EFFICIENCY OF BEATING DEVICE

DPSH Dynamic penetrometers (TG 63-100; TG 63  -150; TG 63-200)

The average measured efficiency (73%) allows to normalize the measured N₂₀ values simplifying the correlations with other in situ measurements and mainly with N_SPT.

The efficiency of the DPSH penetrometer has been measured at two different sites in soils ranging between sand and gravel and unsaturated silty clay .

This preliminary experience has shown that the ratio between the N_SPT value and normalized N_{20 (DPSH Pagani)}  is about 1.5 for gravelly soils, and increases to more than 4 for unsaturatsd silty clayey soils.

The DPSH penetrometer produced and marketed by "PAGANI GEOTECHNICAL EQUIPMENT" (PGE) can carry out continous dynamic penetrometric tests (DP) both according to the methods foreseen by the International (1) and European (2) regulations, and by the Italian rules (3).

The International and European procedures are in practice identical; the Italian one differs in the hammer weight (73,5 Kg instead of 63,5) and in the cone shape (conical instead of cylindric-conical); but in Italy the use of the protecting coating against frictions along the rods is compulsory, while in the other mentioned procedures such an activity is only suggested and, as alternative, a mud injection in the anular space between the rods and the hole wall is allowed, having a lubricating function.

From the mid nineties even in Italy the European procedure is more frequently adopted.

With the PGE penetrometer it is easy to insert a set of steel coating tubes whose outside diameter does not exceed that of the penetrometric cone. The insertion of the coatings requires an extra time of about 60 % in comparison with the time required to infix only the penetrometric rods with the cone.

In the Italian procedure each coating cut down size is inserted in at the end of penetration of each penetrometric rod; the lateral friction corresponding to the length of a single rod just infixed is considered as unimportant.

The average efficiency of the beating device of the DPSH penetrometer by PGE has been measured in two different sites: the first one near the company’s plant located at Campogrande of Calendasco in the province of Piacenza; the second one in the municipality of San Prospero in the province of Mantova.

The efficiency measures of the beating device have been carried out by the company ISMES, appointed by PGE, equipping a standard rod by applying 4 strain gage electrically connected to form a Wheatstone bridge; the rod surface has been previously smoothed by turning and put into the furnace to eliminate all residual tensions due to the steel working.

The position of the loading cell was included between +1.00 and 0.00 m over the field level.

The energy E_a (kgm), transmitted to the rods has then been calculated by ISMES, for each hammer stroke, through the following expression:

where:

E_a = calculated energy (kgm)

K = constant depending on the area of the equipped rod, on the E module and on the steel density

l = distance between the measure sections and the rod base

c = rate of sound propagation into the rods (m / s)

f(t) = strength measured in the rods connected to the measure section (kg)

The efficiency of the beating device, expressed in percentage is:

η = E_a / E_h

where:

the potential energy: E_h = m _* H (kgm)

being:

m = the hammer mass (kg)

H = the falling height of the mass (m)

The soil of this area is made of a stratum of ironing clayey slime, having a thickness of 4-5 metres; then a thick alluvial gravelly-sandy bank of the Trebbia river follows.

The water-bearing stratum is around 9 m under the field level.

Figure 1 shows the stratigrafy of a sampling carried out near the penetrometric vertical; in the stratigrafy is stated the depth of the specimens drawn during the sampling.

In order to obtain representative specimens a nominal boring diameter equal to 160 mm has been used.

Near the sampling area a second drilling (diameter 100 mm) has been effected by standard SPT tests at intervals of 1,5 m. The measured N_SPT values are stated in the stratigrafy del drilling, according to the Italian rules requiring the indication of the strokes necessary to the penetration of each one of the 3 sections of 150 mm.

 Figure 2 and  3 show the granulometric composition of the drawn specimens.

Figure 4 charts the N₂₀ values measured at different depths in a vertical of DPSH dynamic penetrometric test carried out near the above mentioned sampling, under the International method and with the coating to follow. In the same table the N_{20 ( 60% )} values are stated, that is referred to a 60 % efficiency, taking into account both the average measured efficiency and the depth, as well as the SPT tests executed nearby.

The second site is mainly sandy (sands of the Oglio river) but also levels of finer material are found expecially in the first 10 m under the existing field level.

In this location, previously used as field for different types of geotechnical tests, some (ISMES) static penetrometric test results with electrical cone and SPT tests with efficiency measure were already available. Here PGE has effected n. 2 DPSH verticals measuring efficiency of the beating device starting from an 8 m depth from the field level.

Figures 5 and 6 graph the N₂₀ values measured at different depths; on the same chart also the N_{20 ( 60% )} values are stated, that is referred to an average efficiency of 60 %, depending on the depth.

Figures 7, 8, 9  are chartered the values of efficiency measured at different depths in the Santimento site (Figure 7) and in S. Prospero one (Figures. 8, 9).

You can note that variations mainly concern the more superficial measures and that the average value of the efficiency slightly increases with depth:

• 72 % at 5 m
• 73 % at 10 m
• 74 % at 15 m.

In calculation of N_{20 ( 60% )} value an average value of 73 % has been used.

Also for SPT tests executed by ISMES at the time the efficiency of the beating device has been measured, obtaining an average value equal to 55¸ 60 %.