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United Kingdom

While the original test was carricd out In the U.S.A. in 50 to 100 mm diameter wash boring, the test is almost always performed In the U.K. In 'shell* borings of 150 mm to 200 mm, although larger diameters are usod in coarso grained materiale.

This procedurę increases the risk of disturbance of the soli immediately below the borehole due to suction as the shell ls withdrawn (Rodin, 1961;

Meigh, 1964). To minlmize this effect, the British Standard test procedurę requires the shell to be not morę than 90% of tho internal diameter of the casing and that it should be withdrawn slowly from the hole.

After proper cleanlng of the hole, the penetrometer on the end of the boring rods is lowered to the bottom of the hole and driven an i ni tlał 150 mm, and the blow count recorded. The penetrometer is then drivcn a further 300 mm and the blow count for the latter drive is called the penetration resistance ’N\ In good practice today it is usual to record the blow count for each of six 75 mm lncrements. This allows a better assessment of the depth of any disturbance. In very dense materlals, the blow count necessary to achieve fuli penetration may be excessive and tests are generally torminated after reaching a blow count of 50, either during the inltial penetration or the test penetration. The actual penetration achleved is then recorded.

There are no generał reąuirements for casing. It is usually used where the sides of the boreholes need supporting. In stiff clays and weak rocks, tests in unlined holes are generał.

The need to maintain the ground-water level in the borehole during boring and testing ls widely recognised; drilling muds are rarely emptoyed except In exceptionally difficult ground conditions.

In loose non-cohesive soils an extended penetrometer tubę is sometimes used (a further 300 mm is driven after completlon of the standard test and a second ’N’ value is recorded). The results are treated with some caution because of the possible depth of disturbance in "shell" boring in these conditions.

2.2 The Statlc Penetrometer

The mechanical penetrometer as descrlbed by Vermeiden has been used in the U.K. for a number of years. The test proceduro follows standard Dutch practice, with penetration rates of 15 to 20 mm/s. Hand operated machines (2 Mg) were originally introduced, followod by 10 Mg and 17.5 Mg engine-drivon unita betweon 1963 and 1970. As discussed earlier, ground conditions for whlch the system was developed are not widespread, but the techniquc has achieved a wide measurc of acceptance, particularly in loose sands and silts.

Recently, the Laboratorium voor Grondmechanica electric sounding penetrometer as descrlbed by Heijnen (1973) was introduced, and the penetrometer descrlbed by De Ruiter (1971) is also available. As yet, there is little practical experience with electric penctrometors, but their usc ls likely to be morę restrlcted than their mechanical counterparts because of the potential risk of damago to tho expensive cones in all but flne grained soils.

Reactlon is provlded whorevor possible with screw anchors because of the expense and difficultles in handling kentledgo, particularly over difficult ground. This ąuite often leads to inadcąuate reaction being . developed, with consequent limitations in penetration. For soundings from floatlng craft, tho dead weight of the craft is generally used as reaction, and this necessitates that work is carried out only when the craft can rest on the seabed.

Dlfficulty has been encountered in estuarine conditions where the soft or loose upper deposits provide insufficient la te rai resistance to bucki ing and fracture of the sounding rods when the cone reaches dense materiale at depth. This has necessltated the introduction of an assortment of casing techniąues, ranging from pre-boring and insertion of a Steel guide tubę through the soft layers, to pushing a casing over the sounding rods with the machino when denser deposits are reached. It has been found that these guide tubes can absorb a substantial amount of the available reaction in frictional resistance, if they are not stralght and fully concentrlc with the sounding rods.

With the mechanical penetrometer, cone resistance (and where reąuired local friction) is recorded at 200, 250 or 300 mm intervals and 'continuous* cone resistance soundings are also carried out.

Continuous chart recordings of electric soundings are madę together with incllnomcter readings at 1 m intervals.

2.3    Dynamie Sounding

The unsuitability of tho static penetrometer for generał use has led to the occasional use of dynamie sounding techniąues in the U.K. similar to those descrlbed by Terzaghi and Peck (1948) and Rodin (1961). This type of test is largely employed to extend Information betwcen borings where correlationsv*rith S. P.T.*s are generally attempted. It would be used morę often but for the inhibltion set by the common use of standard specifications based on conventional boring and sampllng, and reluctance to extend the specified conventional site investigation into further stages of site lnvestigation.

Correlation of dynamie penetrometers with standard penetration tests appears to depond on tho soli gradlng and lts relative denslty. In practice, it has been found necessary to establish a correlation for the particular deposits on each site.

2.4    Statlc/dynamlc Sounding

The dynamie penetrometer has been found to be rolatively lnsensitive in loose or soft materlals.

A8 theso are often encountered at depth beneath denser deposits which necessitate dynamie sounding, a static/dynamic techniąue has been deviscd. The "static" part of the test is in cssence identical to the