Undrained shear strength Su may be correlated with several parameters, including the following:

Standard Penetration Test:

S_u = 6N    (Eq. 1)
where N is the SPT-N₆₀ number

Table 1 — SPT ‘N’ relative to consistency (HB 160-2006)

SPT ‘N’ blows per 300 mm*	Shear strength (kPa)	Term	Abbreviation	Field guide
0–2	<12	Very soft	VS	A finger can be pushed well into the soil with little effort
2–4	12–25	Soft	S	A finger can be pushed into the soil to about 25 mm depth
4–8	25–50	Firm	F	The soil can be penetrated about 5 mm with the thumb
8–15	50–100	Stiff	St	The surface of the soil can be indented with the thumb, but not penetrated
15–30	100–200	Very stiff	VSt	The surface of the soil can be marked by the fingers, but not indented with thumb pressure
>30	>200	Hard	H	The surface of the soil can be marked only with the thumb nail

Dynamic Cone Penetrometer:

DCP results can be used to estimate the consistency of subgrade soils.

Table 2 — Correlation of fine grained soil consistency to DCP results (HB 160-2006)

Consistency	Blows/100 mm
Very soft to Soft	<1
Firm	1–2
Stiff	3–4
Very stiff	5–10
Hard	>10

Atterberg Limits:

In normally consolidated clays, the undrained shear strength s_u to effective overburden pressure p′_o ratio can be determined as:

1. Bjerrum and Simons (1960) provided Eq. (2) as a best fit to their work:

s_u/p′_o = 0.45(I_P)^1/2,   I_P > 0.5     (Eq. 2)

Using the liquidity index I_L, they derived an approximation of

s_u/p′_o = 0.18(I_L)^1/2,   I_L > 0.5     (Eq. 3)

In both of these equations, use the plasticity index I_P and liquidity index I_L as decimal entries.

2. A linear equation for the s_u/p′_o ratio for nc clays was presented earlier in curve form by Skempton and Henkel (1953) which can be approximated from the plots as:

s_u/p′_o = 0.11 + 0.37I_P    (Eq. 4)

In this equation use I_P in decimal.

3. Karlsson and Viberg (1967) suggest

s_u/p′_o = 0.5w_L,   w_L > 0.20     (Eq. 5)

where the liquid limit w_L is a decimal value.