in this publication:
What are marine clays
The marine clay Problem
Damage Caused by marine clay
Yard Drainage Problems
Diagnosing The Problem
Preventative Maintenance
County Agencies
Selecting an Engineering Firm
Selecting a Contractor
Obtaining a Building Permit
FAQs
WHAT ARE MARINE CLAYS
marine clays consist of a bluish gray, red and yellow, clayey and silty soils that were deposited by rivers flowing into the Atlantic Ocean millions of years ago. Marine clays can be found in discontinuous layers often scattered with thin layers of sand. In Fairfax County, marine clays can be found in widespread areas east of Interstate 95, mostly in the Lee and Mount Vernon Districts.
While Fairfax County maintains soils maps which can locate marine clays the only definitive way to tell if your house is located in marine clay, is to have someone experienced in soils conduct a field investigation and make test borings.
THE MARINE CLAY PROBLEM
Marine clays are a problem because they shrink during dry periods of the year and swell during wet periods. The pressures that the clays exert upon swelling can far exceed the strength of a house foundation and basement walls. The most troublesome areas occur on steeper slopes and where the content of clay and silt is much higher than other soil types.
Some types of damage are more common than others and tend to occur at certain stages in the life of a house with some problems developing slowly at first and becoming more serious over time. However, many houses located within the marine clays may never have problems, while others will undergo foundation distress at sometime in the life of the structure.
DAMAGE CAUSED BY MARINE CLAYS
FOOTINGS
CAUSE
The most common problem in marine clay areas is the settlement and heave of foundation footings which distribute the load of the house to the underlying soils. During dry and shrinking periods, the soil loses moisture, causing a gap under the footings. The buildings settle, usually in an uneven fashion, resulting in broken footings, cracked masonry walls, interior cracks in plaster, and warped door and window frames (Figure 1).
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BUILDING SETTLEMENT CAUSED BY HIGH SHRINK-SWELL CLAYS IS MOST
NOTICEABLE IN DRY SEASONS OF THE YEAR...
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Deeply-rooted vegetation such as trees often contribute to the problem. Trees draw water from the soil through many small roots that extend below and well beyond the farthest-reaching branches, drying out the soil and causing shrinkage. Tree roots can extend well underneath a house and its foundation.
Many of the houses with settlement problems have relatively shallow foundations (frequently less than three feet deep).
Foundations that have settled during dry periods will often return to near the original position after rainfall replenishes the soil moisture causing the soils to swell again. After several cycles, however, the rebound of the foundation may be progressively less, resulting in larger and larger cracks.
RESOLUTION
The purpose of any repair
for damage caused by marine clay is to extend the bearing support for the
house to deeper and more stable soils.
- In new construction footings can be placed four to six feet or more below the ground surface with the exact depth usually determined after a detailed soil investigation.
- Trees should be planted at least 20 feet away from residential structures to minimize drying of the soils by the root system, which can become quite extensive as trees mature.
- On existing construction, underpinning or helical piers are often necessary to reinforce a foundation damaged by settlement. Underpinning a house consists of new footings that extend below the original footings throughout the damaged area (Figure 2). Helical piers are steel anchors that are "drilled" into the ground to a deep depth and attached to the existing footings.
BASEMENT WALLS
CAUSE
Damage to foundation walls often occurs when marine clays are placed as backfill against basement walls. While this practice is now prohibited the Fairfax County, it was a common procedure in houses built before 1975, and may still occur today where the builder is uninformed or careless.
Damage from the pressure of swelling clays in backfill occurs after several yearly cycles of shrinking and swelling before the detrimental effects of the soils are revealed.
Surface water can also accumulate against basement walls where the backfill has settled, increasing the soil moisture and swelling pressure of the soil. The result is the accelerated failure of the basement wall (Figure 3). Failing walls can also exert damaging forces on nearby sewer, water and gas pipes.
RESOLUTION
The most successful repair method for foundation walls damaged from marine clay backfill is to remove and replace the clay with sandy or gravelly soils (Figure 4). Depending on the extent of damage, some walls may need to be replaced while others can be repaired.
Your engineer or contractor should follow the steps below when repairing or replacing a damaged basement wall:
- The house should be temporarily shored if the basement walls are severely damaged or are to be replaced.
- After the underground utilities are marked and located, the marine clays should be removed from behind the basement wall. A four-to-eight-foot separation distance should be maintained between the foundation wall and the natural soils to prevent the future effects of soil swelling pressures against the wall. The separation distance should be approximately equal to the depth of the footings below the ground surface.
- The exterior side of the wall should be waterproofed after the wall is repaired or replaced. Waterproofing should consist of one coat of cement applied to the cleaned basement walls, followed by at least two heavy coats of asphalt sealant.
- Foundation drains should be placed at the footing level on the outside of the house and encased in gravel. The foundation drains should lead to an outlet on a slope away from the house or to a sump pump. A filter fabric should be placed on top of the gravel surrounding the drain.
- The new backfill soils should consist predominantly of coarse-grained sand and gravel soils in order to minimize soil pressures against the walls and to allow good drainage.
- A one- to two-foot thick capping layer of low shrink-swell silty or clayey soils should be placed at the ground surface to reduce the infiltration of surface water. The slope next to the house should be at least five percent (a six-inch drop for each 10 feet in distance away from the house) (Figure 4).
LANDSLIDES
Landslides are more common in landscaped yards and undeveloped areas; however damage to structures from landslides can be dramatic. Slope failures, which usually occur during wet periods of the year, have jeopardized buildings and made yards unusable. Where houses are involved in landslides, movement is usually very slow (fractions of an inch per year) and can be corrected.
Most of the slides are less than one or two feet in depth and can be corrected by flattening the slope or replacing the clays with stable soil materials. Deeper landslides have occasionally occurred, requiring stabilization measures. Slides, shallow or deep, can damage underground utilities, such as gas, water and sewer service.
FLOOR SLABS AND ROADWAYS
Damage to floor slabs and roadways occurs when underlying clays shrink and swell. Road surfaces may deteriorate prematurely. Curbs and gutters may settle excessively. Swelling pressures under floor slabs can cause a damaging uplift of the floor, sometimes requiring replacement of the floor slab. Damage can be minimized by removing high shrink-swell clays under floor slabs and roadways to some depth, usually one to two feet, and replacing the clays with compacted non-swelling soils.
YARD DRAINAGE
Yard drainage problems occur in marine clays because water percolates very slowly through the clays and collects in flat yard areas. Establishing and maintaining lawns and shrubs can be a problem because of poor soil drainage. It is important to maintain positive drainage away from houses and throughout yard areas. Positive grades away from a house help prevent water from soaking into the ground next to the house and causing potential foundation problems. Rainfall runoff may stand for long periods in areas that do not have at least a two-percent slope.
DIAGNOSING THE PROBLEM
As a homeowner, your observations and records of problems over time can help evaluate the problem, as well as the suitability of a particular construction proposal, or even the need for repairs. Use the following as a guide.
- If you see foundation wall cracks, note when they first appeared and how extensive they are. Small settlement cracks (smaller than 1/16 inch) are not uncommon in new construction. They usually appear and stabilize during construction or within the first year. Settlement cracks often appear in houses located in the most stable soils in Fairfax County.
- Cracks that appear abruptly after 5, 10, or 30 years are probably caused by soil problems. Measure and keep a record of the width and extent of the cracks. The appearance of foundation wall cracks does not necessarily mean that you have a structural problem, or even that your house is constructed on marine clay. A structural problem affecting your house can develop, however, if soil-related damage is allowed to progress unchecked for long periods.
| THE APPEARANCE OF FOUNDATION WALL CRACKS DOES NOT NECESSARILY MEAN THAT YOU HAVE A STRUCTURAL PROBLEM... |
- If the cracks open up during the summer months, especially during very dry years, and close up during the moist winter months, they are probably caused by shrinking and swelling of clays below the footings.
- Note the location of trees with respect to damage in the walls. Cracks that extend through masonry walls and follow block and brick mortar lines may indicate advanced settlement due to shrink-swell clays.
- If one or more basement walls is cracked and bowing inward, you may have high shrink-swell clays in the backfill behind the basement wall (Figure 3). Concrete block walls that are bowing inward usually have one or more horizontal cracks about mid-height on the wall. Vertical cracks often develop near the corners.
- Bowed walls may exert pressure against sewer, water and natural gas pipes. Call the gas company immediately if you think that your natural gas pipe or gas meter is under pressure from a damaged wall, because an explosion could occur from a leaking pipe.
- From the outside, check the soil next to your basement wall. If there is a gap one-half to one inch in width between the wall and soil backfill during the summer months, then you may have high shrink-swell clays, at least near the ground surface. The only way to confirm if you have clays throughout the backfill is to excavate or bore down to the footings with an auger.
PREVENTATIVE MAINTENANCE
There are several steps you can take if you think that your house is located in marine clay, or even after you start to see some damage. The steps require maintaining good drainage around the house and keeping the soils under the footings and in the basement wall backfill at stable moisture levels:
- First, make sure that you have positive grades sloping away from the house to help remove rainfall runoff. Backfill next to basement walls normally settles after several years, leaving depressional areas for water to collect. Use a silty or clayey soil rather than sand, gravel or mulch to increase the grade to at least five percent (a six-inch drop in 10 feet) to turn surface water away from the house.
- Make sure that your landscaping does not trap water above the backfill next to the basement wall. Regrading of a yard is sometimes necessary to carry surface water away.
- Make sure the gutters and downspouts are in good condition and that they carry all roof water beyond the backfill or at least 10 feet away from the house on splash blocks or through non-perforated pipe.
- Large trees should be at least 20 feet away from the house.
- If you have a swimming pool, make sure that there are no leaks. Leakage from a swimming pool can cause foundation problems by weakening the foundation soils or increasing swelling pressures against the foundation walls. Excessive watering of plants next to the house may create similar problems.
- Basement sump pumps, if required to maintain a dry basement, should be in good working condition.
- Shallow slides or slope creep may not present a threat to your house, but can be a nuisance in maintaining a yard. Possible methods of stabilizing the creeping slopes include replacing the slide areas with more stable soil materials containing less clay or regrading to a flatter slope. clay areas that have moved because of slope creep can often be stabilized by regrading the slope to 20 percent (a one-foot drop in five feet of distance) or flatter.
Some homeowners with foundation problems add water around the house during dry periods to help increase soil moisture and reduce soil shrinkage under the footings. This might work in theory. We do not generally recommend the practice, however, since uncontrolled addition of water can create more problems than it solves. The addition of too much water can damage foundation walls.
FAIRFAX COUNTY AGENCIES
The following agencies and offices can be contacted for further information:
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Information
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Agency/Office
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Phone
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Soil Information by Address or Tax Identification Number and
Technical Assistance (Problem identification) |
Northern Virginia Soil and Water Conservation District | 703-324-1460, TTY 711 |
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Home Improvement Loan Assistance (Loan Programs) |
Department of Housing and Community Development | 703-246-5000, TTY 711 |
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Consumer Information and Complaints (evaluating private contractors-performance and disputes) |
Consumer Protection Division, Department of Cable Communications and Consumer Protection | 703-222-8435, TTY 711 |
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Location of Underground Utilities (Before soil testing or excavating) |
Miss Utility | 1-800-552-7001, TTY 711 |
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Building and Construction Information, Permit Requirements (Before beginning repairs) |
Land Development Services, Department of Public Works and Environmental Services | 703-222-0801, TTY 711 |
Additional Information on Soils
SELECTING AN ENGINEERING FIRM
A geotechnical engineer -- one experienced in soils and foundation engineering -- has the technical expertise to evaluate and design repairs for marine clay damage. Fairfax County requires a soils report be prepared by a qualified engineer for all new construction and grading work in marine clay. Lists of engineering firms can be found in the phone book.
| Only licensed professional engineers are allowed to perform engineering work in Fairfax County. Ask for proof of licensing. Ask the engineer for qualifications such as training and experience in soil and foundation engineering, and especially about experience with marine clay. Ask for business and professional references and follow up on these. Contact the Consumer Protection Division to see if any complaints have been filed against the engineer and how these were resolved. |
The engineer's investigation should include the following:
- Soil borings or test pits (if foundation settlement has occurred, soils to a depth of at least 10 feet below the bottom of the footings should be investigated).
- Observations on the stability of the surrounding slopes.
- If the problem is confined only to basement wall damage, several auger samples taken throughout the depth of the existing backfill.
- Record of all evidence of damage to the house, and a recommendation for a structural engineer, if needed.
Please note: It is critical, however, that the engineer inspect the soils at the bottom of footings during underpinning construction because of possible variation of the soils in the new backfill soils that are to be placed behind basement walls.
SELECTING A CONTRACTOR
| ...JUST BECAUSE A CONTRACTOR HAS COMPLETED A PROJECT IN A MARINE CLAY DOES NOT MEAN THAT HE OR SHE IS QUALIFIED TO REPAIR YOUR HOME. |
Construction and home repair contractors may advertise their services through a variety of means --phone book (contractors -- general), classified advertisements, flyers and mailing advertisements, and word of mouth.
Some contractors may have experience in house repairs in marine clay. However, just because a contractor has completed a project in a marine clay area does not mean that he or she is qualified to repair your home.
QUALIFICATIONS
Ask contractors for their qualifications:
- Is the contractor licensed to work in Fairfax County or Virginia?
- How long have they been doing home repairs?
- How many projects have they completed in marine clay?
- Who are their business and professional references?
- Do they have liability insurance? (Ask for the insurance carrier and the name of the agent who can verify the policy.)
Find out how reliable the contractor has been in the past. You can contact the Consumer Protection Division to see if any complaints have been filed against the contractor and how these complaints were resolved. Ask the following.
- Is the work completed in a timely fashion?
- Are cost overruns typical?
- Does the completed project look like it was completed by a professional?
- Are the homeowners satisfied with the quality of the work?
ESTIMATE
Ask for a written estimate of the work to be done, including separate statements for materials and labor. Since repair designs will often be prepared by a professional engineer, the contractor will be basing the estimate on the engineering design. Be sure that the contractor understands the scope of the work completely.
A fixed price contract, not an open-ended one, is preferable. The contractor should have enough information from the geotechnical report to develop cost estimates for his or her work. If not, the geotechnical report is probably incomplete and does not address the problems thoroughly. Any change-orders to the contract should also have a fixed price.
A requirement that the bulk of contract payment will be made upon completion of the work, not in advance, will help ensure completion of the project. Consider withholding some portion, for example, one-third, of the payment until all required inspections by the engineer and Fairfax County have been completed and approved.
OBTAINING A BUILDING PERMIT
A building permit is required in Fairfax County prior to any foundation repair work. The purpose of the permit is to ensure that the construction is in accordance with applicable building codes and good engineering practice.
| HELPFUL HINT: You may obtain permits in your own name; however, if a contractor is to perform the work, it is strongly recommended that the contractor secure the permit and be listed as the responsible party. In this way, the county will be in a better position to assist you in gaining compliance with codes if the work is defective. A contractor must be properly licensed in order to obtain a permit. |
The building permit for repair of marine clay damage may be a valuable document when you sell your house because it shows that building codes were followed in correction of the problem. The building permit along with construction drawings and any engineering reports must be reviewed and approved by Fairfax County staff. Fairfax County staff will also inspect the construction. Learn more about the permit process.
| THE BUILDING PERMIT FOR REPAIR OF MARINE CLAY DAMAGE MAY BE A VALUABLE DOCUMENT WHEN YOU SELL YOUR HOUSE... |