Dynamic probing test and empirical formulas
In situ investigations
The purpose of the in situ investigations is to adopt appropriate techniques to allow the determination of the physical-mechanical characteristics of the soils. In-situ investigations include penetrometric tests commonly used for the physical-mechanical characterization of the significant subsoil volume.
The combination of these tests with surveys, laboratory tests on the samples taken during the drilling surveys and indirect investigations (e.g. seismic) is recognized as a valid technique for the investigation of the subsoil because it returns an effective indication of the parameters and the nature of lithotypes. Therefore, laboratory tests and in situ tests are not alternatives to each other but are to be considered complementary.
Classification of penetrometric tests
Among the penetrometric tests are distinguished:
1) static tests: CPT and CPTU (with piezocone); [Detailed descriptions in DIN 4094, 1974; BS 5930 and ISO22476-2, 2005; Table 2]
2) dynamic tests: DP, SPT and dilatometric tests [Detailed descriptions in ASTM6951-03 and ISO22476-2]
In this article, particular attention will be paid to dynamic penetrometric tests.
Dynamic probing and advantages
The dynamic test consist of a tip cone connected to an extension rod and a driving connected to an extension rod and a driving weight for penetration into the ground. The number of
Blows required to successively driving the cone by each 100 mm (or 200 mm depending on the mass of hammer) increment are recorded as a measure of shear strength.
These tests have the following advantages: speed in execution, ease of use, low prices, ability to provide a continuous subsoil profile, identification of thin subsoil layers, distinction between cohesive and non-cohesive soils, allows to obtain geotechnical parameters of the soil by means of correlations and finally, it allows to evaluate the variation in the degree of consistency / thickening of the soils investigated, with decimetric precision.
Previous Study and considerations about undrained cohesion Cu
The dynamic penetrometric test is extremely efficient for the investigation of inconsistent soils, but considering some objective limitations, it is possible to perform the test even in limited cohesive soils, interpreting the shear strength parameters according to undrained conditions through the Cu value (undrained cohesion).
Some authors shown the relationship between different soil characteristics (undrained Cohesion Cu) and the results of dynamic penetrometric tests. In particular, the dynamic point resistance (qd) can be calculated using the following formulas:
qd=M1/(M1+M2)*rd
rd = M1*g*h/A*e
Where:
-
- rd : unit point resistance (Pa);
- M1: mass of the hammer (Kg);
- M2: the total mass of the extension rods, the anvil and the guide rods (Kg);
- g: the acceleration due to gravity (m/s2);
- h: the height of fall of the hammer (m);
- A: the area at the base of the cone (m2);
- e: the average penetration in m per blow (0.1/M from DPL, DPM e DPH e 0.2/M da DPSH; M: is the number of blows per 100 mm penetration)
Butcher et al (1996) presented that dynamic test results obtained from different equipment configurations show similar qd values for the same clay soil profiles. Based on data obtained from investigations carried out on cohesive soils, these authors determined the undrained cohesion Cu (Kpa) as reported in Tab.1.
| Soil type | Correlation | References |
| Soft clay | Cu=qd/170+20 (Cu<50kPa) | Butcher et al (1995) |
| Hard clay | Cu=qd/22 (Cu≥50kPa) | Butcher et al (1995) |
| Clay | Cu=qd/20 | Langton (2000) |
| Fine soil | Log10CBR=0.35+1.06Log10*qd | Amor et al (1999) |
| Fine soil | MR=532.1(DCPI)^-0.492 | Rahim and Georg (2004) |
Tab. 1 – Comparison of correlation for Dynamic probe results and CBR, MR e Cu. MR is the resilient modulus (MPa); CBR, California bearing ratio (%); qd, dynamic point resistance (KPa); DCPI, penetration index of the dynamic penetration test (mm/blow); Cu, undrained shear strength (kPa).
Details of dynamic penetrometric tests
Tab. 2 – Details of dynamic probing test specifications and SPT tools.
Undrained cohesion (Cu) estimation
Some authors proposed a relationship between the results obtained from the dynamic penetrometric test (qd) and the undrained cohesion (cu) as shown below:
logqd=0.637logcu+2.243
the formula can be rewritten as:
cu=qd^1.57/3320
One of the applications of the dynamic penetrometric test could be the control of the degree of compaction for example for cohesive material of the core of a embankment for dams or pavement layers.
Khodaparast et al. (2015) determined an experimental correlation between CP and the qd datum obtained from the penetrometric tests (DPM and DPL, Tab.1) according to the equations:
| CP= 131.27(DCPI)^-0.240 | for DPL |
| CP=155.96 (DCPI)^-0.280 | for DPM |
Where:
-
- DCPI is the penetration index of the dynamic penetration test in (mm/blow)
- CP is the compaction percent.
Innovation
As seen previously, qd is calculated from the various dynamic penetrometric tests, therefore this data can be correlated to CD as follows:
CP = 16.654qd ^ 0.193
The innovation and the advantage that lies in this correlation is as follows: while the other correlation formulas are linked to specific tests (e.g. DPC), this is based on the tip resistance (qd) which can be used for different configurations of dynamic tests. Fig.1 shows the good correlation between the results obtained from the experimental tests.
Fig. 1 – Correlation between dynamic cone resistance and compaction percent for various configurations of the dynamic probe (DPL e DPM). From Khodaparast et al., 2015.
Geostru Dynamic Probing software
The formulas described above have been reported and implemented in the latest Geostru Dynamic Probing software updates, and specifically, the columns with data relating to CBR, MR and CP have also been included in the “Current test” section (Fig. 2).
Fig. 2 – Table and graph of the allowable pressures taken from the Geostru Dynamic Probing software.
Tips on Software and Apps
Considering the text described above the use of the following softwares are recommended:
- DYNAMIC PROBING – Dynamic penetration tests – Software used for Dynamic Penetration Tests, that is the reading, recording, interpretation, storage and the management of any type of penetrometer including new or custom equipment and of in borehole SPT readings.
- STATIC PROBING – Static penetration tests – This program processes and archives penetrometric equipment readings for static penetrometers such as CPT (Cone penetration test), CPTE (Cone penetration test electric) and CPTU (Cone penetration test Piezocone).
Among the apps available on the Geoapp web page (service available for making online calculations) there are several that can be used together with the software mentioned above, for example:
References
American Society of Testing Materials, Standard test method for use of the dynamic cone penetrometer in shallow pavement applications (D 6951-03), ASTM International, West Conshohocken, PA, 2003.
British Standards Institution, Code of Practice For Site Investigations,1999, BS 5930, pp. 59-60.
Butcher AP, McElmeel K, Powell JJM. Dynamic probing and its use in clay soils, In Proceedings of the International Conference on Advances in Site Investigation Practice, ICE London, Thomas Telford, 1996, pp. 383-395.
Deutsches Institut fur Normung, Dynamic and static penetrometers, Dimensions of apparatus and method of operation, DIN 4094, e. V. Berlin, 1974.
ISO 22476-2, Geotechnical investigation and testing, Field testing, Part 2: Dynamic probing, Case postal 56, CH-1211 Geneva 20, 2005.
Khodaparast M., Rajabi A.M. and Mohammadi M., 2015 – The new empirical formula based on dynamic probing test results in fine cohesive soils.
