Significance and Use
4.1 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, 9, 10). 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 (11). Most cone penetrometer rigs are equipped with direct push soil samplers (Guide D6282/D6282M) that can be used to confirm soil types.
4.2 The electronic piezocone penetration test is an in situ investigation method involving:
4.2.1 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, as shown in Fig. 1 (Test Method D5778).
4.2.3.3 Robertson proposed the following equations estimating k from Ic and shown on Fig. 4  (11). These equations are used for some cone penetration testing commercial software for estimates of k based on normalized soil behavior type. However, as shown on Tables 1 and 2, the values estimated from Ic are not very accurate for example, the estimated k value may range over two orders of magnitude.
FIG. 4 Proposed Relationship Between Ic and Normalized Soil Behavior Type and Estimated Soil Permeability, k (Robertson (1))
4.3 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 sediments with low hydraulic conductivity, 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 distinguish between lower and higher permeability zones less than 0.3 m [1 ft] very accurately.
4.4 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 [60 to 120 ft], but depths to over 70 m [200 ft] 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 (D5778).
4.5 Pore Pressure Data:
4.5.1 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 [1 in.] thick.
4.5.2 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 dynamic excess pore water pressure measurements (12, 13, 14).
4.5.3 Dissipation Tests:
4.5.3.1 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. 5 shows a typical dissipation test showing the t50 determined by waiting for 50 % of the highest pressure registered to dissipate. In some soils there can first be a lag before the peak pore pressure occurs. This example also shows that sufficient time was reached to allow the pore pressure to reach full equalization.
FIG. 5 Example Dissipation Test Showing t50 Determination and Equalization of Pore Pressure (Robertson (2))
4.5.3.2 Fig. 6 shows one proposed relationship between t50 dissipation time and horizontal, hydraulic conductivity reported by Robertson (2, 11). This chart uses a tip resistance normalized for overburden stresses in the ground. This requires the estimation of the wet and saturated density of the soil and estimated water table location (2). The data points on the chart are laboratory test data from correlated samples. Figure 6 is developed for 10 cm2 diameter cones and a correction factor is required for 15 cm2 cones (multiply k values by factor of 1.5) (2).
FIG. 6 Relationship Between CPTu t50 (in minutes) and Soil Hydraulic Conductivity (k) and Normalized Cone Resistance, Qtn (After Robertson (2, 11, 15))
4.5.3.3 Included in Fig. 6 is a proposed relationship between dissipation time, soil type, and hydraulic conductivity proposed by Parez and Fauriel (15). This relationship is used in 4.5.3.4 by the high resolution piezocone (HRP) (16) for dissipation tests in sands.
4.5.3.4 A pore pressure decay in a clean sand is almost instantaneous. The 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 hydraulic conductivity of sands in environmental applications. The HRP cone uses special high resolution hardware and software to allow for high resolution data collection even in rapidly dissipating sand formations (16, 17), although recent experience indicates that this might be limited to hydraulic conductivity values less than 10-3 cm/s (18, 19). Partial drainage can also become an issue for cone penetration testing in soils where t50 < 50s and the approximate limits for undrained cone penetration are shown on Fig. 6  (20).
4.5.3.5 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 hydraulic conductivity of confining units. However, long excessive times for dissipation may not be economical in production CPT. Burns and Mayne (21) have developed methods to model the pore pressure dissipations tests in clays considering the stress history of the clays and can predict k and consolidation characteristics. Their method uses a seismic piezocone to measure the soil stiffness using down-hole shear wave velocity measurements.
4.5.3.6 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 [500 lbf/in.2] or more. Since transducer accuracy is a function of maximum range, this provides a relative depth to water level accuracy of about ±100 mm [0.5 ft]. 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 [25-lbf/in.2] transducer would provide accuracy that is better than 10 mm [0.5 in.]. 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.
4.5.3.7 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.
4.6 For a complete description of a typical geotechnical electronic cone penetrometer test, see Test Method D5778.
4.7 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. Onboard CPT single rod soil samplers (D6282/D6282M) are available for short discrete interval soil sampling. Continuous soil cores can be obtained rapidly in a separate location using continuous direct push dual tube samplers (D6282/D6282M). Investigators may obtain soil samples from adjacent locations for correlation purposes, but prior information or experience in the same area may preclude the need for borings for soil samples.
4.8 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 laboratory testing or inspection of soils and rock or both. As such, it is not totally applicable to agencies performing this field 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.
Scope
1.1 The electronic cone penetrometer test often is used to determine subsurface stratigraphy for geotechnical and environmental site characterization purposes (1).2 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 Cone penetrometer tests are often used to locate aquifer zones for installation of wells (Practice D5092/D5092M, Guide D6274). The cone test may be combined with direct push soil sampling for confirming soil types (Guide D6282/D6282M). Direct push hydraulic injection profiling (Practice D8037/D8037M) is another complementary test for estimating hydraulic conductivity and direct push slug tests (D7242/D7242M) and used for confirming estimates. 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 due to specialized monitoring requirements of drilling equipment.
1.6 Units—The values stated in either SI units or in-lb units (presented in brackets) are to be regarded separately as standard. The values stated in each system may not be exact equivalents; therefore, each system shall be used independently of the other. Units for conductivity are either m/s or cm/s depending on the sources cited.
1.7 All observed and calculated values shall conform to the guidelines for significant digits and rounding established in Practice D6026, unless superseded by this standard.
1.8 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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.9 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.
1.10 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
