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 (III)

##### Influence of spacing between the two shear semi-boxes

It is interesting to know the **effect of spacing** between the two semi-boxes of the shear box on the **mechanical behavior of coarse-grained soils**. To this end, we shall present the results of several authors who worked on this subject.

Shirdam [1998] studied the influence of spacing between the two semi-boxes using the direct shear box with diameter Ø = 600 mm and height h = 300 mm on scree reduced to a maximum diameter of 40 mm. He carried out two series of tests with two spacings t = 3 mm and t = 20 mm. He noticed that **soil cohesion** c drops from 106 to 73 kPa when t moves from 3 mm to 20 mm, while the **internal angle of friction** decreases to 1° (Figure I.18). Thus, spacing does have an effect on soil cohesion: Larger spacing leads to decreased cohesion c. However, the angle of friction changes little.

Valle [2001] conducted direct shear tests using the big shear box on coarse-grained soil extracted from the Vallée Heureuse quarry, which produces a natural chalky gravel with a 90 mm maximum diameter. He conducted tests on this gravel refined to 60 mm with 20 mm and 45 mm spacing between semi boxes. He observed that results were identical for both spacings (Figure I.19).

Furthermore, Nichiporovich and Rasskazov [1967], Cruz and Nieble (1971) and Shirdam [1998] showed that the spacing between the two semi-boxes has a significant impact on the value of cohesion below d_{max}/2. Based on this set of results, Valle adopted a spacing of d_{max}/2 between the two semi-boxes to carry out his direct shear box tests.

Afriani [2003] studied the effect of spacing using both the big and small shear boxes on a natural 0/50 mm soil and a reuse 0/25 mm material (originally, a natural material from Criquebeuf-sur-Seine sieved down to 25 mm). She chose her spacing values as a function of maximum grain diameter: d_{max}, d_{max}/2, and d_{max}/4. The results are listed below in table I.3:

We observe that, in the case of the big shear box, an increase in spacing causes a decrease in cohesion while the value of the angle of friction φ remains constant (Figure I.20). However, the results obtained for the small box do not lead to the same conclusions (Figure I. 21).

Besides, in order to study the spacing between the two semi-boxes, Afriani [2003] conducted a series of tests using the 500 x 500 mm direct shear box on the Fontainebleau sand. The results of these tests are given in the following table:

As is the case for coarse-grained soils, the results obtained show that spacing influences the mechanical characteristics of a homogeneous sand. Larger spacing reduces the value of cohesion, and based on figure I.22, we observe that:

- Cohesion drops from 32.1 to 22.4 when spacing increases from 1 to 6 mm. At 12 mm spacing, cohesion drops to 16,4 kPa;
- The internal angle of friction ranges from 34.8° to 37° as spacing increases from 1 to 6 mm. Beyond these values, spacing ceases to have an effect on the internal angle of friction.

Dry sand has a cohesion value close to zero because the material is pulverulent. Afriani’s tests yield cohesion values different from zero and she justified this result by the fact that the sand was humid. Other authors, like Bakir [1993] and Dubreucq [1999], present similar results on a homogeneous sand and they do not get zero cohesion. The results presented above show that spacing plays an important role in the mechanical behavior of soils, mainly on the value of cohesion.

#### Influence of soil-related parameters

##### Influence of inclusion proportions

The amount of coarse elements also plays an important role in the mechanical behavior of coarse-grained soils. The works of Holtz and Gibbs [1956], Donagh and Torrey [1979], Zhang and Si [1982] and Pedro [2004] show that increasing inclusion proportion induces an increase in shear strength of coarse-grained soils. Several authors represented this parameter as the gravel content, defined as the weight percentage of particles bigger than 5 mm in diameter within the** total soil mass**.

In order to illustrate the research done on the effect of this parameter on the mechanical behavior of coarse-grained soils, we present the works of Holtz and Gibbs [1956]. The latter conducted triaxial tests under drained conditions using cylindrical test tubes with a diameter of 228.6 mm and a height of 571.5 mm. Figure I.23 shows that shear strength is low for the gravel-free sample reaching a maximum value of 50% to decrease to 65% of the gravel content.

However, Figure I.23 also shows that shear strength increases with increasing **gravel content** when it passes from 65% to 82%.

Figure I.24 also shows an increase in shear strength when gravel content increases. However, this strength starts to decrease as gravel content exceeds 65% and moves up to 85 % (Zhang and Si [1982]).

Furthermore, Donagh and Torrey [1979] (Table I. 5) equally observed an increase in the internal angle of friction when gravel content increases. In fact, we can clearly see that the angle of friction moves from 33.9^{0} to 38.8^{0} when gravel content increases from 20% to 60% for material with a d_{max} of 19 mm. For material with a d_{max} of 76 mm, the angle of friction moves from 34.6^{0} to 41.3^{0} when gravel content increases from 20% to 60%.

The effect of inclusion proportion was studied precisely by Pedro [2004]. The latter carried out triaxial tests under a drained condition using cylindrical test tubes with a diameter of 100 mm and of a height of 200 mm. The coarse-grained soil studied was composed of a mixture of Fontainebleau and various proportions of 8/10 mm angular gravel. Volume fraction varies between 0 and 35% (Volume fraction of inclusions f_{v} is the ratio of inclusion volume by total volume).

Pedro [2004] studied the influence of this parameter on the characteristics obtained in the form of small and large deformations. These are the main conclusions:

- The value of the
**Young’s modulus**and the isotropic compression modulus increases with an increase in inclusion proportion (Figure I.25); - Shear strength of the soils increases with an increase in inclusion proportion (Figure I.26).

According to his interpretation, gravels are rigid elements mixed in the matrix, and they contribute to the overall resistance of the test tube to shearing. We then observe that all the research that was carried out on the effect of volume fraction of inclusions shows an increase in the shear strength of coarse-grained soils with the increase in volume fraction of inclusions.

##### Influence of the inclusion dimensions

The study of the effect of inclusion size on the observed behaviors is based on tests of similar coarse-grained soils in which the size of inclusions is changed. Holtz and Gibbs [1956] studied the effect of inclusion size on the mechanical behavior of a reconstructed coarse-grained soil. They devised mixtures containing fixed inclusion proportions (inclusion proportion is represented by the parameter f_{m} which corresponds to the mass fraction of inclusions), but variable inclusion sizes. Table I.6 presents the set of results:

Figure I.27 summarizes the results. We observes that the size of inclusions has a weak influence on failure characteristics of the coarse-grained materials.

Donagh and Torrey [1979)] also studied the effect of inclusion size on the behavior of coarse-grained soils. They conducted triaxial tests using test tubes with a diameter of 381 mm on a natural material of d_{max} = 76 mm and on materials whose upper fraction with d_{max} = 19mm was removed and substituted by materials with d_{max} = 19 mm (enl/sub at 19 mm). Table I.7 presents the results obtained by Donagh and Torry [1979]. Figure I.28 shows that, if we consider the difference of the initial void ratio for the tests, the effect of grain size is weak. In fact, we observe that the values of φ’ are higher by 1° to 3° for the natural material that for the substituted materials.

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