How to Select Soil & Rock Properties for Engineering Design?

A rational approach for selecting soil & rock properties for engineering design can be summarized as a logical twelve-step procedure that covers the 3 primary stages i.e.

1. Site Investigation and Field Testing,
2. Laboratory Testing and Interpretation, and
3. Engineering Design

This step-by-step process is presented on the flow chart in fig-1. A brief description of each step of this process is presented below.

Stage-1 Site Investigation and Field Testing

Step-1 [Review Available Information

The best place to start the process of material property selection is to review any and all information that may be available. There are several sources for this information, many of the sources being in the public domain and readily available at modest expense.

Step-2 [Identify Required Material Properties]

No investigation should be initiated without specific goals being established that are related to design and construction issues that must be considered (i.e., performance requirements), engineering properties that are needed, and the type of structure that is to be constructed.

Step-3 [Plan Site Investigation]

Historical information, which will provide anticipated subsurface conditions, coupled with knowledge of the specific design will allow an efficient site specific investigation strategy to be developed. Contingency plans should be considered based on anticipated variabilities in subsurface conditions. Sampling intervals should be identified and an in-situ testing program should be developed.

Step-4 [Conduct Site Investigation and Field Testing]

Once the investigation strategy is developed, it is ready to implement. Findings should be communicated to the geotechnical design engineer during the field work and modifications to the number and types of samples and testing should be determined, as required.

Step-5 [Describe Samples]

Results from the field investigation program and subsequent laboratory identification of samples should be compared to the anticipated conditions based on historical information. Selected laboratory samples can be reviewed by the design engineer to obtain first-hand observations. These samples should be used for performing simple laboratory index tests.

Step-6 [Develop Subsurface Profile]

Using results from the field investigation and the laboratory index tests, a detailed subsurface profile should be developed by the geotechnical design engineer. It is helpful at this step to review the initial site investigation objectives and expectations to be assured that the materials are consistent with expectations.

Step-7 [Review Design Objectives]

An on-going evaluation of field and available laboratory data relative to the design objectives should be performed during the implementation of the site investigation. If adjustments are needed or if additional data needs are identified, procedures should be initiated to obtain the necessary information.

Stage-2 Laboratory Testing and Test Interpretation

Step-8 [Select Samples for Performance Testing]

Prior to initiating the project-specific laboratory testing program, the design engineer should review the recovered samples and confirm the testing that needs to be conducted (i.e., type, number, and required test parameters). If possible, selected samples should be extruded in the laboratory and reviewed by the design engineer.

Step-9 [Conduct Laboratory Testing]

Once the samples have been reviewed and the testing program is confirmed, it is time to continue the index tests and initiate the performance-testing program (with index test correlation for quality assurance). Preliminary results should be provided to the design engineer for review.

Step-10 [Review Quality of Laboratory Data]

If the data and interpreted laboratory test results are not consistent with expectations or if results indicate that the sample was disturbed, it is necessary to review progress and make adjustments. On some projects, results at this stage can be used to plan and initiate a more detailed and focused phase of investigation. A phased investigation approach is particularly helpful on large projects and in cases where there are many unknowns regarding the subsurface conditions or specific project requirements prior to conducting the proposed site investigation program.

Step-11 [Select Material Properties]

The laboratory and field test results should be interpreted and compared to project expectations and requirements. The role of the design engineer at this stage is critical as the full integration of field and laboratory test results must be coupled with the site-specific design. If test results are not completely consistent, the reason(s) should be evaluated, poor data should be eliminated, and similarities and trends in data should be identified. It may be necessary to return to the laboratory and conduct an additional review of sample extrusion, selection, and testing.

Stage-3 Engineering Design

Step-12 [Perform Design]

At this final stage, the design engineer has the necessary information related to the soil and rock properties to complete the design. Additionally, the design engineer also has first-hand knowledge related to the variability of the deposit and of the material properties. Design activities can proceed with knowledge of these properties and variabilities.

Conclusion

In particular, selection of the correct engineering property, laboratory tests, their interpretation, and summarization are often poorly performed. Rigorous attention to this twelve-step procedure is required to assure efficient and thorough investigation and testing programs, especially since many projects are fragmented in which drilling, testing, and design are performed by different parties.