Traduction technique français anglais de la thèse de doctorat de Bassels Seif El Dine intitulée « Etude du comportement mécanique de sols grossiers à matrice »
Université : École des ponts ParisTech (École nationale des ponts et chaussées)
Copyright : Bassels Seif El Dine 2010
French english scientific translation series : Phd thesis entitled : « A study of the mechanical behavior of matrix coarse-grained soils ».
Phd Student : Bassels Seif El Dine
University : École des ponts ParisTech (former : École nationale des ponts et chaussées), FRANCE
Chapter 1 : Bibliographic Elements
Coarse-grained soils occur mostly in mountainous regions. They are composed of a mixture of coarse elements called inclusions (stones, gravel …) and fine soil called the matrix (sand, lime, clay). It is difficult to understand the geomechanical behavior of these soils using classical geotechnical techniques, mainly due to the presence of coarse elements that disrupt or impede test execution. These tests are divided into two types : laboratory tests and in situ tests. In this bibliographic study, we start by defining coarse-grained soils and then describe the various means of characterizing them as well as the results obtained.
The bibliographic study is mainly concerned with the analysis of the mechanical behavior of coarse-grained soils while focusing on the size of coarse elements that are the real source of difficulty. We will then present different possibilities for determining the shear resistance of coarse-grained soils and the factors that influence the experimental results. These factors are as follows:
- Inclusion size.
- Inclusion percentage.
- Inclusion morphological effect.
To these parameters, one should add the effect of scale (test tube dimensions, soil particle size) and the type of device (triaxial, shear box). Finally, we will present the coarse-grained soil characterization methodologies used based on laboratory devices of a classical size.
I.1 Definition of coarse-grained soils – Methods and test devices
Generally speaking, the classifications used in soil mechanics do no use the term “coarse-grained soils”. Several soil classes may correspond to coarse-grained soils. In the research literature surveyed, every researcher has his own definition of what constitutes coarse-grained soils as well as the limits of particle sizes that compose them depending on the in situ and laboratory tests carried out on these soils.
In their laboratory tests on shear strength of coarse materials using a (200 x 200 x 100 mm) shear box, Nichiporovitch and Rasskazov  defined maximum particle size of coarse-grained soils to be 50 mm. Perrot  defined coarse-grained soils to be those containing more than 50% of elements whose size is larger than 2 mm; placing no limit on maximum particle size.
Valle  and Valle  used a natural alluvial gravel with 0/200 mm grain size. He conducted triaxial and direct shear box tests to determine the characteristics of his coarse-grained soil.
In his investigation of the mechanical behavior of coarse-grained soils, Pedro  used alluvial gravel as his natural coarse-grained soil (Figure I.1).
The various coarse materials used in his experimental work were either mountain scree or had alluvial or torrential origin. They were composed of a mixture of elements with different sizes and natures. These coarse-grained soils were thus reconstituted in order to study the influence of the fundamental parameters, among which are size and proportion of coarse elements, on the mechanical behavior of these soils. Of particular importance are the works of Holtz and Willard , Donagh and Torrey  and Pedro .
Holtz and Willard  used as their coarse-grained soil sample, a mixture of gravelly soils (Figure I.2). Clayey soils were mixed with different percentages of gravels with 76.2 mm (3 inches) grain size. Gravel particles were mainly composed of gneiss, granite, and shale coming from river deposits. The density of clayey particles range from 2.66 g/cm3 to 2.70 g/cm3. They also mixed sands with various percentages of gravel with a maximum size of 76.2 mm (Figure I.3).
Pedro  also studied the behavior of benchmark coarse-grained soils made of large-sized inclusions in a sand matrix. He chose inclusions so that the ratio of the average size of matrix elements (d50,mat) to average inclusion size (d50,incl) : d50,mat /d50,incl, is greater than 10; and the ratio of test tube diameter to maximum inclusion size is also greater than 10
We can conclude by saying that, while there is no precise definition of coarse-grained soils, these soils have the following main characteristics:
- A spread grain size.
- Large-sized elements.
I.1.2 Test Methods and devices
In order to study the mechanical behavior of coarse-grained soils and their shear strength, we can use two approaches:
- In situ tests.
- Laboratory tests.
I.1.2.1 In situ tests
The evaluation of the mechanical behavior of coarse-grained soils with in situ tests uses the following devices:
- The pressuremeter.
- The phicometer.
- The in situ shear box.
Several researchers studied the behavior of coarse-grained soils using in situ tests. Most notably, we may cite the works of Ménard (1961), Nichiporovitch and Rasskazov (1967), Jain and Gupta (1974), Philipponnat (1986), Philipponnat and Zerhouni (1993), Combarieu (1995), Bourdeau (1997), Shirdam (1998), Shirdam, Faure, Magnan (1998), Valle (2001). These test showed that in situ characterization of coarse-grained soils requires the use of specific tests as well as large-sized experimental devices. The use of these types of tests poses a particular challenge as to their execution and the interpretation of their results.
In fact, when we try to carry out an in situ test on a significant volume, we face the following challenges:
- Test procedures are lengthy owing to the large size of the test devices.
- The interpretation of these tests seen as a mechanical problem has rather fuzzy conditions and limits. For example, in order to interpret such in situ tests as wave propagation tests, we are forced to assume that the medium is homogeneous. In the case of coarse-grained soils, this assumption is questionable.
- In situ tests are usually costly.
This last difficulty tends to favor the relatively inexpensive laboratory tests. You can find more details on in situ tests and their corresponding results in Pedro .
I.1.2.2 Laboratory Tests
We can study the mechanical behavior of coarse-grained soils using the following two devices:
- Direct shear box.
- Triaxial device.
Direct Shear Box Test
The direct shear box test consists of shearing a soil specimen placed between two semi-boxes according to a shear plane, on which a normal force N and a tangential force T are applied (Figure I.4). The lower semi-box can move horizontally at constant displacement speed. This test enables us to compute the soil’s shear strength as a relationship between the tangential stress τ (T/S) and the normal stress σN (N/S) on the failure surface (S: test tube surface).
Several authors used the direct shear box (Terzaghi and Peck , Nichiprovitch and Rasskazov , LCPC , Bourdeau , Magnan , Shirdam , Valle , Afriani ).
Nichiprovitch and Rasskazov  conducted their tests using a shear box with a length of 1600 mm, a width of 1000 mm and a height of 820 mm. Shirdam  used the large shear box of CETE in Lyon with a diameter of Ø 600 mm and a height of 300 mm (Figure I. 5). Valle  studied the behavior of a coarse-grained soil extracted from an alluvial bank of the Seine river using two shear boxes with dimensions 250 x 250 x 200 mm et 500 x 500 x 300 mm. These same two devices were used by Afriani .
Triaxial device test
The triaxial test, like the direct shear test, allows us to determine the soil’s shear strength. In this test, a cylindrical test tube of a given soil sample is subjected to a uniform stress with the following components:
- An isotropic confining pressure applied through a fluid (usually water) filling the cell.
- An axial deviator stress applied through a piston.
In the test, the deviator stress is increased until the failure of the test tube. The resulting stress state on the test tube is a principal stress with a major principal stress σ1 applied axially and a minor principal stress σ3 applied laterally (the case of compression, Figure I.6).
Several authors used the large-sized triaxial device to study the behavior of coarse-grained soils, among which we cite particularly Holtz and Willard , Valle , and Pedro . The triaxial device used by Holtz and Willard  has a diameter of 230 mm and a height of 570 mm. Valle  studied the shear strength of a coarse-grained soil extracted from an alluvial bank using a triaxial device with a diameter of 152 mm and a height of 304 mm. Pedro  used two devices to characterize coarse-grained soils, a large-sized device (height h = 600 mm and diameter Ø = 300 mm) and a medium-sized device (height h = 200 mm and diameter Ø = 100 mm)(Figure I.7).
I.2 The influence of fundamental parameters on the mechanical characteristics of coarse-grained soils
In order to enrich our knowledge of the mechanical behavior of coarse-grained soils with the goal of providing rational procedures for evaluating them, we have to study the effect of the fundamental parameters on the mechanical behavior of these soils. In what follows, we present the main results obtained in prior research.
I.2.1 Influence of device-related parameters
I.2.1.1 The effect of the size of shear devices
As we saw in I.1.2.2, test devices come into various sizes and soil grain sizes can have narrow or wide distributions. The choice of soil volume is constrained by the available test device sizes. We thus define the size ratio: Ø/dmax for the case of the triaxial device and L/dmax for the case of the shear box, with:
- Ø,D0: Diameter of the test tube used in the triaxial test.
- dmax : Maximum grain size (mm).
- L: Shear box length.
We present some of the research results surveyed in the literature, focusing mainly on the effects of size ratio observed on the mechanical behavior of various coarse-grained soils.