Dynamic probing test (DP)

Essential aspects of the equipment and test procedure

Dynamic probing tests are probably similar to the earliest means by which the characteristics of the subsoil have been investigated by our predecessors.

The test consists of determining the number of blows (N) of a hammer of mass (M) in freefall from a height (H) onto the assembly composed of, from top to bottom, an anvil, a string of rods and a conical tip (the base of which has an area A), so that a certain length of penetration (L) occurs. The diameter of the rods is smaller than the tip cone base, so,

Table 1.10 Type of dynamic penetrometers.

Type

Abbreviation

Moss of the hammer (kg)

Light

DPL

M < 10

Medium

DPM

10 < M<40

Heavy

DPH

40 < M < 60

Super-heavy

DPSH

60

Table l.lI Referential characteristics for penetrometers (ISSMFE, 1989).

Characteristics

DPL

DPM

DPH

DPSH

Hammer mass, M (kg)

10

30

50

63.5

Height of fall, H (m)

0.5

0.5

0.5

0.75

Mass of the anvil and guide rod, 8 (kg)

6

18

18

30

Rod length (m)

1

1-2

1-2

1-2

Maximum mass of the rods, V (kg)

3

6

6

8

Outer diameter of the rods (mm)

22

32

32

32

Inner diameter of the rods (mm)

6

9

9

-

Cone apex angle (°)

90

90

90

90

Base area of the cone, S (cm2)

10

10

15

20

Penetration length, L (cm)

10

10

10

20

Test output

N io

N,o

N io

N20

Range of number of blows

3-50

3-50

3-50

5-100

Specific energy per blow, £, (kj/m2)

50

150

167

238

theoretically, the penetration resistance results only from ground reaction forces on the conical surface of the tip.

Table 1.10 includes a classification of the penetrometers in terms of light, medium, heavy and super-heavy, depending on the mass of the hammer. In turn, Table 1.11 includes some referential characteristics for the four types of penetrometers while Figure 1.28 shows the rods and tips used in some of the dynamic penetrometers.

In order to compare the results of different penetrometers, it is common to adopt the so- called specific energy per blow, Es Dft which represents the kinetic energy of the hammer per unit area of the tip section, expressed by:

Using two systems, I and II, with specific energy values £j>DP and E"dp, the respective results, N, and Nn, corresponding to probing of rod lengths L, and Ln in a given soil, will be related as follows:

This means that the number of blows required to obtain a unitary penetration length is inversely proportional to the specific energy per stroke. This type of comparison involves some reservations, since, as previously discussed, the energy transmitted to the ground also depends on other parameters not included in the previous equation, such as the weight of the anvil, the weight of the rods, etc.

Figure 1.29 illustrates tests using the light and super-heavy dynamic penetrometers.

Rods and tips used in some of the dynamic penetrometers (photo

Figure 1.28 Rods and tips used in some of the dynamic penetrometers (photo: Carlos Rodrigues).

Execution of dynamic probing tests (a) DPL and (b) DPSH (photos

Figure 1.29 Execution of dynamic probing tests (a) DPL and (b) DPSH (photos: Carlos Rodrigues).

 
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