In situ testing

Introduction

In the following sections, the most relevant and frequently used in situ tests will be discussed.

In view of the nature of this book, it has been decided to describe each test in only its essence:

  • (i) the general concept of the equipment and its interaction with the ground under investigation;
  • (ii) the quantities measured in the test, how they are interpreted, and how soil parameters or other results of interest can be derived, including practical examples.

There is abundant information, as well as numerous details related to these tests, which are essential for their proper implementation, which are not included in this text. For each

Table 1.3 Relevant standards for geotechnical investigation and in situ testing.

OperationlTest?

AST/VI

EN ISO

Geotechnical investigation and sampling

D 420, D 1452/D И52М - 16, D 4700

-

Identification and description

D 2488

ISO 14688-1

SPT

D 1586/ D I586M - 18

ISO 22476-3

CPTu

D 3441 - 16 D 5778 - 12

ISO 22476-1

DP

-

ISO 22476-2

PLT

D II95/D II95M-09 D 1 196/ D 1 I96M- 12

CHT

D 4428 / D 4428M - 14

VST

D 2573 / D 2573M - 18

ISO 22476-9

SBPT

ISO 22476-6

PMT

ISO 22476-4

DMT

D 6635 - 15

ISO 22476-1 1

Permeability tests

D 4050

ISO 22282-1 ISO 22282-2

Pumping tests

D 4050

ISO 22282-4

1 The complete designation of each test is included in the subsequent sections.

in situ test, the reader is directed to the extensive normative documents and standards, such as those summarised in Table 1.3.

Standard penetration test (SPT)

Essential aspects of the equipment and test procedure

The approach to geotechnical in situ testing necessarily starts with the SPT, by far the most used field test worldwide. It was introduced to the United States by the Raymond Pile Company in 1902, but its widespread use only began in the 1940s with the publication of the book Soil Mechanics in Engineering Practice (Terzaghi and Peck, 1948).

The test consists of driving a standardized sampler to the bottom of a borehole, shown in Figure 1.13a, by tapping a 63.5 kgf (140 lb) weight hammer falling from a height of 76 cm (30 inches). The sampler is a steel split barrel (with outer and inner diameters of 51 mm and 35 mm, respectively) with a length of about 80 cm and an approximate weight of 6.8 kgf. At the lower end of the sampler, a beveled driving shoe is attached, which facilitates penetration into the ground. At the opposite end, a part is threaded for the connection with the string of rods, and provided with a non-return ball check valve and side vents for purging air and water during penetration of the sampler.

In order to carry out the test, the drilling operation is interrupted, followed by cleaning of the bottom of the borehole (removal of the material disaggregated by drilling and contact with the water used to fill the hole) and descent of the sampler carried by the string of rods. After positioning the sampler in contact with the ground at the bottom of the borehole, the top rod (i.e., at the surface) is adjusted with an anvil that will receive the hammer blows (Figure 1.13b).

The test is performed in two successive phases: in the 1st phase, with a penetration of the sampler of 15 cm; and (sequentially) in the 2nd phase, with 15 cm plus 15 cm, counting the respective number of hammer blows. The number referring to the 1st phase is taken as merely informative, since it is essentially intended to go through the most disturbed ground

SPT test

Figure 1.13 SPT test: a) Terzaghi’s standard sampler; b) view of the test just before the free-fall of the hammer; c) open split barrel sampler after testing (photos: Carlos Rodrigues).

immediately below the bottom of the borehole. The total number of blows in the 2nd phase (i.e., the sum of the two 15-cm sub-phases), termed N, “N-value” or “standard penetration resistance”, is considered the test result.

If, in the 1st phase, the number of blows reaches 50 without a penetration of 15 cm, or the same number (50) is reached in the 2nd phase without a penetration of 30 cm, the test is interrupted and the penetration length reached is registered. In this case, the N value for a penetration of 30 cm is usually obtained by a simple extrapolation.

Before further detailing of some aspects of the test itself, it is important to emphasize that the SPT is, first and foremost, a process for collecting disturbed samples. For this purpose, sampler driving is usually carried out with a spacing of 1.5 m or less. When brought to the surface, the sampler is opened in two barrels, allowing examination of the soil inside (Figure 1.13c). A fraction of the last 30 cm of sampled soil is then stored in a small, properly identified, watertight box, which is later examined for the definition of the stratigraphy and lithology of the ground.

The geological-geotechnical profiles, containing the succession of soil layers or strata used as the basis for the geotechnical studies of a construction site, are usually established based on the disturbed samples collected by driving the SPT sampler. The idea of associating sampler driving with a geotechnical test should indeed be acknowledged, as it provides a basic descriptive parameter of the (mechanical) quality of the ground that can be used in current practice by engineers.

 
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