3. Tests and Procedures
Using the ring of 31.5 radius and 20 mm height for the S2 and S3 soils, they were used for several series of oedometric tests under a K0 condition with null radial strain on saturated specimens. We conducted a test using horizontal orientation for the principal stress (S3I-H) and the other was done in an unsaturated way until it reached an increment of 191 kPa and from now on saturated until the end of the test (S3I-V: Dry Sat), to verify the capacity of collapsibility. In this oedometer study the initial stress stage were 1.2 kPa and the maximum vertical effective stress were 3067 kPa. The subsequent stage was the double of the previous stress stage during 24 h. The first effective vertical stress has to contemplate the development of structural stress (bonding) yielding, which is normally low.
The groups of specimens for oedometer tests were prepared with similar physical parameters. The samples of S2 and S3 soils were obtained from intact (I) soil of embankment and the other remolded (R). The remolded specimens were compacted in a split mold to minimize any distortion, using water contents and dry bulk density approximately equal to natural moisture [
6]. Their physical properties are represented on
Table 2.
Several series of consolidated undrained triaxial compression tests (CU) with pore water pressure and axial strain measurements, were carried out on dump clay. All remolded samples (S3R) were compacted using the same effort. Their physical properties and testing conditions are represented on the
Table 3.
The triaxial set on the cell were saturated, when the reference B is higher than 95% (B = Δu/Δσ3: Δu, pore pressure variation and Δσ3, cell pressure variation) and consolidated for several isotropic consolidation stress or different initial average effective stress. The saturation of each specimen was ensured by water flow followed by application of back pressure. After saturation and isotropic consolidation for different initial average effective stress, CU compression tests were carried out at an axial compression velocity of 0.003 mm/s.
4. Results and Discussion
It is noticeable from oedometric tests an increase of virtual preconsolidation stress for the soils influenced by artificial lateral heart stress (S3I-H: ) and the developed suction for dry-saturated test (S3I-V: Dry_Sat: ). The S3I-V sample, has a preconsolidation stress less than S2I-V , for saturated tests, and explained to be at a lower depth. The virtually intact specimen (S2I-V) and the remolded specimen (S2R-V) have the same preconsolidation stress . This is explained by destructuration processes of the original soil in the construction of the landfill. For this reason, it is preferable to call specimen virtually intact. As for the S3I-V soil, the increase of virtual preconsolidation stress doesn´t has a relation with the proportion of the coal.
The stress-strain-strength performance of soils is described by a set of parameters that are defined in the failure moment. Corresponding parameters were used to the maximum value of shear stress [
], to axial deformation exceeding 10%, to estimate the envelope corresponding to the maximum strength. The values obtained in the failure of specimens for axial deformation exceeding 20% were also considered, to estimate the envelope in its ultimate limit state [
]. This criterion was displayed as fitting the limitation of axial strain allowed by the classical triaxial test.
Figure 3 shows the failure envelope for the maximum shear stress (t
peak) and the ultimate limit state or residual state (t
ult) in the shear stress versus mean effective stress (s’-t) space, for the stress levels used.
In the
Table 4, data, and strength parameters for S1_CZ sample tested are presented. Clay with low plasticity is deposited artificially and at the depth of the sample (49.1 to 66.4 m), the increase in vertical stress will lead to over-consolidation. This is verified in the cohesion that the triaxial test reveal. The increase in mean effective stress during shear, reduces this strength effect.
For the critical level line, a single line is obtained of the type q = мp’ (q
cs = 0.552p’ + 28.269: ϕ’
cs = 14°) in the q:p’ (deviator stress: average effective stress) space that does not pass through the origin since it doesn’t has reached the critical state to the axial strain of 20%. The detailed pieces of information (results and questions) about behavior of specimens were described in the following study [
9]. Yet stress path for p’
0 = 350 kPa reached the structural collapse for extensions close to the maximum stress strength. It is striking that the lines of failure points in the ultimate state (see
Figure 4a) look analogous for the two samples and it is useful to compare these directly. The Isotropic State Line (ISL: ν
CSL = −0.0008ln(p’) + 1.816: R
2 = 0.8785) in ν:lnp’ (specific volume: mean effective stress) space (see
Figure 4b) does not seem to be parallel to the critical state line determined (CSL: ν
CSL = −0.0017ln(p’) + 2.007: R
2 = 0.8785) and these two lines tend to converge at higher stresses. With the rising of mean effective stress, the particles will start crashing which will imply that the dilatancy state parameter (ψ), would probably decrease. We can define a compressible zone and softening by deformation below a zone of instability in relation to liquefaction.
The static liquefaction is true liquefaction, or a high softening deformation is associated with an extreme reduction of the peak strength imbrication for the critical state (
Figure 4a). These peaks have been related to a hypothetical collapse, corresponding to the meta-stable rearrangement of particles. This is the normal shear strength silty/clay behavior of increasing liquefaction potential with an increase of confining pressure. The liquefaction potential index (LPI) is defined by Charles et al. (2004) as:
where q
max is maximum deviator stress (peak) and q
min is minimum deviator stress (quasi steady state). The results of LPI for the CU tests are show in
Table 5.
In stable material or to low confinement stress, both deviator stress [qmax = (σ1 − σ3)peak and qmin = (σ1 − σ3)residual] correspond to the same ultimate shear strength, so the LPI value is zero. The material is more instable when qmax and qmin become different and LPI value increases.
The effective internal frictional angle for peak strength (ϕ’peak) of the clean sample (25°) is basically equal to the soil contaminated with coal and is associated with the development of cohesion effect on the strength.
The cohesion effective or apparent of samples (S2 and S3) may be due to the connections established between fine particles. The cohesion effective explained by the transformation of the material and the connections established.
Table 6 presents the values of mechanical parameters, effective internal frictional angle and cohesion (ϕ’, c’) in terms of (σ
1–σ
3)
peak and (σ
1–σ
3)
residual failure criteria, of the clean samples (S3) and contaminated sample (S2).