The site preparation consists of de-vegetation, lot grading and placement of forms for slabs. The pad preparation is followed by placement of wet and dry utility lines, and cutting of sidewalks and streets. The lots are watered to specs the night before the slabs are poured, however the amount of water added through sprinkling or flooding is not always monitored. ABA is placed and slab is poured, and is post tensioned at latter time.
3.5.3.4 Site Monitoring
The site monitoring consists of three main components: pad preparation, slab construction and lot grading. The dry density and water content of pads is monitored by geotechnical engineers during pad preparation. The quality of concrete used in slab construction is checked by a third party, and the placement of the tendons in the slab is monitored by a structural engineer. The lot grading and drainage are checked by another company. This practice appears to be voluntarily adopted by most builders, although it is not required by law. If grading is found to be inadequate, it is redone at the end of construction. Some builders require the homeowner to sign a contract stating that the grading and drainage cannot be modified, and
plats cannot be planted within 24” from the house. Unfortunately this information is not required to be turned over to the next homeowner.
3.5.3.5 Communication
The builders are communicating a lot with both structural and geotechnical engineers. Some builders have periodic meetings with them. In addition, the geotechnical engineer is present at the site during pad preparation, and the structural engineer is on site during post- tensioning of slabs. Typically the same geotechnical engineer that developed the report is monitoring the site preparation.
The builder also communicates with the homeowner through purchase document which discusses drainage, grading and typical behavior of construction materials. Relevant highlights of some documents are listed below:
• Soils. “The soils in Arizona are known to be expansive in nature. These expansive soils have been analyzed by a soils engineer who has recommended the type and design of the foundation for your home. Any changes in the foundation, the grading and the landscaping of your home and lot can result in severe damage to your property and to neighboring properties. Consult a professional before any such changes are made.”
• Drainage.
o “Do not alter the soil grade”
o “… your lot has been graded to keep water away from your home. The grading plan for your lot has been engineered and graded to local, state and federal standards. Failure to maintain grading can result in damage to your home, your lot and to neighboring property. Any alteration of the established grade plan for your lot may void the landscaping and drainage and termite sections of your warranty.”
o “The soil around each home site is graded to channel storm water away from the home. Please note that the rear yard grading in some communities is designed to retain storm water.”
o “Berms and contours which are designed to direct the flow of water away from the home are especially important and must not be altered.”
o “Keep water ditches or swales open and free of leaves and debris.”
o “Do not build sheds, sidewalks, hot tubs, decks, fences ,pools, or gardens in the swales.”
o “Direct water away from the home to prevent washouts.”
• Pool. “If you choose to have a pool or spa installed, we suggest that you give careful consideration to the eventual drainage problems that could be created.”
• Landscaping.
o “Do not plant along the foundation wall. “
o “Irrigate away from foundation, patio, porch, fence and sidewalks.”
o “Irrigation at or near the foundation will increase the likelihood of soil expansion or settlement resulting in cracking and movement of the fence or home.”
o “We urge you to use drought resistant and drought tolerant plants.”
o “Landscaping can change the grading of your lot. The ground next to your home should always slope away to prevent standing water.”
o “Do not allow sprinklers to wet the house or form puddles near or against the foundation.”
o “Keep plantings in flower beds a minimum of 2’-3’ away from the foundation”
o “Improperly constructed planting beds can result in saturated soil around the perimeter of the building, even when the soil surface nominally has positive drainage away from the building…In this case, improperly constructed planting beds can act to inject water directly into the select fill.”
o “Trees planted near a foundation can upset the soil moisture balance due to the water demand of mature trees, especially during drought cycles. While it may take a number of years before the tree gets large enough to cause structural damage, this will eventually occur if the tree is close enough to the slab. In general, the distance
from the tree to the foundation must be at least half the height of the tree, but the required distance varies with tree species.”
o Do not remove trees near the vicinity of the foundation. “Trees significantly alter the soil moisture balance of the soil, reducing the equilibrium soil moisture in their vicinity.” After tree removal the soil will gain moisture over time and heave causing slab movement.
• Concrete. “Do not allow water to pond near the foundation, patios, walks or driveways. Water can cause soil expansion which can result in fractures to the concrete as well as movement within the home.” “Small cracks, which are a result of contraction and expansion of the concrete are characteristic of concrete and do not affect its performance or durability.”
• Stucco. “Stucco is a cement product and takes approximately 14 days to cure. Stucco is susceptible to cracking due to expansion and contraction. Cracks should be expected during the lifetime of the home due to fluctuating temperatures. This is normal and does not reduce the function of the stucco in any way. Your limited warranty does not cove normal hairline cracks in stucco.”
• Drywall. “Nail pops and minor drywall cracks are normal and are caused by settlement and the normal drying of stud framing and drywall materials.”
3.5.3.6 Mitigation Measures
Builders report that no pre-construction mitigation measures are employed beyond grading and drainage. The practice is to follow geotechnical and structural specs. Sometimes, problems are observed half way through completion of a subdivision; by then it is too late to change. Post-construction monitoring of expansive soils typically does not occur; although builders who encounter expansive soils are beginning to informally check problematic homes (visually inspect the foundation, walls, and driveway for cracks, etc).
3.5.3.7 Sources of Problems
The time lapse between the pad preparation and slab construction is from a few months to few years. During the time lapse, the pads are not covered to maintain the design soil moisture state. In addition, the prepared pads are frequently driven over by construction trucks to access the construction zone. Such practice leads to overcompaction of pads. The site construction is monitored by superintendents who lack the knowledge of how s the dry density and water content of soil influence post construction soil behavior.
3.5.3.8 Litigation
The homeowners rarely sue the homebuilder. Typically the homebuilder resolves issues associated with expansive soils and litigation is avoided. Typically the pre-emptive action on the part of the builder is cheaper than litigation. In most cases litigation is the result of buyer distrust of builders rather than any negligence on the part of the builder. Emotional distress has not been a litigious issue.
3.5.4 Forensic Investigation 3.5.4.1 Failure Modes
Forensic investigation indicates that problems with expansive soils might occur from few months to as many as 20 years after construction. Frequently the problems are associated with a change in landscape irrigation patterns and/or excessive rainfall. Depending on the soil properties and irrigation conditions the following outcomes are possible:
• Soil at center swells up - common with the stem and footer design.
• Soil at center shrinks/consolidates - very uncommon, however was witnessed once. • Soil at the edge swells up (edge lift) - common with PTI method, and
• The soil consolidates or shrinks under the edges (center lift) – observed in stem and footer design.
3.5.4.1.1 Center Lift
Center lift deformation is prevalent in the stem and footer design. It is speculated that the failure occurs under following conditions. The soil stratum consists of layers of expansive soil and more permeable sandy soil. During a wet event, the water may reach the permeable strata through shrinkage cracks in the clay layers and travel horizontally under the slab within the sandy layer. Once it happens, the water is trapped under the slab and the expansive soils start to swell up. At the same time the soil under the footing is consolidating and loses water due to the extremely hot desert conditions; the soil shrinks resulting is center lift slab deformation. Center lift deformation is evident by more observable damage in the interior of the house such as crushed drywall at the top of the wall next to the ceiling – although this same type of damage pattern is consistent with edge drop. It is rare to have benchmarked surveys to determine whether deformations appearing to be center lift are actually due to center lift or edge drop.
3.5.4.1.2 Edge Lift
Edge lift deformation is observed when soil beneath the edges of the structure expand relative to soils in the center, for example, from excessive wetting around the perimeter. It is attributed to poor drainage. Larger stress levels around the perimeter of the structure can help to reduce edge lift. This is one reason why edge lift is observed most commonly in PT slabs having more uniform distribution of loads. Evidence of edge lift movement includes distorted exterior doors, windows and cracks in stairways. Damage is most commonly noticed for edge movement of 2.5" or more per 5'.
3.5.4.1.3 Settlement
Settlement (shrinkage of expansive soils due to drying) of structures is observed in areas of excessive sunshine. It is manifested through cracks in stairways and gaps between a fence and the house. Settling soils (compression) give the same pattern of deformation.
3.5.4.2 Remediation Methods
The following remediation methods are currently used in Phoenix region: • Cut off walls,
• Installation of gutters, • Change watering pattern,
• Intrusion of concrete is common where grout is pumped under sunken structure. This solution does not perform as anticipated. The structures have a tendency to settle after the procedure.
• Chemical treatment with lime or ESLL.
• Drying of soil with hot air. A hole is drilled in the center of the slab where hot air is applied. It flows through the ABC layer and exits on the sides. It is effective for slabs up to 30 ft long.
• Helical anchors or push piers. The expanded soil is removed and the house is allowed to be supported by either push piers or anchors that are installed under the house’s perimeter. Push piers are hydraulically driven into the soil under the slab until it locks up. Once it locks up, it starts to push up the house to the required height. Due to the lifting action, sometimes a significant space between the soil and the slab develops. The space is filled with grout. In the valley the push piers are installed to a depth of 15’- 25’ and are spaced between 6’ to 8’ apart. They usually lock up at 50-60 blow count (helical anchors locks up at 40-50 blow count). This remediation method is frequently used in conjunction with cut off walls.
3.6 Failure Criteria
Soil movement below foundation is associated with foundation movement and structural distress. The American Concrete Institute, ACI, and the Arizona Registrar of Contractors,
AROC, developed acceptable distortion criteria for residential construction, which are commonly
and FL respectively. Flatness refers to slabs waviness or roughness due to random bumps and irregularities “by limiting the magnitude of successive 1’ slope changes when measured along sample measurement lines in accordance with ASTM E 1155” (Standard Test Method for Determining FF Floor Flatness and FL Floor Levelness Numbers). For slab-on-grade, minimum allowable flatness is obtained with FF=15. Levelness refers to the slabs deviation from horizontal over the entire area of the slab “by limiting differences in departure from design grade over distances of 10 ft when measured along sample measurement lines in accordance with ASTM E 1155.” For slab-on-grade minimum local levelness (within 10’) is obtained with
z
F
L∆
=12.5
FL = 10 and global levelness (over the entire floor) with FL=13. The Fl number can be calculated with equation 3.4,
(3.4) where ∆z is the floor level differential. The FL of 10 produces maximum allowable floor level differential of 1.25” per 10’. The Arizona Registrar of Contractors, AROC, provides more stringent requirement of ¼” differential per 12’.
Floor survey elevation (non-benchmarked) alone is not an indicator of direction or magnitude of soil movement. Section 2.6 provides evidence from literature review that newly constructed slab-on-grade can deviate up to 1” from horizontal. Additionally, the structural performance of slab-and-footer foundation system is, in general, independent of the performance of the free floating slab in the areas between footings. The thin slab is designed to act as a separator between the building and the soil below. Developed thermal or shrinkage cracks are not evidence of post-construction structure distress or shoddy construction. Soil movement is associated with distress in superstructure such as diagonal cracks in drywall and stucco, separation of baseboard and wall fixtures from the walls and cracks near soil movement. The AROC document “Workmanship Standards for Licensed Contractors” provides guidance with respect to unacceptable quality of construction, unacceptable post construction deformations due to either settlement or soil movement within 2 years of homeowner occupancy and
homeowner responsibilities which, among others, include: adjustment of doors and windows, maintaining weather-stripping, interior and exterior caulking, leaks from plumbing fixtures and cosmetic repair of hairline cracks on horizontal surfaces. The post construction performance criteria are summarized in Table 3.3.
Table 3.3. Residential construction performance criteria in the first 2 years after homeowner occupancy (AROC, 2004).
Distress
Stemwall > 1/8” wide crack requires cosmetic repair > ¼” wide crack, determine cause of distress and perform appropriate repair
Stoops >1/4” differential
Stucco Excessive hairline cracks or larger then 1/16” wide
Drywall Excessive hairline cracks or larger then 1/16” wide
Bulge or sag in walls and ceilings 3/8” in 8’ is acceptable
Ceiling sag 3/8” in 8’ is acceptable
Flatwork: garage, patio, driveway > 3/32” wide cracks > 1/8” vertical differential, replace effected area > 3/8” wide control joints
Flatwork: sidewalk > 3/16” wide cracks, replace effected area Pool deck > 1/16” wide horiz. and vert. displacements > 1/8” wide control joint separation
Levelness >1/4” in 12’
Concrete spalling unacceptable
Masonry ≥ 1/8” wide stair-step crack
Counter top and wall joint Caulk joint not to exceed 1/8”
Tiles Lose or cracked - Unacceptable > 1/16” joint with other material separation 3.7 Summary
In this section the industry and Arizona practice (based on informal survey data) is presented. Up to about 10 years ago stem-and-footer foundation design was the most commonly constructed foundation system for detached residential construction. Currently post- tensioned slabs are selected to mitigate potential soil movement of low to medium expansive soil typical to the Phoenix metropolitan region. The design is carried out with PTI 3rd
Interviews with industry revealed that professionals involved in construction and design understand the importance of maintaining initial moisture conditions below the foundation by proper grading, drainage and appropriate landscape in the vicinity of the foundation. The 5%
positive slope away from the structure for a minimum of 5’ was adopted long before the governing standards (IBC) required it. It appears that homebuilders make a lot of effort to communicate the significance of moisture control to new homeowners who frequently do not follow the recommendations. When problems occur, remediation methods involve re- establishing of arid moisture conditions around the foundation perimeter by removal of vegetation and regarding the lot. In rare cases, when significant soil movement has occurred and the potential for future soil movement exists, additionally some or all of the following methods are employed: vertical cut-off walls, chemical stabilization, and push-piers. The soil moisture below the structure is assumed to come to equilibrium with the new conditions within 6 months, at which time vertical structure distress remediation is performed consisting of cosmetic repairs, partial removal and replacement of flatwork and walls.
The Arizona Registrar of Contractors, AROC, in “Workmanship Standards for Licensed Contractors” provides guidelines for unacceptable construction quality, unacceptable post construction deformations due to either settlement or soil movement within 2 years of homeowner occupancy and homeowner responsibilities. A number of researchers attempted to identify factors signifying foundation movement such as the angular distortion and crack width, see Section 2.6, though it is not common for geotechnical engineers to present their design recommendations in terms of limited angular distortion. Walsh (2001) writes about newly constructed floor levelness, using angular distortion as a guide. In the Phoenix metropolitan area the AROC document describes the governing standard of practice.
Laboratory testing was performed as a part of this research to identify typical Arizona expansive soil properties and associated parameters employed in current foundation design method (e.g. PTI design procedure), and to obtain transient moisture flow modelling input parameters, which include unsaturated soil functions such as SWCC and hydraulic conductivity and , initial matric suction profile data, and climatic and human-imposed soil surface conditions for establishment of boundary conditions. The unsaturated soil properties, Soil Water Characteristic Curve and unsaturated soil permeability, were either measured or estimated based on measured index properties. The determination of initial and boundary conditions involved the measurement of matric suction beyond the estimated active zone depth and beyond the estimated edge moisture variation distance. To achieve these objectives, soil samples from below slabs of 16 homes were obtained, one next to a residential property and one from undeveloped desert conditions. The descriptions of soil testing performed and a summary data are given below. Detailed soil profile information for each investigated site can be found in Appendix B.
4.1 Field Exploration
The field exploration was aimed at obtaining undisturbed soil samples from under slabs- on-grade of residential construction in Phoenix metropolitan area, Arizona, whose matric suction, beyond that resulting from seasonal variation, had reached equilibrium. To satisfy this requirement, structures five years old or older were chosen for the investigation. The edge moisture variation distance was roughly estimated and the samples were taken at greater distances from the edge of the slab. It was decided that the least inconvenient place to drill was the garage. Before the soil samples were collected from the selected sites, special care was taken in the gathering of related data such as landscape type, existence of gutters, quality of lot grading, and identification of possible water sources such as pool or history of pipe leaks.
4.1.1 Equipment
The field sampling required equipment listed in Table 4.1. The main pieces of equipment included: coring machine capable of coring 4-inch diameter holes in concrete,