4.8.6 Construction

Pier applications require grubbing, and in some cases, blading to prepare the site. The permeability of some soil types can be significantly reduced even with minimal equipment activity; accordingly, the lightest possible tracked equipment should be used for preparing or grading the site. Consult a licensed engineer with soils experience for site specific recommendations.

On relatively flat sites, blading should be limited to shaping the site for the best possible drainage and infiltration (see Figure 4.8.7). Removing the organic topsoil layer is not typically necessary. On sloped sites, the soils may be bladed smooth at their existing grade to receive pier systems, again with the goal of achieving the best possible drainage and infiltration. This will result in the least disturbance to the upper permeable soil layers on sloped sites.

Minimal excavation systems may be installed “pile first” or “post pile.” The pile first approach involves driving or installing all required piles in specified locations to support the structure, and then installing a connecting component (such as a formed and poured concrete grade beam) to engage the piles. Post pile methods require the setting of pre-cast or site poured components first, through which the piles are then driven. Pile first methods are typically used for deep or problematic soils where final pile depth and embedded obstructions are unpredictable. Post pile methods are typically shallower using shorter, smaller diameter piles—and used where the soils and bearing capacities are well-defined. In either case, the piles are placed at specified intervals correlated with their capacity in the soil, the size and location of the loads to be supported, and the carrying capacity of the connection component.

The piles are driven with a machine mounted, frame mounted, or hand-held automatic hammer (see Figure 4.8.8). The choice of driving equipment should be considered based on the size of pile and intended driving depth, the potential for equipment site impacts, and the limits of movement around the structure.

Piling combined with pre-cast walls with sloped bases, or slope cut forms for pouring continuous walls, may be used on sites with limited topography changes similar to the pier applications. Rectilinear wall systems (flat bottom sections), combined with piles, may also be used, but require more site preparation and soil disturbance.

While creating more soil disturbance, sloped sites should be terraced to receive conventional flat-bottomed forms or pre-cast walls. The height difference between terraces will be a result of the slope percentage and the width of the terrace itself. The least impacts to soil will be achieved by limiting the width of each terrace to the width of the equipment blade and cutting as many terraces as possible. Some footprint designs will be more conducive to limiting these cuts and should be considered by the designer.

The terracing technique removes more of the upper permeable soil layer and this loss should be figured into any analysis of storm flows through the site. As with the pier systems, consult a licensed engineer with soils experience for specific recommendations.

With wall systems a free draining, compressible buffer material (pea gravel, corrugated vinyl or foam product) should be placed on surface soils to prepare the site for placement of wall components. This buffer material separates the base of the grade beam from surface of the soil to prevent impacts from expansion or frost heave, and in some cases is employed to allow movement of saturated flows beneath the wall.

Additional soil may remain from foundation construction depending on grading strategy and site conditions. The material may be used to backfill the perimeter of the structure if the impacts of the additional material and equipment used to place the backfill are considered when evaluating runoff conditions.

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Fig4-8-7_Figure 497
Figure 4.8.7 - Flat site construction on piers
Construction of a single-family home on piers on a relatively flat site. Source: Pin Foundations, Inc.

Figure 4.8.8 - Pile driving

Driving of pile with automatic hammer. Source: Pin Foundations, Inc.