The following is the report on the basement of my house.
Project Description … The house is currently occupied and is experiencing visible vertical movements to the floor slab in the basement. This movement is also impacting the stairway to the basement, but Professional Service Industries, Inc (PSI) did not observe any of the upper portions of the structure.
The following lists the material and information provided for this project: 11” x 17” partial sire grade plan showing lot 28 and the milestone schedule.
Purpose and Scope of Services The purpose of this study was to explore the subsurface conditions at the site to investigate the observed slab movements and attempt to determine the cause(s) and recommend repair options.
PSI scope of services included core drilling through the floor slab and advancing 19 shallow soil test hand auger borings at the site, to depths of about 3 feet below the basement floor, select laboratory testing, and preparation of this geotechnical report. This report briefly outlines the testing procedures, presents available project information, describes the site and subsurface conditions and presents recommendations regarding the following:
Subsurface Conditions
The site subsurface conditions were explored with 10 shallow test borings on December 8, 2006. All the boring locations were within the existing building basement area. Boring depths were approximately 3 feet. The approximate locations of the boring are show in a coring location plan.
The boring locations and depths were suggested by PSI and reviewed with the client prior to drilling. Representatives of Pulte Homes and the homeowner were present prior and during the exploration process. PSI personnel marked the borings in the field and measure distances from interior surface features.
The borings were advanced by first using a diamond core drill to cute the concrete floor slab and then thin walled Shelby tubes were advanced to a depth of approximately 24 to 36 inches below the concrete. Soil samples were later tested in the laboratory to obtain soil material properties for the purpose of determining the cause(s) of the movements and repair recommendations. Laboratory testing was accomplished in general conformance with applicable standards.
The subsurface conditions below the 3 to 4 inches of concrete floor slab and approximately 6 inches of gravel, as identified by the 10 borings, primarily include fat clays. These materials are described in more detail in the following paragraphs.
The soils encountered in all 10 borings beneath the gravel primarily included fine-grained soils that extended to the terminal depths of the borings. Based on the results of Atterberg limits and visual classification, these soils were classified as high plasticity silty clay (CH) in accordance with the Unified Soil Classification System (USCS). Visual observations of these fine-grained soils generally indicated stiff consistencies. Overall, the moisture contents of the fine-grained soils varied from 23 to 34 percent. The moisture contents of the fine-grained soils had an average value of 30 percent.
High plasticity “Fat” clays have the potential to shrink and/or swell with changes in moisture content. The impact of these soils is most significant to lightly loaded structures and to floor slabs. Lightly loaded structures are defined as having dead loads of less than 1400 pounds per square foot (PSF) and column loads of less than 150 psf. Most residential and single story structures meet these criteria. Where these soils occur within approximately 2 feet of lightly loaded structures, remedial measures may be warranted.
The following table briefing summarizes the range of results from the field and laboratory testing programs. Please refer to the borings logs and laboratory data sheets for more specific information ( note: I will try to get these on line )
| SOIL STRATA | Standard Penetration N-Value | Moisture Content, % | Dry Unit Weight, pcf | Saturation, % | Liquid Limit, % | Plastic Limit, % |
| Fat Clay | 23-34 | 86-93 | 88-98 | 62-91 | 22-27 |
Water Level Measurements
Free water was observed on the soil in some, but not all of, the borings. No accumulation was observed in any of the borings during the field activities, indicating that groundwater at the site at the time of the exploration was either below the terminated depths of the borings, or that soils encountered are relatively impermeable. Although free water was not encountered at this time, was can be present within the depths explored during other times of the year depending upon climatic and rainfall conditions. However, it should be noted that saturated or nearly saturated soils were identified during laboratory analyses at depths as shallow as one foot below the ground surface. The water level measurements presented in this report are the levels that were measured at the time of field activities.
Field Observations
Visual observations of the movements of the floor slab consisted of differential vertical elevations at column isolation joints and apparent upward movement at the wall perimeter. There was also evidence of some previous and ongoing monitoring of cracks in the floor and on foundation walls. These consisted of dates at various points along the crack length. In general, cracks along the foundation walls and the floor were limited. The floor had been sawcut with control joints and Professional Service Industries, Inc. (PSI) believes this has significantly improved slab performance and appearance. Cracks observed in the foundation walls generally appeared to be typical shrinkage cracks. Displacement of the isolation joints around the interior column pads ranged from approximately ¼ inch to approximately 1 ¼ inches. There was some visual evidence of upward movement of the interior columns as well. No measurements of the columns or related beans were taken. The perimeter foundation walls were free of signs of vertical movements. The stairwell exhibited some signs of distress caused by vertical movements. These included some minor buckling of drywall joints and popping of screw or nail heads. Concrete thicknesses revealed in the cores ranged from 3 to 4 inches and were underlain by generally by 6 inches of gravel. No vapor barrier was observed beneath the concrete slab.
A review of the site grading plan provided indicated that cuts of 2 to 4 feet occurred during the grading process and an estimated addition excavation of 3 to 5 feet occurred to achieve final basement subgrade and foundation elevation. Actual survey of basement floor elevation was not performed.
Geotechincal Evaluation
PSI performed swell testing of the soils obtained during the investigation. These swell tests were performed to determine the swell potential of the in situ soils, the likely swell potential immediately after construction and the swell pressure generated by these soils. All of these parameters are important in evaluating repair options and need to be considered in the evaluation.
The swell tests indicated that the existing on site material is generating swell pressures of approximately 600 psf (0.3 tsf). This swell pressure is well above the 500 to 100 psf generated by the dead load of the weight of the slab and current live loads generated by things resting on the basement floor. The swell pressure is also above the loads likely being imposed on the interior column pads. PSI was not provided these loads but they are typically around 400 to 500 psf of dead load on the soil. What this means is that there is insufficient dead load present to resist the dead load to resist the uplift forces generated by the swelling soils and that is what is causing the movements that are evident. It also explains why the exterior of the structure does not appear to be impacted. The structure will typically exert sustained dead loads in excess of 600 psf on the soils below the foundations. This is supported by the visual observations that the slab has moved more than the column pads have. The floor slab’s control joints and isolation joints at the foundation wall and column pads has prevented a great deal of cracking that typically occurs with swelling soils.
The swell testing also indicated that at the current moisture content the swell potential is relatively low, approximately 0.8%. PSI obtained climatologically data from the National Weather Service and estimates that the soils were drier during construction than they are currently. PSI remolded a swell test with soils at a moisture content of 2 percentage points below optimum and performed a swell test on the material in that state. The results indicated swell potential of approximately 10 percent. Soils typically dry out during construction and Kansas City has been in a dry period for the last two years or more. The excavation for this structure was started in February 2006, which was one of the warmest and driest on record. This could have results in a reduction in moisture content during construction until the slab was placed. Moisture contents below structures almost always increase after construction due to interception of capillary vapors rising and irrigation practices getting landscaping started. Current moisture contents are approximately 2 to 8 percentage points above optimum.
These soils while being above optimum and having only a small potential for future swell are susceptible to future shrinkage if moisture contents are reduced. Drying out of soils below a basement occur much slower and are less likely than wetting of soils in that same location. Drying out can and does occur and the potential can be reduced with positive actions on the outside of the structure. Some of the moisture control measures are listed later in this report. Potential negative volume change (shrink) is theoretically the same as the swell potential except that the forces driving shrinkage are less. If you think of the expansive soils as a sponge, when the sponge is wetted it swelled to absorb the water and if the sponge dries out it will contract, but very little unless there is a force of weight pushing upon it. The soils are the same way; they have a driving force to swell but are more passive about the shrinkage.
The following geotechnical related recommendations have been developed on the basis of the subsurface conditions encountered and PSI understanding of the existing construction. Should changes in the project criteria occur, a review must be made by PSI to determine if modifications to our recommendations will be required.
In considering any repair or remedial action a cost/risk evaluation should be part of the decision process. Sometimes the cost is greater than the future benefit and or risk. The cost/risk evaluation is beyond the scope of PSI services. The primary goal of the remedial repair options is to reduce the impact of slab and residence distress from future soil movements. There are several mechanisms that are capable of accomplishing that. In general the repair mechanisms outlined are intended to remove the problem soils, amend or improve the expansive soils in place to make them behave better, or take the swelling soils out of play with and engineered solution. Listed below are options for repair, with each carrying with it different pros, cons, costs and levels of risk.
Option 1
One option is to remove the floor slab and subgrade soils to depth of 28 inches below the bottom of the slab and replace the soils with low volume change (LVC) soil or a dense aggregate such as AB-III. This would need to occur beneath the interior column foundations as well. After removal and replacement of the soil the slab can be reconstructed. PSI recommends maintaining the isolation joints in the slab at the column pads and perimeter walls. This option is intended to remove the problems soils and replace them with materials with characteristics that are significantly less likely to cause shrink/swell problems to a concrete slab. Removal of soils in a basement condition such as this will be difficult. Existing foundations should be supported during excavation and replacement of expansive soils. Temporary underpinning may be utilized to maintain foundation support. Replacement soils should be tested and documented as placed. If dense aggregate is used as the LVC material, consideration should be given to lowering the sump pit to pick up water collecting in the aggregate.
Option 2
The existing soils have limited remaining swelling potential at their current moisture content and subsequently have limited potential of damage due to swell, but they do still possess shrinking potential. It may be possible to amend the existing soils with hydrated lime or class “C” fly ash to reduce their swell/shrink potential, thus eliminating the need to completely remove the soils and haul in new material. PSI recommends installing an underdrain system to assist in moderating moisture contents. Additional swell testing of the fly ash or lime amended soils assist in moderating moisture contents. Additional swell testing of the fly ash or lime amended soils would be required prior to implementing this option. This would likely require removal of a 3 ft by 3 ft section of the floor slab to obtain a large enough sample to mix with the selected product and then perform swell tests. In this option, PSI would recommend that the interior columns be placed on a deepened foundation designed for a dead load pressure on the soil of at least 600 psf or extended to a depth of at least 3 feet below the floor slab to minimize moisture fluctuations. The depth of this amendment may be as deep as 2 feet depending on the results of subsequent lab testing.
The following recommendations apply to both of the repair options above.
Fill or soils after amendment should be placed in maximum loose lifts of eight (8) inches and compacted to a range of 95 to 100% of the material’s standard Proctor maximum dry density, and within a range of the optimum moisture content of optimum to +3% as determined in general accordance with ASTM procedures. Each life of compacted-engineered fill should be tested and documented by a representative of the geotechnical engineer prior to placement of subsequent lifts. It should be noted that the geotechnical engineer of record can only certify the testing that is performed and the work observed by that engineer or staff in direct support to that engineer.
Tested fill materials that do not achieve either the required dry density or moisture content range shall be recorded, the location noted, and reported to the Contractor. A re-test of that area should be performed after the Contractor performs remedial measures.
PSI should be retained to provide observation and testing of construction activities involved in the foundation and/or earthwork activities of this project. PSI cannot accept responsibility for conditions that deviate from those described in this report, nor for the performance of the repair options if not engage to also provide construction observation and testing of this project.
Option 3
The existing soils can be isolated from the slab by removal of the existing slab and removing a portion of the soils, perhaps as little as 2-4 inches, and placing a structural slab on crush boxes to create a void beneath the slab to allow for future movements in either direction. This option has the advantage of not requiring removal or processing of significant quantities of soil and can be designed to add dead load to the foundations to improve the column foundations resistance to any future swelling. A structural engineer would need to evaluate the interior column foundations, to determine if they might be able to be left in place. It is likely that this option would be less time consuming as well.
Regardless of which repair mechanism is selected there are some general recommendations that apply to portions of the repair process. The geotechnical engineer’s representative should observe the remediation procedures for compliance with the project plans and specifications.
Moisture control
The presence of high plasticity clays around and beneath the structure warrants that consideration should be given to measures that can reduce the long term shrink/swell potential of the clay soils. High plasticity clays expand or shrink by absorbing or losing moisture; aid in the reduction of subsoil moisture content variations. These steps are intended to help reduce the shrink/swell potential, not eliminate it. Some of these measures are:
PSI recommends that a minimum four (4) inch thick free-draining granular mat be placed beneath the floor slab to enhance drainage. The soil surface shall be graded to direct water to the sump without low spots that can trap water prior to placing the granular drainage layer. Consideration should be given to lowering level of the sumppump if granular is used for the LVC zone. Polyethylene sheeting should be placed to act as a vapor retarder where the floor will be in contact with the moisture sensitive equipment or products such as tile, wood, carpet, etc. as directed by the design engineer. The decision to locate the vapor retarder in direct contact with the slab or beneath the layer of granular fill should be made by the design engineer after considering the moisture sensitivity of subsequent floor finishes, anticipated project conditions, and the potential effects of slab curling and cracking. The floor slabs should have an adequate number of joints to reduce cracking resulting from differential movement and shrinkage.
Cracking of concrete slabs is normal and should be expected. Cracking can occur not only as a result of heaving or compression of the supporting soil and/or bedrock material, but also as a result of concrete curing stresses. The occurrence of concrete shrinkage cracks, and problems associated with concrete curing may be reduced and/or controlled by limiting the slump of the concrete, proper placement, finishing, and curing, and by the placement of crack control joints at frequent intervals, particularly where re-entrant slab corners occur. The American Concrete Institute (ACI) recommends a maximum panel size (in feet) equal to approximately three times the thickness of the slab (in inches) in both directions. For example, joints are recommended at a maximum spacing of twelve (12) feet based on having a four-inch slab. PSI also recommends that the slab be independent of the foundation walls. Using fiber reinforcement in the concrete can also control shrinkage cracking.
PSI should be retained to provide observation and testing of construction activities involved in the remedial repair involving earthwork and fill placement operations.
The concept of risk is an important aspect of the geotechnical evaluation. The primary reason for this is that the analytical methods used to develop geotechnical recommendations do not comprise an exact science. The analytical tools which geotechnical engineers use are generally empirical and must be used in conjunction with engineering judgment and experience. Therefore, the solutions and recommendations presented in the geotechnical valuation should not be considered risk-free and, more importantly, are not a guarantee that the interaction between the soils the proposed structure will perform as planned. The engineering recommendations presented in the preceding section constitutes PSI professional estimate of those measures that are necessary for the proposed structure to perform according to the proposed design based on the information generated and reference during this evaluation, and PSI experience in working with these conditions.
The recommendations submitted are based on the available subsurface information. If there are revisions to the plans for this project or if deviations from the subsurface conditions noted in this report are encountered during construction, PSI should be notified immediately to determine if changes in the recommendations are required. If PSI is not retained to perform these functions, PSI will not be responsible for the impact of those conditions on the project.
The geotechnical engineer warrants that the findings, recommendations, specifications, or professional advice contained herein have been made in accordance with generally accepted professional geotechnical engineering practices in the local area. No other warranties are implied or expressed.
After the remedial plans and specifications are more complete, the geotechnical engineer should be retained and provided the opportunity to review the final design plans and specifications to check that our engineering recommendations have been properly incorporated into the design documents. At that time, it may be necessary to submit supplementary recommendations.
The following are the test reports.
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