Subsurface Constructors Inc. Creates New Supports for Dover High School
A geotechnical ground improvement system was used to boost the bearing capacity of poor soils and mitigate settlement under structural footings for a large portion of an $87 million high school under construction in Dover, New Hampshire.
Subsurface Constructors Inc., an engineering and construction company with offices in St. Louis and Boston, installed numerous aggregate piers at the site of the new Dover High School, which is being built under the construction management of PC Construction, headquartered in Vermont.
"We installed about 1,050 aggregate piers in poor soils under a significant part of the school building site," said Kurt Amidon, P.E., who heads the Northeast Region office of Subsurface Constructors. Soil conditions varied widely at the school site according to Amidon, who has 20 years' experience as a geotechnical consultant in New England.
"Soils ran from shallow bedrock to as much as 10 feet of poor quality fill, to 26 feet of very soft-to-stiff sensitive marine clays over dense glacial till and bedrock. The clays we encountered during construction proved to be deeper and softer than anticipated, but due to the flexibility of aggregate pier ground improvement, we were able to lengthen the stone columns through the clay layers."
Improving Weak Soils
Aggregate piers, also known as stone columns, are a type of ground improvement system recommended for a portion of the school site by geotechnical consultants McPhail Associates. After studying the nature of site soil deposits, the consultants noted that a conventional footing foundation system would be most economically feasible in certain areas. In a geotechnical foundation report to architects HMFH, the consultants said they anticipated that it would be necessary to employ a combination of conventional footings bearing either directly on the undisturbed glacial till deposit, or on the existing fill and alluvial soils that have been improved with a ground improvement system such as aggregate piers.
Amidon explained that ground improvement is the use of mechanical techniques to improve weak or marginal soils and fills in order to support a structure. There are a number of methods to accomplish this, but aggregate piers or stone columns are considered to be the most cost-effective option if the alternatives are 1) removing unsuitable material and replacing it with suitable fill, or 2) constructing deep foundations down to suitable load-bearing strata.
Simply stated, aggregate piers are columns of compacted stone that are installed in groups in poor soils to increase their load-bearing capacity. Piers are formed by placing lifts of stone into a hole in the soil and compacting the stone using high-energy densification equipment such as a high-frequency vibratory probe or a vertical tamper, depending on the installer.
The Importance of Modulus
The installation of aggregate piers densifies the surrounding soils into material with a high Young's modulus, which is a measure of the stiffness of a material and its resistance to deformation under loads. In general, a high modulus indicates a high load-bearing capacity.
Before production installations began, crews successfully completed two modulus tests at the start of construction, which confirmed design strength and estimated soil properties. Piers were designed for an allowable footing bearing pressure of 4,000 psf.
During production, installations were continuously monitored using the company's onboard computer system by the subcontractor's own superintendent and an independent geotechnical engineer provided by the project team, Amidon said. He said each pier was installed with the company's Stratocaster rig, which uses a down-hole vibrating (vibro) probe to construct a column of stone. During operations, the vibro-probe advances to the desired depth while densifying and improving the surrounding soil. When the required depth is reached, stone is placed at the bottom of the hole using a 7-inch diameter tube running along the front of the vibro-probe, and the stone is compacted in lifts. The vibro-probe further improves the surrounding soils laterally, yielding a high modulus material.
Massive Downward and Lateral Forces
The mass of the installation rig adds substantially to the downward pressure.
"This rig set up weighs about 90,000 pounds and can induce over 40,000 pounds of downward pressure to the vibro-probe," noted Amidon. "Supplementing that downward pressure is approximately 45,000 pounds of centrifugal, horizontal force induced in soils at a high frequency of 50 to 60 Hertz by the vibro-probe," he said.
Amidon added that stone columns were 26 to 30 inches in diameter and extended through the clay to a depth of 10 to 26 feet below construction grades. Aggregate used for the process was a clean, three-quarter-inch stone.
Some of the piers were predrilled to a depth of 5 feet in the fill with an 18-inch O.D auger. The site contractor, S.U.R. Construction Inc., of Rochester, New Hampshire, disposed of the spoils produced by the pre-drilling operation.
Subsurface Constructors began installing the aggregate piers around mid-August 2016 and completed the installation in late October. At the same time, S.U.R. Construction was busy excavating site utilities including storm, sewer and water lines, and had begun excavating foundations for the new school.
From Conception to Construction
The new Dover High School is located next to the existing 1967 high school building. Designed by HMFH Architects Inc., the new school consists of a two- to three-story building with a footprint of approximately 185,000 square feet, and can accommodate about 1,500 students and 150 teachers. The facility has a total project cost of roughly $87.4 million, with the State of New Hampshire providing about $13.5 million of that to cover the costs of a Career Technical Center (CTC) included in the new building. The CTC will provide auto, electrical, welding and construction teaching spaces, as well as a culinary kitchen and a photography studio. Once the new school is completed, crews will demolish the old school and create a new parking lot and playing fields in its place.
Dover's new high school has been a works in progress since 2012, when a Visioning Committee was formed to assess how the existing Dover High School and CTC could meet the demands of 21st century education. The original high school was built in 1967, followed by construction of the CTC in 1989. Both facilities were considered outdated and under-equipped by today's educational standards.
After receiving considerable feedback from community groups, city residents, and students and staff of the High School and CTC, the committee decided that either a new facility should be constructed or the existing facility updated to modern standards.
In 2014, a Joint Building Committee (JBC) was formed to help guide the project. Comprised of six voting members (two City Councilors, two School Board members and two community members) plus several advisors, the JBC selected HMFH Architects Inc. to conduct a needs assessment, site selection and design services for the project. As part of this process, PC Construction worked with the Dover School District and HMFH Architects to analyze and evaluate three construction plans ranging from total renovation to the demolition of the existing school and construction of a new facility.
The current plan for building an entirely new school and demolishing the old one was determined to be the most cost-effective choice for the long term. Consequently, in May 2015 the JBC selected PC Construction to provide Construction Management Services. Since that time the JBC has stewarded the project through the feasibility study, design phase and construction phase. The new facility is expected to be ready for the 2018-2019 school year.