Monday, June 17, 2013

Consider the Angle of Repose

     So often people feel the need to fight the grades that are natural to their site.  They think that they should have a large flat area where the natural grade of the site is sloping, or they want a slope to feature some aspect of what they intend to plant or showcase.  They want to put natural streams into culverts, or they want to create a stream where one does not exist.
     Obviously, these changes can be made and are made on a regular basis.  Sometimes the solution to a desired flat area is to create a steep slope above and below the flat area.  Sometimes the solution is to construct a retaining wall or two.  Sometimes the solution is to remove the top of a hill and use that soil to fill a hollow.
     I have frequently seen the constructed solution  take the form of reshaping the land to create steep slopes on one or more sides of a site.  This can be a very effective way to create privacy, by creating berms, or to create a flat central lawn.  All to often I have seen this done, though, without the aid of a trained designer.  In fact, many 'garden centers' and 'landscapers' offer grading services.  Frequently these outlets have people who might know how to operate a backhoe or bulldozer, but do not have any design knowledge or training.  Although some equipment operators have an inherent sense of how to form the soil, many do not.  As a result, I have seen a good deal of disastrous consequences.
     So just what can happen if a mistake is made?  To the equipment operator, it can mean his life!  I have seen equipment flip and roll trapping the operator in the wreckage.  More often, though, the disaster occurs after the grader is done and has moved on.  The berm or grade change is completed and maybe even seeded and then a storm rolls in.  Suddenly the soil that was barely staying in place has the added pressure of saturation and begins a downhill tumble.
Don't let this happen to you!
     Designing the grading of a site requires that the designer know the soil type, the rainfall for the area and the subsequent angle of repose.  Without this information, slope failure is a distinct probability.  The soil type determines the angle of repose which is the key to most failures.
     All soils are made up of small particles of minerals - the result of the breaking down of  rock that formed the site millions of years ago - and organic matter - the result of the breaking down of plants in very recent times.  Soils are generally classified as sand, silt and clay (with a fourth classification of gravel that is not considered to be soil) based on the size of the particles that make up the soil.  Sand is the largest of the particles and clay is the smallest.  When these soils are built up to create a steep slope, they will naturally only stay at the steepness defined by their specific angle of repose.  This angle is directed by the individual particle density, surface area of the particles, their shape and the coefficient of friction as they relate to the force of gravity that acts on them.  Attempts to form a slope steeper than the angle of repose of the specific soil of a site will result in that soil reverting back to its natural angle.  That angle becomes flatter when you add water to the mix because water reduces the coefficient of friction allowing the particles to slide past each other.  For example, a dry sand has an angle of repose of 35 degrees, but a wet sand has an angle of repose of 25 degrees.  That is why the contractor can walk away from a completed site and the disaster of slope failure will not occur until it rains.
     Make sure you know who is designing your grades and how much training they have had.  It is also helpful to get a soil analysis if a great deal of grading is being proposed.  Clients that refuse to pay the 'extra' for a soil analysis often take the burden of failure on themselves and find that the 'extra' paid for that analysis might have been a wise investment.  Don't let a lack of knowledge about your site and your soil cause you to be the next person facing disaster.

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