5a_Subsurface_Exploration_Addendum_1.pdf

February 3, 2014 J017065.11 Revised Ms. Janis Cooper CAR 2015 Foundation One Memorial Drive Gateway Tower, Suite 700 St. Louis, Missouri 63102

ADDENDUM NO. 1 REVISED

SUPPLEMENTAL DESIGN RECOMMENDATIONS

CITY ARCH RIVER 2015

JEFFERSON NATIONAL EXPANSION MEMORIAL

ST. LOUIS, MISSOURI

Dear Ms. Cooper:

Presented herein are supplemental design recommendations for the referenced project.

Design recommendations are provided for remediation of existing fill below proposed retaining walls and other structures based on structural drawings provided by David Mason & Associates.

This report is prepared as an addendum to our April 26, 2013, subsurface exploration report1 for the proposed City Arch River 2015 project. Only items specifically mentioned are modified from the original report; conclusions, recommendations, and limitations contained in the report remain unchanged and apply to this addendum.

PROJECT DATA

Geotechnology reviewed Structural Drawings for retaining walls shown on Sheets S100 through S111. The drawings include details for remediation of existing fill below foundations for strip and spread footings (S101), cheek walls (S102), security walls and bollards (S103 through S106), concrete benches (S107 through S109), plaza fountain and drainage basin (S110), and light poles (S111).

SUPPLEMENTAL DESIGN CONSIDERATIONS AND RECOMMENDATIONS

SITE GRADING

Site Preparation. Cut areas and areas to receive fill and backfill should be stripped of topsoil, soft soil, rubble fill, and other deleterious materials. Topsoil can be stockpiled for later use during landscaping or removed from the site. Existing fill materials should be entirely removed and replaced from within and to a 5-foot horizontal distance beyond (outside of) 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 No. J017065.09 dated April 26, 2013.

CAR 2015 Foundation J017065.11 February 3, 2014 Addendum No.1 Revised footprint of the proposed museum expansion and any other buildings. Existing fill should also be removed and replaced to a depth of 2 feet below pavement subgrades. The overexcavations can be backfilled with compacted, well-graded, crushed rock.

We understand that sections of the east slope will be remediated using geogrid reinforcement and compacted soil. Pavements and lightly loaded structures in these areas do not require additional remediation.

SHALLOW FOUNDATIONS

Structures that can be supported on shallow foundations include security walls, concrete benches, the plaza fountain and drainage basin, and light poles. The extent of remediation performed can be determined based on the net applied load and susceptibility of the structure to settlement.

However, the risk of detrimental settlement must be accepted by the owner as balance for reduced construction costs.

General Bearing Pressure. Strip and spread footings can be proportioned for net allowable bearing pressures of 1,500 and 2,000 psf, respectively, provided they bear on firm, natural soil or remediated fill. Fill remediation shall consist of removal and replacement with compacted, well-graded crushed rock to a depth of 2 times the footing width below the design bearing elevation.

The remediation should extend one footing width and at least 2 feet minimum beyond the footing area on each side. The minimum lateral dimensions for strip and spread footings should be 18 and 24 inches, respectively. Exterior footings and footings in unheated interior areas should be embedded 30 inches below the lowest adjacent exterior grade to provide protection from seasonal moisture variations and frost penetration.

Lightly Loaded Structures. For lightly loaded items where settlements in excess of 1-inch can be tolerated, Geotechnology recommends remediating existing fill to a minimum depth of 2 feet below the footing bearing level and replacing with compacted, well-graded crushed rock. The lateral extent of fill remediation should be at least 1 foot beyond the foundation perimeter.

Geotechnology should observe the exposed subgrade soils in the overexcavations, to confirm that loose or open rubble fill is not present. If the excavations are large enough, the subgrade should be proof-rolled prior to placing backfill materials. The overexcavations should be backfilled with compacted, well-graded, crushed rock as recommended in our original report.

We understand that sections of the east slope will be remediated using geogrid reinforcement and compacted soil. Lightly loaded structures in these areas do not require additional remediation.

Bollard Walls. Geotechnology reviewed the design requirements for the bollard walls, including structural modeling of impacts and limitations on settlement. We understand that collision forces are resisted by passive pressure from the soil behind the walls and not from the bearing soil below the walls. We also understand that the bollard walls are entirely below-grade