Pre-installed cable bolts for two-pass installation roads

Ross Seedsman

Outline • Previous controversial ideas. • Accepted wisdom on cable

bolts. • Ground characteristic and

support reaction. • Case study - Two-pass

longwall installation road. – Application of some

simple tools. • Recommendation - debond.

Top of potential spanning unit

Height of compressive failure

Some 2014 controversies

0 2 4 6Height of softening (m)

0

20

40

60

80

100

Dis

plac

emen

t (m

m)

0 20 40 60 80 100Displacement (mm)

0

2

4

6

Hei

ght o

f sof

teni

ng (m

)1. Preinstalled cables may rupture

2. Roof displacement is the dependent variable

Stress-induced boundary crushing

• Cover page, • Page 254

It should be noted in any case that cablebolts are unlikely to arrest the onset of rock failure under high stress, and may do little to alter the progression of such failure into the rockmass. The objective here is to hold the failed material in place so that the broken rock itself can generate the necessary confinement to reduce the extent of progressive damage and instability. In highly plastic (deformable) rockmasses under high stress, it is also unlikely that cables will be effective in arresting the progression of failure. In addition, in these environments, the induced displacements may be too great for the system to handle and cable strand rupture may be inevitable in pre-installed systems.

Stress shadowing and relaxation Low stresses can pose as much a risk as high stresses in a fractured or jointed rock mass. While even modest stresses across a back or sidewall can serve to clamp the rockmass blocks in place…. gravity will dominate once these stresses decrease. In an elastic model….. Zones of “tension” or zones of near-zero stress in, for example in a stope wall indicate potential problem areas. Zones of relaxation pose additional hazard for cablebolting. … stress decreases across a cable array can seriously impair the bond strength of plain strand cablebolts. Rockmass stiffness is also dependent on confinement in fractured rockmasses and decreases with relaxation. This has a compounded detrimental effect on cable capacity - just when bond strength is needed the most. It is for this reason that plating and the use of modified strand cablebolts are recommended in fractured-destressed rock

The relaxing/debonding problem

Har

d ro

ck

Soft

rock

Hei

ght

Imposed stress normal to borehole wall

Gro

ut

Gro

ut

Ground reaction curve in the Logical Framework

Displacement

Supp

ort p

ress

ure

Preinstalled

Partial debond

Pretensioned, fully grouted

Gravity collapse – suspension – horizontal line

Suspension

Possible application to installation roadways

…..the cablebolts installed normal lo the laminations covering the span area should be designed as stiff reinforcement within the zone of rock equivalent in thickness to a self supporting beam as calculated in this analysis…..Beyond this limit, an optimum cable array should have a more ductile response to allow the beam to deflect a small amount to generate the required compression for stability . Beyond this should be a suitable anchorage length. If the cables can hold the weight of this beam then stability should be assured. This result is usually more efficient than a pure deadload estimate on a relaxing hangingwall (no beam formation)

Case study

• 260 m depth. • In-situ horizontal/vertical stress = 1.8. • Stress-relieving roadway at 14 m

offset. • 11 of 2.1m X grade bolts @ 1m. • 3 of 8m fully grouted 60 tonne cables

on first pass @ 2m. • No movement on the first pass. • 1 additional fully 8m cable @2m on

second pass. • >100 mm surge, 30 mm - 40 mm at

1.8 m horizon on the second pass. • No loads on cable plates. • Loads on some bolt plates. • Loud, sharp bangs in the roof.

Phase2 model suggests failure of the low strength siltstones

2.823

Strength Factor0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

86

42

0

-2 0 2 4 6

3.800

Strength Factor0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

86

42

0

-2 0 2 4 6 8

Stress-relief assisted on the first pass

Stress relief

First pass

Second pass

Possible explanations (#1 & #2)

Contours - either breaking of cables due to strain incompatibility during compressive failure

or de-bonding due to stress relaxation after compressive failure

Cross hatch indicates

suspended load

Hei

ght

Imposed stress normal to borehole wall

Enough debonded length to accommodate displacements

A 0.35 m thick layer of the medium-grained sandstone can explain alot

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8Beam thickness (m)

0.1

1

10

100

Cen

trelin

e de

flect

ion

(mm

)

0.603

1.08

3.34

8.6

16

1.64

0.637

1.63

0.898

3.73

8.75

1.19

5.2 m spans9 m spans

Numbers are stability factors

The ground characteristic of a voussoir beam

0

10

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30

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60

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0 10 20 30 40 50 60 70 80

Supp

ort p

ress

ure

(kPa

)

Deflection (mm)

0.4m thick unit

0.2m thick unit

0.3m thick unit

Cable reaction depends on un-bonded length

INCR

EASI

NG

EN

ERGY

AB

SOPT

ION

Support efficiency when cables loaded eccentrically

First pass Second pass

Ground reaction patterns for installation roadway

Recommendations for cables: Plate and debond

Top of potential spanning unit

Height of compressive failure