Ground pile design and management
June 1, 2007
With grain production rising in many parts of the world, companies that store grain have been busy expanding their permanent storage capacity.
But even after they have completed their expansion projects, the amount of grain taken in sometimes exceeds the amount of permanent storage space available. In such instances, facility managers may consider using temporary piles to store the excess grain. The degree of success with outdoor grain piles depends on variables within an operator’s control — site preparation, storage design, use of aeration and storage management — and some that can’t be controlled, such as weather.
There is more risk of storage losses with ground piles than with bins, so ground piles should be considered short-term storage and must be monitored frequently.
The first step is site selection. Select a naturally occurring high point a site near the existing grain storage. Run the pile north and south to allow the sun to dry off the sloping sides. Nearby trees provide wind protection but can also be a home for birds and might result in snow drifting on the pile.
Consider how big of an area is required. Approximate pile heights, diameters and volumes for maize and wheat are shown in the table on page 54. Also consider that trucks need a diameter of about 130 feet (40 meters) to turn around.
Frequently, grain is piled outdoors by portable augers or belt conveyors, resulting in elongated triangular-shaped piles. Burial of a conveyor or the undercarriage may result in damage during movements, so overhead conveyors should be considered for deeper piles. Circular piles typically use stationary conveying equipment for placement and removal.
Preparation of the ground pad involves stabilizing the existing base by adding strength and decreasing permeability. A crown should be created at the center point of the pile along with a gradual slope away from the center. Good drainage is provided with slopes of 1% to 2%. The area surrounding the pad should be well drained to remove water running off the pile and pad.
Reduced water permeability of the pad can be accomplished by mixing lime, fly ash or cement in the soil prior to compaction.
The amount of compression necessary for a good pad should approach 98% of the standard proctor density. Another option is to place plastic (6 mil minimum) on the ground to keep soil moisture from wetting the grain.
Make sure that water drains away from the storage to reduce the wetting of soil under the pile.
A concrete pad or paved surface should be installed if the location will be used for several years.
Storage capacity is increased by using a retaining wall. Design and build the wall to support the force of the grain exerted on the wall. The force on a vertical wall is the grain equivalent fluid density multiplied by the grain depth.
Maintenance of a retaining wall includes examining the wall for anything that is out of alignment. Examine connections for separation or movement, check wall anchors and look for deterioration of materials.
Place only cool, dry, clean grain in outdoor piles. Maximize the pile size, height and diameter to reduce the ratio of grain on the surface, which is exposed to weather damage, to the volume of grain in the pile. Installation of an aeration system is critical unless the pile
is very small. Temperature at the center of an 80-foot (24-meter) wide non-aerated pile of maize (corn) will stay warm, and grain deterioration is likely.
Build the pile uniformly to achieve a maximum grain surface slope. This can be accomplished by keeping the drop distance from the spout to the pile at a minimum. The maximum angle of repose and pile height occurs when grain rolls down the side of the pile.
It is important to avoid creating hills, valleys, folds and crevices that will collect water. Sprouting and mold growth occurs first in these areas due to collecting moisture. Keep people and animals off the grain pile since divots in the pile collect water. Placing a temporary fence around the pile helps mitigate this problem.
Grain cleanliness also determines the success of outdoor piles. Segregation occurs during freefall filling. Weed seeds and fine material accumulate in the center and lighter material flows to the outside. An accumulation of wet material at any point in the pile can result in the grain heating and quality deterioration.
The pile should be placed rapidly and immediately covered to minimize exposure to moisture. If rain falls on the pile before it is completed, a wet layer will get buried within the grain, resulting in a potential spoilage area.
Covering the pile will reduce exposure to rain and snow and minimize damage by wind and birds. A one-inch (2.5-centimeter) rain increases the moisture content of one foot (0.30 meters) of maize by nine percentage points. The top surface should be smooth to aid in drainage. Condensation under the cover may cause problems unless controlled with aeration. Move airflow under the cover to carry the moisture away. A drainage tile under the cover serves as an air intake duct when the aeration fan exhausts air from the bottom of the pile. This assists with providing uniform air distribution under the cover.
Negative pressure holds the cover. A restricted air intake for vacuum aeration systems creates the suction required to hold the cover down when it is windy. The amount of wind exposure determines the amount of suction needed. The amount of suction can be varied by operating more fans when it is windy. Back-up power may be desired for electrical outages that occur during storms.
The cover, which should extend over the pile edge, carries the water away from the piled grain to prevent wetting the grain. Make sure that water drains away from the storage by sloping the ground around the storage.
Examine the cover for perforations from rodents, wear points, wind, ice or vandalism, and repair any perforations.
Cooling the grain with aeration is vital for proper storage. Cool temperatures minimize mold growth, limit moisture migration and control insects. Operate aeration fans until piled grain temperatures are uniform and equal to the average outdoor temperature. The grain should be cooled to about 20 to 25 degrees F (minus 6 to minus 4 degrees C) or as cool as is possible for winter storage, if 20 to 25 degrees F cannot be obtained. Use of an inexpensive aeration controller that turns fans on and off based on outside temperature will facilitate rapid cooling.
An aeration airflow rate of about 0.10 cubic feet (0.03 meters) per minute per bushel is frequently recommended for dry grain. However, dry grain can be aerated using lower airflow rates and operating fans for a longer period.
The time in hours required to cool stored grain can be estimated by dividing 15 by the airflow rate. For example, about 150 hours of fan operation will cool grain aerated with an airflow rate of 0.1 cfm/bu. Due to airflow variations in the pile, longer aeration times may be required. Using a smaller airflow rate will reduce the size of ducts and fans required, and using less aeration will reduce the amount of grain shrink that occurs if aerating with low humidity air.
Aeration ducts are positioned to provide uniform airflow through the grain and to provide suction to hold the grain
IMPORTANCE OF MANAGEMENT
Quality deterioration in outdoor grain piles can occur rapidly. Check frequently for heating, moisture accumulation and general condition of the grain. Check the grain temperature and moisture content at several locations every two to three weeks. Exposed grain pile tops get trampled, and they can suffer wind and moisture damage.
Coverings can be loosened by temperature changes, changing winds, animals and vandalism. Walls can suddenly burst open from extra pressure caused by wetted grain.
Grain should be unloaded from the center to prevent uneven wall loading, which may lead to wall collapse. During grain removal, spoiled grain becomes commingled with sound grain, contaminating the entire lot with damaged kernels and commercially objectionable odors.
Leaving spoiled grain along the base and edge of the pile on the ground as the rest of the pile is removed may help grain handlers avoid having to blend all the grain stored in the outdoor piles. Likewise, piles with the long axis pointing east and west may experience greater quality deterioration along one face of the pile. Segregating grain from the half of the pile with less damage limits the amount of grain that requires additional conditioning via cleaning, drying and blending.
Kenneth Hellevang is an agricultural engineer-post harvest/structures for the North Dakota State University Extension Service, Fargo, North Dakota, U.S. He can be reached at 1.701.231.7243 or Kenneth.firstname.lastname@example.org.