Rock Mechanics- Ch 5

Front 1

What is initial stresses ?

Back 1

Initial stresses, the stresses that exist without the influence of excavation, including tectonic stress andgravity stress.

When choosing the orientation for a cavern , we hope to avoid aligning the long dimension perpendicular to the greatest principal stress

* Vertical

* Horizontal

Front 2

Vertical stress

Back 2

The average vertical stress must equilibrate the downward force of the weight of overlying rock

Front 3

Ka, K, Kp,

Back 3

Ka corresponds to conditions for normal faulting, in which the vertical stress ismajor principal stress and failure is by horizontal tension,

Kp corresponds to conditions for reverse faulting, in which the vertical stress isminor principal stress and failure is by horizontal compression,

qu is the unconfined compressive strength

Ka<K<Kp

Front 4

Horizontal stress direction

Back 4

For normal faulting σ1 is vertical and σ3 ishorizontal and perpendicular to the fault trace.

For reverse faulting σ3 is vertical and σ1is horizontal andperpendicular to the fault trace.

For strike-slip faults σ1 is horizontal and inclined about 30° with the fault trace, closewise or counterclockwise as dictated by the sense of motion on the fault

Front 5

What’s the most used techniques to get initial stresses?

Back 5

Hydraulic fracturing ,

Flat jack method, Overcoring

Hint 5

H F O

Front 6

Characteristics of initial stress

Back 6

1. it’s a relatively stable and unstable stress field that varies with time and space.

2. The measured vertical stress is basically equal to the weight of the overburden.

3. The shallow horizontal stress is generally greater than the vertical stress

4. The ratio of horizontal stress to vertical stress decreases with increasingdepth.

5. The relationship between the maximum horizontal principal stress and the minimum horizontal principal stress also increases linearly with depth

6. The ratio of the minimum horizontal principal stress and the maximumhorizontal principal stress generally differ greatly, showing a strongdirectivity

7. The ground stress is obviously affected by the disturbance of topography,rock mass structure and fault.

Hint 6

R M S R^2 R G

Front 7

High ground stress criterion

Back 7

Only when the ratio of the maximum in-situ stress to the strength of surrounding rock reaches a certain level can it be called high in-situ stress or extremely high in-situ stress. (commonly used method to judge )

Front 8

High ground stress phenomenon

Back 8

Core pancake phenomenon.

Rock burst.

The wall of the underground tunnel is disjointed,

Uplift of rock foundation pit bottom.

Hint 8

C R W U

Front 9

The necessity of studying the problem of high ground stress

Back 9

1.Compared with other disciplines, the biggest difference of rockmechanics lies in the existence of initial ground stress.

2. High ground stress is an important factor affecting project safety inhydropower project development and mine energy exploitation