Products, Vendors, CAD Files, Spec Sheets and More...
Sign up for LAWeekly newsletter
Roads, buildings, parking areas, etc. all need to be established or built on level ground. What follows are some things every Landscape Contractor should know about grading – from a basic definition of the term; pros and cons of cut and fill; recommended retaining walls for slopes and more. Grading is changing the landform to accomplish a desired shape, because rarely is the ground perfectly level.
When grading, one must define the slope. For instance, a 10-foot drop along a horizontal distance of 100-feet is a 1:10 gradient. This can also be defined as a percent slope: 10/100=10% slope. Angles are also used but they are not as easy to work with.
Slope is especially important for proper drainage. If you know the percent slope required for proper drainage and the length of land to be drained you can calculate the vertical drop needed.
As with everything there are positives and negatives, so let?EUR??,,????'???s take a look at the pros and cons of cutting into the soil and removing the soil. The pros are the resulting soil surface will be stable, or less prone to erosion, because the remaining soil has not been disturbed. The compaction and natural stability of the soil are still in tack. The cons include the high cost of disposing of soil. Also, top soil is removed and unfertile subsoil is now at grade.
With fill, which is basically adding soil to the slope, it is the only option when low spots need to be brought to a higher elevation. Another positive of fill is that it is the only option when the grade needs to be raised to match an existing structure. The cons are it is often expensive to haul in, and it even more money to have it compacted, unless you want to wait for it to settle, which can take a long time. Fill is also prone to erosion and foundations built on fill must extend beyond the fill to the undisturbed soils below.
The most economical solution is a combination of cut and fill, as the soils from the cut can then be used as fill. The volume of soil to be moved in a cut or fill can be calculated by: volume = height x width x length. The volume of soil to be moved in a cut or fill can be calculated from a topographic map in plan and section view. Height and width can be estimated from the section or sections. Length can be estimated from the plan. If the grade changes over the cut or fill, draw sections for each different slope and calculate the length for each different section, then add the volumes together. The total grade will stay the same, so making one area more level means that another has been made more steep.
One common way to create level areas is to terrace the land using walls. If terracing without walls, one must pay attention to the angle of repose, which is the angle a soil will naturally form when piled loosely. Regardless of the terrace system, water should be diverted behind the terrace not over the top or front.
Retaining walls provide the maximum usable space on slopes. There are two types of walls – solid and porous. Solid walls are solid concrete or mortared joints. For instance, mortared brick, block, and stone walls are solid. Solid walls require footings (underground foundations of poured concrete), which must extend below the frost line. Footings stabilize the wall and prevent it from moving during freezing and thawing. Poured concrete should contain rebars to strengthen it and keep it from cracking. Footings should also be wider at the base and wider than the wall they support.
Water pressure builds behind solid walls, therefore all solid walls must have weepholes that should be placed every 6-8 feet. The entire back of the wall can be filled with coarse gravel, but at least gravel should be placed from the bottom of the wall to the weepholes to allow water to escape. Solid walls are usually perfectly vertical and there are different ways to keep them strong. Some of those ways are with ?EUR??,,????'??Dead men,?EUR??,,????'?? which are perpendicular supports that extend back into the grade behind the wall. Dead men might extend 6 feet back into the grade and be placed every 12-15 feet. Also a buttress can be used. A buttress braces a wall from the outside. There are different types of buttress?EUR??,,????'??? including stepped buttress, and arched buttress. Dead men and buttresses should be interlocked with the wall not just leaning against it. Porous walls are walls that water can seep through. Dry rock, railroad ties, landscape timbers, post walls and wooden walls are porous. Porous walls should lean back against the soil or batter. A common formula to figure out how to set up a porous wall is 2-3 inches of batter per 1 inch in height. A gravel pack behind the wall will help with drainage. Here are some specifics about different types of porous walls: Railroad ties ?EUR??,,????'??? batter and Dead men will help stabilize the wall. A stake ties together with rebar or spikes. Usually each tie is anchored near the ends and in the middle. Landscape timbers which are usually 4-inches x 5-inches x 10-feet are too narrow when laid, so they are best used as vertical supports. The timbers are nailed to one another and to the uprights. The uprights are spaced 4-5-feet apart. Post walls use posts that are rounded pieces of wood or other material placed vertically in a trench. The posts must be placed as much below ground as they are above ground. Wheepholes are advisable with a gravel pack.
Water pressure builds behind solid walls, therefore all solid walls must have weepholes that should be placed every 6-8 feet. The entire back of the wall can be filled with coarse gravel, but at least gravel should be placed from the bottom of the wall to the weepholes to allow water to escape. Solid walls are usually perfectly vertical and there are different ways to keep them strong.
Some of those ways are with ?EUR??,,????'??Dead men,?EUR??,,????'?? which are perpendicular supports that extend back into the grade behind the wall. Dead men might extend 6 feet back into the grade and be placed every 12-15 feet. Also a buttress can be used. A buttress braces a wall from the outside. There are different types of buttress?EUR??,,????'??? including stepped buttress, and arched buttress. Dead men and buttresses should be interlocked with the wall not just leaning against it.
Porous walls are walls that water can seep through. Dry rock, railroad ties, landscape timbers, post walls and wooden walls are porous. Porous walls should lean back against the soil or batter. A common formula to figure out how to set up a porous wall is 2-3 inches of batter per 1 inch in height. A gravel pack behind the wall will help with drainage.
Here are some specifics about different types of porous walls: Railroad ties ?EUR??,,????'??? batter and Dead men will help stabilize the wall. A stake ties together with rebar or spikes. Usually each tie is anchored near the ends and in the middle. Landscape timbers which are usually 4-inches x 5-inches x 10-feet are too narrow when laid, so they are best used as vertical supports. The timbers are nailed to one another and to the uprights.
The uprights are spaced 4-5-feet apart. Post walls use posts that are rounded pieces of wood or other material placed vertically in a trench. The posts must be placed as much below ground as they are above ground. Wheepholes are advisable with a gravel pack.
With berms it is important to pay special attention to the angle of repose. It is suggested that a 1:3 maximum slope be used for mowed surfaces. Berms can also act as dams so be conscious of how drainage patterns change. For modular concrete stones, it is not recommended for walls over 3-feet high. For walls higher than three courses, bury the first course 3-inches below grade and level the stone in both directions. It is advisable to stager courses (ie ?EUR??,,????'??One-on-two ?EUR??,,????'??? two-on-one?EUR??,,????'??). Landscape fabric behind the stones will stop soil from seeping out. How do you Choose?
With berms it is important to pay special attention to the angle of repose. It is suggested that a 1:3 maximum slope be used for mowed surfaces. Berms can also act as dams so be conscious of how drainage patterns change. For modular concrete stones, it is not recommended for walls over 3-feet high. For walls higher than three courses, bury the first course 3-inches below grade and level the stone in both directions. It is advisable to stager courses (ie ?EUR??,,????'??One-on-two ?EUR??,,????'??? two-on-one?EUR??,,????'??). Landscape fabric behind the stones will stop soil from seeping out.
Here are some good tips on how to choose the right material after everything is graded and work is ready to proceed. If you need to build a wall over 4-feet, do not use wooden walls, small-diameter post walls, or landscape timbers. Certain municipalities may require a building permit and engineering specifications for walls over 3-feet high. Small walls should have smaller or shorter materials, while large or long walls should have larger or longer materials, not only for structural strength, but also for proper scale.
The landscape should take its cue from the surrounding architecture or natural elements. For instance, a brick wall may not look right with a stone house. Remember, indigenous materials that don?EUR??,,????'???t have to be trucked a long way may be cheaper. It all depends on the size of the project, and budget for that project. These are just some tips to help you with a successful grading project.
Lou Anella teaches Horticulture at Oklahoma State University.
There are three ways that a Landscape Contractor can change the grade:
A Serene Escape in Uptown Charlotte
Raleigh, North Carolina
Francisco Uviña, University of New Mexico
Hardscape Oasis in Litchfield Park
Sign up to receive Landscape Architect and Specifier News Magazine, LA Weekly and More...
Invalid Verification Code
Please enter the Verification Code below
You are now subcribed to LASN. You can also search and download CAD files and spec sheets from LADetails.