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Dynamic Cone Penetrometer Specs

The CPT report is in the attachments section above. 
 
The diagram to the right is of the dynamic cone penetrometer we used in Bolivia.
 
But before we had the booklet, the CPT was measured before it was returned to Trigon.

  • long rod: 1 5/16" od, 7/8" od (threads), 1/2" id, 30 3/4" long
  • tip: 13 1/8" long, reaches point through 1 5/8", same barrel characteristics as rod
  • drop mass: 4 1/4" od, 1 3/4" high on first disc; 6 1/2" od and 1" high on second disc (impactor)
  • mass drops over 20" on a 1 1/2" rod
  • all steel

Trigon told us the hammer was 35lbs, though we think it weighed about 15 pounds (and we were right!), and the whole thing weighed 35 pounds. They also have information on interpreting CPT results; email Joe for info.

Email to Joe Doane

Anna,

The number of blows obtained for each increment are averaged to obtain the DCP reading at the test depth. Based on previous research and when properly evaluated, the DCP readings approximate the Standard Penetration Test (SPT) resistance within firm to stiff residual soils, and can also be utilized to evaluate the variability within fill soils. The DCP test is commonly utilized to verify the allowable bearing capacity for shallow foundations in the Piedmont region of North Carolina. The test procedure is generally described in ASTM Special Technical Bulletin #399 (I believe we gave a copy of this to Patrick Ye when he picked up the DCP).
 
For the Piedmont region of North Carolina we generally use the following values:
5 blows - 1,500 PSF
6 blows - 2,000 PSF
7 blows - 2,500 PSF
8-9 blows - 3,000 PSF
12 blows - 3,500 PSF
>15 blows - 4,000 PSF with approval of the Engineer
 
It's important to understand this test is considered qualitative and not quantitative, meaning that in order to properly evaluate the test results you should have SPT values obtained from a drill rig and a knowledge of the local geology. Also, the values listed above should not be considered reliable for alluvial soils. This is because the properties of alluvial soils will vary greatly within short distances and depths.
 
The best advise I could give you is to assume a low bearing capacity (1,000 to 1,500 PSF) with large amounts of settlement.  
 
Joe

Source: George F. Sowers and Charles S. Hedges,
"Dynamic Cone for Shallow In-Situ Penetration Testing," Vane
Shear and Cone Penetration Resistance Testing of In-Situ Soils,
ASTM STP 399, Am. Soc. Testing Mats., 1966, p. 29.

CPT Preliminary Calculations

W = s*q*A

where

  • W = energy released from dropping cone penetrometer hammer in one blow (J)
  • s = distance driven down (m)
  • q = average punching or penetration stress (N/m^2)
  • A = (perpendicular?) area of cone to direction of distance (m^2)

W = 15 lb * 20 inches * (1 ft / 12 inches) * 32.174 ft/s^2 = 804.35 lb-ft-ft/s^2
A = pi*r^2 = pi*(.75 inches * (1 ft / 12 inches))^2

Using our data for the number of blows and the distance driven (s), you can calculate values for q. However these values are off by two orders of magnitude compared to the values Joe cites. Joe uses an s value of 1.75 inches as reported as the official increment of depth in the manual.

Potential Errors:

  • The area is defined differently
  • The potential energy from the hammer dropping is not the same as the kinetic energy transferred to the penetrometer
  • Using English units

References:

CPT Conclusion

Conclusions can be found in attachments, titled "Cone Penetrometer Data Interpretation."