Section 3: Interpretation of Soil Data
Overview
A critical step in foundation design is determining strata and reasonable strengths to be assigned to each stratum. Selecting design parameters that exceed those existing at the project site will result in increased risk of unacceptable performance. Selecting design parameters that are substantially less (or overly conservative) than those that exist will lead to increased costs from excessively conservative design or construction issues.
Divide the subsurface materials into strata based on material description and test values. Review all tests within each stratum to evaluate the variability of the data. If a single, unusually high strength test is present among a group of distinctly lower test values, disregard the anomalous test value. An average strength may be assigned for an entire layer(s) broken down into Engineering Stratigraphic Units (ESUs) when the test values are reasonably similar.
Avoid defining very thick strata with widely variable test values. Subdivide thick strata with test values varying from soft near the top to distinctly harder toward the bottom into two or more strata with compatible values. Failure to subdivide may result in an unconservative average strength being applied to foundations that terminate in the upper zone of that stratum.
In-situ vs. Lab Data
Geotechnical data reports including boring logs and in-situ and lab testing will be used in collaboration to assign design parameters to stratigraphic units. As previously mentioned, SPT provides an acceptable means to gage strength of sands, but hits refusal in bedrock and Intermediate Geomaterial (IGM) and results are highly variable in cohesive material. Lab results (in accordance with Chapter 4) are essential to isolate compressive and shear strength parameters when in clay or rock.
Disregard Depth
Disregard surface soil in the design of deep foundations, i.e., drilled shafts and driven piles. The disregarded depth is the amount of surface soil that is not included in the design of the foundation due to potential erosion from design flood or check flood, future excavation, seasonal soil moisture variation (shrinkage and swelling), lateral migration of waterways, and other factors. Amount of disregard may be different based on the design check being performed. For axial and compressive loading, disregard a minimum amount of 5 ft. over non-water crossings and 10 ft. over stream crossings. For abutments, disregard the portion of foundation passing through embankment fills. Note that the length of disregard may differ depending on the design check being performed.
When permanent casing is used for deep foundation installation, disregard side resistance with respect to axial design checks.
For projects where the existing ground line is at an elevation considerably higher than the proposed grade line (roadway is to be depressed) soil softening, swelling or heave must be accounted for in design of embankment slopes, roadways, retaining walls and foundation elements. Soils in these conditions respond to the removal of overburden (unloading). This response could have a dramatic impact on the design approach taken.
Additional considerations for disregard depth are required when encountering downdrag or scour. See Section 4 of this Chapter for downdrag. Information regarding disregarded depth and scour methodology at bridge foundations can be found in Section 6 and within the TxDOT Scour Analysis Guide.
Drilling Data, Laboratory Data, and Subsurface Classification
Acquire geotechnical borings in accordance with Chapter 2 and 3. Perform laboratory testing, borehole logging, soil/rock identification, classification and reporting in accordance with Chapter 4.
Identify rock type and characteristics in accordance with Chapter 4. Perform core recovery such that at least one (1) unconfined compressive test can be performed per bedrock unit or group with similar characteristics and per each boring.
Capacity From Texas Cone Penetration Test
Do not use Texas Cone Penetration Test for new designs. Refer to Appendix A for procedure to evaluate existing structures using TCP data.