6585844979

Israel 187

DRILLING OPERATIONS

For borings in soil two main types of drilling equipment in soil strata are being used. Diameter of hole is ordi-narily 4 to 6 inches. When the strata inyestigated is above ground water le-vel, the auger type is usually used either with or withoug Steel casing. On the other hand, under water level, the wash boring system is applied. In order to prevent caving-in of materiał into the borehole either casing or bentonite slur-ry is being used. Drilling with bentonite slurry is being used. Drilling with bentonite has become morę popular in re-cent years. Washing is being carried out by forcing dri11 water down through the drill pipę and through sideways and up-ward inclined holes in the chopping bit. Care is taken to avoid bottom disturbance from any upward gradient of water into the hole be maintaining a fluid level in the borehole at least 1 1/2 ra higher than ground water level. The operations of washing and driving of the casing are done simultaneously. A test is carried out every 1.5 meters in depth and when-ever a change in the soil strata is ob-served.

TESTING PROCEDURĘ

After the bottom of the borehole has been cleaned, the spoon sampler is Iow -ered carefully to make contact with the soil. Testing is then started and the number of blows per each 15 cm during 45 cm of penetration is recorded. The finał SPT result is ordinarily reported as the sum of the last two 15 cm inter-vals. Whenever irregularities occur during the penetration, such as change in the/resistance, they are recorded and added to the log as a remark. Where ma-terials are encountered with SPT values of greater than 100, the penetration achieved by 100 blows is noted. Ali ma-terials sampled in the spoon are recove-red and stored in polyethelene bags.

DUTCH CONE

Use is occasionally madę of a raotorized Dutch Cone aparatus, built by N.V. Goud-se Machinę Fabriek (Gouda, Holland), which has a capacity of 10,000 kg. The penetrometer is of the moveable cone type,.with an apex angle of 60° and a 10 cm² area (diameter of 36 mm). Resis tance of the cone and of the friction loads are raeasured by oil manometers. A penetration ratę of about 2 cm/sec is em-ployed. Since most of the sites investi-gated are in areas where upper layers of dense sand or Kurkar may be encountered, this apparatus is not generally utilized for routine foundation exploration pro-grams. However, it has been very help-ful in some cases where fili were to be built on soft foundations, particularly for evaluating the depth, strength of layers and extent of the clay and silt deposits. In one case it was mounted on a fiat car and utilized for determining the characteristics of both erabankment and foundation for a portion of a rail-way linę in a clayey soil area which had been reguiring extensive raaintenance.

On several occasions the Dutch Cone point resistance has been compared with vane shear strength, the latter being measured in cased borę holes. The penetration resistance, divided by the vane strength, may be considered as a bearing capacity factor, N_c*. The values of N_c' have been

found to vary with shear strength, being about 30 at vane shear strengths of 0.2 kg/cm² and decreasing to about 10 for vane shear strengths higher than 1.0 kg/ cm². Fig. No. 4 is a plot showing N '

w

values versus vane shear strength for several sites in Israel.

hN/m²

%

i

o.

*6

f.o^(	)	50	100 ISO	200 250	300
		i	I
				K-Raiłway localion		-
30		***		• -Hobora *<(•		-
		V K>		v-Acr* *it»		-
		O O
20			■			“
10		V		V V V	W
			* '"l			.
		05	10 1.5	20 25	10	i

^ vont kQ/cm

Fig. 4 - N_c* Plotted against Vane

Shear Strength

It should be observed that the Dutch cone values in themselves are not sensitiye to changes in shear strength.

Use has been madę in Israel of hand ope-

rated penetrometers with fiat disc tips

yarying from 1 cm to 4 cm diameter for

clay soils. Load is measured directly

on proving rings of 100 kg. capacity.

The load reąuired for a bearing capacity

failure is observed, and interpreted as a

ąuick shear strength by using a N ¹ value

c

of 10. Discussion of this method is in-cluded in a paper by Wiseman and Schiff-man (1967).

The Soiltest type of pocket penetrometer is used extensively as an aid in field and laboratory logging of saraples, as well as in inspection of field test pits.

PROCEDURĘ FOR CLAYEY STRATA

Where clayey strata are encountered, in the course of the usual soil investiga-tions, penetrometer testing is not em-ployed independently. Instead, vane shear test raethods are ordinarily spe-cified. In special cases, as described above, where correlations with vane shear strength are available, the Dutch Cone apparatus has been very useful.