Project Description

This project was developed on a construction spoil fill site located in the northern part of Pittsburgh, PA along Interstate 279 and included a two-story building and satellite dish farm totaling about 40,000 square feet. Maximum column loads ranged from 40 to 200 kips. The building was constructed using a slab-on-grade and spread footings approach. The fill at the site originated from the construction of I-279. Excavated rock and soil were dumped in a valley located at the current WPXI building location.

The initial design called for the building to be placed on concreted drilled shafts. Removal and replacement of the fill was not practical due to the depth that the fill extended. A ground improvement approach - Dynamic Compaction - was ultimately suggested by the owner's engineer and was determined to be the most economical.

Ground Conditions

The proposed building site was underlain by up to 35 feet of loose soil and rock fill. The fill material was heterogeneous both in composition and in-situ density. It varied in composition from fine-grained silty clay, with some rock fragments and construction debris, to predominantly coarse-grained rock fragments in a matrix of silty sand and clay. Based on the standard penetration test resistance values (SPT-N values), the fill was medium-dense to dense. However, it was recognized that some of the higher blow counts may have been attributed to the presence of larger boulders and may erroneously have suggested higher densities. Accordingly, the fill had the potential for experiencing large differential settlements with time. The fill layer was deemed not suitable to support a slab-on-grade and footings structure.

Ground Improvement Solution

The Dynamic Compaction approach required 3,468 prints distributed across the building footprint. The compaction was achieved by using a 15 ton weight dropped from a height of 70 feet to densify the underlying fill and soils through the generation of high-energy waves.

Owner – WPXI Channel 11
General Contractor – Mascaro Construction
Owner's Engineer – Geomechanics, Inc.