). The test results are presented on Figure 1 adjacent to the soil boring profiles, at the depth from which the samples were recovered.

5.0 ENGINEERING EVALUATION AND RECOMMENDATIONS

5.1 General

Since the soils are in a very loose to soft condition in the upper 5’, we recommend the overall footprint should densified by compacting exposed soils with a heavily loaded dump truck. Within footings, it will be very important to perform intensive vibratory compaction of footing bottoms to increase bearing capacity and reduce the potential for excessive settlement. These compaction/densification actions should also aid in detecting possible “weak” areas that could require improvement. We do not recommend use of large vibratory rollers since these could damage existing nearby foundations. Details of the necessary densification and proof-compacting are included in the next section.

5.2 Site Preparation

The following are our recommendations for site soil preparation and foundation design for a shallow foundation system.

These recommendations should be incorporated into the project specifications.

Subsurface Exploration and Geotechnical Evaluation – Downtown Restrooms – 131 S. Broad St., Thomasville, GA -4-

Alpha Geotechnical and Testing Services, Inc. 4778-A Woodlane Circle, Tallahassee, FL 32303 (850) 514-4171 www.alpha-geotech.com

1. The entire structure area "footprints" and planned pavement areas, plus a minimum margin of five feet laterally, should be stripped and grubbed of all surface vegetation, debris and other deleterious material, as encountered.

Although the existing concrete could remain in place beneath the general building footprint, it should be removed at footing locations such that new foundations bear on soils, not existing concrete. Excavated soils may be stock-piled for later reuse. Most of the near surface site soils at the southwest portion of the site are sands with silt to clayey sands and should be well suited for structural fill. However, the lean clays at the northeast contain higher clay contents and would be difficult to obtain compaction. An off-site borrow source may need to be considered.

In general, structural fills should comply with item 5 below.

Any existing utility pipelines (e.g. storm sewer lines) that currently lie within new building footprints should be removed and back-filled, not just abandoned in place.

2. The cleared and/or cut surface in building construction areas must be proof-rolled using a heavily loaded dump truck. We recommend multiple passes with the objective of achieving a minimum 95% percent Modified Proctor maximum dry density (ASTM D 1557) to a depth of at least 18” below the compacted surface.

Within footing bottoms, intensive vibratory compaction should be performed to effectively densify the deeper fill soils that may exist below footing bottoms. Again, a minimum 95% of the Modified Proctor maximum dry density should be attained.

We recommend performance of at least one field density test for each 5,000 square feet of prepared area (but a minimum of three tests, regardless of the size). It is important to contact the testing laboratory at least a few days prior to proof-rolling, so that they can obtain proctor test samples, and perform the proctor tests in the laboratory, so that the maximum proctor dry density values will be available at the time of proof-rolling and density testing. We recommend at least three compaction tests in footing bottoms as well.

3. If any areas yield during proof-rolling, they must be explored in a few small test pits to evaluate the condition of the soils. Should yielding result from excessive soil moisture, two corrective alternatives may be considered.

a. If the existing soils are sands or clayey sands (less than 50% clay), dry the soils until the moisture content is 2 to 3 percent below the optimum moisture content as determined from the Modified Proctor test. The soils may be harrowed and air-dried to obtain the desired moisture for compaction.

b. Replace the wet material with soils conforming to that stated in Item 5, below.

Replace any materials, if determined to be deleterious, in areas that "yield" during the proof-rolling operation, with suitable fill material conforming to that stated in Item 5, below.

4. After satisfactory proof-rolling of the cleared and/or cut surfaces in accordance with the above, filling with suitable, well-compacted soil may proceed. Fill material should conform to that stated in Item 5 below, and should be placed in level lifts not exceeding 12 inches in uncompacted thickness. Each lift should be compacted by repeated passes with appropriate compaction equipment, to achieve at least 95 percent of the Modified Proctor maximum dry density. The filling and compaction operations should continue until the desired elevation is achieved. Again, at least one field density test for each 5,000 square feet of prepared fill area should be performed (minimum 3 tests).

5. Fill materials required to elevate the slab area should consist of select fills, which are uniformly graded clean sands to slightly silty or slightly clayey sands, free of organics and other deleterious materials, with less than 35 percent passing the No. 200 sieve. These soil types are less sensitive to moisture problems and are less likely to experience time related settlement than more silty or clayey soils, so the use of select fill tends to reduce earthwork delays caused by seasonal rains and minimize the potential for differential settlement of foundations. Much of the

Subsurface Exploration and Geotechnical Evaluation – Downtown Restrooms – 131 S. Broad St., Thomasville, GA -5-

Alpha Geotechnical and Testing Services, Inc. 4778-A Woodlane Circle, Tallahassee, FL 32303 (850) 514-4171 www.alpha-geotech.com naturally occurring near surface sand with silt soils encountered in our southwest borings do comply with these recommendations but those at the northeast do not. Alternatively, an off-site borrow source should be considered.

5.3 Shallow Foundation Design

Foundation soils prepared in accordance with the above recommendations (natural soils or fills) should be suitable for supporting the proposed structures with a design soil contact pressure of 2,000 pounds per square foot (psf) or less. The weight of the concrete may be neglected when computing the contact pressure. Footings should be embedded at least 18" below surrounding ground. Isolated footings should be at least 18" on each side to prevent punching shear failures.

A modulus of sub-grade reaction of 250 pci may be used in design for slabs.

Based on the information gathered during our exploration and the loading conditions previously estimated, along with satisfactory completion of the recommended soil and foundation improvements discussed above, the recommended soil contact pressure will yield a minimum factor of safety greater than 2.0 against bearing capacity failure. The total load related settlement is estimated to be one inch or less.