Geotechnical Engineering

DHARM

284 GEOTECHNICAL ENGINEERING

+t

o

4

1

2

3

Loading (s 3 constant)

Unloading (s 1 constant)

A

Loading (s 1 constant)

Un loading (s 3 constant)

Failure envelope

ss1v=

ss3h=

s 1

s 3

Vertical compression

(>)ss 13

ss1v=

ss3h=

s 1

s 3

Vertical extension

(<)ss 13

s

Failure envelope

• t

Fig. 8.24 Typical stress paths for triaxial compression and extension tests (loading/unloading)
A-1 is the effective stress path for conventional triaxial compression test during load-
ing. (∆σv = positive and ∆σh = 0, i.e., σh is constant). A typical field case is a footing subjected to
vertical loading.
A-2 is the unloading case of the triaxial extension text (∆σh = 0 and ∆σv = negative).
Foundation excavation is a typical field example.
A-3 is the loading case of the triaxial extension test (∆σv = 0 and ∆σh = positive). Passive
earth resistance (Ch. 13) is represented by this stress path.
A-4 is the unloading case of the triaxial compression test (∆σu = 0 and ∆σh = negative).
Active earth pressure on retaining walls (Ch. 13) is the typical field example for this stress
path.

Figure 8.25 shows the typical stress paths for a drained test. Point A corresponds to the
stress condition with only the confining pressure acting (σ 1 = σ 3 and τ = 0). Point F represents
failure. Stress paths for effective stresses, total stresses, and total stresses less static pore
water pressure are shown separately in the same figure.

t

o

Failure envelope

F

AB

TSP

uo ss,

TSSP

ESP

Fig. 8.25 Stress paths for drained test

[Note : TSP to the right of ESP indicates a positive pare water pressure.]