Description
Dowels are short untensioned steel bars inserted and grouted into holes drilled across the potentially unstable block or slab down to the underlaying stable rock; they are usually about 25 mm in diameter, embedded 0.5 to 1.0 m into the sound rock below and spaced about 0.5 to 0.8 m apart. A typical example is shown in Figure 1.
Dowels are generally adopted in situations characterized by:
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presence of isolated potentially unstable blocks or slabs of rock located on an otherwise stable slope of parent rock, with clearly identifiable discontinuities separating the potentially unstable mass from the underlying stable slope;
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situations where the removal of the potentially unstable mass (scaling and trimming) is impractical, for example because it would interfere unacceptably with existing structures or infrastructure.
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situations where geomorphological conditions and/or the presence of structures or infrastructure at the toe of the slope do not allow the installation of passive barriers.
Dowels are installed approximately perpendicular to the sliding surface, to provide additional shear resistance across the potential failure surface. They are used to support blocks or slabs of rock with thicknesses up to 1 to 2 m. They are most effective when there has been no prior movement of the rock so that there is interlock on the potential sliding surface. For blocks or slabs thicker than 1 to 2 m or where there has been previous movement, the required support may be provided more reliably by rock bolting (un-tensioned or tensioned)
When istalled at the toe of the block or slab, dowels are provided by a cap of reinforced concrete which encases the exposed steel; in these cases the concrete shall be in contact with the rock face so that movement and loss of interlock on the potential rupture surface are minimized.
Where the mass to be supported is fractured into blocks which are too small to be dowelled individually and/or rests on material which is not sufficiently competent to provide adequate anchorage to the dowels, the potentially unstable mass may be harnessed by structural netting (or, more rarely, ropes) of adequate stiffness and resistance, anchored by dowels along the edges of the potentially unstable mass. A typical example is shown in Figure 2.
Design methods
Dowels operate on the following basic principles:
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The dowels restrict movement along the potential failure plane so as to preclude possible reductions in available resistance that could arise from loss of interlocking;
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Sufficient additional shear strength is provided by the dowels at small deformation such that, together with the resistance already available (and preserved) along the discontinuity, sufficient overall resistance is provided to guarantee the stability of the potentially unstable mass with an adequate factor of safety.
The objective of the design is to define the number and charateristics of dowels necessary to ensure that the principles decribed above are satisfied.
The additional shear resistance to be provided by the dowels in static and seismic conditions may be evaluated on the basis of planar and/or wedge limit equilibrium analyses of the type amply discussed in fact-sheet 2.0 on “General aspects of mitigation by changes to slope geometry and/or mass distribution”.
The additional shear resistance provided by the dowels with respect to a specific discontinuity and dowel configuration may be evaluated for example as proposed by Panet (1987), taking into account both the shear resistance of the dowel and the additional resistance associated with the tension which is induced in the dowel by dilatancy on the discontinuity and/or by geometrical effects. For simplicity, these additional contributions may be ignored, considering the shear resistance of the dowel alone, especially where the dowels are close to perpendicular to the potential failure surface.
Where dilatancy along the discontinuity and/or geometrical effects are taken into account, the length of embedment in the potentially unstable mass and especially in the underlying stable material must be sufficient to provide adequate longitudinal anchorage to the dowel.
The results of the analyses depend critically on the precise modelling of the geometry of the discontinuities which represent the potential failure surfaces, as well as the shear resistance and dilatancy along the potential failure surface.
In order for the dowels to operate as anticipated, both the potentially unstable mass and the underlying stable material must provide sufficient lateral resistance to the dowel, which is normally the case in competent rock but may need to be verified in highly weathered rock, weak rocks and/or rocks susceptible to weathering.
The design should include careful consideration of access and operating conditions and the associated safety precautions.
Functional suitability criteria
Type of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Fall | 8 | Typically most suitable to prevent sliding of individual blocks; in special circumstances may be used also to prevent rotation/toppling of individual blocks. |
| Topple | 2 | |
| Slide | 0 | |
| Spread | 0 | |
| Flow | 0 | |
Material type |
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| Descriptor | Rating | Notes |
|---|---|---|
| Earth | 0 | Requires both potentially unstable mass and underlying stable material to be competent rock; harnessing when potentially unstable mass is highly fractured. |
| Debris | 0 | |
| Rock | 8 | |
Depth of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Surficial (< 0.5 m) | 8 | Typically suitable for blocks/slabs up to 1 to 2 m depth only. |
| Shallow (0.5 to 3 m) | 2 | |
| Medium (3 to 8 m) | 0 | |
| Deep (8 to 15 m) | 0 | |
| Very deep (> 15 m) | 0 | |
Rate of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Moderate to fast | 0 | Bloks must be stable at time of construction. |
| Slow | 0 | |
| Very slow | 0 | |
| Extremely slow | 8 | |
Ground water conditions |
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| Descriptor | Rating | Notes |
|---|---|---|
| Artesian | 0 | Suitable for all groundwater conditions; ”artesian” not applicable to the type of situation treated by dowels and harnessing. |
| High | 6 | |
| Low | 6 | |
| Absent | 6 | |
Surface water |
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| Descriptor | Rating | Notes |
|---|---|---|
| Rain | 8 | Not practical within or close to water courses. |
| Snowmelt | 8 | |
| Localized | 6 | |
| Stream | 0 | |
| Torrent | 0 | |
| River | 0 | |
Reliability and feasibility criteria
| Criteria | Rating | Notes |
|---|---|---|
| Reliability | 8 | Simple schematization and analysis. Possible pitfalls in the systematic identification of blocks or slabs to be treated. |
| Feasibility and Manageability | 8 | Well established technique, widely used where applicable. Often insufficent attention paid to durability. |
Urgency and consequence suitability
| Criteria | Rating | Notes |
|---|---|---|
| Timeliness of implementation | 6 | Requires access on steep slopes, implying specialist equipment. Works must be planned carefully to avoid exposing workers to rockfall from above. |
| Environmental suitability | 4 | will be updated |
| Economic suitability (cost) | 6 | Typically moderate; access conditions may have a strong impact on cost. |
References
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Panet M. (1987). ”Reinforcement of rock foundations and slopes by active and passive anchors”. Proc. 6th Int. Conf. on Rock Mechanics, Montreal, Balkema, 1411-1420.