EVALUATING SLOPE, SOILS, DRAINAGE & RADON

IN THIS ARTICLE
Site Problems vs. Assets
Slope
Surface Water
Subsurface Water
Soil Types
Filled Land
Radon
Difficult Sites                    View all SITE PREP articles

A piece of land that looks gorgeous to the buyer’s eye may look like trouble to an experienced builder. Issues such as soil type, the presence of ledge, high water tables, and poor drainage are just some of the issues that can complicate construction and drive up costs.

Some issues, such as the slope of the site are obvious. If the site drops about six feet over the width of the house, you will need a deeper foundation on the low side (or a stepped-down foundation).  Soil types are less obvious, but certain types of problem soils can make a site challenging and expensive to build on.

Water is often the most difficult site issue to understand and predict. Surface water, including runoff from rain and snow, can cause erosion and flooded basements if not properly managed. Subsurface water can interfere with septic systems and flood basements, often in the spring or rainy season when the water table is highest. Both issues can be dealt with using proper grading and drainage techniques. However, managing subsurface water with curtain drains, extensive subsurface drainage, and – if all else fails – sump pumps may be a good reason to walk away and look elsewhere to build.

TURNING SITE PROBLEMS INTO ASSETS

Most site problems can be overcome by an experienced builder with a big enough budget, and some can be turned into assets with a little creativity. A moderately sloped site allows for a walk-out basement and can be regraded to a terraced yard.  A rock outcropping can become a focal point in landscaping. A low, wet spot may have potential as a year-round pond. But it’s best to go into a project with your eyes open.

When you are evaluating a building site, think ahead to how the land will work with your general house design. Where would you like to site the house? What areas should be cleared and what left natural?  On a sloped site will you build a walk-out, step the building down the slope, or move a lot a earth around to create a level yard with a terraced slope or retaining wall? Keep an open mind and you can often turn a lower-cost “difficult” site into a special home environment.

SLOPED SITES

A little slope is nice for aesthetics and site drainage (drains around a full foundation can drain to daylight), but a steeply sloped site can substantially raise site development costs a lot.

  • Moderate slopes. Gentle grades of less than about 10% (a one foot rise over 10 feet) are the easiest to build on. Between 15% and 20%,  you should plan to spend more on grading and the foundation, which will need to be higher or stepped down on the low side. This is a good slope for walk-out basement.  See Building on a Slope.
  • Steep slopes. Much above about 20%, you may have to do more extensive earth moving and soil stabilization to create workable grades and prevent erosion. Cut and fill operations can get costly, especially if you have to truck in additional fill. The cost of fill varies greatly depending on what is nearby and accessible, so don’t make assumptions here — get estimates.
  • Erosion during the building process can lead to fines (for polluting streams) and extra costs to regrade eroded areas.  Some combination of terracing, retaining walls, and special foundation and drainage techniques may be needed to keep the  soil and the building itself from sliding down the hill.
  • Hillside foundations. Houses are routinely built on extreme grades of 50% or more in places like California, where hillside home sites are coveted. These buildings all have complex engineered foundations installed by specialists in hillside work, which can be dangerous and is always very expensive. Foundations for these homes cost more than many houses built on flatter sites.
  • Drainage of steep slopes. On steep sites, pay special attention to soils and drainage. Wet, dense soils on the uphill side of a foundation can exert excessive force on the uphill foundation wall – like a dam holding back a lake of mud. And fast-running water on a steep slope can lead to excessive erosion if not managed properly. Areas of the country subject to heavy winter rains and mudslides are of special concern. When in doubt, get input from a civil or geotechnical engineer familiar with local conditions.

SURFACE WATER

Water can be a wonderful amenity on a building site. Few people don’t enjoy a water view, a swimming pond, or a babbling brook. However, water can also be your biggest enemy on a building site and can make or break a project. Most problems can be solved with enough money, but it may not be worth the expense and not all efforts work as planned. Engineers can figure out a way to build on wettest of sites, but you should  understand the costs and risks before proceeding.

Unless you are trained in civil engineering, understanding the effects of water on your site in both dry and wet seasons can be challenging. If possible, view your site in the wettest season – after the snow melts in New England, for example. The cute little brook that runs across the site could turn into a raging river, a low spot on the site might become a pond. On the other hand, a lovely pond in the spring might become a mud-caked pit in the dry season.

If your site has drainage ditches, culverts, or other man-made efforts to manage site drainage, ask when they were installed, for what purpose, and how well they work. Look for evidence of high water in any ditch, stream, or other natural or man-made water course. If possible visit the site after a heavy rain to see watch the drainage patterns in action. Does your chosen house site sit high and dry or look more like a duck pond? Most issues can be corrected with grading and planned drainage, but a very wet site may require expensive engineering to get right.

It is especially important to understand drainage patterns on a hillside site or one at the bottom of a hill. Runoff from rain and snowmelt can usually be channeled around your building by some combination of grading, swales, and culverts.

Plant swales with grass or water-tolerant plants

Swales offer a low-cost approach to surface drainage that is easy to integrate with the landscaping.

Swales. On sites with extensive surface water, for example, at the bottom of a hill, surface water can be directed away from the house by shallow ditches, called “swales”. These are often located between house lots  or along roads and often drain toward the front or rear of the lots. They are typically covered with lawn and  may be incorporated into the landscaping with water-tolerant plants.

The grass slows the flow of water, promoting infiltration back into the soil and evaporation. The grass and other plantings are also critical to stabilize the soil and prevent erosion along the drainage path.

See also Managing Roof & Yard Runoff

 

Erosion. Depending on soil type and vegetation, erosion can also be a problem on disturbed areas of the site that conduct water or where water is discharged. Stones, plantings, or landscaping elements that slow down the flow of water will help prevent erosion in these areas.

SUBSURFACE WATER

If surface water is challenging to understand, subsurface “groundwater” can be downright mystifying. The basics of aquifers and water tables is discussed in the section on well drilling. From a building standpoint, you don’t want the lowest floor in your house below the seasonal high water table. This is primarily a problem with full basements. It’s possible to keep water out a basement built below the water table, but it’s expensive to build a house that’s a tight as a boat and prone to failure. In the end, you may end up with a sump pump running throughout the wet season. Also, excavating and pouring concrete footings in a wet hole is tricky and can lead to poor quality concrete or excessive settling.

It’s not impossible to build a dry house on a wet site. The water table can be lowered around the house site by “dewatering” techniques such as curtain drains (also called “French drains”). This may involved drainage ditches around the property as well as a large bed of gravel or crushed stone below the foundation that drains to daylight or a storm sewer. In one house I designed, water ran continuously in the springtime through a deep gravel bed under the foundation, but the basement was dry as a bone. The land developer used a sophisticated engineered solution. To trust the average builder to get this right without the input of a civil or geotechnical engineer would be risky.

One way to avoid basement water problems is to build on a concrete slab-on-grade, as is commonly done in some regions. Crawlspace construction is another option as long as the crawlspace is safely above the seasonal high water table and built with an effective vapor barrier on the ground. A chronically wet crawlspace is worse than a wet basement, since mold and rot are often undetected until extensive damage is done.

See also:     Building a Dry Basement           Basement Leakage Q&As   

SOIL TYPES

Soils have a mix of particles from extremely fine clays and fine silts to course sand and gravel. You can get a pretty good idea of the soil type by examining a handful of soil taken from below the topsoil, the dark, organic matter that’s usually no more 6 to 10 inches deep. You cannot see the individual particles in soils high in clay. When squeezed, this soil feels smooth and can be rolled into thin shapes. Granular soils with more silt, sand, and gravel have visible particles that feel more gritty. They will not hold a shape when squeezed.

  • Clays. Soils with high clay content tend to hold a lot of water and drain poorly. Soft clays have limited load bearing ability, required foundation modifications. In cold climates, clay soils are prone to frost heaves, which can damage foundations, slabs, and piers that support decks and porches. Expansive clays, common in some parts of the country, expand when wet with enough force to destroy foundations, and shrink when dry causing excessive settlement. Most non-expansive clay soils are OK to build on, but will require more granular fill to be brought in for backfilling, reducing problems with drainage and frost heaves. Expansive clays require expensive engineered foundations.
  • Granular soils. These are ideal for building and drainage. They have high load-bearing capacity and drain well. Areas where the soil is pure sand are easy to excavate and build on, but may need some extra help with landscaping and grading to avoid erosion.
  • Ledge. Sites with solid or fractured ledge near the surface present problems with excavation, drainage, and septic systems. Blasting is expensive and carries liability in the event that a neighbor claims that your blasting cracked their foundation (blasting contractors routinely photograph and document surrounding properties as a safeguard). Foundations can formed around and pinned to solid ledge, perhaps leaving a granite outcropping in your basement. Sealing between the foundation and the ledge can also be difficult and prone to leakage. When possible, it is much less costly to leave the ledge in place and work around it. Since blasting is often bid by the cubic yard, and the amount of stone is unknown, the cost can be difficult to estimate.

FILLED LAND

Ever seen those signs reading “Clean Fill Wanted.” Well that “clean” fill usually ends up filling in the low spots on someone’s land. If you have the misfortune of buying that land and building on top of the fill, your foundation could settle and crack significantly, causing damage throughout the house.

If the fill was, in fact, clean – that is, free of tree stumps, garbage, topsoil, toxic materials, and organic material – and was properly compacted when it was placed (compacting one shallow layer or “lift” of fill at a time), then it would be fine to build on. However, these are big if’s. If you have reason to believe that the land you are viewing has been substantially altered, you may need to examine the soil with a core sample or test pit. In general, uncompacted fill needs to be removed and compacted properly, or replaced with properly compacted granular materials, such as sand, gravel, or stone, before building. A similar problem exists if the excavator digs too deep and the foundation is placed on disturbed soil. Unless the replaced soil is properly compacted, the foundation may settle and crack.

RADON

While you don’t hear much about radon these days, it’s still down there in the soil in some areas of the U.S, and still responsible for a large number of lung cancer deaths, according to health experts. Some regions, such as the Reading Prong area in the mid-Atlantic states, are known “hot spots” and are identified as Zone 1 (highest risk) on the EPA Radon Map.

Click to enlarge. Source: US EPA

Zone 2 is considered moderate risk and Zone 3 low risk. The mapping is not very precise and state authorities, whom you can find through the EPA website, may have better information about your particular state.

Radon Testing. Unfortunately, there’s really no way to know about an individual house without testing the building  after construction. Testing of the soil prior to building does not reliably predict household levels. Testing after construction is inexpensive and easy to do yourself. You can purchase a test kit at any good hardware store or online. Also, many state health departments provide high-quality test kits to homeowners at discount prices. A short-term test of 2 to 3 days can identify a problem, but since levels may vary from day to day, a longer term test is needed for an accurate assessment.

Closely follow the instructions that come with the test kit. The test kits are typically placed in a frequently used room on the lowest level of the home that is used regularly (typically the basement level or first floor). The kit should be placed in a room that is used regularly such as a den or bedroom, but not a kitchen or bathroom. For a short-term test,  keep windows and outside doors closed and do not run ventilation systems or exhaust fans for any length of time.  Windows and doors should not be left open for at least 12 hours before the test. You can learn more about radon testing from the EPA’s Citizen’s Guide to Radon.

Radon Removal. In a new home in Zone 1, the EPA recommends that you install a passive radon mitigation system when you build– essentially a PVC pipe run from the gravel bed under the slab or basement floor through the roof, like a pluming vent (in additional to sealing around the slab edges). If radon levels are found to be high, you add an inexpensive inline fan and, voilà, you’ve got an effective radon mitigation system.

If you’re building in Zone 2 or 3, but are still concerned, you can safeguard your new home pretty cheaply by running a PVC pipe through the slab or basement floor into the gravel bed, and then cap the pipe. If the house is later found to have radon, the pipe can be extended through the roof to create a passive or active system.

If properly installed, these systems can reduce indoor radon levels by as much as 99%.

Read more on Radon Risks and Mitigation

DIFFICULT SITES

Other than sites that are illegal to build on, there are few sites that are truly unbuildable. Just take a look at the hill towns in Europe, still standing after hundreds of years of clinging to precipitous hillsides – and built without the benefit of steel, reinforced concrete, and construction cranes.

In many areas of the country, the best building sites are take or prohibitively expensive, so if you have a tight budget, you are probably going to see a lot of less than optimal sites. They may be steep, rocky, or wet, hard to access, or require alternative septic systems or special engineered foundations. The costs of developing a site can vary enormously depending on its specific characteristics and your construction plans, so any site you are seriously considering needs to be individually assessed.

If you can buy the land at the right price, a “difficult” site may be a good deal. With a little creativity, you can often turn a difficult site into something unique and appealing. Handled properly, water, rocks, and steep slopes can become important landscaping features that make your building site special.

The key is to understand the costs and risks ahead of time. Hiring a civil or geotechnical engineer for a couple of hours can be money well spent. If necessary bring in a foundation contractor, blasting contractor, or septic system installer familiar with local conditions to give their opinion and maybe a ballpark estimate. If a septic system plan is already completed, get an estimate from a contractor.

Use the Typical Site Development Costs checklist as a starting point to work up a realistic estimate of site development costs. Then use the first five sections of the  Estimating Worksheet  (General Requirements, Site Prep, On-Site Water/Sewerage, Utilities, and Excavation/Earthwork)  to work up a detailed estimate, which you can use to make your  bid. Or, if it’s too expensive or will never be right for you, walk away. Don’t fall in love with a piece of land and proceed based on wishful thinking. Too many projects get abandoned when the owners discover that the real costs of going forward are so much higher than they imagined.

See also  Foundations & Sitework    Sitework Q&As






Comments

  1. Should You Build in Bowl?

    We recently came into the possession of a plot of land about .82 of an acre. It has a “bowl”. The sides of the lot gradually lower to the middle on all four sides. Is it possible to build a house here or are we going to be looking at a lot of grading?

    • buildingadvisor says:

      Building in a bowl is not ideal, but since you already own the property, you might as well see what you can make of it. In general, bowls collect water and snowmelt, so they can be pretty soggy in wet seasons. On the other hand, if you have a granula soils that can readily absorb water – or you are in an arid area that doesn’t get a lot of rain, then this may not be a concern.

      Also being the lowest spot around, the bowl may be closer to the water table, and could possibly even form a pond during the seasonal high water table. All these things you can determine with a little excavation and observation of the soil profiles by an engineer (civil, soils, or geotechnical).

      Assuming the lot is a collection point for runoff from the surrounding properties, you will need to effectively manage the surface water if you wish to develop the site. If the land rises on all sides of the site, you may not be able to grade yourself out of the problem. Instead, you may be able to install a subsurface drainage system using some combination of surface grates, catch basins, curtain drains, and underground drainage pipe to conduct the water to somewhere where it won’t cause problems for you or your neighbors. Some communities let you tie into storm sewers under certain conditions.

      Subsurface drainage may work in conjunction with siting the house near the highest point on the land and bringing in some fill as needed around the house. The house could, perhaps have a walk-out in back and the “bowl” refashioned into a lowered back yard.

      I have seen downslope lots and even bowl-shaped lots developed successfully, but it takes a little creativity and extra expense. However, it may be less expensive than you think to develop the lot. You may want to contact an architect or landscape architect to do a feasibility study and conceptual design to show you possible solutions that would work on the site.

      Best of luck with your project!

  2. Site Suitable for Walk-Out?

    We are looking at a site described as “gently rolling”. Should we have a contractor take a to look to see whether it is suitable for a walk-out basement?

    • buildingadvisor says:

      For a standard 8 ft. foundation wall, the lot needs to slope downward about 7 ft. from the front to the walk-out entrance in back. A little less slope of about 4-6 feet works well as the grades can be adjusted in the finished grading around the walk-out. A 7 foot drop over 35 feet is a 20% grade. For steeper sites, you can expect to spend extra money on the foundation and earthwork, as well as retaining walls and landscaping, to stabilize the slope and create usable yard space. See also Building on a Sloped Site.

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