Site Grading, Sloping & Erosion

Sloping

Some developments will present sloping and is important to follow the same guidelines to establish the best topography for the building structure. The focus will be in lots with low, medium, and steep slopes which will still follow the models presented before for better drainage.

  • Rear to front drainage according to 2-3 % slope
  • Rear to front drainage according to 3-6 % slope
  • Rear to front drainage according to a >6% slope
  • Standard split surface drainage overall slope 1.5 %

Learn more at  https://www.edmonton.ca/business_economy/documents/ResidentialGuidelines.pdf

Lot or Site Grading

Lot or site grading is the practice of reshaping the surface within a lot to create more suitable building topography (arrangement of the natural and artificial physical features of an area) that controls and directs surface water to a water discharge instead of toward a building. Proper contouring ensures water will drain away from the building. An important advantage of lot grading is the reduction of soil erosion. Generally, the gradient heading away from the building is between one and ten percent.

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Grading issue at side of house in Palau, Lombok

Grading in the lot should be properly maintained to avoid over saturation of the soil in any given area. A routine checkup of all the graded surfaces as well as all any erosion control mechanisms should be done periodically. Heavy rainfall tends to accumulate sediment and other debris in low or hollow places (known as swales) and water diversion channels. To function as intended, all paths should be clear. Any washout or break that interrupts the flow of the surface water to the main gutter should be repaired as soon as discovered and before it can cause major erosion.

Types of Grading

To better manage the water runoff away from the building, it is necessary to establish properly graded open spaces (contouring) around it to direct water to the intended drainage path. Below are three recommendations including split drainage, drainage to a swale, and rear to front drainage.

Split drainage: The building is at a higher level than in front of and behind it so water runoff is discharged to the front or back of the property.
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Drainage to swale: the water runoff is directed to a swale
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Rear to front drainage: The highest elevation is in the back of the property as well as where the building is located, causing any water runoff to go to the side or front of the property.
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For each of the above the slope directs water away from the home.

Erosion and Slope Control

Erosion is a natural process that refers to the loss of topsoil from the earth due to wind and water runoff. A slope on a lot can significantly contribute to the degradation of the soil. Slopes accelerate the water movement and the land when is stripped of the top layer is significantly more likely to a landslide. Water runoff is damaging to the foundation of a building and should be directed away. To prevent soil erosion in a property with a slope there are some things to consider. One of them is the inclination, as a smaller inclination will be easier to control. less inclined slopes are more likely to be controlled with more natural methods. Medium slopes can be successfully managed with locally produced mats or other nettings. Steeper slopes would have to be controlled with more thoughtful techniques.

A further look at the slope can give the builder a good idea of the technique that will be used to control erosion from taking place. Up to 2 inches of wood or bark mulch as well as river rock or stones can be used when the slope is less than 33%. Vegetation is another method effective for this type of slope. Slopes between 33% and 50% can be controlled with locally produced coconut fiber mats or other nettings as well as rip rap. Vegetation can be planted but the use of netting is important as it will hold the soil while the plants establish their roots. Stepper slopes such as those with inclinations of more than 50% can be controlled with the construction of wood, concrete or block retaining walls, rip rap, interlocking concrete blocks or even treated wood.

When a slope is oversaturated with water, pressure can build up easily resulting on a fail and following collapse of the slope. One of the most important parts of a slope protection mechanism is a properly designed drainage system.

Up to 2 inches of wood or bark mulch as well as river rock or stones can be used when the slope is less than 33%. Vegetation is another method effective for this type of slope. Slopes between 33% and 50% can be controlled with locally produced coconut fiber mats or other nettings as well as rip rap (rubble or loose rock). Vegetation can be planted but the use of netting is important as it will hold the soil while the plants establish their roots. Steeper slopes such as those with inclinations of more than 50% can be controlled with the construction of wood, concrete or block retaining walls, rip rap, terraces, interlocking concrete blocks, or even treated wood.

Learn more at https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs144p2_063808.pdf

Common Components of Erosion Control on Sloped Property

Vegetation

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This is the most natural and less costly method to prevent slope failure.
  • Hydro seeding – The spraying of fertilized grass seeds over the surface soil. Grass will eventually grow roots that will prevent the movement of the soil surface by acting as a natural reinforcing and compressing mat.
  • Turfing - Direct application of already grown grass mats that have roots long enough to start incorporating to the soils and act as a strengthening cover on the overall slope.
  • Trees - The planting of specialty trees that develop deep roots to strengthen and compress the slope. This method is used along turfing or hydro seeding [1]

Erosion Control Blankets or Mulch

Erosion control blankets (also known as mats) are permeable (water can go through them) and flexible covering placed on the surface of the ground. They can be made of grass, jute, hay, bamboo fibers, or similar. If blankets or mats are not available, a person can also spread mulch like material (compost) to prevent erosion.

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Rigid Cover

Rigid cover is solid and impermeable (not allowing liquid to pass through. The most common of these covers are the sprayed concrete, which is applied by spraying mortar to the face of the slope. The other cover is plastering, which is a mix of clay and cement as thick as 40 to 50 millimeters. These covers act as wrapping material which keep the slope from moving. This method can create a weird appearance and can be treated to blend with the surrounding. Also, this material is impermeable thus the need of weep holes to drain the ground water which might cause pressure behind the slope. The most common of these covers are the sprayed concrete, which is applied by spraying mortar to the face of the slope. The other cover is plastering, which is a mix of clay and cement as thick as 40 to 50 millimeters. This covers act as wrapping material which keep the slope from moving

Drain Systems

A surface drain system is able to discharge all the water run off or storm water within the area affecting the treated slope. An effective system includes a surface channel, catch pits, and sand traps. A subsurface drain system is laid below the surface to discharge all the ground water and keep the pressure of the water at a safe level. It includes a layer behind the slope to lead the ground water to weep holes, a drain pipe, and cut off drain.

Surface Drain System

A surface drain system able to discharge all the water run off or storm water within the area affecting the treated slope. An effective system includes surface channel, catch pits, and sand traps.

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Subsurface Drain System

Laid below the surface to discharge all the ground water and keep the pressure of the water at a safe level. It includes a layer behind the slope to lead the ground water to weep holes, drain pipe, and cut off drain. Other Components can be for slope reinforcing and tightening. A wire math is for surfaces with higher inclinations and reinforcement mats can be in lower slopes in bigger areas.

Terracing

Terracing is when you convert sloping land in to a number of level flat areas that resemble a series of steps in a staircase. Terraces break up the rate of water decent and reduce both the amount and the velocity of water moving across the soil surface. This reduces soil erosion and often increases the usability of the land for farming or other purposes.

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Learn more

References

[1] http://www.winnipeg.ca/waterandwaste/drainageFlooding/lotgrading/lotDrainage.stm

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