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The design and engineering of overall site features of a residential development or sub-division is the responsibility of qualified Architects, Civil Engineers and other professionals. They must determine the layout of lots, the overall surface drainage patterns, roadway locations and details, utility systems (water, gas, sewer, electricity, etc.), the general locations and positioning f buildings to assure conformance with zoning requirements of density, setbacks, heights, etc., and other similar project-wide considerations. It’s therefore beyond the scope and intent of this guide to discuss those development-level planning factors. Instead, our comments regarding the site will focus on the immediate lot or site for an individual residence project, to provide some guidelines and factors to consider.
If adjacent abutting property in its natural state is higher but pitched towards yours, some modifications such as a drainage swale (ditch) or other grading re-configuration on your property may be necessary to divert surface water so it won’t be a hazard or detrimental factor to the use of your land. Conversely, any alterations to the topography of your site must not thereby divert water flow to your neighbor. If the topography of either property must be deliberately altered (re-graded) because of the development or building process, the resulting configuration must not divert surface flow or drainage to the other property if that flow did not exist in the natural state.
It’s important to be sure that placement of buildings and grading of land adjacent to them be such that surface water flow is directed away from—instead of towards—the buildings. Elevations of any floor slabs at grade should, at the absolute minimum, be 6 inches above the highest adjacent grade. Any slab at grade which ends up with its top elevation at or below the adjacent ground has a very high potential for receiving standing or flowing water during storm conditions.
Sloping or hillside land may require special treatment in the form of cuts, fills and /or terracing to divert and control run-off water. On the sides of the building which face rising slopes, a drainage swale (ditch) should be created at the base of the main slope, and fill material then be placed so that there is a rise of grade from the swale back up to the building, thus forcing surface drainage away from the building, towards the swale. See ( ill. 6).
ill. 6: Drainage Swale Along Hillside
Grading, Fill and Backfill:
It’s highly desirable to minimize changes in the topography of a building site. Whenever possible, the building should be built on natural grades rather than modified conditions. This is because all modifications alter the load carrying capacities and drainage characteristics of the soil.
In the area to be occupied by the building—including all outside walls, paved areas, driveways, etc.—all plant stumps and roots should be removed to a depth of at least 12 inches below ground surface.
(CAUTION) Be especially concerned about areas within the actual profile of the building which are proposed to receive fill or disturbed soil. Once disturbed or excavated, the compaction and density of soil is dramatically reduced, because when loosened the void-to-solid ratio of the soil is increased. Filled or backfilled areas must therefore be placed and compacted by special means, with special care. Fill/backfill material should not contain clay, topsoil, loam or construction debris. It’s especially important that scraps of lumber, form boards, grade stakes or wood debris of any kind not be placed into, or left in, fill and backfill areas. This is to prevent voids left by rot and decayed material, and especially to prevent attraction of subterranean termites.
Backfill material should be granular in texture and reasonably permeable to water. Fill/backfill must be placed in layers (lifts). Depending on the type of soil, each lift should generally not exceed 6” to 12” in thickness, or depth, and each lift then be compacted by special mechanical equipment to achieve at least 90% to 95% of its original density when dry. Unless this procedure is followed, filled or backfilled areas will eventually settle, often severely and usually uneven in amount at different locations, with very unfortunate impacts on the building. These can include broken/failed footings and foundation walls, cracked floors, ruptured pipes, warping, distortion and cracking of walls and ceilings, etc. When fill is placed in thick lifts, any efforts to compact the material will produce results only in the upper portions of the material, not throughout the entire depth of the lift. It’s simply not possible to achieve true compaction to the proper density when placed in a single lift. Unfortunately, this is a too common practice, especially in the backfilling of trenches or against foundation walls. The consequences of subsequent settlement are obvious.
The contractor or builder should be required to have all areas of fill tested for proper compaction by an independent testing laboratory, starting at the bottom of the excavation. You have every right to insist on seeing the results of those compaction tests; and , if you have any doubts or concerns about filled conditions in your building site, insist on testing and on a professional’s interpretation of the results.
Topsoil is the top few inches (usually not more than 6 to 8 inches) of the ground surface, rich in organic materials, having good drainage characteristics. If the building site has topsoil it should be saved for future use. Topsoil definitely should not be used for fill, backfill, nor be removed from the site. Have the builder stockpile it out of the way of construction, for future re use for lawn areas, plantings, gardens, etc. He simply bulldozes it off into a storage pile. Nature has taken millions of years to create the unique materials composing topsoil, and it should not be ignored nor wasted. However, not all sites have topsoil of the quality and characteristics we are referring to. If you are personally unable to make an identification, have a Landscape Architect or specialist look at it and advise you.
Clay can be the most troublesome material encountered on a site. Clay is impermeable to water and so makes drainage and percolation of water into the ground difficult; it’s usually compressible and so allows more or less settlement than other soils. It also can be expansive, i.e., subject to an increase in volume when saturated with water. Clay should not be used for fill or backfill because of these characteristics. Generally, building footings and foundations should not be placed directly on clay. However, clay can be useful as a means to control and limit water percolation into the ground, by its placement as the top layer over backfill. This can be effective along basement walls to keep water from collecting in the former excavation pocket and thus become a problem for the basement spaces.
RETAIN and PROTECT PLANTS:
Try to retain and maintain existing trees and other major plants, even if it requires slight adjustments in building placement to do so. These trees and plants are almost always an asset to the surroundings; useless destruction is always unfortunate. Plants are expensive, and take substantial time to re-grow to maturity. A good landscape plan will utilize existing major plants rather than destroy them.
Be alert to the builders’ efforts to physically protect trees and their root systems from construction activity. If they are close to construction areas they may require wrapping or other physical buffers to keep machinery and equipment from physically damaging them. Roots, if exposed, should be re-covered as soon as possible before dry-out occurs, and then be kept moist.
If public sewers are not available, you will likely have an on-site septic system for sewage disposal. The physical location and relationships of a septic system to the buildings, to other site improvements, and especially to any water wells on the site or on adjacent property, requires special consideration, and has legal constraints. Local laws will usually be enforced which dictate minimum distances from various features, especially water wells. This is to minimize the potential for contamination of ground water and wells, due to seepage of the effluent (waste water) from disposal field or pits.
Be sure that the sizing of the septic system tank and disposal field has been based on local regulations, and on percolation tests done at the site. These are simply holes dug to a certain size and depth in the earth, filled with water several times, and the time for the water to drain down recorded. This information is then used as a basis for calculation of the required surface area of the disposal system. This process measures the permeability, or resistance, of the soil to water absorption/percolation, a key factor to the functional life of a septic system.
A grave misconception about septic systems is that they are permanent systems that, once installed and operating, never need to be maintained, checked on or cleaned out. This is not true. The septic tank is a receiving and holding vessel in which certain bacterial action takes place on the contents, and from which fluids drain out to the disposal system for percolation into the soil. However, the bacterial action in the tank does not completely decompose all material that enters it, converting it all into liquid which the disposal field can leach away. Instead, a scum or foamy mixture of gasses, bacteria, and suspended solids forms and floats on top of the tank contents; heavier solids, called sludge settle to the bottom of the tank. The scum and sludge gradually accumulate and fill the tank. If these are not periodically removed from the tank they eventually flow into the outgoing (effluent) and incoming (supply) piping, clogging and ruining the system. These materials must be removed by pumping out the tank every 3 to 5 years, maximum, depending on the number of building occupants. This is why septic tanks have large removable access covers, and why a tank location must be documented, be accessible and not be too deep in the ground. Here is a diagram of a typical septic system.
ill. 7: Diagram Of Typical Septic System: perforated distribution lines in special absorption trenches
TREATMENT FOR SUBTERRANEAN TERMITES:
Subterranean or ground-nesting termites, are the most destructive insect pests of wood. They are found in all States except Alaska, but are most common and aggressive, and hence most destructive, in the more temperate parts of the country. See ( ill. 8) for an indication of the relative density of termite activity throughout the United States. Subterranean termites thrive in moist, warm soil containing an abundant supply of food in the form of wood or other cellulose material. They often find such conditions beneath buildings where the space below the first floor is poorly ventilated and where scraps of lumber, sawdust, form boards, stumps or roots are left in—or on—the soil. Most termite infestations in a building occur because wood used in its construction touches, or is close to, the ground. This could be at sills, porches, steps, wood posts, terraces or decks. In addition, cracks or voids in foundations and concrete floors make it easy for termites to reach wood that does not actually touch the soil. Any crack 1/32” or greater will permit the passage of termites. Termite activity can exist even in colder northern areas when soil within or adjacent to heated basements is kept warm throughout most of the year.
FIG 8: Density Of Stippling Indicates Relative Hazard Of Termite Infestation In The United States
There are a series of construction practices and details which can seriously impact termite access and damage to residential buildings, as follows:
1) Don’t allow wood products to be left, or buried, in the soil under or near the building.
2) Keep drainage away from the building and from collecting near or under it.
3) Make footings and foundation (or basement) walls as impervious as possible to termites by striving to eliminate voids, cavities and cracks thru which the insects can penetrate and use as paths for access to wood members.
Reinforced poured concrete footings and foundation walls offer the best chances of achieving this. Walls of hollow masonry block should have solid poured concrete caps and ! or have the hollow cores of the top course completely filled with cement grout.
4) Consider the use of metal termite shields at the top of foundation walls, just below any wood construction.
5) Maintain certain minimum distances between finished grades, or soil levels in crawl spaces, to all wood construction above. See ( Figs. 9 and 10).
6) Chemically treat the soil around and under foundations and grade-supported floor slabs of buildings.
The chemical treatment approach is mandated in certain western and southern states. Certain toxicant chemicals are approved and in use for this purpose. Specific solutions and concentrations of these chemicals must be applied to the exposed soil under concrete floor slabs, along and under the foundations of wood framed buildings.
Specific details and recommendations regarding these preventative measures are well described in a publication of the United States Department of Agriculture, Home and Garden Bulletin Number 64, entitled : SUBTERRANEAN TERMITES, THEIR PREVENTION and CONTROL IN BUILDINGS. This document is the authority referenced for chemical treatment requirements by certain governmental jurisdictions in the state of Arizona which use the UNIFORM BUILDING CODE.
ill. 9: Clearances Of Wood Floors To Soil, For Termite Inspection and Control
ill. 10: Wood Construction On Concrete Slab, For Termite Inspection and Control.