4.8.2 Design

Based on the type of structure to be supported and the specific site or lot topography, a pier type foundation or perimeter wall type foundation must first be selected (see Figures 4.8.5 and 4.8.6). Soil conditions are determined by a limited geotechnical analysis identifying soil type, water content at saturation, strength and density characteristics, and in-place weight. However, depending on the pile system type, the size or scale of the supported structure, and the nature of the site and soils, a more complete soils report including slope stability and liquefaction analysis may be required.

Piers using pin piles can be used for various structure types, including residential and light commercial buildings. When designing with piers, the engineer or vendor supplies the structural requirements (pile length and diameter and pier size) for the pier system. The structural engineer then determines the number and location of piers given the structure size, loads, and load bearing location.

Roof runoff and surrounding storm flows may be allowed to infiltrate without using constructed conveyance when selection of the foundation type and grading strategy results in the top layers of soil being retained and without significant loss to soil permeability and storage characteristics.

Where possible, roof runoff should be infiltrated uphill of the structure and across the broadest possible area. Infiltrating upslope more closely mimics natural (preconstruction) conditions by directing subsurface flows through minimally impacted soils surrounding, and in some cases, under the structure. This provides infiltration and subsurface storage area that would otherwise be lost in the construction and placement of a conventional “dug-in” foundation system. Passive gravity systems for dispersing roof runoff are preferred; however, active systems can be used if back-up power sources are incorporated and a consistent and manageable maintenance program is ensured.

Garage slabs, monolithic-poured patios, or driveways can block dispersed flows from the minimal excavation foundation perimeter and dispersing roof runoff uphill of these areas is not recommended (or must be handled with other stormwater management practices). Some soils and site conditions may not warrant intentionally directing subsurface flows directly beneath the structure, and in these cases, only the preserved soils surrounding the structure and across the site may be relied on to mimic natural flow pathways.

Fig4-8-5_Figure 495
Figure 4.8.5 - Pier type minimal excavation foundation
Pier type foundation. Source: AHBL, Inc.

Fig4-8-6_Figure 496
Figure 4.8.6 - Wall type minimal excavation foundation
Wall type foundation. Source: Pin Foundations, Inc.