ASTM D6067-10

Environmental site characterization projects almost always require information regarding subsurface soil stratigraphy and hydraulic parameters related to groundwater flow rate and direction. Soil stratigraphy often is determined by various drilling procedures and interpreting the data collected on borehole logs. The electronic piezocone penetrometer test is another means of determining soil stratigraphy that may be faster, less expensive, and provide greater resolution of the soil units than conventional drilling and sampling methods. For environmental site characterization applications, the electronic piezocone also has the additional advantage of not generating contaminated cuttings that may present other disposal problems (2, 3, 4, 5, 6, 7, 8, 15). Investigators may obtain soil samples from adjacent borings for correlation purposes, but prior information or experience in the same area may preclude the need for borings (1). Most cone penetrometer rigs are equipped with direct push soil samplers (Guide D6282) that can be used to confirm soil types.

The electronic piezocone penetration test is an in situ investigation method involving:

Pushing an electronically instrumented probe into the ground (see Fig. 1 for a diagram of a typical cone penetrometer). The position of the pore pressure element may vary but is typically located in the u₂ position (Test Method D5778).

Recording force resistances, such as tip resistance, local friction, and pore water pressure.

Data interpretation.

The most common use of the interpreted data is stratigraphy based on soil behavior types. Several charts are available. A typical CPT soil behavior type classification chart is shown in Figs. 2 and 3 (9). The first step in determining the extent and motion of contaminants is to determine the subsurface stratigraphy. Since the contaminants will migrate with groundwater flowing through the more permeable strata, it is impossible to characterize an environmental site without valid stratigraphy. Cone penetrometer data has been used as a stratigraphic tool for many years. The pore pressure channel of the cone can be used to determine the depth to the water table or to locate perched water zones.

Hydraulic conductivity can be estimated based on soil behavior type (Figs. 4 and 5). These estimates span two to three orders of magnitude. Alternately, pore pressure data (4.5) can be used for refined estimates of hydraulic conductivity.

When attempting to retrieve a soil gas or water sample, it is advantageous to know where the bearing zones (permeable zones) are located. Although soil gas and water can be retrieved from on-bearing zones such as clays, the length of time required usually makes it impractical. Soil gas and water samples can be retrieved much faster from permeable zones, such as sands. The cone penetrometer tip and friction data generally can identify and locate the bearing zones and nonbearing zones less than a foot thick very accurately.

The electronic cone penetrometer test is used in a variety of soil types. Lightweight equipment with reaction weights of less than 10 tons generally are limited to soils with relatively small grain sizes. Typical depths obtained are 20 to 40 m, but depths to over 70 m with heavier equipment weighing 20 tons or more are not uncommon. Since penetration is a direct result of vertical forces and does not include rotation or drilling, it cannot be utilized in rock or heavily cemented soils. Depth capabilities are a function of many factors including:

The force resistance on the tip,

The friction along the push rods,

The force and reaction weight available,

Rod support provided by the soil, and

Large grained materials causing nonvertical deflection or unacceptable tool wear.

Depth is always site dependent. Local experience is desirable.

Pore Pressure Data:

Excess pore water pressure data often are used in environmental site characterization projects to identify thin soil layers that will either be aquifers or aquitards. The pore pressure channel often can detect these thin layers even if they are less than 20 mm thick.

Excess pore water pressure data taken during push are used to provide an indication of relative hydraulic conductivity. Excess pore water pressure is generated during an electronic cone penetrometer test. Generally, high excess pore water pressure indicates the presence of aquitards (clays), and low excess pore water pressure indicates the presence of aquifers (sands). This is not always the case, however. For example, some silty sands and over-consolidated soils generate negative pore pressures if monitored above the shoulder of the cone tip. See Fig. 1. The balance of the data, therefore, also must be evaluated. There have been methods proposed to estimate hydraulic conductivity from excess pore water pressure measurements (10, 14, 16).

Dissipation Tests:

In general, since the groundwater flows primarily through sands and not clays, modeling the flow through the sands is most critical. The pore pressure data also can be monitored with the sounding halted. This is called a pore pressure dissipation test. A rapidly dissipating pore pressure indicates the presence of an aquifer while a very slow dissipation indicates the presence of an aquitard. Fig. 6 shows one proposed relationship between dissipation time, soil type, and hydraulic conductivity (11).

A pore pressure decay in a sand is almost instantaneous. The permeability (hydraulic conductivity), therefore, is very difficult to measure in a sand with a cone penetrometer. As a result, until recently the cone penetrometer was not used very often for measuring the permeability of sands in environmental applications. Newly developed hardware and software now allow for high resolution data collection even in rapidly dissipating sand formations (12).

A thorough study of groundwater flow also includes determining where the water cannot flow. Cone penetrometer pore pressure dissipation tests can be used very effectively to study the permeability of aquitards (13).

The pore pressure data also can be used to estimate the depth to the water table or identify perched water zones. This is accomplished by allowing the excess pore water pressure to equilibrate and then subtract the appropriate head pressure. Due to high excess pore pressures being generated, typical pore pressure transducers are configured to measure pressures up to 3.5 MPa. Since transducer accuracy is a function of maximum range, this provides a relative depth to water level accuracy of about ±150 mm. Better accuracy can be achieved if the operator allows sufficient time for the transducer to dissipate the heat generated while penetrating dry soil above the water table. Lower pressure transducers are sometimes used just for the purpose of determining the depth to the water table more accurately. For example, a 175-KPa transducer would provide accuracy that is better than 10 mm. Incorporation of a temperature transducer and appropriate calibration allows for high precision and rapid data collection. Caution must be used, however, to prevent these transducers from being damaged due to a quick rise in excess pressure. Some newer systems allow for large burst pressure protection without hysteresis, which enables users to collect data in highly stratified environments without as much concern for transducer damage.

When coupled with appropriate models, three dimensional gradient can be derived from final pressure values collected from multiple CPT locations. Once gradient distributions have been derived, and hydraulic conductivity and effective porosity distributions have been generated, seepage velocity distributions can be derived and visualized. This type of information is critical to environmental investigations and remediation design. If contaminant concentration distributions are known, the same software can be used to derive three dimensional distributions of contaminant mass flux.

For a complete description of a typical geotechnical electronic cone penetrometer test, see Test Method D5778.

This practice tests the soil in situ. Soil samples are not obtained. The interpretation of the results from this practice provides estimates of the types of soil penetrated. Investigators may obtain soil samples from adjacent borings for correlation purposes, but prior information or experience in the same area may preclude the need for borings.

Certain subsurface conditions may prevent cone penetration. Penetration is not possible in hard rock and usually not possible in softer rocks, such as claystones and shales. Coarse particles, such as gravels, cobbles, and boulders may be difficult to penetrate or cause damage to the cone or push rods. Cemented soil zones may be difficult to penetrate depending on the strength and thickness of the layers. If layers are present which prevent direct push from the surface, rotary or percussion drilling methods can be employed to advance a boring through impeding layers to reach testing zones.

Note 1—The quality of the result produced by this standard is dependent on the competence of the personnel performing it, and the suitability of the equipment and facilities used. Agencies that meet the criteria of Practice D3740 are generally considered capable of competent and objective testing/sampling/inspection/etc. Users of this standard are cautioned that compliance with Practice D3740 does not in itself assure reliable results. Reliable results depend on many factors; Practice D3740 provides a means of evaluating some of those factors.

Practice D3740 was developed for agencies engaged in the testing or inspection of soils and rock or both. As such, it is not totally applicable to agencies performing this practice. However, users of this practice should recognize that the framework of Practice D3740 is appropriate for evaluating the quality of an agency performing this practice. Currently there is no known qualifying national authority that inspects agencies that perform this practice.

FIG. 1 Electronic Cone Penetrometer (Test Method D5778-07)

FIG. 2 Simplified Soil Classification Chart for Standard Electric Friction Cone (Robertson and Campanella 1986) (9)

FIG. 3 Normalized Soil Classification Chart for Standard Electric Friction Cone (Robertson 1990) (9)

FIG. 6 Proposed Relationship between T50 Dissipation Time, Soil Type, and Hydraulic Conductivity (12)

1.1 The electronic cone penetrometer test often is used to determine subsurface stratigraphy for geotechnical and environmental site characterization purposes (1). The geotechnical application of the electronic cone penetrometer test is discussed in detail in Test Method D5778, however, the use of the electronic cone penetrometer test in environmental site characterization applications involves further considerations that are not discussed. For environmental site characterization, it is highly recommended to use the Piezocone (PCPT or CPTu) option in Test Method D5778 so information on hydraulic conductivity and aquifer hydrostatic pressures can be evaluated.

1.2 The purpose of this practice is to discuss aspects of the electronic cone penetrometer test that need to be considered when performing tests for environmental site characterization purposes.

1.3 The electronic cone penetrometer test for environmental site characterization projects often requires steam cleaning the push rods and grouting the hole. There are numerous ways of cleaning and grouting depending on the scope of the project, local regulations, and corporate preferences. It is beyond the scope of this practice to discuss all of these methods in detail. A detailed explanation of grouting procedures is discussed in Guide D6001.

1.4 The electronic cone penetrometer may be be combined with other direct push sampling and testing methods. Estimated soil types can be confirmed by soil sampling (Guide D6282). Cone penetrometer tests are often used to locate aquifers for installation of wells (Practice D5092, Guide D6274). Cone penetrometers can be equipped with additional sensors for groundwater quality evaluations (Practice D6187). Location of other sensors must conform to requirements of Test Method D5778.

1.5 This practice is applicable only at sites where chemical (organic and inorganic) wastes are a concern and is not intended for use at radioactive or mixed (chemical and radioactive) waste sites.

1.6 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.

1.7 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

1.8 Standard Practice—This practice offers a set of instructions for performing one or more specific operations. This document cannot replace education or experience and should be used in conjunction with professional judgment. Not all aspects of this practice may be applicable in all circumstances. This ASTM standard is not intended to represent or replace the standard of care by which the adequacy of a given professional service must be judged, nor should this document be applied without consideration of a project's many unique aspects. The word "Standard" in the title means only that the document has been approved through the ASTM consensus process.