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Pin foundations for Low Impact-Clearwater Commons

Case Study by Eliott Frank and Chris van Daalen
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Pin foundations for Low Impact-Clearwater Commons
Site and Stormwater
Structure
Low-Impact Development (LID)
Pin Foundations
Snohomish County
27051800401300
Tom Campbell
Multifamily
Banyon Tree Design
Cascade Built
Pin Foundation inc.

Abstract

This sustainable community development of 16 homes used innovative Pin Pile precast pin style foundations to virtually eliminate soil disturbance and storm-water impacts on their hydrologically sensitive site, as a result, no excavation & fill permit was required. By implementing this technology as part of a low-impact development strategy fdiaor the whole development, Clearwater Commons navigated a path to compliance that protected their watershed, while allowing more homes, that were more affordable than would have been possible with traditional foundations or alternative LID strategies.

Permitting Process

Pin Pile Foundations keep topsoil and natural drainage intact so the home’s entire footprint is considered to be pervious. Normal excavation and soil disturbance permits are not needed as Pin Pier foundations sit on the top soil, eliminating all excavation and soil compaction.  To meet the intent of code requirements for foundations (IRC Section R401 Foundations), they supplied code official with a Washington state-certified engineer’s stamp of approval after onsite geotechnical (soil analysis) data was collected, and loading and design calculations used to prescribe necessary pin length and placement.  After the concrete footings and steel pipes are installed, an inspector checks depths and bracket orientation with the engineered plan before construction begins.

Code RequirementCompliance Path
IRC R401 General: Foundations and subsequent sections; drainage, soil compression, load , etc.

Code officials require a geo-technical site evaluation and engineer calculations and stamp of approval to allow the use of pin pile foundations per PDS Rule 5660

30.63B.110 Standards for cuts and excavations

30.63B.120 Standards for fills and embankments

Pin pile foundations do not require excavation or filling, leaving minimal disturbance.  They are exempt from these requirements.  Compliance achieved via Snohomish County Pin Pile Rule

Project Description

The Clearwater Commons community design approach virtually eliminated soil disturbance and stormwater impacts on this hydrologically sensitive site as part of an overall LID strategy/approach. The foundations used at Clearwater Commons were largely responsible for this. I visited Clearwater Commons on an open house weekend to see the site, as well as a chance to meet with the designer and producers of the Diamond Pier foundations . The foundations, which were used on around 90% of the buildings on the property, were used to achieve the Common's LID goals and the requirements of the county. The county would not allow below ground foundations due to the ecologically sensitive buffer zone building site. The property is located between a wetland and a stream, leaving a long strip of semi-buildable land in the middle. The pinfoundationcloseup.jpgnatural water drainage patterns from the wetland to the river pass directly under the construction area. This posed a problem for building with a traditional slab on grade foundation or crawl space foundation, as a special land disturbance assessment and permit would be required, costing more money and time. A traditional foundation would also hinder the travel of water to the creek that the Clearwater Commons community aimed to restore. Due to these considerations the designers' solution was Pin Foundations, Inc. pin-pile foundations. These foundations are designed to remain stationary in saturated and marginal soils, as well as resist frost and clay heaving. Recently, these foundations were used for a mile long boardwalk on an estuary restoration project on the Nisqually National Wildlife Refuge.

Pin pile foundations are precast concrete blocks with galvanized steel pipes inserted at opposing angles. “The precast concrete head is installed at the ground surface, and steel bearing pins are driven through the head and into the ground“.  They act as a simple and effective building foundation that does not require excavation or soil compaction, therefore exempting the structure from excavation and fill permits. The structure is built on top of the diamondpierdp50.jpgconcrete footing using large steel brackets and pressure treated wood. By not creating a continuous barrier below ground, sub-surface water maintains its natural flow path under the house.

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Design / Build Process

Clearwater Commons utilizes three pre-designed house plans to minimize the need for engineering, as the placement of these piers depends on the building's wind sheer, soil conditions, and load weight. These problems are accounted for by approved geo-technical and structural engineers. Once the calculations have been submitted and approved to the county official, construction can start. Pin Foundations can be used on mild slopes and marginal soils, and require little to no excavation or filling in either case. This makes installment easy, needing only site lines, a level, a sledgehammer, and the pier components. Directly after installing all of the pins and testing the pin depths, framing of the floor structure happens. Installation of a single pier can take as little as 5 minutes with the correct application tools.

Cost / Benefit

For a single custom designed home, pin foundations are more expensive than traditional foundations. Although, there are trade offs, they keep the footprint and yard of the property natural, do not require an excavation and fill permit, they reduce or eliminate a detention pond, and are the most simple and low-maintenance technology that complies with upcoming storm-water management codes. However, as the number of houses go up, the benefits follow. Possibly complying with storm water codes at a lower cost than traditional foundations. As water quality degradation is a widely recognized problem among new building developments, Low Impact Development regulations will become more strict.

The crux of their implementation, says Rick (the owner of Pin Foundations inc.), is the conventional builder and developer. For the correct application of the pin foundation the soil must not be compacted or excavated (usually done by the developer). The problem is many big builders are not familiar with pin foundations, and when building a housing development, familiarity with materials governs labor cost and efficiency. This creates an incentive for the developer to excavate and plat the land for the big builder’s conventional foundation building practices. Although not drastically more expensive than a conventional foundation (5-10%+), the engineering of the house, and the use of pressure treated wood add cost. Elements such as wind shear and roof load determine the placement of each piling, requiring the stamp of an engineer. This makes pin foundations not as cost effective as traditional foundations for one-off buildings, with the exception of building in ecologically sensitive areas. Starting the design of a house with a simple pin pile configuration can greatly reduce costs of retrofitting and engineering, making the Pin Foundation system more competitive. However, on a commercial scale, using pre-approved designs can offset costs by reducing the need for an engineer and architect, as well as reducing the size of a detention pond (less impervious surfaces), producing more build-able lots with dramatically less excavation.  This allows whole housing developments to retain their natural beauty, drainage, and worth, with less carbon emitted from high amounts of concrete, less transportation cost of materials (burning fossil fuels), less labor, and higher building density. The last benefit is they are designed for dis-assembly, i.e. they can be easily removed and potentially reused, as all houses have life spans (approx. 75 years in the U.S) and demolition costs can be quite high, especially for a poured concrete foundation. These foundations can be removed with a simple pipe wrench, eliminating more labor, dangerous drilling equipment, transportation, eliminating a lot of solid waste, and so on, preserving the natural soil for generations to come.

Lessons Learned

Not every green innovation can be used universally, however some are better suited to large projects and have hidden benefits when looking at it as a system instead of a single component.

Project Contacts
Owner: Tom Campbell Clearwater Commons Designer: Banyon Tree Designs Builder: Sloan Ritchie Cascade Built 206 354 3455
Subcontractor: Rick Gagliano Pin-Foundation Inc. 253 858 8809
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