The main reason for performing a geotechnical investigation is to provide a structural engineer, architect, owner, and/or contractor with information on site subsurface conditions. The geotechnical report is required to provide structural foundation design, grading criteria, and other soil-related issues including, but not limited to: surface water and subsurface groundwater control, compaction criteria, and future performance determinations.
However, without sufficient tests and calculations, the geotechnical reports can lack vital information if they fail to provide proper documentation, testing or analysis of the site, and the future probable behavior of the soils on the site. In the Colorado Front Range and nationwide, there has been some work to standardize the investigation of sites with unstable soils. Even with this standardization of protocols, the risk of performance failures for the development increases significantly when proper verification, testing, and analysis of the geotechnical issues on the site is not provided.
There are many requirements for a geotechnical report to provide good analysis of the site for the intended construction. Primarily, there is a need for a comprehensive understanding of the geology of the site and the surrounding area. In many instances, reports are provided in which the geology of the site is not fully explored. This can lead to many false conclusions. For example, by not understanding basic geology, such as the dipping bedrock planes along the Front Range of Colorado, Wyoming, or other areas, one could conclude that a single test result would provide proper data for the design of the foundation.
However, only a few dozen feet away, a very large difference in soil parameters could exist. Also, the active zone (zone of seasonal groundwater fluctuation or depth of wetting) and depth of potential soil movement cannot be adequately addressed when the site’s geological profiles are not completely understood. In other instances, the geology of the site may indicate past landslides or other important issues. This basic need of a site investigation should not be based on the judgment of the geotechnical engineer and must be determined by adequate site testing.
We must also be realistic in understanding that the geotechnical community in everyday work is not sampling all of the soil on the site. For example, site samples are taken one per lot on a “postage stamp” 75 by 100 foot lot, using a California Sampler. The diameter of the sample is approximately 2 inches, and the length of the sample is 4 to 8 inches. Therefore, only 0.02 square feet and 0.013 cubic feet per sample are tested per lot, and then extrapolated to represent the entire lot size. This accounts for a representative sampling of approximately 1 to 343,000 thousandths of the entire site.
Without enough data points, the engineer is providing improper extrapolation in determining the conditions on the site; this type of extrapolation is found to be common in the review of geotechnical reports in forensic engineering work. Thus, the need for data bounding the site allows for interpolation testing in lieu of testing every square foot and foot depth of soil.
Good engineering practice would require the review of neighboring lots, a comparative analysis, and better interpolation of data. For example, a site may indicate a low expansive soil based on volumetric heave and swell pressure, but a neighboring lot only 50 to 100 feet away indicates a high swell and high swell pressure. The designer surely would not consider that somehow the Civil Engineer laying out the lot lines magically knew that the soil profiles changed at the lot line! Yet, we as engineers practicing in foundation engineering and forensics often see the designs between adjoining lots vary based on the use of a single sample of soil. Standard geotechnical textbooks recommend that the most conservative data obtained for the area be used for development of the geotechnical design parameters on the property. It is not good geotechnical engineering practice to design for a low swell situation on a lot that is adjacent to a lot with higher swell.
Another issue is the use of soil samples being taken at limited depths. A standard basement floor will be located approximately 8 to 9 feet lower than grade. Custom homes can be as deep as 14 feet with the garage floor located on the soils at or near grade. The report prepared for the site indicates testing depths of 1 foot, 4 feet, and 9 feet. An immediate question comes to mind: “How did the engineer providing the geotechnical parameters for the structural design and risk analysis determine the characteristics of the soils for the piers or footings below the basement floor?”
The soils report will indicate that the depth of the pier shall be a minimum length of 25 to 40 feet, yet there is not one test of the soil in the zone of penetration for the pier. Without that data, no realistic foundation design parameters, such as minimum dead load and depth of penetration or qualitative heave analysis, could be conducted to determine what the depth of the pier should be to stabilize the structure. The fact is that the basement is now where the originally tested soils used to be. In this scenario, the owner simply paid for the testing of the soil that was deposited off-site, and not the soil used to support the lower foundations. The testing should extend at least 10 feet beyond the depth of foundation bearing. Thus, if the piers are 30 feet in length, the testing should be greater than 40 feet, considering the depth of the basement walls and the length of the piers together.
A third key issue in the soils data is the discovery and the prediction of groundwater levels. As engineers, we know that the groundwater will increase from the direct influence of site surface flows, offsite flows, utility trenching, and pipeline or surface water leaks. We have now put habitants on previously uninhabited soils, increased the water flows into these areas, and minimized evapotranspiration.
According to Colorado statutes, groundwater represents a hazard that must be identified prior to the design and development of the site. The geotechnical report must correlate the need for either capturing and disposing of or managing the water. These systems can either be localized, such as dendritic drain systems under the basement level; or dealt with utilizing a regional system such as wells, interceptors, or dewatering drains.
The understanding of the groundwater is critical to the performance of the site. The soils report needs to address the current and future groundwater conditions so the design and construction can incorporate the expected groundwater. The widely used method of dealing with expansive soils with over excavation and replacement techniques, in most cases, will create a bathtub effect. Basically, a large diameter well is created by cutting a hole in tightly embedded clays and/or claystone (relatively impermeable materials), and replacing it with manmade fill (usually relatively more permeable materials). Due to this fact, the use of the over excavation and replacement technique has failed on numerous sites and has not been completely successful in mitigating the effects of expansive soils.
Foundations supporting the lightly loaded structures for residential and low-rise buildings are susceptible to damage when the foundations move differentially. In order to deal with this, system foundation design options must be closely related to the understanding of the future performance of the soils. The geotechnical engineer needs to provide enough information to determine how the soils will behave. Without the testing, it would be analogous to the design of steel beams by only visual selection of size and material. It would be below the standard for a structural engineer to size the members in the frame by visual methods, yet we see many geotechnical engineers practice such a craft by providing open hole inspections without any testing of the materials at the bottom of the hole, or below the hole into the bearing and active soils. Again, the engineer or technician does not possess the powers to deduct the soils material products through the use of their vision alone.
Foundations can be designed for either non-movement or for movement, depending on the type of materials utilized in the construction. The spacing of footings, piers, control joints, and cold joints can be implemented to deal with anticipated movement. The reinforcement of the system’s walls can also be designed and installed to resist lateral, upward, or downward pressures. The geotechnical report must provide this data to the structural engineer for the foundation to perform adequately. If not called out or specifically stated, then one should not design for zero movement, but request that proper design parameters be provided by the engineer specifying the soil mechanical properties.
The provisions for foundation movement can also be specified in the finish veneers of the upper floors. These can include control joints, expansion joints, proper veneer types, or attachment methods. Regardless, good design and construction practices require proper understanding of the behavior of the foundation resting on unstable soils. Therefore, in the review of thousands of soils/geotechnical reports for sites under forensic investigation in projects across the state of Colorado, it has become very clear that the majority of these reports fall short of providing the owner risk-based design parameters. In addition, these reports fall short of providing the designer and contractor predictions of future performance, interpretation of the site geology, consideration of adjacent existing site or future development, as well as tests at the proper depths for analysis of the soils for the determination of the interaction of the foundation and floor systems with the soils that support them.
One should stress the importance of covering all of these issues with the geotechnical engineer early in the project, specifically the need for a complete geological understanding of the site and surrounding areas, the proper testing depths and analysis methods, the provisions for future probable movement analysis (providing a predicted range of movement is preferred), and an overall risk assessment. With this information obtained on the front end, the developer, builder, owner, engineers, and architects can provide a product suited to the parameters of the site chosen for construction.
© Copyright 2006 Professional Investigative Engineers. All rights reserved.
Ed holds a Bachelor of Science in Civil Engineering, and a Master of Science in Civil Engineering with an emphasis in structural engineering. He is a Certified Level 2 Infrared Thermographer, a Certified 3rd Party EIFS Inspector, and a Certified Forensic Claims Consultant (CFCC.) Ed's background includes geo-hydrology, hydrology, hydraulics, civil engineering, structural engineering and extensive work in construction forensics for building envelope sciences including asphalt work to the roof. Clients include construction companies, management companies, insurance companies, attorneys, homeowners, and homeowner associations. Ed's work has included deposition testimony, expert witness testimony, and mediations and arbitrations. He is a registered engineer in the States of Colorado, Florida, North Dakota, Wyoming, New Mexico, Texas, Nevada, Mississippi, and Nebraska. Ed's dedication to the community is demonstrated by being an active member of over 25 professional organizations. He is a published writer in "The Journal of RCI," "Building Integration Solutions," "The Forensic Engineering Task Force," "The Investigative Engineer," "The I-ENG-A Report," “Inframation, Proceedings Volumes 4, 5, 6, 7, 8,” “Colorado Claims Guide,” and "Building Integration Solutions."