Geotechnical engineers often use the Natural Resource Conservation Service maps, and geological maps for investigation planning. They also rely on experience from previous investigations. The time lapse between the site investigation and construction varies substantially. For public agencies the time lapse is up to five (5) years while for the private sector, it is from two (2) months to three (3) years. It is not uncommon for the soil investigation to continue after construction has begun. The focus of this survey was on residential construction practices.
The total number of samples taken per area varies widely and depends on project size and uniformity of soil encountered. The soil investigation consists of preliminary and final stages. During the preliminary investigation, one soil sample is taken every 20 acres. Three values were obtained for the number of borings obtained during the final investigation. They are 1) one boring per 5 acres; 2) 8 borings per 100 homes, and 3) one boring per 600’ x 600’ area.
The depth of soil investigation varies from 5’ to 20’ depending on the company performing the investigation. It is the industry consensus, however, that the top 4 to 5 feet of soil are the most important for the slab design with active zone depth assumed to be within the upper 3’ to 8’ of the soil profile. Ring samples (56% area ratio samplers) are collected samples at 2’, 5’ and 10’. The soil samples are stored for one week to three months after the field investigation. The soil samples may be either sent back to the client or stored for some period of time, up to two years.
Soil testing is performed on bulk samples obtained with an auger and undisturbed samples collected in brass rings at the site. The dry density of soil is estimated from blow count (STP) or determined from undisturbed ring specimens. The moisture content and percent of silt and clay is visually estimated in the field from disturbed soil samples obtained with a split spoon sampler.
The bulk samples are used to obtain gradation, Atterberg Limits, Proctor compaction test, and reconstituted (re-compacted) sample swell tests – EIAZ (see section 3.5.1.4 for description). A few companies were found to perform additional soil testing including direct shear, pH, resistivity, percent of chloride solvent, and soluble solids. For the reconstituted specimen swell test (EIAZ), a token load of 100 psf is placed on top of the compacted specimen that is contained in a ring. The specimen is given free access to water and is allowed to swell for 24 hours.
The undisturbed soil samples are used for moisture content determination and, in some cases, density determination and response-to-wetting test. The soil with the lowest blow count is commonly selected for the undisturbed response-to-wetting test wherein the ring specimen, at
in-situ moisture content, is loaded to overburden stress and then given free access to water. Depending on the geotechnical firm, the response-to-wetting test specimen may be allowed to dry some from in-situ moisture conditions prior to testing.
Most of the laboratory work is performed by the same company that completed site investigation. Contract labs are only used in exceptional cases when in-house lab is too busy or for tests not performed in-house, such as hydrometer testing. Soil suction is never measured.
3.5.1.2 Site Monitoring
Pad preparation is typically monitored by a geotechnical technician for moisture content and dry density; the soil samples obtained are typically not stored by the firm. A builder representative is typically present during the pad preparation process. The constructed pads are certified for construction of slab-on-ground for three (3) months. It is not uncommon for the builder to recondition the pads in order to obtain new pad certification.
3.5.1.3 Communication
Communication between geotechnical engineers and structural engineers/builders is generally limited to clarifications and problems. The structural engineer needs em, ym, soil bearing capacity, subgrade modulus, and friction angle values from the geotechnical engineer. The builder needs to know in general what kind of soil is present at the site and prevalence of various materials, or in other words, if there is a problem with expansive soils. Additionally, the builder needs to know and how to deal with expansive soils.
3.5.1.4 Geotechnical Report
The geotechnical report briefly describes washes and general topography of the site. The location of underground utilities is not identified by the geotechnical engineers and their location is not included in the report. Lot grading is discussed, and special consideration is given when expansive soils are found at the site. General recommendations relative to expansive
soils are to not “overconsolidate” (or densify) the soil. The soil moisture is commonly specified to remain between -1% to +4% of optimal water content as determined by standard Proctor. Sometimes recommendations are given to avoid irrigation or to keep vegetation away from the structure.
The swell potential of the soil is determined and it is reported using either ASTM D 4829 EI test or the Arizona modified Expansion Index, EIAZ test. The EIAZ test is performed on reconstituted soil samples with water content decreased by 2 from optimum water content and dry density 95% of the maximum dry density as determined with the standard Proctor compaction test. The results are presented as a percentage of swell. The swell potential is categorized as small when EIAZ is smaller than 2.5%. For EIAZ between 2.5% and 4.5% the swell potential is defined as moderate, and for EIAZ larger than 4.5%, the swell potential is referred to as high. All surveyed companies indicated that for sites with high swell potential, only PT slab design is recommended post approximately 2003.
3.5.1.5 Design Procedure
Depending on client needs, both PTI (1996) and stem-and-footer, or only PTI (1996) design recommendations are given. For PTI method, additional alternative recommendations include deepening of the perimeter beam from 12” to 24”, and lime stabilization. The PTI design method indicates that center lift (which is actually edge drop) is the governing mode of slab failure.
3.5.1.6 Mitigation Measures
The builder who uses a city-approved soil report is responsible for soils mitigation (except for custom builders, who do not even need a soils report). Pre-construction mitigation for expansive soils generally involves limiting compaction and trying to achieve the appropriate water content (-1% to +4% of optimal moisture content is acceptable) and 95% of maximum dry density by standard Proctor (+/- 2% is acceptable) to minimize swelling. Prior to construction,
the pads are reconditioned by applying an undetermined amount of water through a sprinkler system or by flooding. A less common mitigation strategy is the removal of one to two feet of problematic soil and replacing it a with non-expansive compacted fill material. Lime treatment is generally dismissed as ineffective, although there are cases of use of lime treatment.
Post-construction monitoring has commonly been limited to drainage inspections. Houses are typically investigated only after problems occur. The investigation consists of a manometer reading (level survey) to estimate differential swell. Post construction soil investigation and testing is done only in connection with litigation.
3.5.1.7 Areas of Problems
The main cause of excessive soil swell is the initial moisture state of pads prior to construction. The time lapse between pad preparation and vertical construction is from a few months to a few years. Typically, the pads are not reconditioned prior to construction, resulting in over-dried pads. The most common mode of failure observed is edge lift. Center lift is observed only when there has been a pluming leak; however, edge drop (center lift) could also occur in response to excessive drying at the perimeter of the structure.
3.5.1.8 SWCC and Suction
Familiarity with the soil water characteristic curve was not extensive. This was also the case for the terms matric, osmotic, and total suction. Units used for soil suction are those of pressure – psi, psf, and kPa. Engineers indicated that they do not work with suctions at all and are therefore not familiar with these terms.
3.5.2 Structural Engineering Interviews