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decreaslng. For method 8 it 1s stipulated in the standard of 1971 that the levels where the rods are rotated and where the anvil is reset should be recorded. Knowledge of these levels 1s necessary for an accurate interpretation of the test diagram. A diagram is shown in Fig.18.

The traditional ram-sounding method was investiga-ted 1963-1969 as reported by Bergdahl & Dahlberg (1973).

LIFTING HEAD

GUIDE ROD (HOHOW) TOTAL LENGTH *.7Sm

DROP HAMMER 63.5kg {HEIGHT OF FREE FALL 50cm t 3cm )

RUBBER CUSHION £jXE,p_ANV!L

ROO (SOUP) i 32mm

Fig.19

Improved ram-sounding device (Method A).

way than that recommendcd ahove. It is recommen-ded to record the level of the point when the cushion is changed. A diagram is shown in Fig.18.

The effects of the height of fali on the penetra-tion resistance were investigated in 1966. Test results indicated that the number of blows re-quired to reach a depth of 10 m increased by 50% at a free fali of 40 cm in comparison with a 76 cm free fali (Fig. 20). It has been observed that the penetration resistance by the traditional procedurę with a height of fali of 76 cm agreed well with that by the improved method with a height of free fali egual to 40 cm. This diffe-

ACCU MULAT ED NUMBER Of BlOWS

Fig. 20

Ram-sounding, Method A and Method B, used in sand at different(controlled) heights of fali. Source: U Bergdahl & R Dahlberg, 1973.

An 1mproved penetrometer with a free falling ram (drop hammer) has been in use sińce 1966 (Fig.19). This penetrometer has the advantage that the height of free fali can be regulated accurately (50 * 3 cm), that the friction in the equipment is smali and that the drop hammer strikes a fixed anvil, attached to the top of the penetrometer rods. Errors caused by var1ations in the height of fali and by slip of the anvil have thus been eliminated. Test results show that the raaximum intensity of the shock wave for the new penetrometer corresponds to that for the traditional method, when the height of free fali is reduced from 60 to 50 cm and a rubber cushion is used to reduce further the intensity of the blow. Keasu-rements indicate that the maximum intensity of the blow in that case is 50 to 60 kN.

In method A the sounding rods are driven mechani-calTy by a 63.5 kg drop hammer. The penetration resistance is registered as the number of blows requ1red for every 20 cm of penetration. The ratę should not exceed 60 blows/min. The rod should be rotated two tums every 20 cm. When the penetration resistance is less than 5 blows/20 cm the penetrometer should be rotated only when a new rod is added. When the penetration resistance exceeds 50 blows/20 cm the rod should be rotated two turns every 50 blows.(The penetrometer should not be rotated when the rod is struck.) It should be noted on the diagram when the rod cannot be rotated or when the rotation is done in some otFer rence in performance 1s for the traditional method mainly due to friction in the wires.

An 1nvestigation has shown that even a rotation of the penetrometer rod of 20 degrees before each blow significantly reduces the skin friction resistance, especially in sand. Without rotation the skin friction 1n loose sand will account for 70 to 80 percent of the total resistance when the depth is large. Then the variations in the point resistance will morę or less be concealed.

or^ram^soundi ng

When the relative density is investigated of sand and gravel at large depths (> 10 m) the skin friction can be reduced further by increasing the rotation of the rod to 45 degrees before each blow. At this rotation the whole rod will rotate down to a depth of at least 30 m. When the penetration depth exceeds 30 m, a larger rotation than 45 degrees before each blow will be requi-red. This large rotation should be used also in stiff clay and sllty soils when the pile length is estimated.

It is the opinion of the Swedish Conmittee on Penetration Testing that method A gives morę reliable test results than method B. Method A should be used especially when the penetration test is used as a control meTFod and reproducab-le results are required and when firm bottom for