A comprehensive subsurface exploration plan is necessary to communicate the intent and level of testing that may be required. Effectively communicating these requirements not only ensures that required data is obtained, but it serves as a plan to minimize resources expended.

Communication with lab personnel can help determine the volume of material that might be required to perform the type and number of tests desired. Since there are limited in-house resources and funding often defines outsourcing, it will be necessary to minimize the number of tests and still obtain the level of data required to fully describe project site characteristics. Costs are typically 0.5%-1.0% of the project estimate.

As simple as it seems and obviously critical, locations for sampling have to be specifically identified and communicated to field personnel. Identify not only the geographical location of samples to be taken, but the depth schedule of sampling at each location as well.

The two sampling methods most often used are disturbed sampling, sometimes called bulk sampling, and undisturbed sampling. Each has its advantages depending on the tests being performed. Since bulk sampling rapidly provides sufficient material for laboratory testing, it is most commonly used. Undisturbed sampling is most commonly used to identify existing engineering properties and to make recommendations to the designer.

The two primary sampling techniques used in pavement material analysis are disturbed and undisturbed. Each is descriptive of the amount of disruption of the soil matrix from its natural or in situ state.

Sampling frequency depends on the level of investigation, uniformity of soils, and the potential for detrimental reaction from chemical stabilization. General recommendations for various soil conditions are listed in Table 3-5.

Sample materials continuously to a depth of at least 15 ft. in areas with high moisture fluctuations. Where excavations will exceed this depth, sampling should be conducted to finished subgrade depth plus 2 additional feet.

When materials change physical characteristics, a new bulk sample should be taken.

TxDOT’s laboratory testing procedures contain the methods and processing requirements to accomplish each procedure. It is not the intent to repeat those methods in this document, but procedures used frequently are listed in Table 3-6 and briefly discussed.

It is essential for the design engineer evaluating laboratory data to set minimum acceptable criteria. From a pavement design standpoint, any material in place should be either suitable or modifiable to a suitable state; additional thickness of pavement layers will be able to compensate for most soils. Since there are time constraints, political influences, costs, and other such criteria that often influence the judgment regarding a soil’s suitability, this approach is often not feasible. There is not one criterion that can determine what is acceptable. All factors must be weighed and trial designs made with each alternative considered.