Introduction

This report presents a summary of the geotechnical issues regarding the design and construction of the two schools on Springfield Street and Hartford Avenue in the City of Providence. This report is based on my recollections and a recent review of the project files at Northeast Engineers & Consultants, Inc.(NE&C). I provided geotechnical consultation during design and construction of the project while serving as an outside consultant to NE&C prior to my joining the firm full-time in August 1999.

NE&C was part of the design build team for the project led by O. Ahlborg & Sons, Inc, and provided the geotechnical and civil engineering consultation and design. The project was designed and constructed in the spring and summer of 1999. It consisted of the construction of an elementary school and a middle school on the site, plus paved parking areas and landscaping.

Subsurface Conditions

The site was known to be a previous dumping area. Test borings were drilled in March and April 1999 to explore the subsurface conditions in order to determine the most appropriate foundation design for the two schools. The borings indicated that the middle school site was underlain by a layer of fill that was roughly between 10 and 20 feet thick. The fill was found to consist of soil, debris, rubble, brick, glass, ash, cinders, and wood. In most areas, there was a 5 to 10 foot thick layer of organic silt and peat beneath the fill. Beneath the peat was dense to very dense native sandy soil. The depth to the top of the dense sand layer ranged from about 10 to 25 feet within the building footprint.

Test pits excavated in the elementary school footprint encountered fill to a depth of 12 feet or less, underlain by dense native sands. No organic silt or peat was found beneath the elementary school.

Groundwater was encountered at depths of between 8 to 12 feet below the ground surface.

Foundation Design

Various options for support of the two schools were evaluated. The existing fill and the organic soil and peat were considered compressible and likely to cause settlement of any buildings placed on them. Removal of the fill and replacement with compacted engineered fill was determined to be cost effective for the elementary school due to the shallow depth of the fill, but not for the middle school since the fill was much deeper and extended well below the groundwater table. Removal and replacement of the fill with engineered compacted fill would allow the elementary school to be constructed with conventional shallow footings and a slab-on-grade floor.

It was determined that a pile foundation would be the most appropriate and cost effective design for the middle school foundation. Piles transfer the structural loads of the building through the compressible fill and organic soil to the dense native soils, thus preventing settlement of the structure. The floor of the middle school was selected to be a structural slab, being totally supported by the pile foundation and not deriving any support from the ground surface. This is in case the ground surface undergoes any settlement after construction.

Different pile types were considered. Steel pipe piles filled with concrete were selected over timber piles due to their higher load bearing capacity and their ability to be installed at the correct lengths by cutting off or splicing. The piles were designed to achieve capacity by friction in the dense native sands. The selected design was a 12 inch diameter closed end steel pipe pile filled with concrete with an allowable vertical downward capacity of 40 tons. To achieve the design capacity, a minimum required embedment into the native dense sand below the fill was determined. The total pile depth would be the depth to the top of the bearing layer plus the embedment depth.

The middle school foundation was designed as a system of pile caps interconnected with grade beams and topped by a structural slab. Each pile cap supported concentrated column loads and included between 2 and 6 piles. The grade beams around the perimeter of the building extended below the final exterior grade for frost resistance.

Construction

A representative from NE&C monitored the pile installation on a full-time basis. Blow counts were recorded to allow us to verify that the piles were driven into the bearing layer to the minimum embedment depth. A total of about 750 piles were driven in April, May, and June 1999. Total pile depths ranged from about 31 to 70 feet.

Load tests were performed on three of the piles to confirm the pile design. The piles were loaded to twice the design capacity and no signs of failure were detected. Based on our observations and load test results, we concluded that the piles were driven to the design capacity and would provide a suitable foundation for the building.

Settlement

The middle school was designed with a pile foundation and structural slab due to the potential for settlement from the imposed load of the building. The elementary school was constructed on engineered fill. The amount of settlement of the building foundations that may possibly occur from the structural loads would not be considered significant from a structural viewpoint. No distress or significant cracking to the buildings and floors would be expected.

Fills can also compress over time due to their own weight or due to decomposition of organic materials. This could lead to settlement of the ground surface beneath the middle school or the surrounding area. However, due to the age and nature of the fill, we would expect this type of settlement to be relatively small after the building construction. The organic soil beneath the fill and the fill itself would have been compressed by the surcharge weight of the overlying fill over the 30 or more years since it was placed, so the amount of additional consolidation in the future would be expected to be minor. The possibility of settlement of the ground surface exposing the underside of the middle school building is considered remote, since the exterior grade beams extend below grade. In any case, there would be plenty of time to correct any significant settlement around the building if it occurred well before it exposed the bottom of the foundation to the outside. At the present time, over 3 years after construction, we are not aware of any reported settlement of the building or the surrounding ground surface.

Limitations

This report is based on my recollections and a review of currently existing project files. This information is solely about the geotechnical issues of the project. Our geotechnical consultation was performed in general accordance with standards of local geotechnical engineering practice at the time that they were performed.