The Project
The structure is a single-story grocery store in south-central Louisiana with steel frame construction and brick and masonry block exterior walls. Maximum loads are 70 kips for the columns and 4 kips/lin ft for the walls. The structure is supported on a shallow foundation and has a uniform thickness slab-on-grade floor system.
The Problem
In 1984, about 1 yr after construction was finished, the owner and building occupant saw building distress on the interior and exterior of the structure. More specifically, the exterior walls on the building's north and east side developed cracks and the interior floor slab became uneven. The parties initially involved with the project suspected either foundation settlement or soil shrinkage was the cause of the distress. The building's tenant had concerns about the structural integrity of the building and the safety issues associated with uneven flooring. Survey data later showed that upward movements caused by soil expansion caused the distress.
The Background
In June 1982, an owner/developer retained a local geotechnical firm to perform an investigation for the project. The work consisted of 11 undisturbed-sample borings and four auger borings. The undisturbed-sample boring extended to depths of 16 to 30 ft while the auger borings extended to 6-ft depth. The undisturbed-sample borings were advanced to between 16- and 20-ft depth using dry-auger techniques; wet-rotary methods were used below the dry-auger depth. Water was encountered at about 18-ft penetration in only one of the open boreholes.
At the time of the study, site grade varied between about El 25 and 40 ft Mean Sea Level (MSL). The site was covered with grass, brush, and trees. The trees (gum, pecan, elm, and oak) had trunk diameters ranging between about 12 and 24 inches.
The firm's evaluation and recommendations can be summarized as follows:
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Stratum I, the upper 2 ft, usually contained a surface crust of dense clayey silt. |
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Stratum II was a 4-ft-thick stratum of stiff to very stiff silty clay. |
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Most borings terminated in Stratum III, a stiff to very stiff clay. |
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The firm identified the clays below about 4- to 6-ft as having a 'slight' potential for swelling. |
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Shallow foundations were found to be suitable. The firm gave recommendations for shallow foundations to be placed in new compacted clay fill, the clayey silts (Stratum I), or the Stratum II or III soils. The minimum footing depth was at least 18 in. below grade, but had to be deep enough to reach the desired bearing material. |
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The firm recommended using deep foundations (drilled shafts or drilled-and-underreamed footings) if the existing trees were left near the finished structure. The recommended foundation depth for this approach was at least 10 ft below grade. |
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Since the shallow soils below a depth of about 4 to 6 ft had a 'slight' potential for swelling, the firm noted that a ribbed slab could be used to allow some movement to occur and to minimize floor slab cracking caused by normal settlement or nearby trees. |
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Recommendations for structural fill in the building area included using clean sand or a low plasticity clay compacted to 90 percent of the modified Proctor (ASTM D 1557) maximum dry density. |
The owner/developer elected to use shallow foundations placed in the Stratum II or III soils and a uniform thickness slab-on-grade floor system. The existing trees were removed, and the ground surface in the building area was lowered about 3 to 4 ft so that the ground surface was at about El 34. The owner/developer also retained the geotechnical firm to provide construction materials testing (CMT) services.
Construction began in January 1983 and it was finished in the fall of 1983. The firm's CMT services consisted of conducting soil density tests and observing foundation excavations prior to concrete placement. From January to April 1983, the firm made 13 site visits. All the earthwork and footing installations were completed within this time.
The Details of the Problem
The author became involved with the project in April 1985 to assess the problem, identify possible causes and potential solutions, and to represent the owner/developer in the event of litigation. At that time, visual observations indicated that movements had occurred that resulted in cracks in the exterior brick and masonry block walls. Crack widths varied from hairline to as wide as 0.5 to 0.75 inch. On the interior of the store, floor tiles were loose and cracked and there was distortion of the floor board molding and wall covering. Some dampness was observed under some of the floor tiles. This portion of the store contained refrigerated vegetable coolers with water misters. Utility lines below the floor slab consisted of water supply lines, drain lines, and a garbage disposal line.
The Approach for Assessing the Problem
To assess the problem, a series of steps were undertaken. These steps included:
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reviewing the existing geotechnical data and CMT data furnished by the owner/developer |
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surveying the walls, slabs, and grade beams to determine the direction, magnitude, and rate of movement |
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collecting additional data about the physical soil properties and post-construction soil conditions through test pits and soil borings |
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obtaining information about groundwater conditions at the site by installing open piezometers and observing the test pits |
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conducting physical and chemical studies to determine possible sources of water |
Soil Properties. The regional geology consists of soils belonging to the Prairie Formation, a Pleistocene age terrace deposit. Typically, these soils are overconsolidated, strong clays. Some of these soils are low plasticity clays while others are highly plastic.
At the site, the Stratum II and III soils had liquid limits (LLs) that ranged from 32 to 95 percent and averaged 56 percent. The average plasticity index (PI) was 33 percent and individual PIs ranged between 6 and 69 percent. The average plastic limit (PL) was 23 percent. The natural moisture content (Wc) of these soils at the time of the initial geotechnical study varied between 14 and 42 percent and averaged 24 percent. The investigative post-construction testing showed some increases in Wc, on the order of 2 to 3 percentage points.
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The shallow soil's undrained soil shear strength (Su) was between about 1.3 and 4.6 ksf and it was typically about 1.5 to 2 ksf. The unit dry weight (UDW) of these soils was usually between about 90 and 95 pcf; however, some samples had UDWs of 105 to 110 pcf.
Clay Mineralogy. X-ray diffraction tests performed as part of the investigation showed that a sample from about El 30 contained 42 percent smectite, 22 percent illite, and 36 percent kaolinite. This sample had a LL of 46 percent and a PI of 28 percent.
Swell Potential. The clay mineralogy information and swell test data combined with the previously described Wc, plasticity, UDW, and Su data suggested these soils had a high to very high swell potential if soil moisture contents were allowed to increase.
Survey Data. Movement data collected in May 1986 showed the east wall had experienced about 2 in. of upward vertical movement; the north wall had about 0.8 in. of upward vertical movement and some outward movement to the north; and the floor slab heaved about 3 in. in some areas. Heave plots and contours showed the upward movements were concentrated over a buried utility line trench.
Groundwater Data. Test pits, shallow borings, and open piezometers installed to various depths were located along the exterior of the structure. The test pits showed water seeping out from below the footings that were about 2 to 2.5 ft below grade. The piezometers showed static, long-term groundwater levels were below about El 26 or about 8 ft below the finish floor elevation. However, several shallow piezometers along the east side of the structure showed a water surface at about El 31.
Source of Water. To determine possible sources of water, the owner/developer conducted an extensive testing program. This work consisted of using pressure tests and fluorescein dye to test all known drain, supply, and sewer lines. Vent-to-roof lines were tested with water. Chemical analyses were performed on water samples collected from the test pits and piezometers and the results were compared to the contents of the drain, supply, and sewer lines. These tests were inconclusive in that no leaks were identified and the chemical analyses did not clearly identify the source of the water.
The Selected Solution
The investigative work clearly indicated that soil heaving of the potentially expansive clays was occurring. Soil moisture contents had increased and water was seeping out from below the shallow footings. However, the water source could not be positively identified. While the movement rate had slowed considerably, the owner/developer still had a dissatisfied tenant. Since slight movements greatly reduce swell pressures and some soil moisture content increases had occurred, the decision was made to begin making building repairs.
Repair Scheme. In general, the repair concept required breaking out the existing concrete floor on the building's interior and rebuilding portions of the north and east exterior walls. All this had to be performed while the store remained open for business.
Observations. During the floor slab breakout and removal operations, soil moisture content tests were performed in various areas. In one of the utility trench lines, unusually high moisture contents were measured. This trench contained a water line that supplied water used to mist produce and for other purposes. Further observations and investigation revealed that water was leaking from a slab-level fitting on this valve. Although the line had been tested, the valve only leaked when it was open. When the valve was closed (as it was during pressure testing), no leakage occurred. Finally, the source of water had been identified! The repair work proceeded and it was completed in late 1986. Since then, no additional problems have developed and the structure is performing according to the tenant's and owner/developer's expectations. No litigation occurred.
Out-of Pocket Repair Costs. The owner/developer incurred substantial costs to investigate and repair the problem. The out-of-pocket costs were about $160,000.
The Lessons Learned
The lessons learned from this case history can be somewhat difficult to identify. However, there seem to be lessons for design professionals as well as for owners and developers. Some of these are:
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Open communications are required by all parties throughout the project. The geotechnical engineer must be aware of site conditions that exist before development begins. The geotechnical engineer also must have a clear and complete understanding of the planned changes caused by the development so that the influences can be considered. |
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Word usage is important. Be careful of words such as can, could, or may when words such as recommend, must, shall, or should are intended. Also, words such as slight can mean different things to different readers. |
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Owners and others using the geotechnical report should have a clear understanding of the geotechnical recommendations. They should ask questions to aid in their understanding of the recommendations. They also should understand and be willing to accept the consequences of not following the geotechnical engineer's recommendations. |
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The geotechnical report must clearly identify key assumptions and it must clearly state the geotechnical engineer's areas of concern. |
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Appropriate laboratory tests must be performed to adequately characterize the important soil properties. |
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When active (shrinking and swelling) clays are present, the geotechnical report should identify the importance of preventing or at least minimizing changes in soil moisture content. It also would be appropriate to include recommendations about measures to prevent or accommodate soil movements caused by changes in soil moisture content. |
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When active soils are present, the project's design professionals should be aware of the need to limit the potential for external sources of water and they may need to change standard building details to adequately fit site-specific conditions. |
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When repair work or investigative work is being undertaken, the on-site presence of experienced engineers trained to use observational methods can be very helpful for identifying problems and solutions. |
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Sometimes, in spite of taking positive measures, unusual or unlikely events can cause or contribute to problems at a site. |
© Lourie Consultants, September, 1995
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