A3__2012_Geotechnical_Evaluation_Report_14Mar2012_-_Chamblle_IRS_Deck__VGA00039.pdf

1841 West Oak Parkway, Suite F

Marietta, Georgia 30062 Telephone 770-427-9456

Fax 770-427-1907 www.atcassociates.com

IRS Parking Deck Geo & SW Rpt.docx

March 14, 2012

Hermosa Construction Group 2000 RiverEdge Parkway, Suite 740 Atlanta, Georgia 30328

Attention: Mr. Jason Herndon

Project Manager

Subject: Report of Geotechnical Evaluations and Shear Wave Velocity Testing Parking Deck Renovations at IRS Service Center Chamblee, Georgia

ATC Project Number 066.43047.5948

Dear Mr. Herndon:

ATC Associates Inc. has completed the authorized Geotechnical Evaluation and Shear Wave Velocity Testing for the subject project. The purposes of this work were to provide geotechnical design recommendations for the upcoming parking deck renovations, and to establish a Site Class for use in the design of the renovations. The following letter report discusses our understanding of the project, and provides our recommendations for design.

PROJECT INFORMATION

Information about this project was provided by Mr. Christopher Hector of the GSA, and by the structural engineers for the project, Mr. Sunil Puri and Mr. Nick Saenz of Walter P. Moore. We were also provided with a preliminary layout plan for the new foundations.

The project site is located along the south side of Chamblee Tucker Road, just to the west of the intersection with Buford Highway, in Chamblee, Georgia. The existing 3-story, rectangular parking structure has 2 elevated floors. The overall footprint is roughly 240 feet by 360 feet.

The structure was originally built in the late 1990s and has been in service since that time.

Along the northern and southern edges of the deck, the elevated floors are cantilevered beyond the last row of columns. We understand these cantilevered sections have experienced significant sagging, accompanied by noticeable cracking within the floor along the column line.

These cantilevered sections are to be re-supported by installing new foundations and columns http://www.atcassociates.com/

IRS Parking Deck Geo & SW Rpt.docx 2 along the outboard edges of the structure. Additionally, other new foundations and shear walls will be installed throughout the deck.

The new foundations along the north and south edges of the structure may be grouped into two typical configurations. Most of the new foundations will be approximately 6 feet square, and will be installed adjacent to existing columns. A second foundation type will support new shear walls. These rectangular foundations will occur at two locations, and will be about 23 feet long by 7 feet wide.

We also note that several interior existing foundations spread throughout the deck will be enlarged to accommodate new shear walls.

We understand that the anticipated new foundation loads will be relatively light. For the new individual columns along the edges, the maximum anticipated vertical loading will be 70 kips.

Assuming the new foundation will be 6 feet square, the resulting net bearing pressure would be approximately 2 kips per square foot. We also assume the new shear wall loadings will produce light bearing pressures. We also anticipate that elevations of the new foundations will match those of existing foundations.

The original design and construction of the deck was based (in-part) upon a geotechnical engineering evaluation completed by ATC (formerly ATEC Associates Inc.), dated March 19, 1996. This current evaluation and report uses the information from that report for new design.

We believe the existing parking deck is supported on spread footing foundations as previously recommended.

GENERAL SUBSURFACE CONDITIONS

The following paragraphs present a brief summary of the subsurface conditions encountered in the soil test borings as part of the original geotechnical study for the parking deck. We have attempted to account for the grade changes that occurred during construction in summarizing the subsurface conditions. We have attached Test Boring Records relevant to the renovation work. Please refer to the ATC (formerly ATEC Associates Inc.) report dated March 19, 1996 for a more thorough discussion of the subsurface conditions.

Area and Site Geology

The site is located in the Piedmont Physiographic Province, an area underlain by ancient igneous and metamorphic rocks. The upland soils in this area are the residual product of in-place weathering of the parent rock. A typical residual soil profile consists of clayey soils near the surface, where soil weathering is more advanced, underlain by sandy silts and silty sands that generally become less weathered and denser with depth to the top of parent bedrock. The boundary between soil and rock is usually not clearly defined.

A transitional zone called “partially weathered rock” is normally found above the parent bedrock.

Partially weathered rock is defined for engineering purposes, as residual material with standard

IRS Parking Deck Geo & SW Rpt.docx 3 penetration resistances in excess of 100 blows per foot and which can be penetrated by soil drilling equipment. Weathering is facilitated by fractures, joints, and the presence of less resistant rock types. Consequently, partially weathered rock and the hard rock profiles are irregular, and zones of partially weathered rock or rock may occur within the soil mantle well above the general bedrock level.

Local geologic mapping indicates the site is underlain by the Norcross Gneiss formation, which consists of light gray gneiss locally containing amphibolite.

General Subsurface Conditions

Fill Materials

Fill is any material that has been transported and deposited by man. Soils described as fill were encountered in several borings across the site to depths ranging from about 3 to 15 feet below current grades. The fill materials were described as silty clays (CL), sandy silts (ML), and silty sands (SM). Standard Penetration resistance values ranged from 5 to 13 blows per foot (bpf). The deeper fills appear to be located towards the southeast quadrant of the parking deck.

Residual Soils

Residual soils, formed by in-place weathering of the parent rock, were encountered in all the borings to depths ranging from 15 to 30 feet below current grades. The residuum consisted of sandy silts (ML) and silty sands (SM). Standard Penetration resistance values ranged between 3 to 73 bpf.

Partially Weathered Rock

Partially weathered rock (PWR) is a transitional material between soil and rock, which retains the relic structure of the rock and has very hard or very dense consistencies. Partially weathered rock is locally defined as material with standard penetration resistances greater than 100 blows per foot, and which can be penetrated by the power auger. Partially weathered rock was encountered in a majority of the borings below the residual soils. The PWR generally extended to auger refusal depths. The partially weathered rock was described as very dense silty sands (SM).

Auger Refusal

Refusal is a designation applied to any material that cannot be further penetrated by the power auger and is normally indicative of very hard to very dense material, such as boulders, lenses, or the upper surface of bedrock. Auger refusal was encountered at depths ranging from about 20 to 35 feet below current grades.

Groundwater

Groundwater levels were measured in the previous borings conducted during the 1996 exploration.

The top of the groundwater table at that time generally ranged between EL 935 feet up to EL 940

IRS Parking Deck Geo & SW Rpt.docx 4 feet. We emphasize that groundwater levels fluctuate, and may be higher at later times.

Consequently, the installation of new foundations could encounter groundwater. Careful attention to temporary groundwater control will be a necessity during new construction.

SHEAR WAVE VELOCITY TESTING AND RESULTS

Procedures and Analysis

Shear wave velocity testing was performed along one traverse at the site using SeisOpt ReMi shear wave testing equipment. The SeisOpt ReMi system uses refraction micro-tremor recordings from standard refraction equipment to estimate average shear wave velocities at the site. Velocity evaluation depths can range up to 300 feet, but are typically on the order of 150 to 200 feet based upon soil conditions and the traverse configuration.

The shear wave velocity traverse was located along the south side of the parking deck. The traverse was approximately 286 feet long with 12 geophones spaced at approximately 26-foot intervals. Twenty 30-second recordings of ambient ground vibrations were obtained along the traverse.

The recorded ground vibrations were returned to our office where analysis was performed. The ambient ground vibration records were put through a wave-field transformation to obtain a shear wave dispersion curve. Then, manually selected points along this shear wave dispersion curve were made to produce the best fit shear wave velocity profile. The resulting Average Shear Wave Velocity Profile is attached. The average shear wave velocity within 100 feet of the existing ground surface was then established along the traverse per Section 1613.5.5 of the 2006 IBC.

Test Results and Recommended Site Class

The results of the shear wave velocity testing indicate an average shear wave velocity within 100 feet of the existing ground surface of 1,280 ft/s. This corresponds to a Site Class “C” as defined in Table 1613.5.2 of the 2006 IBC. Therefore, we recommend a Site Class “C” be used for the project.

GEOTECHNICAL DESIGN RECOMMENDATIONS

The following conclusions and recommendations are based upon our understanding of the project, upon the soil boring data collected during past site explorations, and from our experience with similar site conditions and structural plans. If our understanding of project information is not complete, please allow us to review our report and provide supplemental comments.

Shallow Foundation Alternative

Settlement Estimates: Based upon our discussions with the design team, the preferred new foundation type is shallow foundations. These new footings should be installed to near the same

IRS Parking Deck Geo & SW Rpt.docx 5 bearing elevation as the adjacent foundations. We have estimated that the bearing elevation of the new foundations will be near EL 941 feet.

Most of the new foundations will likely be supported in competent residual soils. We note that the anticipated footing bearing elevations are generally 5 to 10 feet lower than the original ground elevation before construction. Consequently, there has been a slight “pre-load” from soils that were excavated from the site.

We have calculated approximate values of anticipated settlement for the new foundations. For the 6-foot square footings that will bear directly upon the residual soils, we estimate total settlements will likely range between 0.3 to 0.4 inches. However, there are some fill materials towards the southeast corner of the deck that will underlie several new foundations (reference soil test boring N-8). In this vicinity, we estimate total settlements will be slightly more. We project total settlement will range from 0.4 to 0.5 inches, subject to a thorough field examination of the bearing soils by the geotechnical engineer.

We have also estimated settlements for two shear wall footings along the exterior walls (reference the new footings between 7 and 8 lines). Due to the larger size of these foundations, we recommend the maximum allowable average bearing pressure be 1500 psf. This should result in total settlements of less than ½ inch.

The renovation program will also include enlargement of certain interior footings to accommodate new shear walls. We anticipate these new foundation segments will not increase the average bearing pressure of the foundations. We can provide additional comment regarding these enlarged foundations upon request.

Footing Design Recommendations: Design of the new footing foundations may include the use of static friction to resist lateral loading. We recommend a coefficient of static friction of 0.35 between the footing concrete and the underlying silty soils. This is an ultimate value, without any safety factor.

During transient loading conditions, such as temporary wind and seismic events, the recommended net bearing pressure may be increased by 33 percent.

Footing seismic ties are only needed if the structure is assigned to Seismic Design Category D, E, or F.

Earth Pressures: There may be a need to utilize passive earth pressures to help resist lateral loading conditions. We recommend that either residual soils, or well compacted fills that surround foundations, be designed for a maximum allowable equivalent fluid pressure of 150 pounds per cubic foot. The uppermost two feet should be neglected. This recommendation only utilizes about one-half of the anticipated ultimate passive value.

Recommended earth pressures for below grade walls are provided in the original geotechnical report. Since that report was issued, the applicable IBC 2009 code now requires seismic loading on below grade walls.

IRS Parking Deck Geo & SW Rpt.docx 6

Basement walls are commonly designed using at-rest earth pressure. During seismic events, the additional earth loads are normally applied to the active earth pressure distribution. Depending upon the methods of design and construction of the walls, a seismic event may not produce an appreciable earth pressure increase over the at-rest earth loads, provided the maximum applied seismic energy is less than about 0.5 times gravity. In the event that the new design involves design of buried walls, we can provide additional input as needed.

Alternative Deep Foundations

Our estimates of new footing settlements are described above. In the event that the design and construction team considers these projected settlement values excessive, we recommend you consider two alternative approaches.

One method to reduce settlement of new foundations would be to utilize compacted aggregate piers before construction of the spread footings. This ground improvement technique is accomplished by first drilling one or more open holes to a prescribed depth, and backfilling with layers of open graded stone, or aggregate. Each layer is heavily vibrated to improve overall ground conditions. This approach is commonly a design-build alternative, performed by specialty geotechnical contractors. It is common for well-designed compacted aggregate piers to reduce settlements by approximately one-half. We note that the drilling for compacted aggregate piers at this site would penetrate the groundwater table, and may be problematic. However, most specialty geotechnical contractors have methods to install these piers below the groundwater table at a small premium.

You may also consider the use of various deep foundations to limit settlements. We recommend you consider the use of either augered, cast-in-place concrete piles, or micro piles.

Augered, cast-in-place concrete piles should be designed with a minimum diameter of 12 inches to permit installation of reinforcing steel. The tips of the piles should be embedded at least 5 to 10 feet into the partially weathered rock that underlies the site, to achieve allowable capacities of 40 to 60 tons or more per pile. Projected pile lengths would 25 to 40 feet. One full-scale load test should be conducted to verify capacity. In the event that this alternative is the selected approach, we can provide additional design detail as needed.

Micro-piles are another possible deep foundation alternative. Micro-piles are commonly constructed as grout-filled boreholes with a steel bar or casing inserted, to permit load transfer to the bearing stratum. Micro-piles are typically a design-build foundation system, adapted to site specific requirements. For this project, we estimate that 8-inch diameter micro-piles would be capable of achieving allowable design capacities of 4 to 5 kips per foot of embedment into the partially weathered rock. The support within the overlying soils should be neglected. If the micro-piles are subjected to lateral loads, a surface casing will also be required. Again, the specialty contractor should design and construct these types of foundations.

IRS Parking Deck Geo & SW Rpt.docx 7

Construction Considerations – Footings

At the beginning of construction of new footing foundations, the precise location of the groundwater table needs to be established. This should be done prior to any excavation. We anticipate the shallowest groundwater occurs in the northwest corner of the deck. A geotechnical engineer should drill several open boreholes using hand equipment to identify the elevation of groundwater.

Temporary groundwater control will be necessary if the planned footing excavations extend to the phreatic surface. Any excavation should be preceded by temporary dewatering. The top of the groundwater table must be maintained at least 3 feet below the deepest point of excavation, in order to maintain the integrity of the exposed soil bearing surfaces. We anticipate trenches, sumps and pumps could be used for temporary groundwater control.

Upon excavation, the geotechnical engineer should carefully examine all foundation bearing surfaces to confirm adequate bearing conditions. If the excavation will remain open for more than one day, a thin mud-mat of concrete can be used to protect the bearing surface.

Should there be loose soils in any excavation, we recommend the bearing surface be densified using motorized compaction equipment such as a ram-x self-propelled compactor, or hand operated wacker-packer. The contractor should use caution near shallow groundwater. Excessive compaction could be detrimental.

QUALIFICATIONS

This report presents the results of a Geotechnical Evaluation and Shear Wave Velocity testing specific to the parking deck at the IRS Chamblee Facility in Chamblee, DeKalb County, Georgia, and applies only to this project and areas discussed.

Our professional services have been performed, our findings derived, and our recommendations prepared in accordance with generally accepted geotechnical and earthquake engineering principles and practices. This warranty is in lieu of all other warranties either expressed or implied. This company is not responsible for the conclusions, opinions or recommendations of others based on these data.

IRS Parking Deck Plot.xlsx

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Average Shear Wave Velocity Profile Parking Deck Renovations

IRS Chamblee Facility Chamblee, Georgia

ATC Project Number 066.43047.5948