Allen Owings, | 1/11/2016 1:10:26 PM
Some sand bunkers are a nightmare to maintain. Those with steep sides aggravated by other problems may have washed out areas after a rainstorm.
The structural stability of sand on a slope is a function of slope, type of sand and the amount of water. Knowledge of the following principles will show you where your problem lies and suggest what to do about it.
Limiting the steepness of the bunker face can dramatically decrease the erosive forces that produce sand movement during heavy rains. If renovating, try to develop a milder face slope so gravity and water flow have less force over that area. A steeper slope wall requires a deeper layer of sand beneath it.
The quality of the sand is critical, too. It relates to particle stability (interlocking), water percolation rate and hydraulic load (saturated pore space).
Sand particle size and uniformity are important to internal drainage. Sand is defined by USDA as soil particulates that range in size from 0.05 mm (very fine) to 2 mm (very coarse). Larger particles stack together, creating larger gaps or macropores. Smaller sand particles will stack tighter and leave less macropore space. This assumes a relatively uniform particle size distribution. If particle sizes in the sand mix differ by much, the smaller ones fit into the macropores created by the larger grains. The denser result is like the image of making bricks with sand and clay.
Sand particles can range in shape from very angular, sub-angular, sub-rounded and on up to well rounded. Any particle will also range in shapes like spherical to elliptical. The more rounded, smooth and spherical the sand grains (on average), the tighter (more dense) they pack. A tight pack is not good for drainage and high perc. A tighter packing has less air space between particles. These smaller pores tend to hold onto water longer and saturate faster. Introduced water is slower to perc through this profile. The resulting hydraulic load increases the ability of the sand particles to slip and migrate (wash out).
Angular and irregular shaped grains create many more macropores (large spaces). Macropores move water through the sand profile much better and thus minimize time of saturation. This reduced hydraulic load allows the grains to stay in place better as long as drainage is adequate to keep the internal water moving out of the bunker.
Another option in reducing hydraulic load in the bunker slope is to divert surface runoff water from entering the bunker. Instead of allowing a grassed slope to drain into the bunker and erode a bunker face, create a lip to divert it elsewhere: away from going into the bunker.
The other part of water flow management to reduce the hydraulic loads is drainage. It is essential to have good percolation through the sand. Turgeon and Vargas call for a bunker sand to perc a hydraulic conductivity of at least 20 inches/hour. That soil solution has to go somewhere or it will back up and saturate even the macropores. This hydraulic load will encourage sand grain movement (erosion).
There are several options for bunker drainage that will not be fully covered here. They may include drain pipes, gravel and/or a profile system similar to USGA specs for a green's construction. Geotextile liners, properly installed, will help in maintaining the integrity of a system longer by keeping clay and silt particles from migrating into the sand profile and plugging up the larger pores.
If your bunkers wash out too regularly, consider the reasons this happens. Sometimes changing the geometry will be a good fix and some will need more appropriate materials.