Homeowner’s Responsibility of Drainage Construction and Maintenance Procedures Manual for Post Tensioned Slabs on Ground:
(This information comes directly from the Post Tensioning Institute)
Post Construction Site Considerations for Foundation Performance:
The performance of residential structures built on ground supported con Crete foundations depend not only on proper design and construction. But also on proper foundation environment maintenance performed by the occupant or owner of the property. Many residential foundations have experienced foundation problems as a result of improper installation, maintenance or alterations of the drainage system and landscaping. A properly designed and constructed foundation may still experience distress from soils which undergo volume changes caused by non climatic Moisture sources such as leaking pipes or irrigation.
The purpose of this section is to present recommendations for site conditioning of residential foundations. It is recommended that the builder Submit this information to the property owner in disclosure form at the time the property title is transferred.
Initial site grading shall provide positive drainage away from the foundation perimeter. The site drainage plan developed by the civil engineer should be maintained during the design life of the structure. Positive Drainage, to prevent water from ponding next to foundations, is imperative in minimizing soil related foundation problems. Drainage or other Discharge channels should be kept clear at all times of all debris, in Order, to allow water discharge away from the building footprint. The most commonly used technique for positive drainage is grading Away from the foundation to promote rapid runoff and to avoid ponding Water near the foundation. Poor drainage or ponding water can cause a Change in soil moisture content, resulting in swelling of the supporting Soils, causing foundation movements. Recommendation for positive Drainage is 3% to 5% slope for a minimum distance of 10 ft. (3.1 m) from the edge of the foundation. Berming of landscaped beds, while visually appealing, can create a damming effect between the berm and the foundation that may prevent water from draining away. Special Attention must be paid to these areas by providing additional precautions, such as area drains. Area drains must be checked periodically to insure that they are functional.
Should the site drainage be inadequate, property compacted select fill Material can be provided to reestablish positive drainage. The builder can be contacted to obtain information from the geotechnical engineer’s Report regarding the type of select fill material and the degree of compaction Necessary to provide adequate drainage. Proper compaction is required to Minimize sub grade settlements near the foundations and to prevent the Subsequent ponding of surface water.
Improper fill material and/or compaction may result in the appearance of Positive drainage; however, the drainage may not be effective as in the Case of permeable sands placed on top of an expansive clay layer that is Not sloped away from the foundation, If the reestablishment of positive Drainage is not possible, an alternate area drain system may be provided. Foundation design for sites with greater than 9% slope should insure that Ground water is not trapped on the cut (uphill) side of the foundation and That the drainage provided to remove this water from around the structure is far enough away, (minimum of 5 ft. (1.5 m) from the edge of the Structure) as to prevent the undermining of the foundation from the water Flow. This drainage can also minimize the 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, f issued or highly permeable soil; however, drains cannot stop the migration of Moisture into the soil beneath the foundation. Moisture barriers, while expensive, can be effective if placed near the edge of the foundation to minimize Moisture migration. The geotechnical engineer can recommend the proper Depth for a moisture barrier system depending upon the type of soil and the Climatic conditions prevalent in the area where the foundation is constructed. Roof drains should be tied into the storm drain or direct water away from the foundations. Property owners should also be aware of the potential Hazard of leaky pools or plumbing. A noticeable increase in monthly water Bills can indicate a problem that should be corrected immediately. It is important to note that consistent moisture content of supporting soils is the key to proper foundation performance. In areas where Silty and sandy material is present, excessive water can cause the soil to Lose bearing capacity in areas where expansive clays are present. Excessive water can increase swelling and moderate moisture will cause the shrinkage of the supporting soils.
The following is a list of items to be considered when planning proper Foundation maintenance:
- Maintain positive drainage away from the foundation and Install drain pipes (if applicable). Never allow water to pond near or against foundation slabs.
- Replace and compact any loose fill adjacent to the foundation with native soil; do not use sand or a granular material.
- Check gutters and downspouts to be sure that they are Clear and that the water is discharged away from the foundation area.
- Avoid seasonal drying around the perimeter of the foundation.
- Existing vegetation near the foundation typically draws Added water from the adjacent soil towards the foundation, thus causing added soil movement.
The objective of a proper maintenance program is to maintain as near constant moisture content, as possible, for the soil around the perimeter and under the foundation. It is recommended that all property owners conduct a yearly survey of their foundation and perform any maintenance necessary to improve Drainage and prevent the ponding of water adjacent to these structures. This is especially important during the first ten (10) years after construction because this is usually the time when the most severe adjustment between the new foundation and it’s support soil occurs. Property owners should also be made aware of the precautions that are to be taken when modifying or cutting holes in foundation slabs reinforced with unbonded post tensioning tendons.
Ground supported slabs constructed using proper foundation design, construction techniques and adequate drainage systems can stilt experience Distress if the site slope, type of vegetation, surrounding landscape and irrigation water supply is not properly selected and maintained. One of the most critical aspects of landscaping is the continuous maintenance of properly designed slopes. Installing flower beds or shrubs next to the foundation And keeping the area flooded will result in localized soil swelling. This Expansion may result in added edge lift of the foundation system. It is recommended that initial landscaping or hardscape be done on all Sides and that drainage away from the foundation be provided and maintained. Partial landscaping on one side of the foundation may result in Swelling on the landscaped side due to added non climatic irrigation Water. This can cause differential movements resulting in non serviceable Slabs or foundations. Landscaping is often overlooked by property owners as an area that May contribute to possible foundation problems. When planning flower Beds or locations of trees and shrubs, consideration must be given to the Effect that vegetation may have on existing drainage patterns. Landscaping should be installed so as to avoid water “ponding” or “standing” at any location around the perimeter of the foundation. Positive Drainage away from all foundations and off the property is critical to the Performance of any slab foundation supported on ground. Landscaping And ground cover can help prevent erosion and, if properly maintained, protect the ground from losing moisture.
Caution must also be taken when new patios and fences are installed. Water must at all times drain away from such structures and follow the Drainage pattern previously established. REMEMBER: Any changes in The exterior layout of the property, flower beds, decks, patios, fences, Trees and shrubs, must be planned such that positive drainage away from Any foundation structure and off the property is provided at all times. Sprinkler systems are beneficial in maintaining uniform moisture content in the soils that surround the foundation slab; however, they should be placed around the entire perimeter of the foundation. Precautions, Such as the proper backfilling of excavations for the sprinkler lines monitoring for leaks and setting controls so that a uniform amount of water is achieved for all areas are important factors to consider if a sprinkler system is to be beneficial.
Trees located near a foundation can be a potential contributing factor to Foundation distress Experience has shown that the presence of or the Removal of large trees that are in close proximity to residential foundations can cause long term problems. Depending on the type of tree, proximity to the edge of the foundation and its size vertical movements in the Foundation of as much as 3 5 in. (75 125 mm) are not uncommon. This problem can be aggravated in most areas by cyclic wet and dry seasons; however, the condition will be most severe during extreme droughts. Trees that require large amounts of water or that have large surface root Systems such as willow, elm or oak are the most detrimental to foundation Performance. It is recommended that trees not be planted closer than half Of the anticipated canopy diameter or ?O ft. (6 m) from the edge of a foundation Existing trees that are closer than this should be thoroughly soaked At least twice a week during dry periods and once a week during periods Of moderate rainfall. Close monitoring of the surface root system may indicate that more frequent watering is required. Root barriers are effective in protecting foundations while preserving the beauty of mature trees The System should be placed adjacent to the foundation is constructed of Monolithic concrete or other impermeable solid material be a minimum of 36 in. (915 mm) deep and extend the full length of the tree canopy. Whether the barrier will be truly permanent is questionable because the Roots may be able to grow around or under the trench however it should At least increase the time it takes for the roots to grow back.
STANDARD BUILDING CODE© 1997 EDITION Copyright 1997 FIRST PRINTING
In areas with expansive clay soil conditions, the root system of trees and large bushes tend to dry up the soil. When they are removed soil swelling (or heaving) may occur. Studies have shown that this swelling can last as long as 20 years depending upon the size and extent of the root system. Foundations that are built in heavily wooded areas on expansive clay soil should be designed with this anticipated additional vertical expansion Considered. Alternatively, the site can be left alone for several years after Removal of the trees and/or large bushes to allow the moisture regime of the desiccated area to stabilize; however, this option is generally not considered practical. Tree removal can be safely accomplished provided that the tree is no older than any part of the house since the subsequent Heave can only return the foundation to its original level. There is no Advantage to a staged reduction in the size of the tree; therefore, if a tree is to be removed, it should be removed completely at the earliest possible opportunity. When a tree is older than the foundation, it is not considered advisable to remove the tree because of the danger of inducing Damaging heave, unless the foundation was designed for the total computed vertical movement. This process does not occur for foundations Built on non expansive sandy soil conditions.
If the anticipated heave caused by the removal of a tree is too large Some kind of pruning, such as crown thinning, crown reduction or pcilarding can be considered Pollarding where most of the branches are Removed and the height of the main trunk is reduced is often mistakenly Specified. Most published advice links the height of the tree to the likelihood of damage when in fact it is the leaf area that is most important; therefore crown thinning or crown reduction in which some branches are shortened or removed is the preferred method. Pruning should be done in such a way as to minimize future growth while maintaining shape and without leaving the tree or bush vulnerable to disease. In some cases this should be done by a reputable tree surgeon or qualified contractor working under the instructions of an arbor culturist in some cases there may be some opposition to the removal or reduction of size of an offending Tree. The property owner, a neighbor, local authorities or a Tree Preservation Order may require that alternate methods, such as the root Barriers be utilized In this case the property owner needs to be made Aware of the risk of property damage that can result by leaving the tree. Every property owner should conduct a yearly survey of the foundation and perform any preventative maintenance necessary to improve Drainage and minimize the effects of landscaping and existing vegetation on the foundation. Special attention is important during the first 10 years after ~a foundation is constructed as this is the time of the most severe Adjustment between the new construction and the environment; however, this condition can change yearly for the life of the foundation. 1804.1.6 Where water impacts the ground from a roof Valley, downspout, scupper, or other rain water collection Diversion device, provisions shall be made to prevent soil Erosion and direct the water away from the foundation. 1804.1.7 Finish grade shall Foundation .for drainage. Be sloped away from the Cabo One and Two Family Dwelling Code 1995 Edition First Printing 401.3 Drainage. Surface drainage shall be diverted to a Storm sewer conveyance or other point of collection so as To not create a hazard Lots shall be graded to as to drain Surface water away from foundation walls. The grade Away from foundation walls shall fall a minimum of 6 Inches (153 mm) within the first 10 feet (3048 mm).