In-situ testing is used in most site investigation programs. After in situ investigation, sampling is done and then laboratory tests. With a tight budget, it is important to strike the balance between in situ investigation and laboratory testing. The depths and number of boreholes, the types of labs, and the types of samples and in situ tests done determine the cost of site investigation programs. All these can cost up to one percent of the total budget of the project. It is important to know what you are looking for when planning a site investigation program.
In-situ or lab tests, which is better?
In simple terms, it is not advisable to compare lab and in-situ tests. Both of them have important roles to play. Lab tests have controllable and well-defined boundary conditions, more theory behind the interpretations but test small volumes of soil.
When it comes to in-situ tests, you will benefit from the quickness of the test and larger volumes of soil will be tested. This means that there is a more representative. Geophysical surveys can complement a traditional investigation program of a site. Today, technology has improved and we have drones to do certain aspects of the work.
About 80% to 90% of the in-situ tests worldwide consist of SPT or standard penetration or cone penetration (CPT). Vane shear has a special place for determining the undrained sensitivity of soft clays and shear strength. DMT or dilatometer test and pressuremeter tests are also becoming very popular and any in-situ testing company can use them. DMT or CPT are used for determining the shear wave velocities that can be used in determining the dynamic properties.
Standard penetration testing
This is a very dynamic option. It is also cheap and has been recognized as a simplified method that provides very useful information. While it is reasonably consistent with the fine-grained sands results, it is not as consistent with clays and coarse sands, which can be hard for gathering accurate data and soil conditions analysis.
Dynamic cone penetrometer
With a dynamic cone penetrometer or DCP, the cone is typically driven by a standard force from the hammer. How far the cone will move with each blow determines the density of the soil and the properties at the level. It gathers an estimate of soil properties.
Since dynamic cone penetrometer is hand-powered, it is more portable and cheaper than CPT equipment but the possibility of error makes it trickier to get accurate and consistent data. One of the largest difficulties of DCP is the effects of friction on the rods and the depth of investigation (DCP should not be used for greater than 1m).
Fortunately, handheld electronics has alleviated these issues and improved the consistency of results in in-situ testing. Smartphone apps and laser rangefinders can be used to count the blows and record accordingly. From a smartphone, the data can be graphed easily in the field and recorded on a computer or sent to a client for analysis.
