Subsurface_Exploration_Addendum_2.pdf

December 13, 2013 J017065.12

Ms. Janis Cooper CAR 2015 Foundation One Memorial Drive Gateway Tower, Suite 700 St. Louis, Missouri 63102

ADDENDUM NO. 2

SUPPLEMENTAL BORINGS AND LATERAL LOAD ANALYSIS

ELEVATED WALKWAY

CITY ARCH RIVER 2015

JEFFERSON NATIONAL EXPANSION MEMORIAL

ST. LOUIS, MISSOURI

Dear Ms. Cooper:

This letter is prepared as an addendum to the previous subsurface exploration report1 and modifies only the items specifically mentioned. As such, the conclusions, recommendations, and limitations contained in the original reports remain unchanged and apply to this addendum. The purpose of our current services was to provide supplemental subsurface information and perform lateral load analysis for the proposed deep foundations supporting the elevated walkway.

PROJECT INFORMATION

We understand that an elevated walkway will be constructed close to the northeast corner of the arch grounds (along Washington Avenue and east of the Arch Garage) as part of the redevelopment of the Jefferson National Expansion Memorial in St. Louis, Missouri. The 11 span walkway will be approximately 336 feet long. Based on data provided by Mr. John Kildea of David Mason Associates, the axial and lateral loads acting at the interior bents will be in the range of 152 and 30 kips, respectively. At the abutments the axial and lateral loads are anticipated to be 76 and 15 kips, respectively.

FIELD EXPLORATION

The field exploration consisted of drilling five borings, designated as Borings W-1 through -5, at the approximate locations shown on Plate 1. The borings were located in the field by Geotechnology by measuring distances from existing site features. The elevations at the boring locations, as shown on the

1 Subsurface Exploration, City Arch River 2015, Jefferson National Expansion Memorial, St. Louis, Missouri.

Prepared for City Arch River 2015 Foundation, by Geotechnology, Inc.; Report Number J017065.09 dated April 26, 2013.

CAR 2015 Foundation J017065.12 December 13, 2013 boring logs, were estimated using the elevations shown on a site plan provided by the client. The locations and elevations are approximate. If more precise data are required, the client should retain a registered surveyor to establish boring locations and elevations.

The borings were drilled to auger refusal using CME 55 and 750 rotary drill rigs equipped with hollow stem augers. Standard Penetration Tests (SPT's) were performed using an automatic hammer.

Split-spoon samples and relatively undisturbed Shelby tube samples were obtained at the depths indicated on the boring logs presented in Appendix A. Rock was cored in the borings using double-tube NX wireline methods. Rock core photographs are included in Appendix B. A legend of the terms and symbols used on the borings and rock core descriptions are included in Appendix B.

SUBSURFACE STRATIGRAPHY

The soil stratigraphy consists of a single stratum of previously placed fill which extends to the depths of auger refusal (6 to 35 feet). The fill generally contains silty clay, sand and varying concentrations of masonry rubble.

Auger refusal was encountered in the borings between depths of 6 and 35 feet (El 407.52 to 429).

Auger refusal elevations at the boring locations are shown on Plate 2. It should be noted that hollow stem auger borings often penetrate several feet of weathered rock and boulders prior to refusal. Drilled pier augers often refuse on these obstructions and rubble fill at a higher elevation. The cored rock consists of moderately hard to hard, gray, aphanitic to medium crystalline limestone. Rock core samples had recoveries of 13 to 100 percent, but typically were above 90 percent. Rock quality designation (RQD)3 values ranged from 0 to 83 percent, but were generally in the fifties and sixties suggesting a fair quality rock.

DESIGN CONSIDERATIONS

Based on the boring data, it appears that rock may be encountered at or above the planned pier cap base within the north half (approximately Bents 7 through 12) of the walkway. Hence, the loads at these bents can be supported on shallow foundations bearing on rock as recommended in the previously referenced report. The south half of the walkway (approximately Bents 1 through 6) can be supported on drilled piers bearing on the underlying bedrock. Rock sockets maybe required for some pier groups as outlined in the subsequent lateral load analysis section. Recommendations for drilled piers are provided herein. Weathered rock in foundation excavations can generally be excavated by conventional equipment with rippers. Removal of solid rock may require drilling and blasting or splitting. Funds should be provided in the construction budget for rock removal.

2 All elevations herein refer to the mean sea level (msl) in feet.

3 Rock quality designation is the ratio of the sum of the pieces of core measuring 4 inches or larger to the total length of the cored interval, expressed as a percentage.

Drilled Piers. Drilled piers bearing on the underlying bedrock can be designed for a net allowable bearing pressure of 20 kips per square foot (ksf). The depth of a pier foundation should be determined during construction based on observation of drilling and the prepared bearing surface. Rubble, boulders or ledge rock (stringers) may be encountered at elevations above the required bearing stratum. Rock augering and/or coring will be required to advance the piers through large pieces of rubble, boulders and stringers when encountered. Rock should be present over 90 percent of the bottom of the pier. The drilled pier construction budget should include a contingency for rock and rubble removal. Considerable variation in the final elevation and quantity of rock excavation should be anticipated.

Additional capacity can be obtained by socketing the drilled piers into sound bedrock. In this case, the pier capacity would be equal to the summation of the following: (1) the end area of the shaft multiplied by the allowable end-bearing pressure and (2) the competent rock socket surface area multiplied by the allowable skin friction value. Allowable side frictional resistance of 15 and 50 psi can be applied to weathered and unweathered limestone, respectively.

The allowable contact pressure on the bedrock given herein is based on the inherent strength of the intact rock and on the influence of jointing, fracturing and solution features. The bedrock at the site is variable with respect to quality and also contains seams of lower-strength rock. Variation in the elevation of pier bearing surfaces will occur. The depth to rock and final bearing elevation may vary significantly within short distances. Pier drilling contractors should be made aware of the potential presence of boulders and ledges of rock in the overlying soil, and rubble material in the fill.

Drilled pier installation becomes more difficult below the groundwater level due to the influx of soil and water into the shaft excavation. Consequently, in the event groundwater is encountered, the piers will need to be temporarily cased to prevent caving of soft or cohesionless soils and to seal off groundwater. In the event groundwater cannot be sealed off, the contract specifications should include notification that pumping equipment may be required to control groundwater inflows prior to concrete placement. Concrete may be placed below water using tremie methods.

LATERAL LOAD ANALYSIS

Based on data provided by Mr. John Kildea, we understand that the interior bents and abutments will be supported on 24-inch to 42-inch (single piers) diameter piers, (four and two pier groups) respectively. Foundation details were provided in drawings titled “Elevated Walk Plan Elevation” and “Elevated Walk Sections and Details”, sheets numbered S200 and S201, respectively and dated October 25, 2013. These sheets are reproduced and included in Appendix C. Prior to the current exploration, Geotechnology performed a preliminary lateral load analysis based on 30-inch diameter piers, an assumed soil stratigraphy and structural details provided in the referenced drawings. Based on the preliminary results, Mr. Kildea requested Geotechnology perform the same analysis with 24-inch diameter piers in lieu of the 30-inch piers, and a single 42-inch diameter pier at all bents. The current analyses are based on the revised criteria.

Group Pier Resistance. Lateral load can be resisted by the soil resistance adjacent to the pier. A complex nonlinear analysis is required to appropriately estimate the lateral load capacity of deep foundations from soil resistance because modulus of subgrade reaction of soil is not a constant property and varies nonlinearly with deflection. We used the computer program GROUP to analyze the lateral capacities of the group of piers. Structural data at each bent is given in the table below:

STRUCTURAL DATA

Pier Number of Piers in Group

Diameter (in)

Pier Length

(ft)

Axial Load (kips)

Lateral Load (kips)

Remarks

Bent 1 2 24 36 76 15 Pier top 1 foot below surface

Bents 2, 3 and 4 4 24 24 152 30 Pier top 7 feet below surface

Bents 5 and 6 4 24 8 152 30

Pier top 7 feet below surface.

Pier embedded 3 feet into rock

We assumed that the pier tops are in a fixed state condition and embedded 1-foot into the pier cap. The analyses were performed for static conditions. Groundwater was assumed to be present at the soil/rock interface for the analysis. The following parameters were assumed for the subsurface strata:

SUBSURFACE PARAMETERS

Parameters Fill Weathered Limestone

Total Unit Weights (pcf) 120 140 Cohesive Strength (psf) 300 - Compressive Strength (psi) - 300 Strain e50 0.02 - Young’s Modulus (psi) - 100,000

The plots of variation of deflection, moment and shear with the embedded depth for the piers are provided in Appendix D. Appropriate safety factors should be applied for the structural design. Also, no excavation around the shafts should not be permitted when lateral loads are present on the shafts.

Single Pier Resistance. The previously described lateral load analyses for groups of piers were repeated for a single 42-inch diameter pier at all bents. We used the program LPILE to analyze the lateral capacities of the drilled piers. The subsurface properties and loads on the piers were the same as described in the previous section. Exceptions are that for the single pier analyses we assumed that all piers will be socketed into the rock to a depth of 3 feet and the pier tops will be in a pinned state condition.

TABLE I

ROCK COMPRESSIVE STRENGTH DATA

Boring No. Sample Compressive Strength

(psi) Type of Rock

Depth (ft)

Elevation (msl)

W-1 10.3 426.7 12,055 Limestone W-3 19.8 405.2 20,182 Limestone W-5 36.3 411.7 11,231 Limestone

APPENDIX A

LOGS OF BORINGS W-1 THROUGH -5

BORING LOG: TERMS AND SYMBOLS

ROCK CORE: TERMS AND SYMBOLS

APPENDIX B

ROCK CORE PHOTOGRAPHS

APPENDIX C

STRUCTURAL DRAWINGS

APPENDIX D

GROUP ANALYSIS RESULTS

24-INCH DIAMETER PIERS

APPENDIX E

LPILE ANALYSIS RESULTS

42-INCH DIAMETER PIERS