cerun empangan tanah

8/4/2019 cerun empangan tanah

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CONSTRUCTION OF SLOPES

By adopting, the most appropriate construction methods within the overall design the long term

stability and hence the whole-life costs of the slope can be optimized. Construction may be considered

under the following headings:

1. Scaling and trimming2. Blast design3. Benching

Scaling and trimming

Scaling refers to the removal of loose, overhanging or protruding blocks using hand pry bars,

hydraulic splitters or jacks, and explosives. Trimming involves drilling, blasting and scaling to

remove small ragged or protruding rock in overhang areas.

Blasting procedures

The amount of rock mass is broken and loosened during excavation is dependent upon the method

used. The ultimate in damage control is machine excavation. This is used in weak rocks and highly

fractured hard rock. Where the combination of fracture spacing and rock strength preclude machine

excavation, explosives must be used to break the rock. The objective here should be to break up the

rock within excavation and preserve that beyond it in the soundest possible condition.

Blast-hole patterns and powder loads must be properly balanced so that advantage is taken ofthe energy released by the explosive and the desired blast effects are obtained with minimum damage

to the rock. Control of the degree of fragmentation can also facilitate handling the muck and ensure

that the blasted rock is suitable for use as fill.

Benches

The geometry of the benches is governed by the physical and mechanical characteristics of the rock

mass. Bench height should provide a safe and efficient slope and an optimum overall slope angle.

Design of the bench face angle is governed to a large extent by the attitude of unfavourable

structures in the slope and away from problem areas. Berm surfaces should be graded to assist the

collection of water in ditches and also to facilitate general drainage in a direction away from potential

areas of instability.

Benches appear to have no effect on the slope with respect to deep seated failure. Although

shear stress increases with slope height, the direction of the maximum shear stress is supposedly

independent of slope height and bench geometry.

Stepped benches may be used on slopes cut in highly weathered rock material to control

erosion and to establish vegetation. The design objective is that the material weathering from each risewill fill up the step of the bench and finally create the practically uniform overall slope. The steps are


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constructed horizontally to avoid the longitudinal movement of water, which could cause considerable

erosion. Seeding and mulching or other suitable methods of slope stabilization can be readily applied.

However, for rapidly ravelling slopes, about half of the bench or step width should be filled before

seeding is done to prevent smothering of the seed.

STABILIZATION

The stabilization of rock faces can be achieved by a combination of the following methods:

1. By removing water by provision drainage channels and by preventing water from entering therock mass

2. By preventing movement by use of retaining walls, anchors, dowels, gabions, etc3. By providing benching to give access to all sections of the slope for maintenance, protection

from rockfalls, and provision of drainage channels.

Surface and sub-surface drainage

Method that can be applied to improve either the surface or sub-surface drainage conditions and,

hence, increase the stability of the slope should be given high priority in the design of any slope.

Drainage measures, as compared with other possible measures, frequently result in substantialbenefits at significantly lower cost. Often large failures, involving several thousand cubic metres of

material, cannot be controlled within a practical limits by any means other than some form of

drainage.


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Sprayed concrete

Slopes prone to spalling, rockfalls and sliding of small volumes of rock can be stabilized effectively

by spraying the face with concrete. Shotcrete is the term used for a sprayed concrete comprising

mortar with aggregate as large as 20 mm in size whereas the term gunite is used to describe a similar

material that contains smaller aggregate. The shotcrete or gunite is normally applied to an irregularrock surface, the resulting surface configuration is smoother. The sprayed concrete helps to maintain

the adjacent rock blocks in place by means of its bond to the rock and its initial shear and tensile

strength acting as a membrane. The result is that a composite rock-concrete structure is developed on

the surface of the rock. There is no transfer of load from the rock mass to the sprayed concrete lining.

In that the interlocking quality of the surface blocks is improved, the sprayed concrete acts as

reinforcement and not as support. The more quickly the concrete is applied after excavation, the more

effective the results are.

Rock bolts

Rock bolts are generally regarded as a short, low-capacity reinforcement comprising a bar or tube

fixed into the rock and tensioned to a predetermined load. For most rock bolts the free length is fully

bonded using grout or resin soon after the bolt has been stressed. Rock bolt are used to :

Tie together blocks to increase the effective base width in order to prevent toppling or toincrease the resistance to sliding on discontinuity surfaces.

Anchor structures such as retaining walls and catch nets.The reinforcement is usually designed by using the limit equilibrium method of analysis. For the

reinforcement to be effective the bolt or anchor must be fixed beyond the assumed failure surface. The

anchor force required for stability is applied partly as an increased compressive normal stress on the

failure plane, thus increasing frictional resistance, and partly as a force resisting the driving force that


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is causing instability of the slope. The value of the component forces will depend on both design

geometry and orientation of the anchors and the characteristics of the assumed failure surface.

Rock bolts and anchors are often installed in arrays. Varying rock mass conditions within a rock

slope may require rock bolts of different capacities to be used. In weak or soft rocks beams may be

required to spread the load at each anchor/bolt head.

To minimize the loosening affects associated with recently excavated rock slopes, rock bolts shouldbe installed be installed and tensioned as soon as possible after each lift of an excavation and

preferably before the next lift is blasted.


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REFERENCES

Noel Simons, Bruce Menzies And Marcus Matthews, A Short Course In Soil And Rock SlopeEngineering, Thomas Telford

Muni Budhu, Soil Mechanics & Foundations

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