Wet soil conditions and hydraulic pressure may lead to pit failure

Iowa Manure Management Action Group
November 20, 2018
By Iowa Manure Management Action Group
This fall has been exceptionally wet and that has led to saturated soil conditions around much of Iowa, and while this has made the primary focus on manure delayed harvest of corn and soybeans and thus limited area for manure application.

These soil conditions may also lead to saturated soils around the manure storage, as the hydrostatic pressure of the saturated soils places high amounts of pressure on both the side walls of manure storages as well as up lift pressure on the floor. When a manure pit is full, these pressures are equalized by the manure inside, but when emptied damage to the concrete can occur.

Seasonal high water tables and saturated soils can exert water pressure on the liquid manure storage foundation walls and floor slab, and have the potential to cause significant structural damage if not adequately controlled. Structural damage can include cracking of the walls, heaving of the floor or side walls.

Saturation of the soils around the facility can occur from either precipitation or from a high water table. Rain can cause water to pool on the ground surface around the exterior of the concrete storage and saturate the soil. This excess soil water can't be held by the soil and can create significant pressure on the structural walls and the floor.

Ensuring the soil around the structure slopes away from the building can help direct this water away from the building. In cases where a seasonal high water table is present around a manure storage a perimeter drain tile system is generally recommended to help remove water and reduce water pressure on the exterior of the manure storage.

While these tile systems should usually be adequate to prevent manure storages, during exceptional weather conditions this tiles may not be able to remove water due to the tile system outlet being below the stream level or alternatively, if the perimeter tile is connected into an existing tile network, flow limitations of the tile system due to full pipes from large drainage areas.

There are a few things you can check to help determine if your manure storage is facing high hydraulic pressures.

1. Evaluate the grade of the soil around your facility. If water isn't draining away consider trying to regrade the landscape around your buildings this fall or next spring to improve site drainage. In the meantime, allow the soils around your facility several days to dry before removing manure from the storage.

2. If a tile system surrounds your storage, check to see if the tile system is still flowing. If the tile system is flowing, especially at levels higher than normal, it would indicate that the soil around the facility still has excess water conditions around the storage that could put hydrostatic pressure on both the walls and floor of the storage and lead to potential concerns about pit damage.

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