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Geotechnical Guidelinesfor Design, Construction, Materials and Maintenance of Residential Projects in the Houston Area.
D. Eastwood, P.E.
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The variable subsoil conditions in the Gulf Coast area has resulted in very special design requirements for residential and light commercial foundations. The subsurface conditions should be carefully considered when a subdivision or a residence is to be built. Proper planning from the stand point of environmental conditions, subsidence, faulting, soil conditions, design, construction, materials, quality control and maintenance program should be considered prior to any development.
The purpose of this document is to recommend the scope of geotechnical work to develop soils and foundation data for a proper and most economical design and construction of foundations in the Houston area. It is our opinion that portions of these studies should be performed prior to developing the subdivision or buying the lots in order to minimize potential future soils and foundation problems. These problems may arise from the presence of hazardous waste, faulting, poorly compacted fill, soft soil conditions, expansive soils, perched water table, presence of sand and silts, tree roots, etc. This guideline is divided into six segments, including Pre-Development Studies, design, construction, materials, quality control, maintenance program and foundation stabilization. Our recommendations are presented from a geotechnical stand point only and should be complemented by a structural engineer.
Environmental Site Reconnaissance Study
Environmental site assessment studies are recommended on the tracts of land for subdivision or commercial developments. A study like this is generally not required for a single lot in an established subdivision or an in-fill lot in the city. This type of study is used to evaluate the potential risk of environmental contamination that is on or used to be on a project site prior to development. The study is divided into phases, Phases I through III.
The scope of Phase I includes a preliminary site reconnaissance, including: (a) document search, (b) site walk through, (c) review of aerial photographs, (d) historical ownership report, (e) regulatory data review and (f) a report of observations and recommendations.
In the event that the results of the Phase I study indicates the potential for the presence of contaminants, a Phase II study is performed. The scope of Phase II study may include: (a) soil and groundwater sampling, (b) chemical testing and analysis, (c) site reconnaissance, (d) contact with state and federal regulatory personnel, and (e) reporting.
A Phase III study involves implementing the recommendations given in the Phase II study; including remediation and monitoring.
Potential subsidence problems should be considered when developing subdivisions in the coastal areas, such as Clear Lake, Seabrook, Baytown, etc. Also, other parts of Houston, subject to groundwater removal are also subject to subsidence. This type of study is generally not needed for a single lot in an established subdivision or an in fill lot in the city.
Subsidence is the sinking of the land surface caused by the withdrawal of groundwater. The land elevation lost to subsidence is generally permanent and irreversible. In the Harris-Galveston region of Texas, subsidence poses the greatest threat in the coastal areas susceptible to flooding due to high tides, heavy rainfall and hurricane storm surge. Because of low elevation, any additional subsidence in the coastal areas results in a significant increase in potential tidal flooding or permanent inundation.
The rate of land subsidence in Harris County has been reduced significantly due to changes in water development from the surface water instead of groundwater.
A review of recent subsidence data available from Harris County Subsidence District indicates that the subsidence in areas such as Pasadena, Southwest Houston, etc. have slowed down significantly. However, the subsidence rate in the Addick Area (West, Northwest Houston) is about one-inch per year.
Many faults have been observed within the Gulf Coast Region of Texas. In general, faults are caused by groundwater and oil removal from the underlying surface. Faults originate several thousand feet below the ground surface and can often cause displacement of the ground surface, causing broken pavement and damage to residential and commercial structures.
Faults are studied in several phases. A Phase I fault study will include the first step in identification of faulting. The scope of a Phase I investigation includes the following elements:
1. Literature Review. This includes a search for, and study of, published data on surface faults in the area of the site.
2. Remote Sensing Study. Aerial photographs, infra-red imagery, where available, should be studied.
3. Field Reconnaissance. This includes a visit to the study area and vicinity by a qualified engineer to examine the area for physical evidence. Physical evidence includes, but is not limited to, (a) natural topographic scarps, (b) soil layer displacements that may be recognized in ditches, creek banks and trenches, (c) breaks in pavements, (d) distress in existing buildings, and (e) vertical offsets in fences.
Once a residence is built on an active fault, the foundation for the residence will be subject to a continual movement and subsequent distress. Foundation stabilization of structures built on active faults can be difficult, but possible. A study of geologic faulting is recommended prior to development of any subdivision in the Gulf-coast area.
The Houston area is located on the Gulf of Mexico Coastal Plain, which is underlain largely by overconsolidated clays, clay shales and poorly cemented sands to a depth of several miles. Nearly all soil of the area consists of clay, associated with moderate amounts of sand. Some of the formations in the Houston area consist of Beaumont, Lissie, and Bentley.
The Beaumont formation has significant amounts of expansive clays, resulting in shrink/swell potential. Desiccation of this formation also produces a network of fissures and slickensides in the clay that is potential plains of weakness. The Beaumont formation generally occurs in South, Southwest, East, and Central Houston. The Lissie and Bentley formations generally occur in North and part of West Houston. These formations consist of generally sands and sandy clays. These soils are generally low to moderate in plasticity with low to moderate shrink/swell potential.
General Soils Conditions
Variable soil conditions occur in the Houston area. These soils are different in texture, plasticity, compressibility, and strength. It is very important that foundations for residential and light commercial structures be designed for subsoil conditions that exists at the specific lot in order to minimize potential foundation and structural distress. Details of general subsoil conditions at various parts of the Houston area are described below. These descriptions are very general. Significant variations from these descriptions can occur. The General soil conditions are as follows:
Water Level Measurements
The groundwater levels in the Gulf Coast area vary significantly. The groundwater depth in the Houston area generally ranges from 8- to 30-feet. Fluctuations in groundwater level generally occurs as a function of seasonal rainfall variation, temperature, groundwater withdrawal, and construction activities that may alter the surface and drainage characteristics of the site.
The groundwater measurements are usually evaluated by the use of a tape measure and weight at the end of the tape at the completion of drilling and sampling.
An accurate evaluation of the hydrostatic water table in the relatively impermeable clays and low permeability silt/sands requires long term observation of monitoring wells and/or piezometers. It should be noted that it is not possible to accurately predict the pressure and/or level of groundwater that might occur based upon short-term site exploration. The installation of piezometers/monitor wells is beyond the scope of a typical residential geotechnical reports. We recommend that the groundwater level be verified just before construction if any excavations such as construction of drilled footings/underground utilities, etc. are planned.
The geotechnical engineer must be immediately notified if a noticeable change in groundwater occurs from the one mentioned in the same report. The geotechnical engineer should then evaluate the affect of any groundwater changes on the design and construction of the facilities.
Some of the groundwater problem areas in Houston include Southside Place, parts of Sugar land, etc. One should not confuse the perched water table with the groundwater table. A perched water table occurs when bad drainage exists in areas with a sand or silt layer, about two- to four-foot thick, underlain by impermeable clays and sandy clays. During the wet season, water can pond on the clays and create a perched water table. The surficial sands/silts become extremely soft, wet and may lose their load carrying capacity.
Foundations and Risks
Many lightly loaded foundations are designed and constructed on the basis of economics, risks, soil type foundation shape and structural loading. Many times, due to economic considerations, higher risks are accepted in foundation design. Most of the time, the foundation types are selected by the owner/builder, etc. It should be noted that some levels of risk are associated with all types of foundations and there is no such thing as a zero risk foundation. All of these foundations must be stiffened in the areas where expansive soils are present and trees have been removed prior to construction. The following are the foundation types typically used in the area with increasing levels of risk and decreasing levels of cost:
The above recommendations, with respect to the best foundation types and risks, are very general. The best type of foundation may vary as a function of structural loading and soil types. For example, in some cases, a floating slab foundation may perform better than a drilled footing type foundation.
Residential structures in the Houston area are supported on drilled footings, post-tensioned slabs, or conventionally reinforced slabs. In general, properly designed post-tensioned slabs or conventionally-reinforced slabs perform satisfactory on most subsoils. Drilled footings may provide a superior foundation system when large slabs, significant offsets or differential loading occurs on the foundations.
The selection of foundation is a function of economics and the level of the risk that the client wants to take. For example, a structural slab foundation is not used for a track home that costs about $100,000. This type of foundation is used for houses that cost usually much more expensive. In general, floating slab type foundations are used with houses with price ranges of less than $200,000 or when subsoil conditions dictates to use this type of foundation.
Geotechnical Foundation Design Criteria
Foundations for a residential structure should satisfy two independent design criteria. First, the maximum design pressure exerted at the foundation base should not exceed the allowable net bearing pressure based on an adequate factor of safety with respect to soil shear strength. Secondly, the magnitude of total and differential settlements (and shrink and swell) under sustained loads must be such that the structure is not damaged or its intended use impaired.
It should be noted that properly designed and constructed foundation may still experience distress from improperly prepared bearing soils and/or expansive soils which will undergo volume change when correct drainage is not established or an incorrectly controlled water source becomes available.
The design of foundations should be performed by an experienced structural engineer using a soils report from an experienced soils engineer. The structural engineer must use a lot/site specific soils report for the foundation design. The structural engineer should not use general subdivision soils reports written for underground utilities and paving for the slab design. Furthermore, he should not design slabs with disclaimers, requiring future soils reports to verify his design. The designers or architects should not provide clients with foundation design drawings with generic foundations details. All of the foundation drawings should be site and structure specific and sealed by a professional structural engineer.
Recommended Scope of Geotechnical Studies
Soil testing must be performed on residential lots before a foundation design can be developed. The recommended number of borings should be determined by a geotechnical engineer. The number of borings and the depths are a function of the size of the structure, foundation loading, site features, and soil conditions. As a general rule, a minimum of one boring for every five lots should be performed for subdivision lots. This boring program assumes that a conventionally-reinforced slab or a post-tensioned slab type foundation is going to be used. Furthermore, many lots will be tested at the same time so that a general soils stratigraphy can be developed for the entire subdivision. In the event that a drilled footing foundation is to be used, a
a minimum of one boring per lot is recommended. In the case of variable subsoil conditions, two or more borings per lot should be performed. A minimum of two borings is recommended for custom homes or a single in-fill lot. A minimum boring depth of 15-feet is recommended for the design of post-tensioned or conventially-reinforced slabs. The boring depths for the design of drilled footing foundations should be at least 15-feet deep. In the event that the lot is wooded and expansive soils are suspected, the boring depth (if drilled footings are to be used) should be increased to 20-ft. On the wooded lots, when the presence of expansive soils are suspected the borings should be drilled near the trees, if possible. Root fibers should be obtained to estimate the active zone depth. The active zone depth is defined as the depth within which seasonal changes in moisture content/soil suction can occur. In general, the depth of active zone is about two-feet below the lowest root fiber depth.
The borings for the residential lots should be performed after the streets are cut and fill soils have been placed and compacted on the lots. This will enable the geotechnical engineer to identify the fill soils that have been placed on the lots. All fill soils should have been tested for compaction during the placement on the lots. A minimum of one density test for every 2500 square feet per lift must be performed once a subdivision is being developed. Fill soils may consist of clays, silty clays, and sandy clays. Sands and silts should not be used as fill materials. Typical structural fill in the Houston area consists of silty clays and sandy clays (not sands) with liquid limits less than 40 and plasticity index between 10 and 20. The fill soils should be placed in lifts not exceeding eight-inches and compacted to 95 percent of the maximum dry density (ASTM D698-91). On-site soils with the exception of sands can also be used as structural fill under floating slab foundations. A floating slab foundation is defined as a conventionally-reinforced slab and a post-tensioned slab.
In the case of a subdivision development, the developer should perform only the borings for the streets and underground utilities. The borings for the lots should wait until all fill soils from street and underground utility excavations are placed and compacted on the lots. In general, the geotechnical testing of the soils for the lots should be the builders responsibility. We recommend that all of the foundations in the subdivision be engineered by a registered professional engineer specializing in residential foundation design.
In the areas where no fill will be placed on the lots prior to site development, the borings on the lots can be performed at the same time as the time as the borings for streets. The soils data from the street and underground borings should never be used for the slab design. This is due to potential in variability in the soil conditions, including soils stratigraphy, compressibility, strength, and swell potential.
Soil borings must be performed prior to foundations underpinning for distressed structures. This is to evaluate the subsoil properties below the bottom of the drilled footings. The depth of drilled footings for foundation underpinning should be determined by a geotechnical engineer. Unfortunately, this is not always followed, and many "so called" foundation repair jobs are performed incorrectly, causing significant financial loss for the client.
In the event of building additions, a minimum of one boring is recommended on residential additions of less than 1,000 square feet. A minimum of two borings is recommended for additions greater than 1,000 square feet.
In general, a scope of typical geotechnical exploration does not include the evaluation of fill compaction. These studies should have been performed at the time of fill placement.
Foundation Design Considerations
In the areas where highly expansive soils are present, the drilled footings should be founded in a strong soil stratum below the zero movement line. This depth is defined as the depth below which no upward movements occur. It is possible to found a drilled footing below the zero movement line and within the active zone depth. The active zone is defined as the zone within which seasonal changes in subsoil moisture can occur. This is shown on Plate 1. Drilled footings in the area with deep active zones, where trees are present, and subsoils are expansive can be as much as 18-feet deep. The depth of drilled footings should also be determined such that the uplift along the pier shafts be resisted by the presence of bells or shaft skin friction below the zero movement line. The depth of the active zone should be verified by a geotechnical exploration. The evaluation of active zone depths and zero movement line should be performed using the techniques provided in the 1996 Post-Tensioning Institute Slab-on-Grade Design Manual. Drilled footings founded at shallower depths may experience uplift due to expansive soils. In the areas where non-expansive soils are present, the footing depth can be as low as eight-feet.
The grade beams for a floating slab foundation should penetrate the clay soils a minimum of 12-inches. The grade beam penetrations for a floating slab foundation into the surficial sands should be at least 18-inches to develop the required bearing capacity. A minimum grade beam width of 12-inches is recommended in sands and silts.
In the event that a floating slab (post-tensioned slab or a conventionally-reinforced slab) is constructed in sands or silts, the geotechnical engineer must specify bearing capacity, assuming saturated subsoil conditions. This results in bearing capacities in the range of 600- to 900 psf in a typical sand or silt soils in the Houston area. Higher bearing capacity values can be used if the sands/silts do not get saturated during the life of the residence. This assumption is generally unrealistic due to the presence of sprinkler systems, negative drainage, and cyclic rainfall in the Houston area.
Design parameters for a post-tensioned slab on expansive clays must carefully evaluated by a geotechnical engineer. It should be noted that the 1996 post-tensioned slab design manual does not directly model the poor drainage, the effect of the trees, and the depth of the active zone. The geotechnical engineer must modify the design parameter presented in the manual to come up with the proper design parameter. It should be noted that it is currently very difficult (to impossible) to design economical floating slab foundations on expansive soils on wooded lots where trees are to be removed prior to slab construction.
The floor slabs for foundations supported on drilled footings may consist of (a) structural slabs with crawl space, (b) slab-on-fill or (c) slab-on-grade.
A structural slab should be used when a minimum risk foundation is to be used. This type of floor slabs are generally expensive. A slab-on-fill will be less expensive than a structural slab with crawl space. The fill thickness in areas where expansive soils are present should be about 18-to 48-inches. The higher fill thickness should be used in areas such as Bellaire, Tanglewood, New Territory, etc, where highly expansive clays exists (plasticity indices above 50).
In the event that a structural slab foundation is used, the crawl space area should be properly drained so that any water would drain towards the exterior grade beams. Furthermore, the area should be properly vented.
The floor slabs can be supported at grade on drilled footings if the subsoils are non-expansive. All of the subgrade soils should be prepared in accordance to the soils report site preparation section prior to fill placement.
Void boxes are historically used under the grade beams to separate the expansive soils from the grade beams. The void boxes collapse once the underlying expansive soils swell up; thereby minimizing uplift loads as a result of expansive soils on the grade beams. This can be an effective feature for reducing potential pressures on grade beams.
In areas of poor drainage, void boxes may act as a pathway for water to travel under a foundation system. This condition may result in an increase in subsoil moisture contents and subsequent swelling of the soils. This may result in uplift loads on the floor slabs, and subsequent distress to the foundation and structural system.
We recommend that the decision on whether or not to use void boxes be made by the owner/builder after both the positive and negative aspects of this issue are evaluated. Based on our and other experts personal experience with void boxes, it is our opinion that they will not provide an effective feature for reducing swell pressure on the grade beams. In general, the validity of void box usage is presently being questioned because of the frequency of observed negative effects which may outweigh its benefits.
It is recommended that site drainage be well developed. Surface water should be directed away from the foundation soils (use a slope of about 5% within 10-feet of foundation). No ponding of surface water should be allowed near the structure.
Residential Structures Constructed near the Bayous
Many large residential structures are being build near the bayous. Portions of the slopes on the bayous are very steep with slopes steeper than 3(h):1(v). The foundations for residences near the bayous must be provided by the use of deep drilled footings/piling. The geotechnical boring depths should be at least twice the depth of the bayou.
Any foundation which falls within the hazard zone which extends from the toe of the slope, extending backward on a 4(h):1(v) slope to the existing grade should be supported on deep foundations. Foundations outside the hazard zone may be supported on shallow piers. The floor slabs in the hazard zone should consist of a structural slab. The floor slabs outside the hazard zone may consist of slab-on -fill or slab-on-grade. No skin friction should be used for piers within the hazard zone from the surface to the toe of the slope elevation.
We recommend the stability of bayou slopes be evaluated using a slope-stability analyses, using computer solutions. The house should be placed on top of the slope and the stability of the slope for global stability should be evaluated. The slopes should then be flattened and covered with erosion protection to minimize potential sloughing and erosion problems.
Our recommendations on site preparation are summarized below:
1. In general, remove all vegetation, tree roots, organic topsoil, existing foundations, paved areas and any undesirable materials from the construction area. Tree trunks under the floor slabs should be removed to a root size of less than 0.5-inches. We recommend that the stripping depth be evaluated at the time of construction by a soil technician.
2. Any on-site fill soils, encountered in the structure and pavement areas during construction, must have records of successful compaction tests signed by a registered professional engineer that confirms the use of the fill and record of construction and earthwork testing. These tests must have been performed on all the lifts for the entire thickness of the fill. In the event that no compaction test results are available, the fill soils must be removed, processed and recompacted in accordance with our site preparation recommendations. Excavation should extend at least two-feet beyond the structure and pavement area. Alternatively, the existing fill soils should be tested comprehensively to evaluate the degree of compaction in the fill soils.
3. The subgrade areas should then be proofrolled with a loaded dump truck, scraper, or similar pneumatic-tired equipment. The proofrolling serves to compact surficial soils and to detect any soft or loose zones. Any soils deflecting excessively under moving loads should be undercut to firm soils and recompacted. The proofrolling operations should be observed by an experienced geotechnician.
4. Scarify the subgrade, add moisture, or dry if necessary, and recompact to 95% of the maximum dry density as determined by ASTM D 698-91 (Standard Proctor). The moisture content at the time of compaction of subgrade soils should be within -1 to +3% of the proctor optimum value. We recommend that the degree of compaction and moisture in the subgrade soils be verified by field density tests at the time of construction. We recommend a minimum of four field density tests per lift or one every 2500 square feet of floor slab areas, whichever is greater.
5. Structural fill beneath the building area may consist of off-site inorganic silty clays or sandy clays with a liquid limit of less than 40 and a plasticity index between 10 and 20. In the event that a floating slab foundation system is used, on-site soils (with the exception of sands or silts), free of organics, can be used as structural fill. Other types of structural fill available locally, and acceptable to the geotechnical engineer, can also be used.
These soils should be placed in loose lifts not exceeding eight-inches in thickness and compacted to 95 percent of the maximum dry density determined by ASTM D 698-91 (Standard Proctor). The moisture content of the fill at the time of compaction should be within +2% of the optimum value. We recommend that the degree of compaction and moisture in the fill soils be verified by field density tests at the time of construction. We recommend that the frequency of density testing be as stated in Item 4.
6. The backfill soils in the trench/underground utility areas should consist of select structural fill, compacted as described in Item 4. In the event of compaction difficulties, the trenches should be backfilled with cement-stabilized sand or other materials approved by the Geotechnical Engineer. Due to high permeability of sands and potential surface water intrusion, bank sands should not be used as backfill material in the trench/underground utility areas.
7. In cut areas, the soils should be excavated to grade and the surface soils proofrolled and scarified to a minimum depth of six-inches and recompacted to the previously mentioned density and moisture content.
8. The subgrade and fill moisture content and density must be maintained until paving or floor slabs are completed. We recommend that these parameters be verified by field moisture and density tests at the time of construction.
9. In the areas where expansive soils are present, rough grade the site with structural fill soils to insure positive drainage. Due to their high permeability of sands, sands should not be used for site grading where expansive soils are present.
10. We recommend that the site and soil conditions used in the structural design of the foundation be verified by the engineer's site visit after all of the earthwork and site preparation has been completed and prior to the concrete placement.
Other Construction Considerations
1. Grade beam excavations should be free of all loose materials. The bottom of the excavations should be dry and hard.
2. Surficial subgrade soils in the floor slab areas should be compacted to a minimum of 95% of Standard Proctor Density (ASTM D 698-91). This should be confirmed by conducting a minimum of four field density tests per slab, per lift.
3. Minimum concrete strength should be 3,000 psi with a maximum slump of 5-inch. Concrete workability can be improved by adding air to the concrete mix and the use of a concrete vibrator. The concrete slump and strength should be verified by slump tests and concrete cylinders.
4. The Visqueen, placed under the floor slabs, should be properly stretched to maximize soil-slab interaction.
5. In the areas where expansive soils are present, the backfill soils for the underground utilities under the floor slabs should consist of select fill and not sands or silts. The cohesionless backfill can act as a pathway for surface water to get under the foundation and resulting in subsoil swelling. In the event that a floating slab is used, on-site soils (not sands or silts), free of organics, can be used as structural fill.
6. Tree stumps should not be left under the slabs. This may result in future settlement and termite infestation.
The use of proper materials is crucial to the performance of a foundation system. Some of the relevant material issues is as follows:
Construction monitoring and quality control tests should be planned to verify materials and placement in accordance with the project design documents and specifications. Earthwork observations on the house pad, pad thickness measurements, drilled footing installation monitoring, and concrete placement monitoring should be performed. Details of each of these items is described in the following paragraphs.
The subgrade and fill soils under the floor slabs should be compacted to about 95 percent of maximum dry density (ASTM D 698-91). Furthermore, the fill soils should be non-expansive. Atterberg limit tests should be performed on the fill soils, obtained from the borrow pit, to evaluate the suitability of these soils for use as structural fill and their shrink/swell potential. Expansive soils, of course, should not be used as structural fill. In the event that a floating slab foundation is used, on-site soils with the exception of sands/silts can be used as structural fill.
Field density tests should be conducted on the subgrade soils and any borrow fill materials in the floor slab and pavement areas. In the areas where expansive soils are present, about 18- to 36-inches of structural fill is placed under the floor slab areas. Laboratory proctor tests will also be performed on the on-site soils as well as off-site borrow fill materials to evaluate the moisture-density relationship of these soils.
Fill Thickness Verification
Fill soils may have to be placed on the lots to raise the lot or to provide a buffer zone in between the on-site expansive soils and the floor slabs. We recommend that the required thickness of the fill be verified after the completion of the building pad. This task can be accomplished by drilling two borings to a depth of two-feet in the building pad area, examining and testing the soils to verify the fill thickness.
Drilled Footing Observations
In the event that the structure is supported by drilled footings, we recommend that the installation of the footings be observed by a geotechnical technician.
The technician will conduct hand penetrometer tests on the soil cuttings to estimate the bearing capacity of the soil at each footing location. He will make changes to the foundation depth and dimensions if obstacles or soft soils are encountered. Therefore, minimizing costly construction delays. In addition, the technician must verify the bell size by a bell measurement tool. One set of concrete cylinders (four cylinders) will be made for each 50 yards of pour. Two cylinders will be broken at seven days, and two cylinders at 28 days.
Concrete Placement Monitoring
The concrete sampling and testing in the floor slab and placement areas will be conducted in accordance with ASTM standards. A technician will monitor batching and placing of the concrete. At least four concrete cylinders should be made for each 50 yards floor slab pour. Two concrete cylinders are tested at seven days and two cylinders at 28 days.
HOMEOWNER MAINTENANCE PROGRAM
Performance of residential structures depends not only on the proper design and construction, but also on the proper foundation maintenance program. Many residential foundations have experienced major foundation problems as a result of owner's neglect or alterations to the initial design, drainage, or landscaping. This has resulted in considerable financial loss to the homeowners, builders, and designers in the form of repairs and litigation.
A properly designed and constructed foundation may still experience distress from vegetation and expansive soil which will undergo volume change when correct drainage is not established or incorrectly controlled water source becomes available.
The purpose of this document is to present recommendations for maintenance of properly designed and constructed residential projects in Houston. It is recommended that the builder submit this document to his/her client at the time that the owner receives delivery of the house.
The initial builder/developer site grading (positive drainage) should be maintained during the useful life of the residence. In general, a civil engineer develops a drainage plan for the whole subdivision. Drainage sewers or other discharge channels are designed to accommodate the water runoff. These paths should be kept clear of debris such as leaves, gravel, and trash.
In the areas where expansive soils are present, positive drainage should be provided away from the foundations. Changes in moisture content of expansive soils are the cause of both swelling and shrinking. Positive drainage should also be maintained in the areas where sandy soils are present.
Positive drainage is extremely important in minimizing soil-related foundation problems.
The homeowners berm the flowerbed areas, creating a dam between the berm and the foundation, preventing the surface water from draining away from the structure. This condition may be visually appealing, but can cause significant foundation damage as a result of negative drainage.
The most commonly used technique for grading is a positive drainage away from the structure to promote rapid runoff and to avoid collecting ponded water near the structure which could migrate down the soil/foundation interface. This slope should be about 3 to 5 percent within 10-feet of the foundation.
Should the owner change the drainage pattern, he should develop positive drainage by backfilling near the grade beams with fill compacted to 90 percent of the maximum dry density as determined by ASTM D 698-91 (standard proctor). This level of compaction is required to minimize subgrade settlements near the foundations and the subsequent ponding of the surface water. The fill soils should consist of silty clays and sandy clays with liquid limits less than 40 and plasticity index (PI) between 10 and 20. Bank sand or top soils are not a select fill. The use of Bank sand or top soils to improve drainage away from a house is discouraged; because, sands are very permeable. In the event that sands are used to improve drainage away from the structure, one should make sure the clay soils below the sands have a positive slope (3 - 5 Percent) away form the structure, since the clay soils control the drainage away from the house. The on-site soils (not sand or silts), free of organics, can be used as structural fill.
The author has seen many projects with an apparent positive drainage; however, since the drainage was established with sands on top of the expansive soils the drainage was not effective.
Depressions or water catch basin areas should be filled with compacted soil (sandy clays or silty clays not bank sand) to have a positive slope from the structure, or drains should be provided to promote runoff from the water catch basin areas. Six to twelve inches of compacted, impervious, nonswelling soil placed on the site prior to construction of the foundation can improve the necessary grade and contribute additional uniform surcharge pressure to reduce uneven swelling of underlying expansive soil.
Pets (dogs, etc.) sometimes excavate next to the exterior grade beams and created depressions and low spots in order to stay cool during the hot season. This condition will result in ponding of the surface water in the excavations next to the foundation and subsequent foundation movements. These movements can be in the form of uplift in the area with expansive soils and settlement in the areas with sandy soils. It is recommended as a part of the foundation maintenance program, the owner backfills all excavations created by pets next to the foundation with compacted clay fill.
Grading and drainage should be provided for structures constructed on slopes, particularly for slopes greater than nine percent, to rapidly drain off water from the cut areas and to avoid ponding of water in cuts or on the uphill side of the structure. This drainage will also minimize seepage through backfills into adjacent basement walls.
Subsurface drains may be used to control a rising water table, groundwater and underground streams, and surface water penetrating through pervious or fissured and highly permeable soil. Drains can help control the water table in the expansive soils. Furthermore, since drains cannot stop the migration of moisture through expansive soil beneath foundations, they will not prevent long-term swelling. Moisture barriers can be placed near the foundations to minimize moisture migration under the foundations. The moisture barriers should be at least five-feet deep in order to be effective.
Area drains can be used around the house to minimize ponding of the surface water next to the foundations. The area drains should be checked periodically to assure that they are not clogged.
The drains should be provided with outlets or sumps to collect water and pumps to expel water if gravity drainage away from the foundation is not feasible. Sumps should be located well away from the structure. Drainage should be adequate to prevent any water from remaining in the drain (i.e., a slope of at least 1/8 inch per foot of drain or 1 percent should be provided).
Positive drainage should be established underneath structural slabs with crawl space. This area should also be properly vented. Absence of positive drainage may result in surface water ponding and moisture migration through the slab. This may result in wood floor warping and tile unsticking. Furthermore, The crawl space area should be properly vented.
It is recommended that at least six-inches of clearing be developed between the grade and the wall siding. This will minimize surface water entry between the foundation and the wall material, in turn minimizing wood decay.
Poor drainage at residential projects in North and West Houston can result in saturation of the surficial sands and development of a perched water table. The sands, once saturated, can lose their load carrying capacity. This can result in foundation settlements and bearing capacity failures. Foundations in these areas should be designed assuming saturated subsoil conditions.
In general, roof drainage systems, such as gutters or rain dispenser devices, are recommended all around the roof line when gutters and downspouts should be unobstructed by leaves and tree limbs. In the area where expansive soils are present, the gutters should be connected to flexible pipe extensions so that the roof water is drained at least 10-feet away from the foundations. Preferably the pipes should direct the water to the storm sewers. In the areas where sandy soils are present, the gutters should drain the roof water at least five-feet away from the foundations.
If a roof drainage system is not installed, rain-water will drip over the eaves and fall next to the foundations resulting in subgrade soil erosion, and creating depression in the soil mass, which may allow the water to seep directly under the foundation and floor slabs.
The home owner must pay special attention to leaky pools and plumbing. In the event that the water bill goes up suddenly without any apparent reason, the owner should check for a plumbing leak.
The introduction of water to expansive soils can cause significant subsoil movements. The introduction of water to sandy soils can result in reduction in soil bearing capacity and subsequent settlement. The home owner should also be aware of water coming from the air conditioning drain lines. The amount of water from the condensating air conditioning drain lines can be significant and can result in localized swelling in the soils, resulting in foundation distress.
A house with the proper foundation, and drainage can still experience distress if the homeowner does not properly landscape and maintain his property. One of the most critical aspects of landscaping is the continual maintenance of properly designed slopes.
Installing flower beds or shrubs next to the foundation and keeping the area flooded will result in a net increase in soil expansion in the expansive soil areas. The expansion will occur at the foundation perimeter. It is recommended that initial landscaping be done on all sides, and that drainage away from the foundation should be provided and maintained. Partial landscaping on one side of the house may result in swelling on the landscaping side of the house and resulting differential swell of foundation and structural distress in a form of brick cracking, windows/door sticking, and slab cracking.
Landscaping in areas where sandy, non-expansive soils are present, with flowers and shrubs should not pose a major problem next to the foundations. This condition assumes that the foundations are designed for saturated soil conditions. Major foundation problems can occur if the planter areas are saturated as the foundations are not designed for saturated (perched water table) conditions. The problems can occur in a form of foundation settlement, brick cracking, etc.
Sprinkler systems can be used in the areas where expansive soils are present, provided the sprinkler system is placed all around the house to provide a uniform moisture condition throughout the year.
The use of a sprinkler system in parts of Houston where sandy soils are present should not pose any problems, provided the foundations are designed for saturated subsoil conditions with positive drainage away from the structure.
The excavations for the sprinkler system lines, in the areas where expansive soils are present, should be backfilled with impermeable clays. Bank sands or top soil should not be used as backfill. These soils should be properly compacted to minimize water flow into the excavation trench and seeping under the foundations, resulting in foundation and structural distress.
The sprinkler system must be checked for leakage at least once a month. Significant foundation movements can occur if the expansive soils under the foundations are exposed to a source of free water.
The homeowner should also be aware of damage that leaking plumbing or underground utilities can cause, if they are allowed to continue leaking and providing the expansive soils with the source of water.
Effect of Trees
The presence of trees near a residence is considered to be a potential contributing factor to the foundation distress. Our experience shows that the presence or removal of large trees in close proximity to residential structures can cause foundation distress. This problem is aggravated by cyclic wet and dry seasons in the area. Foundation damage of residential structures caused by the adjacent trees indicates that foundation movements of as much as 3- to 7-inches can be experienced in close proximity to residential foundations.
This condition will be more severe in the periods of extreme drought. Sometimes the root system of trees such as willow, elm, or oak can physically move foundations and walls and cause considerable structural damage. Root barriers can be installed near the exterior grade beams to a minimum depth of 36-inches, if trees are left in place in close proximity to foundations. It is recommended that trees not be planted closer than half the canopy diameter of the mature tree, typically 20-feet from foundations. Any trees in closer proximity should be thoroughly soaked at least twice a week during hot summer months, and once a week in periods of low rainfall. More frequent tree watering may be required.
Tree roots tend to desiccate the soils. In the event that the tree has been removed prior to house construction, subsoil swelling can occur for several years. Studies have shown that for certain types of trees this process can last as much as 20 years in the areas where highly expansive clays are present. In this case the foundation for the house should be designed for the anticipated maximum heave.
Furthermore, the drilled footings, if used, must be placed below the zone of influence of tree roots. In the event that a floating slab foundation is used, we recommend the slab be stiffened to resist the subsoil movements due to the presence of trees. In addition, the area within the tree root zone may have to be chemically stabilized to reduce the potential movements. Alternatively, the site should be left alone for several years so that the moisture regime in the desiccated areas of the soils (where tree roots used to be) become equal/stabilize to the surrounding subsoil moisture conditions.
Tree removal can be safe provided the tree is no older than any part of the house, since the subsequent heave can only return the foundation to its original level. In most cases there is no advantage to a staged reduction in the size of the tree and the tree should be completely removed at the earliest opportunity. The areas where expansive soils exist and where the tree is older than the house, or there are more recent extensions to the house, it is not advisable to remove the tree because the danger of inducing damaging heave; unless the foundation is designed for the total computed expected heave.
In general, in the areas where non-expansive soils are present, no foundation heave will occur as a result of the tree removal.
In the areas where too much heave can occur with tree removal, some kind of pruning, such as crown thinning, crown reduction or pollarding should be considered. Pollarding, in which most of the branches are removed and the height of the main trunk is reduced, is often mistakenly specified, because most published advice links the height of the tree to the likelihood of damage. In fact the leaf area is the important factor. Crown thinning or crown reduction, in which some branches are removed or shortened, is therefore generally preferable to pollarding. The pruning should be done in such a way as to minimize the future growth of the tree, without leaving it vulnerable to disease (as pollarding often does) while maintaining its shape. This should be done only by a reputable tree surgeon or qualified contractor working under the instructions of an arboriculturist.
You may find there is opposition to the removal or reduction of an offending tree; for example, it may belong to a neighbor or the local authority, or have a Tree Preservation Order on it. In such cases there are other techniques that can be used from within your own property.
One option is root pruning, which is usually performed by excavating a trench between the tree and the damaged property deep enough to cut most of the roots. The trench should not be so close to the tree that it jeopardizes its stability. In time, the tree will grow new roots to replace those that are cut; however, in the short term there will be some recovery as the degree of desiccation in the soil under the foundations reduces.
Where the damage has only appeared in a period of dry weather, a return to normal weather pattern may prevent further damage occurring. Permission from the local authority is required before pruning the roots of a tree with preservation order on it.
Root barriers are a variant of root pruning. However, instead of simply filling the trench with soil after cutting the roots, the trench is either filled with concrete or lined with an impermeable layer to form a "permanent" barrier to the roots. Whether the barrier will be truly permanent is questionable, because the roots may be able to grow round or under the trench. However, the barrier should at least increase the time it takes for the roots to grow back.
Every homeowner should conduct a yearly observation of foundations and flat works and perform any maintenance necessary to improve drainage and minimize infiltrations of water from rain and lawn watering. This is important especially during the first six years of a newly built home because this is usually the time of the most severe adjustment between the new construction and its environment. We recommend that all of the separations in the flat work and paving joints be immediately backfilled with joint sealer to minimize surface water intrusion and subsequent shrink/swell.
Some cracking may occur in the foundations. For example, most concrete slabs can develop hairline cracks. This does not mean that the foundation has failed. All cracks should be cleaned up of debris as soon as possible. The cracks should be backfilled with high-strength epoxy glue or similar materials. If a foundation experiences significant separations, movements, cracking, the owner must contact the builder and the engineer to find out the reason(s) for the foundation distress and develop remedial measures to minimize foundation problems.
Several methods of foundation stabilization are presented here. These recommendations include foundation underpinning, using drilled footings or pressed piling, moisture barriers, moisture stabilization, and chemical stabilization. Some of these methods are being used in the Houston area. A description of each method is summarized in the following sections of this document.
Foundation Underpinning, using drilled footings or pressed piling has been used in the Houston area for a number of years. The construction of a drilled footing consists of drilling a shaft, about 12-inches in diameter (or larger) constructed underneath the grade beam. The shaft is generally extended to depths ranging from 8 to 12-feet below existing grade. The bottom of the shaft is then reamed with an underreaming tool. The hole is then backfilled with steel, concrete, and the grade beams are jacked to a level position and shimmied to level the foundation system.
In a case of pressed piling, precast concrete piers are driven into the soils. These pier attain there bearing capacity based on the end bearing and the skin friction. In general, the precast concrete units are about 12-inches in height, six-inches in diameter and jacked into the soil. It is important the precast pier foundations are driven below the zero movement line to resist the uplift loads as a result of underlying expansive soils. Some of these jacked piles may consist of perma-piles, ultra piles, cable lock piles, etc.
The use of drilled footings/pressed piles should be determined by a geotechnical/structural engineer. Each one of these foundation systems have their pluses and minuses. Neither of these foundations can resist upward movement of the slabs. In general, they only limit the downward movement of the slabs. The pressed piles may not resist uplift loads as a result of skin friction of expansive soil if they are not connected with a cable or reinforcement. Therefore, if the units are not properly connected, they will not provide any tensile load transfer. The construction of each method should be monitored by an experienced geotechnical/structural engineer.
Helical piles which consist of Helical auger drilled into the soils provide a good method for underpinning, especially in the areas where sand, silts, shallow water table or caving clays are present. The helical piles are drilled into the soils until the desired resistance to resist the compressive loads are achieved. The augers are then fitted with a bracket and jacked against the grade beams to lift and to level the foundations.
Interior foundations may be required to level the interior of the residence. This can be accomplished by installing interior piers, tunneling under foundations and using pressed piling, or the use of polyurethane materials injected at strategic locations under the slab. The use of tunneling to install interior piers may introduce additional problems, such as inadequate compaction of backfill soils under the slab. However, the author has never encountered such a problem with pressed piling.
Partial underpinning is used in the areas where maximum distress is occurring under a slab. The use of full underpinning which includes placement of piers/pressed piling underneath all foundations is not necessarily a better method of stabilizing foundations. Many foundations are performing satisfactorily with partial underpinning. In the event that foundation underpinning is used, the home owners should put into place a foundation maintenance program to prevent additional foundation distress as a result of changes in subsoil moisture content.
Moisture Stabilization can be an effective method of stabilizing subsoil shrink swell movements in the ares where expansive soils are present. This method of stabilization is not effective in the areas where sands are present such as north of Harris County in areas such as Kingwood, Fairfield and The Woodlands. This method could be effective in the areas of highly expansive soils such as Tanglewood, Bellaire, West University, River Oaks, South Houston, and Southwest Houston. The method uses a porous pipe that is placed around the perimeter of the foundation and is connected to a water pressure system. A timer turns the water on and off depending on the subsoil moisture conditions, the moisture conditions around the perimeter of the house are monitored by moisture sensors. In general, the purpose of the system is to stabilize the moisture content around the slab to a uniform condition; therefore, minimizing the extremes of shrink and swelling problems. As it was mentioned earlier, the use of this method can result in major problems in the areas where sandy soils are present.
Moisture barriers can be used to isolate subsoil moisture variations in the areas where expansive soils are present. This can be as a result of surface water, groundwater, and tree root systems. In general, a moisture barrier may consist of an impermeable filter fabric, placed just outside the grade beams to depths ranging from five- to seven-feet. The moisture barriers can be horizontal or vertical. A horizontal moisture may consist of a sidewalk attached the exterior grade beams. The waterproofing between the moisture barrier and the exterior grade beams are very important. The connection should be completely sealed so that surface water can not penetrate under the horizontal moisture barrier. In general, it may take several years for the moisture barriers to effectively stabilize the moisture content underneath the floor slabs. A minimum vertical moisture barrier depth of five-feet is recommended.
This method of foundation stabilization has not been used in the Houston area routinely; however, it has been used for many projects in Dallas and San Antonio areas. The purpose of chemical stabilization is to chemically alter the properties of expansive soils; thus, making it non-expansive. In a chemical stabilization technique, the chemicals which may consist of lime or other chemicals are injected into the soil to a depth of about 7-feet around the perimeter of the structure. The chemical stabilization may (a) chemically alter the soil properties, and (b) provide a moisture barrier around the foundation. In general, this type of stabilization is effective when the chemicals are in intimately mixed with the soil. This can occur in soils that exhibit fissured cracks and secondary structures. This method of stabilization is not effective in the areas where soils do not experience significant cracking.
Regardless of what method of foundation stabilization is used, the homeowner maintenance with respect to proper drainage and landscaping is extremely important for success of any method.
RECOMMENDED QUALIFICATIONS FOR THE GEOTECHNICAL ENGINEER
We recommend that the geotechnical engineer should have the following qualifications: