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Penetration testing in United Kingdom    esopt

S. Rodin, B.O. Corbett, D.E. Sherwood & S. Thorburn    State-of-the-art report

1. GEOLOGICAL CONDITIONS

Ali the małn geological periods and epochs are well represented in the United Kingdom by a great variety of rock and soil types ranging from strong igneous and metamorphic rocka, mainly in the mountainous arcas, to recent soft or loose alluvial sedlments.

These recent deposits are largely found in the major post-Pleistocene eatuaries and fenlanda where the bu lid up of considerable thicknesses has been possible because of the post glacial rise in sea ievel. They may include peat deposits which are also found in poorly dralned upland arcas.

There are extensive areas, including much of the industrial lowlands, where the so called "solid" geological strata consist of weak sedlmentary rocks such as Triassic Bunter Sandstone and Keuper Marł, the Cretaceous chalks and weak sandstones and the Tertiary overconsolidated clays and sands.

There are few areas in the U.K. which have escaped the effects of glaciation. Ali the highlands show land-forms resulting from glacial erosion and associated processes, while most valleys and much of the lowlands contain hlghly variable soils which are the results of glacial deposition. These glacial deposits (Figurę 1) which may be very thick, can vary laterally and vertically within smali areas from lacustrine clays and 8llts to fluvioglaclal sands and gravels or to tills which may be dominantly cohesive or gra nu lar.

Because of the wide variation in the type and strength of soils and rocks, it has been necessary to develop a rangę of sito lnvestigation techniąues, some of which

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are applicable only to limited areas. This is particularly true of static penetrometers, which can only penetrate the very soft and soft soils which form the alluviai and estuarine deposits and the bog areas In the mountainous parts of the country. The combined dynamic/statlc penetrometer referred to In section 2.4 has a wlder use where both soft and hard layers are present.

Because of the limited use of the sophisticated types of penetrometers, there is considerable dependence on the standard penetration test, which, although crude, is inexpensive and simple to carry out. In the U.K., data is available for empirical interpretatlon of the standard penetration test in granular soils and soft to medium hard rocks.

2. DESCR1PTION OF PENETROMETERS

Three main types of penetrometer are in common use in the U.K.; the standard penetrometer (S. P.T.), the dynamie penetrometer and the static or pushed penetrometer. The S.P.T., although intended for use in sands, is often emplpyed in stiff clays and weak rocks particularly when driven undisturbed samples are difficult to recover. The static penetrometer has a relattaely limited application because of the variabillty of the majority of our recent deposits.

Dense or very dense sands or gravels overlying firm clay are not uncommon. For some ground conditions, however, the static penetrometer is our most satisfactory investigation tool. Dynamie and combined static/ctynamic penetrometers attempt to extend the role of penetration testing into the denser granular deposits and have many special applications (for example, proving the thickness of a gravel layer underlylng soft deposits for piling design).

Fig. 1 Glacial deposits

2.1 Standard Penetration Test

The basie procedurę for the S.P.T. as published by Terzaghi and Peck (1948) was adopted as a British Standard test (BS 1377 - test 18). Since its introduction, significant variations have been developed in the U.K.

In grayels and weak rocks, the driving shoe is replaced by a solid 60° cone. This modified penetrometer is referred to as the dynamie cone penetration test. The "standard" and "dynamie cone" penetration tests have been compared by Palmer and Stuart (1957) who concluded that the solid cone tended to givc slightly higher results. This view has been confirmed by Rodin (1961).

Although eariy tests and correlations were carried out using sąuare boring rods, it is now current practice to use "A" size drill rod (41.3 mm dlameter; 5.7 kg/ m). Automatic trip hammers of standard specification (63.6 kg dropplng through 762 mm) are genorally used and other forms of hammer are actively discouragcd.

It has been suggested (Serota, 1971) that the blow count using a trip hammer is less than that obtained when using a slip ropę hammer, on which Terzaghi and Peck based their work.