Approach for SPT

What is a SPT?

The Standard Penetration Test (SPT) is a very widely used in-situ geotechnical test often applied in combination with other tests during the advance of a borehole by various rotary drilling techniques. The boreholes may also be advanced by sonic or direct push techniques, but these are far less common. The test is typically conducted at discrete intervals following separate drilling operations.

The test is valued for its ability to retrieve a sample and a simultaneous measurement of soil strength, as well as its relatively low cost, simple test protocol, wide-availability, and compact data set which can be easily manually recorded and transmitted. The sampler is relatively robust and can be used in nearly any geo-environment: soft to hard soils, cemented soils, regional geomaterials, urban fill, and soft rock.

The test is commonly used to evaluate the strength of soils through correlations to the angle of internal friction of granular materials and soil relative density. Over the decades of use of SPT, researchers have also amassed correlations to most every commonly used engineering parameter, with variable reliability, precision, and accuracy. Caution is advised when making interpretations about strength and other behaviors in environments where the test is not well suited, as described in governing standard test methods and other manuals of practice. While the SPT can collect samples in most materials it is not well suited to determining material strength properties of complex geotechnical materials such as peats, clays, and relatively large aggregates (as compared to the sampler size).

The SPT test is typically conducted across an interval of 18-inches [450 mm], creating a measurement associated with that sample length. Extremely soft or extremely hard materials may influence the test interval and measurement reporting. While back-to-back (continuous) SPT sampling is possible, the test is commonly conducted at intervals specified by the owner or agency. Often 2.5 ft., 5 ft. or 10 ft. intervals (or SI equivalent) during the advance of a borehole and is typically conducted at increasingly wider intervals as borings advance deeper owing to the need to alternate the drilling bit or tools with the sampler by pulling the complete string of drilling rods in and out of the hole.

The SPT test is considered a “disturbed sample” test as the sampler has a comparatively narrow aperture to collect the sample and thick outer wall. The basic mechanics of the test involve driving the soil sampler, at the base of the drill string, through the ground by a series of dynamic hammer drops applied to the top of the drill rods. The number of blows taken to drive the rods in measured depth intervals is recorded.

A variety of hammers and hammer systems can be used. Mechanical trip hammers, donut hammers, winch hammers, and safety hammers are in use worldwide. The test method prescribes a weight of 140 lbs [63.5 kg] and a drop height of 30 inches [750 mm]. Some hammer systems, such as rope and cathead hammers are operatory dependent and energy can vary widely. For improved performance and consistency, hammer systems should be calibrated to determine the energy transfer ratio (ETR), with the energy reported on boring logs for use in design [this is optional in the US standard].

For a typical test, a boring is advanced to the desired test depth (or elevation). The drilling tooling is pulled from the hole and the drill bit replaced with the SPT sampler. The sampler is lowered back into the borehole and the test begins. The top of the drill rod is marked off in three (3) 6-inch [750 mm] increments. The test consists of recording the number of blows to advance the sampler in each of three (3) consecutive 6-inch [150 mm] intervals. The sample tube is driven the first 6-inches and the number of blows needed to penetrate that interval is recorded. The procedure similarly repeats for the second 6-inch [150 mm] and third 6-inch [150 mm] intervals. After the sampler has advanced to a depth of 18-inches [450 mm] the test concludes. The three penetration “blow” numbers are recorded. A calculated value, the sum of the number of blows associated with the second 6-inch [150 mm] and third 6-inch [150 mm] intervals is reported as the "standard penetration resistance", “N-value” or “SPT blow count”

Notably, there are a number of “special cases” where the N-value may be unusual. If the sampler is driven less than the full extent the number of blows for each full increment or partial increment is recorded. The number of blows in a given increment are counted until a test termination condition is reached:

• The sampler penetrates 6 inches [150 mm]
• The sampler is driven 50 blows in any of the 3 increments
• The sampler is driven a total of 100 blows
• The sampler does not advance after 10 consecutive blows.

Additionally, if the sampler advances under the weight of the sampler and rods (or the sampler, rods, and hammer assembly) conditions of “weight of rods” or “weight of hammer” are noted rather than a “N-value.”

An additional component of the test is the “Sample Recovery” which is a visual observation and manual measurement of the sample recovered in the SPT sampler once the sampler is brought to the ground surface for recovery of the obtained sample within the SPT sampler.

A variety of metadata is required by the US and other standards. Applicable standards also specify the minimum precision associated with the reported values among other aspects of the testing procedures and equipment. Basic measurement output from a SPT test includes:

Basic measurement output from a SPT test includes:

• Depth of each test location along the extent of the borehole [measured by the operator based on the amount of drilling tooling in the ground and the ground elevation],
• The increment blow values of the 1st, 2nd, and 3rd increments, in addition to any special notations associated, such as the distance penetrated if less than 6-inches [150 mm]
• Notations if any of the termination criteria are reached other than penetration or if the sampler advances under the weight of rods or hammer.
• The % recovery of each sample within the SPT sampler
• Other driller notations such as equipment and tooling, method of keeping the borehole open, depth of the water surface, size of casing, hammer systems used, etc.

In addition, the calculated N-value is the primary output from the test:

• The "standard penetration resistance", “N-value” is calculated as the sum of the number of blows associated with the second 6-inch [150 mm] and third 6-inch [150 mm] intervals.
• If the sampler is driven less than 18-inches [450 mm], the number of blows per each complete 6-inch [150 mm] increment and each partial increment is recorded. In this circumstance there is not an N-value associated with that depth.

The N-value is often reported along with a calculated value, N60, where the measured N-value is adjusted to a 60% drill rod energy transfer ratio. Depending on owner and agency test procedures, SPT measurements can further be corrected prior to their use in engineering calculations based on a variety of variables. Corrections may be applied based on:

• Rod type (A rod vs. N rod)
• Use of liners within the SPT sampler
• Short rod lengths
• Boring diameter
• Other factors

The SPT measurements (N-value or other partial penetration and blow measurements) are often recorded in the field on a boring log in a discrete column of data. This information is then supplemented with other field or lab data which may include the drilling operations, soil type by observation or identification or classification system evaluation techniques, and frequently moisture content evaluated from a portion of the sample recovered in the soil sampler. SPT measurements are commonly reported and associated on a boring log with other depth-based data at the same location:

• Soil moisture contents
• % sample recovery
• A text-based soil identification or classification value derived from the sample
• Symbology associated with a soil identification or classification
• A drilling operation (casing, plug drilling, etc.)

A challenge for this common test is that while the basic data is often relatively simple and concise, there are a large number of supporting informational elements either associated with the site, the borehole, or the test. Many of these details are required by the test standard in the report for a proper and complete understanding of the measurement data.

Driller and lab technician comments can be important to interpreting otherwise unusual values, when compared with other in-situ tests or laboratory tests on collected samples conducted in association with the SPT testing. As such, the numeric measurements associated with the SPT test are frequently associated with supplemental text-based notes and comments, which may not be consistent among organizations, drilling staff, or even an individual driller.

A SPT drill rig on a project site; a work table with a SPT sampler is in the foreground. Photo courtesy of Minnesota Department of Transportation.

A SPT sampler rests on a work table, ready to be attached to the drill rods when the drill tooling is removed at the desired sample depth. Photo courtesy of Minnesota Department of Transportation.

The applicable US Standard is ASTM D1586/D1586M-18e1 Standard Test Method for Standard Penetration Test (SPT) and Split-Barrel Sampling of Soils.

Exposing SPT with the FROST Geotech Plugin

To demonstrate how the final results, interim test information and related metadata are stored and organized within the FROST server, we offer the following example SPT test for a single test interval. Relevant data are:

GENERAL TEST INFORMATION

1. Test procedure used: ASTM D1586/D1586M-18e1 Standard Test Method for Standard Penetration Test (SPT) and Split-Barrel Sampling of Soils
2. Location: Test run from 1.5 to 3 ft depth in borehole B-001-0-20
3. Borehole is is 41 ft deep and its collar is located at lat/lon 39.47466/-81.796858, elevation 249.50928 meters (WGS84)
4. Hammer used: CME Automatic
5. Hammer Efficiency: 84%

OBSERVATIONS MADE DURING THE TEST

Drive Set Number	Blow Count	Penetration (ftUS)
1	9	0.5
2	8	0.5
3	9	0.5

FINAL REPORTED RESULTS

• N-Value: 17
• N_1-60: 24

Note: This example focuses only on the in-situ test procedure and its results; it does not provide any information regarding the recording of any material samples that may have been recovered as a result of the test. Use of the FROST plug-in for recording material sample information is discussed in Approach for Atterberg limits

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:

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. In this example, the hammer information and hammer efficiency data would be held in the properties object of the Sensor. One Sensor instance is needed for all SPT tests conducted in one or more boreholes 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 SPT test. There are 5 ObservedProperty instances (top of diagram, below Sensor):

• 2 properties observed that constitute the final reported summary results of the test:

• 1. n_value (standard penetration resistance)
• 2. n1_60 (standard penetration resistance corrected for hammer energy and overburden conditions

• 3 properties observed as the test is being run

• 3. driveSetIndex (a counter for each set of hammer blows to drive the rod 150 cm)
• 4. blowCount (number of blows during the drive set (increment)
• 5. penetration (the distance the sampler travels during the drive set.

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 borehole.

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

BhCollarThing, BhTrajectoryThing and Location

The DataStreams all link to the borehole via its BhTrajectoryThing object instance. BhTrajectoryThing (left edge of diagram) represents the borehole's geometry and contains the borehole 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 borehole as a whole. All general metadata about the borehole is contained in the BhCollarThing object instance; it's geometry is represented by a 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 an SPT test or any in-situ test is the act of observing properties in a segment of the hole. The single BhSampling object instance (below and to the right of the BhTrajectoryThing in the diagram) holds the depth information of the test run (fromPosition=1.5, toPosition=3) 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 borehole (below 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.

This SPT test consists of 11 individual observations. Two of them (results for n_value and n1_60) are reported summary results that derive from the driveSet (blowCount and penetration) results that are observed directly during the test. The links shown in the diagram that relate observations to each other provide the means for distinguishing between the driveSet and summary/derived results.

Each driveSet increment is a set of three observation results. These results are useless independently - for example, the blowCount in a driveSet has no meaning without the associated penetration result for that drivesetIndex, and vice versa. To model the driveSet, the driveSetIntex result is linked to its associated blowCount and penetration results, and the blowCount result is linked to its associated penetration result. These three observations, as a set, contribute to the determination of the final n_value result, and to model that association, the driveSetIndex observations are linked to the n-value observation. In this way, one can traverse from the n_value observation to access those blow count and penetration observations that contributed to the n_value result.

Finally, the n1_60 observation instance is linked to the n_value observation instance to demonstrate that the n1_60 result relies on n_value.

To provide the most flexibility for querying, Datastream object instances associated with the Observations are linked in the same manner as their associated Observations, as seen in the diagram.

The current STA model does not provide for one-way links where an association role can be assigned. Such capability would be useful in modeling evem more complex geotechnical test results.