What is a CPT?

Cone Penetration Testing (CPT) is a technique used to evaluate subsurface stratigraphy and material properties. An instrumented probe is advanced into the soil by direct-push techniques from a reaction system, which is often a heavy vehicle but may be an independent frame with soil reaction anchors. Measurements from probe sensors are recorded on the probe or transmitted in real-time to an up-hole data collection and processing system at frequent intervals. Unlike many other in-situ geotechnical investigation techniques, the system does not use rotary drilling. CPT is frequently used both independently and as a companion to borehole drilling. Cone penetration testing is used worldwide but is generally less common than rotary drilling and related testing (such as SPT) due to the ubiquity of drill rigs and the ability of drills to penetrate nearly all geomaterials for site characterization. CPT rigs may be mounted on skids, wheeled or tracked vehicles, or reaction frames which can be moved into place manually or using light vehicles and small equipment. Vehicles and frames may use earth anchors to provide reaction for the probe push system. CPT systems can also be deployed in marine environments using specialty systems.
Most modern CPT probes are configured with sensors to measure the force acting on the cone tip, force acting on a nearby friction sleeve, pore water pressure, and probe vertical tilt (used primarily to evaluate the probe position and inclination for terminating a push if verticality is not well maintained).

The technique has several advantages including:

• Probe sensors are generally robust and easy to check for proper calibration; measurements are often highly repeatable and less operator dependent than other techniques.

• Data is collected electronically at frequent time/depth intervals creating a near-continuous record.

• The technique is relatively fast with a standard advance rate of 20mm/sec.

• Probes are available in several shapes and sizes for targeted applications.

• The technique is well suited for both land and marine applications.

• Many systems are automated to varying degrees.

• Probes may be configured with a variety of additional sensors:

• o Multi axial or multiple location geophones (seismic wave speed; pressure and shear wave measurement)

• o Video cameras

• o Electrical conductivity or resistivity

• o Fuel fluorescence

• o Gamma radiation detectors

• o Magnetometers

• o Multiple friction sleeves

• Relative safety.

• o Rigs are often enclosed with protection from weather, traffic, dust, and other conditions.

• o There are no rotating parts in the operational area greatly minimizing operator hazards

• Reaction force may come from dead-weight or a variety of anchoring systems

• Rigs exist in a wide range of sizes and configurations

• The operation results in relatively little disturbed ground and spoil and is well suited to environmentally sensitive sites.

CPT systems record near-continuous readings of measured sensor output. These readings are then processed using calibration conversions to engineering units, usually in real time during the advance of the probe for operator observation and reaction. Some probes have an onboard memory system and the probes are pushed and retrieved without real- time information [limiting issues related to wires and wireless commination]. Proprietary output files usually contain a variety of metadata, calibration information, and probe measurements. Additional supporting files may be created if standard testing is paused to conduct specialty time-domain tests at specified depths, such as pore water pressure dissipation tests or shear wave velocity tests. Recorded measurements are usually post-processed to develop a variety of engineering parameters based on calculations, theoretical, or empirical relationships.

Basic measurement output from modern electronic CPT systems includes:

• Time
• Depth [measured by an up-hole sensor]
• Tip stress
• Sleeve friction
• Pore-water pressure*
• X-Y Inclination [measured by a pair of accelerometers]

Other measured real-time values could include the measured down pressure of the drive system from pressure transducers included in the hydraulic system. A common calculated real-time value is the Friction Ratio. The pore pressure is typically measured immediately behind the tip element at the cone shoulder (the u2 position). Some specialty cones have the pore pressure element at the cone nose tip (the u1 position) or behind the friction sleeve (the u3 position), though these are far less common in non-research [production] applications. The location of the pressure sensor element is usually included in the test metadata. The CPT test measurements are usually post-processed and information about the site location (coordinates and elevation) is often added into the file along with other site information, such as the sounding number, time, weather, rig idendification, etc. Complete depth-based post-processed output frequently includes corrected measurements and calculated parameter values:

• Corrected Tip stress
• Friction Ratio
• Resistivity/Conductivity
• Seismic arrival times (P, S1, S2 waves at intervals)
• Correlated parameters (by one or more methods)

• Soil Behavior Type (SBT)
• Soil Material Index (Ic)
• Effective Friction Ratio (phi prime)
• Undrained Shear Strength (su)
• Unit weight
• Preconsolidation Stress (sigma prime sub p)
• Yield Stress Ratio (YSR)
• Lateral Stress Coefficient (Ko)
• Constrained Modulus (D’)
• Drained Young’s Modulus (E’)
• Bulk Modulus (K’)
• Subgrade Reaction Modulus (ks)
• Resilient Modulus (ks)
• Small Strain Shear Modulus (Gmax)
• Coefficient of Permeability (k)
• Coefficient of Consolidation (Cv)

Some specialty tests are performed at intervals during a regular CPT advance. Pore water pressure dissipation tests may include the time and a series of pore pressure measurements taken at a specific depth. These time-domain tests are typically conducted at selected depths (rather than at regular intervals) and may take a few minutes to several hours to complete. The test data is typically recoded in a separate CPT data file, at each depth where a test is conducted, with a different data structure, associated with the main data file by test ID.

Similarly, seismic tests may include the time and geophone response at a specific depth. Other specialty measurements (or data streams) may be recorded with the basic measurements with additional sensors on the probe, such as electrical conductivity or magnetometers. Similar to pore water pressure dissipation, the test data is typically recoded in a separate CPT data file at each depth where a test is conducted, with a different data structure, associated with the main data file by test ID.

A video feed from a camera mounted within the probe may be recorded as a separate data stream. Video cones are outfitted with depth encoders to include the depth on the video recording (which may be recorded separately as an analog or digital format companion file). Typically CPT logs contain the basic test data and dissipation and seismic data is processed separately and the calculated “results” (such as permeability (k) or pressure or shear wave velocity) are indicated on the log.

CPT operators often also have the capability to advance a soil sampler using the same reaction, advance system, and rods in an adjacent sounding. Samples are obtained and companion information from samples, such as photographs, moisture assessments, stratigraphic notes, soil identification and classification may accompany the digital record.

In addition to the advantages described for data acquisition in the field, the relative robustness of the data is useful for correlations and assessments of site variability. CPT data is often used to characterize site stratigraphy [with a high degree of confidence], model liquefaction potential, or to model the design performance of both deep and shallow foundation systems. Often, CPT measurements are correlated to traditional Mohr-Coulomb (phi-c) engineering design parameters, but some modern design techniques use direct-design from original measured CPT engineering values (without the correlation step). There are dozens of direct CPT design methods, making the exchange and interoperability of CPT data especially valuable for use when importing values into design software.

CPT rods being added as a probe is advanced at a project site. Photo courtesy of Minnesota Department of Transportation.

Operator controlling the push rate as a CPT probe is advanced and data is automatically recorded by a data acquisition system. Photo courtesy of Minnesota Department of Transportation.

A medium-sized truck-mounted CPT direct-push rig characterizing soils near an existing culvert. The truck is leveled on a set of jacks and the cone is pushed from a location near the center of gravity of the carrier. Photo courtesy of Minnesota Department of Transportation.

A typical CPT output plot showing Sleeve Stress, Tip Stress, Friction Ratio, Pore Pressure and a calculated Soil Behavior Type (SBT). Figure courtesy of Minnesota Department of Transportation

The applicable US Standard is ASTM D5778-20, Standard Test Method for Electronic Friction Cone and Piezocone Penetration Testing of Soils

Exposing CPT with the FROST Geotech Plugin

A typical CPT sounding includes the measurement of several soil properties at numerous closely spaced points as the CPT probe is advanced into the subsurface. To demonstrate how CPT measurements and and related metadata are stored and organized within the FROST server, we offer the following example of a simple CPT sounding where three propertis are measured:

GENERAL TEST INFORMATION

1. Test procedure: ASTM D3441-16, Standard Test Method for Mechanical Cone Penetration Testing of Soils
2. Sounding info: C-10, 15.03 ft sounding depth, top of sounding located at lan/lon 39.475026/-81.795909, elevation 252.61824 meters (WGS84).
3. Cone Serial Number: 128.074
4. Tip area: 15 cm2
5. Distance between tip and sleeve transducers: 15 cm
6. Probe Penetration Rate: 1 cm/s
7. Net Area Ratio Correction: 0.8

OBSERVATIONS MADE DURING THE TEST

Below is a sampling of the data generated. There are 263 sounding points total

Depth (ft)	Tip Resistance (tsf)	Sleeve Friction (tsf)	u2 Pore Pressure (tsf)
0.153	16.1	0	0.06
0.211	26.2	0	0.06
0.292	41.8	0.35	0.07
..	...	...	...
3.357	32.2	1.2	-0.1
3.41	32	1.19	-0.1
3.461	31.9	1.19	-0.09
3.516	31.6	1.16	-0.09
...	...	...	...
14.765	84.1	0.47	0.22
14.824	84.6	0	0.23
15.034	78.1	0	0.23

Instance Diagram

Object instances and the associations required to properly expose the example test data with the FROST Geotech Plug-in are shown in the following object diagram. This shows data for only sounding point at depth of 3.41 ft:

The following summarizes the various entities in the diagram:

Sensor

The Sensor object serves as the observing procedure in STA. One object instance is needed for this example (top center of diagram), and in this example holds the information about the test procedure, test parameters, equipment used and other metadata about the test, The general test data above (except for item 2) are all stored in the properties object of this Sensor instance. One Sensor instance is needed for all CPT tests conducted in one or more soundings provided that the same procedure, test parameters and test equipment is used for all tests.

ObservedProperty

The ObservedProperty object instances identify the properties that are observed by the CPT test. These are:

• tip_resistance
• sleeve_friction
• pore_water_pressure_u2

As with Sensor, the ObservedProperty instances can be reused for multiple tests.

DataStream

All of the object instances in the diagram are linked to the Sensor and ObservedProperty instances via Datastream instances (below the ObservedProperty objects on the diagram), which serve to associate observation results obtained from a feature of interest to its observed property, observing procedure, and the sounding's trajectory.

3 Datastream instances are needed, one for each ObservedProperty instance.

BhCollarThing, BhTrajectoryThing and Location

The DataStreams all link to the souinding via its BhTrajectoryThing object instance. BhTrajectoryThing (left edge of diagram) represents the sounding's geometry and contains the sounding length and information for linear referencing. The trajectory's geometry is given in the associated Location instance. BhTrajectoryThing is associated with a BhCollarThing instance, which represents the sounding as a whole. All general metadata about the sounding is contained in the BhCollarThing object instance; it's geometry is represented by an associated point Location object instance.

More detail about properties of BhCollarThing and BhTrajectoryThing can be found in the Borehole log discussion.

BhSampling and BhFeatureOfInterest

Sampling in the context of a CPT test or any in-situ test is the act of observing properties a a point or segment of the BhTrajectoryThing. The single BhSampling object instance (below and to the right of the BhTrajectoryThing in the diagram) holds the depth information of this sounding point (atPosition=3.41) and links to BhTrajectoryThing in order to affix the linear referenced sampling positions to the trajectory geometry.

BhSampling produces a BhFeatureOfInterest object, which represents the sampling location within the sounding (to the right of the BhSampling object instance in the diagram).

Observation

The remaining entities on the diagram are Observation instances that provide the results for their associated observed properties. Each Observation instance links to the BhFeatureOfInterest (sampling location) and to the Datastream instance associated with the appropriate ObservedProperty. As the results of each observed property are independently measured by sensors on the CPT probe, they are independent and therefore there are no related observations.

To add data from more sounding points, this requires adding a new BH_Sampling and BH_FoI in addition to the three new Observations for each sounding point. In the diagram below, we have added an additional sounding point (the next one in the depth series). These additions are shown in grey to distinguish them from the objects already displayed in the simpler diagram above.