Introduction to In Situ Testing

Introduction

Over 40years ago, Mitchell et al. (1978) gave a number of reasons for the growing interest in the use of in situ testing techniques:

• 1. Ability to determine properties of soils such as sands and offshore deposits that cannot easily be sampled in the undisturbed state;
• 2. Ability to test a larger volume of soil than can conveniently be tested in the laboratory;
• 3. Ability to avoid some of the difficulties of laboratory testing, such as sample disturbance, the proper simulation of in situ stresses, temperature, and chemical and biological environment; and
• 4. Increased cost effectiveness of an exploration and testing program using in situ methods.

Engineers should not expect a single in situ test to provide the answer to all geotechnical problems. Just as different laboratory tests are used to obtain specific soil properties, different in situ tests have been developed for the same purpose.

Role of In Situ Testing In Site Investigations

Like all soil tests, in situ tests provide a way of obtaining additional information about subsurface conditions at a site. They are used to give a more complete picture of site conditions and soil behavior and reduce uncertainties inherent in most projects. Geotechnical engineering often requires the use of many tools, and Figure 1.1 shows the various tools available for geotechnical design. In situ tests are rarely used as a complete replacement for test borings and laboratory tests for a site investigation but are typically used to compliment a traditional subsurface exploration program in order to enhance the information regarding site conditions.

Advantages and Limitations of In Situ Tests

In situ tests can provide a number of advantages over the traditional drilling, sampling, and laboratory testing approach used in many geotechnical projects. However, like all tests, in situ tests also have a number of limitations. It is important that engineers understand both the advantages and the limitations of in situ tests.

Figure l.l Tools used in the practice of geotechnical engineering.

Advantages of In Situ Tests

Advantages of in situ tests include the following:

• 1. Tests may be conducted in soil deposits that are difficult or impossible to sample or test;
• 2. Soil properties that cannot be easily determined by conventional laboratory tests can be determined from in situ tests;
• 3. A larger total volume of soil that may influence the design can be tested;
• 4. In situ tests avoid some of the difficulties inherent in a conventional lab testing program;
• 5. Some tests provide a near continuous record of vertical variations in soil conditions;
• 6. There is often a reduction in the time of the site investigation;
• 7. Some tests allow for real time or rapid data reduction;
• 8. In situ tests may be used to assess the influence of scale and macrofabric on soil behavior;
• 9. Tests are performed in a field environment; and
• 10. There is almost often substantial cost savings to a project.

Testing Soils that are Difficult to Sample

Often times, subsurface explorations encounter soils that are difficult or impossible to sample using conventional drilling and sampling methods. Typical examples include loose sands and silts below the water table, very soft or highly sensitive clays, and highly weathered or structured materials, such as surficial crusts or residual soils. In some cases, artesian or other unusual groundwater conditions may also create drilling and sampling difficulties. In these cases, the use of Cone Penetration Test (CPT)/Piezocone (CPTU) or Dilatometer Test (DMT) may provide results that are more reliable than laboratory tests conducted on samples of poor quality.

Determining Soil Properties that are Difficult to Measure by Laboratory Methods

Using common laboratory testing techniques, it is sometimes difficult to obtain accurate measurements of certain key soil properties that may be essential in a geotechnical engineering design. For example, the small-strain shear modulus or horizontal stress can be extremely difficult to measure in the laboratory and require special equipment and high-quality undisturbed samples. In these cases, an alternative may be to select an appropriate in situ test that can provide these measurements.

Testing a Larger Volume of Soil

For most typical geotechnical problems, a traditional subsurface exploration is usually set up to obtain samples at some preselected depth interval (often 5 ft (1.5 m)) or whenever a stratigraphic change occurs. For routine projects, the number of test borings and the number of samples are relatively small, which gives a limited view of the engineering behavior of the soil. A stratigraphic profiling test, such as the CPT, CPTU, or DMT, can increase the amount of soil investigated by two to three orders of magnitude. Important strata that might otherwise be missed during conventional drilling and sampling may be identified.

Avoiding Difficulties with Sampling and Laboratory Testing

One of the limitations of obtaining soil samples from the field, transporting them to the laboratory, and performing tests is that there are always some unavoidable changes in the environment relative to field conditions. Changes in pore water pressure, stress field, composition of the pore fluid, temperature, and sample disturbance may have differing degrees of influence on the behavior of different soils. In situ tests avoid many of these issues. In addition, laboratory tests are not without their own set of problems, related to differences in test equipment, test procedures, sample sizes, and rate of loading.

Obtaining Near Continuous Profiling

It is possible to obtain a near-continuous record of the vertical variations in soil conditions using penetration tests such as the CPT/CPTU and DMT. Data acquisition systems used with the CPT/CPTU can provide detailed information about changes in stratigraphy that occur with depth. The use of stratigraphic profiling tools is one of the main areas where in situ tests can enhance site investigations for geotechnical engineers.

Reduced Testing Time

The use of in situ tests often provides a substantial reduction in the time necessary to complete the site investigation in comparison to conventional site investigation practice. Rapid determination of specific soil properties during field exploration also allows the engineer to make a preliminary assessment of the subsurface conditions. There is also the opportunity to evaluate any problematic areas, e.g., unstable or soft ground conditions, excavation problems, etc.

Rapid Data Reduction

Some in situ tests provide rapid and automatic data acquisition and reduction in the field, often as the test proceeds. This means that the technician or engineer can track the progress of the test and troubleshoot any difficulties that may arise during the test. Judgement may then be used to decide whether more tests need to be conducted at the site.

Assessing the Influence of Scale or Macro-Fabric on Soil Behavior

In some soils, such as highly fissured or fractured clays or highly weathered residual deposits, the macrofabric can exert a significant influence on soil behavior. These soils present a problem to the engineer in obtaining high-quality undisturbed samples for laboratory testing and in evaluating how the laboratory response reflects the behavior of the soil in the field. In these materials, it is important to test a sufficiently large volume of soil in order to obtain a reasonably accurate indication of soil behavior.

Conducting Tests in a Field Environment

In situ tests are performed in the field under conditions that are closer to “undisturbed” than laboratory tests. That is, the vertical and horizontal stress conditions, pore water pressure, temperature, and pore fluid chemistry are more representative than conditions that are typically used in laboratory tests.

Cost Savings

The reduction in testing time and increase in information provided by many in situ tests usually result in substantial economic benefit to many projects. The conventional approach to geotechnical design involves a high degree of uncertainty that evolves as a result of the highly variable nature of geologic deposits and the small level of investigation generally possible. This in turn leads to a design approach that is often more conservative than if more detailed information could be obtained. A reduction in the level of uncertainty that can be achieved by incorporating in situ tests into the subsurface exploration program may result in cost savings.