Strength parameters can be assigned to soils or rocks based on: • Descriptions of the strength in the soil profile.
For the purposes of shear strength evaluation soils have been divided into two broad categories :
• Cohesionless soils (sands, silty sands, slightly clayey sands) for which the shear strength is assumed to be represented by drained conditions in terms of the angle of shearing resistance (φ').
• Cohesive soils (clays, silty clays, sandy clays etc.) for which the strength can be defined in terms of undrained (φ' = 0 and Cu equal to a finite value) and drained shear strength (φ' equal to a finite value and C' = 0 or a finite value).
In-situ Description of Soil Consistency
The in-situ description of soil consistency can be used to obtain a rough estimate of φ' of a cohesionless soil and of the undrained cohesion (Cu) of cohesive soil. The undrained cohesion (Cu) is equal to half the unconfined compressive strength (UCS). It is recommended that consistency be described in accordance with Jennings et al (1973). Guidelines in this regard are given in Tables 3.3.4 and 3.3.5 for cohesive and non-cohesive soil respectively.
Table 3.3.4 -Shear strength parameters for slow draining cohesive materials Consistency Rule of thumb Field identification Unconfined
Compressive Strength (kN/m²) Approximate SPT (N)
S.l V. Soft Easily moulded by fingers. Distinct heel marks left on freshly exposed surface (Heel mark = approx: 150 kN/m2). Geologists pick can be easily pushed in up to its handle
< 40 < 2
S.2 Soft Easily penetrated with thumb. Moulded with strong pressure. Feint heel marks on freshly exposed surface. Geologist's pick can be pushed in up to 30 to 40mm (sharp end).
40 to 80 2 to 4
S.3 Firm Indent by thumb with effort. Very difficult to mould with fingers. Geologists pick (sharp end) can be pushed in up to 10mm. Slight penetration with hand spade
80 to 160 4 to 8
S.4 Stiff Penetration by thumb nail. Cannot be moulded with fingers. Geologists pick (sharp end) makes slight indentation when pushed. Hand pick required for
Table 3.3.5 -Shear strength parameter for quick draining non-cohesive materials Consistency Rule of thumb Field
identification Approx. CPT(MPa) Approx SPT. (N) Approx. φ' Typical Dry Density (kN/m3)
Very Loose Almost no resistance to
shoveling 0 to 2 0 to 5 26 to 28 < 14.5 Loose Easily penetrated with
12mm bar pushed by hand. Small resistance to
shoveling
2 to 4 5 to 10 28 to 30 14.5 to 16.0 Med Dense Easily penetrated with 12
mm bar driven with 2kg hammer. Considerable resistance to shovelling
4 to 9 10 to 30 30 to 35 16.0 to 17.5 Dense Hard penetration with 12
mrn bar to 300mrn driven with 2kg hammer. Hand pick required for
excavation
9to12.5 30 to 50 35to40 17.5 to 19.5
Very Dense Penetration only up to 75mm with 12mm bar driven with 2kg hammer. Power tools required for excavation
> 12.5 > 50 40 to 50 > 19.5
Index Property Tests
Many researches have published typical ranges and correlations between the results of index property tests (Atterberg Limits and Particle size distribution) and the effective shear strength of soils (mainly cohesive soils with plasticity index greater than 7). The following procedures are recommended:
• The effective angle of friction can be obtained using the relationship given by Kenney (1959). This relationship is given in Figure 3.3.4. Although this relationship is for normally consolidated soils the effective angle of friction should not be much different for over consolidated soils.
Figure 3.3.4- Plasticity index vs sin φ' after Kenney (1959)
Table 3.3.6- after NAVFAC DM7 (1971)
Typical strength characteristics Group
Symbol Soil Type Max γd OptimumMoisture (%) Cu (kPa) C' (kPa) φ' (deg.) tan φ' GW Well-graded clean gravels,
gravel-sand mixtures 19.7-21.2 11-8 0 0 >38 >0.78 GC Clayey gravels, poorly
graded gravel-sand-clay 18.1- 20.5 14- 9 0 0 >31 >0.6 SM Silty sands, poorly graded
sand-silt mix
17.3-19.7 16-11 50 5 34 0.67 SC Clayey sands poorly graded
sand-clays
16.5-19.7 19-11 75 10 31 0.6 CL Inorganic clays of low to
medium plasticity
15.0- 18.9 24- 12 85 12 28 0.54 ML Inorganic silts and clayey
silts
Shear Strength Parameters from In-situ and Penetration Tests
Empirical relationships between soil shear strength (φ' for non-cohesive and Cu for cohesive) and penetration or in-situ test values have been put forward by many authors in different parts of the world.
For non-cohesive soils two approaches have been adopted where relationships have been developed (i) dependent on vertical effective stress and (ii) independent of vertical effective stress. Figures 3.3.5 (a) and (b) give values of φ' independent of Po' while Figures 3.3.6 (a) and (b) show values of φ' dependant on Po' for both SPT and CPT tests.
(a) (b)
Figure 3.3.5- φ' Independent of Vertical Stress after (a) Peck et al (1974) and (b) Kahl et al (1968)
The undrained cohesion Cu for sensitive and normally consolidated clays has been studied by several authors and correlations with SPT results show a wide scatter as outlined by De Mello (1973) and Navfac DM7 (1971) and shown in Figures 3.3.7 (a) and (b) below. If accurate values are required for design, direct correlation should be obtained during the geotechnical investigation or measured on undisturbed samples in the laboratory.
(a) (b)
Figure 3.3.7- Cu vs N correlation for soft sensitive clays after (a) De Mello (1973) and
(b) Navfac DM7 (1971)
Stroud (1974) gives correlations of SPT vs undrained shear strength for stiff insensitive clays which are considered to be applicable to a wide range of residual and transported clay soils in Southern Africa. These are shown in Figure 3.3.8.
Undrained shear strength correlations to CPT cone resistance values (qc) for normally and overconsolidated clays are well covered in the literature and, like the SPT test, a reasonably wide scatter is evident and is dependant on whether the clay is sensitive, normally consolidated or overconsolidated. The equation qc = Nk Cu + σvo governs the relationship of qc with undrained shear strength. For normally consolidated clay Nk ≈ 15 while for sensitive clays Nk can be as low as 5.
Undrained shear strength correlations to the Dynamic Probe Light (DPL) test (outlined in SECTION 2.2.3) have been given by Brink et al (1982) and are summarised in Table 3.3.7.
Table 3.3.7 -Undrained shear strength correlations with DPL test after Brink et al (1982)
Sandy Materials
Description SPT N (blows per 300mm) Dynamic Probe Light (DPL) (mm per blow) Very loose < 5 > 75 Loose 5- 10 30- 75 Medium dense 10- 30 12.5- 30 Dense 30 –50 5 -12.5 Very dense > 50 2- 5 Clayey Materials Very soft < 2 > 110 Soft 2-4 55-110 Firm 4 –8 30- 55 Stiff 8-15 15-30 Very stiff 15- 30 7- 15