Creep behaviour of structured clays in triaxial stress space: theory and experimental investigation

Figures & data

Figure 1. Curve of void ratio versus log (time) and definition ‘secondary: compression coefficient (adapted from Yin and Feng, 2016).

Figure 2. Schematic diagram of incremental stress and strain ratios.

Figure 3. Stress conditions applied in creep test on a lightly overconsolidated clay (Le et al., 2019, modified from Tavenas et al. 1978).

Figure 4. One-dimensional compression curves of structured and unstructured gypsum–kaolin specimens (Le and Airey 2021).

Table 1. Material properties.

Material	Depth	Specific gravity	Plastic limit [%]	Liquid limit [%]
Gypsum		2.39	Non-plastic	62
Kaolin		2.64	24	55
London Clay Unit B	26.40–28.50	2.75	25	79

Figure 5. One-dimensional compression curves of undisturbed and reconstituted specimens of London Clay.

Table 2. Test summary.

Test no.	Type	Stress ratio	Creep Stage	Creep stress
		$\mathrm{\Delta }q/\mathrm{\Delta }p{ }^{\mathrm{\prime }}\left(\mathrm{d}\mathrm{e}\mathrm{g}\right)$		$p{ }^{\mathrm{\prime }};q\left[kPa\right]$
GK1	Iso-DC	3 (71.5)	1	52; 45
			2	60; 72
GK3	Iso-DC	3 (71.5)	1	62; 62
			2	83; 130
			3	90; 153
GK4	CA-DC	0.5 (26.5)	1	27; 18
LC1 (28.2–28.35 m)	Iso-DC	15.6 (86.3)	1	444.2;12.6
			2	446.4; 62.7
			3	455.5; 198.8
			4	458.1; 248.2
			5	464.2; 346.8
LC2	Iso-DC	6.12 (80.7)	1	450.7; 51.4
(28.35–28.5 m)			2	458.9; 102.8
			3	469.5; 164.0
			4	475.9; 204.9
			5	489.6; 285.9
			6	500.8; 355.0
LC4 (26.4–26.7 m)	Iso-DC	−3.87 (102.5)	1	367.7; 288.1

Figure 6. Effective stress paths for tests on gypsum-kaolin (GK) specimens.

Figure 7. Effective stress paths for tests on London Clay (LC) specimens.

Figure 8. Development of strain for gypsum-kaolin (GK) specimens (a) stress-strain behaviour. (b) time of strain development.

Figure 9. Large stress-strain behaviour of intact London Clay (LC) with intermediate stages of constant stress.

Figure 10. Development of volumetric and shear strain with time for samples of London Clay with time.

Figure 11. Development of strain increment for gypsum kaolin (GK) specimens.

Figure 12. Development of strain increments for LC specimens.

Figure 13. Ratios of incremental creep viscous strain components for GK specimens in $p{ }^{\mathrm{\prime }}:q$ stress space.

Figure 14. Ratios of incremental viscous creep strain components for LC specimens in $p{ }^{\mathrm{\prime }}:q$ stress space.

Figure 15. Typical strain development at constant stress. Development of strain for Sample LC1 at Creep Stage 3: Volumetric (a) strain against time (b) strain-rate against time; Shear (c) strain against time (d) strain-rate against time.

Figure 16. Influence of incremental stress ratio ($\mathrm{\Delta }{\sigma }_v/{\sigma }_v)$ on the settlement time curves (after Leonards and Altschaeffl 1964) with permission from ASCE.

Table

$A$	parametric creep constant
$e$	void ratio
${\sigma }_1^{\prime }$	axial effective stress
${\sigma }_3^{\prime }$	radial effective stress
${\sigma }_v^{\prime }$	vertical effective stress
${\dot{\sigma }}_v$	rate of change of vertical stress
$q$	deviator stress
$K_0$	coefficient of earth pressure at rest
$M$	Critical state stress ratio
$m$	rate of change of strain-rate with logarithmic time
$p^{\prime }$	mean effective stress
$t$	time (linear)
$t_1$	reference time
$t_e$	equivalent time
$t_{EOP}$	time to end of primary
$u$	excess pore pressure
$W$	work (strain energy)
$C_c$	normal compression index
$C_{c\epsilon }$	normal compression index normalized for strain
$C_{c\epsilon }^{\ast }$	instantaneous normal compression index normalized for strain
$C_{\alpha }$	secondary compression index
$C_{\alpha \epsilon }$	secondary compression index normalized for strain
$\overline{\alpha q}$	stress-dependent creep parameter
$\mu$	coefficient of secondary compression
${\rho }_s$	portion of undeveloped secondary compression
$\epsilon$	strain (true)
${\epsilon }_{vol}$	volumetric strain
${\epsilon }_s$	shear strain
${\epsilon }^{vp}$	viscoplastic strain
${\epsilon }_T$	total strain
$\dot{\epsilon }$	strain-rate
$\mathrm{\Omega }$	strain-energy stress function
$\mathrm{\Delta }{\epsilon }_s/\mathrm{\Delta }{\epsilon }_{vol}$	Incremental strain ratio
$\mathrm{\Delta }q/\mathrm{\Delta }{p}^{\prime }$	Incremental stress ratio in $q-p{ }^{\mathrm{\prime }}$ space
$\mathrm{\Delta }{\sigma }_v/{\sigma }_v$	Incremental vertical stress ratio

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