Description
When a differential weathering process occurs due to variations in adjacent rock strata, some rock segments can be destabilized by the erosion of the underneath rock band and a rock protection must be provided to avoid further degradation (Howell, 1999). Dentition consists of applying a covering facing to control the weathering, and degradation of a rock (McMillan et al., 2011). This measure provides a support from toppling for harder blocks with softer rock bands underneath. The material normally used includes mass concrete or shotcrete that may be held into the slope face by short steel dowels. Masonry is sometimes used as dentition to fill cavities in weathered rock cut slopes along roads (Hearn, 2011). Generally, the dentition is built up from the toe of the slope: the bands of soft material are trimmed back, and the resulting slots packed with stockings filled with draining material (gravel) and then faced with rough masonry or reinforced concrete (Fookes & Sweeney, 1976). A grout pipe may be provided for subsequent grouting to ensure good contact between the overhang and the supporting concrete dentition (GEO, 2003).
Advantages
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Appropriate on very steep slopes where erosion or localized weak zone may cause further instability;
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If local stones are used, the impact is reduced with a consequent improvement in the appearance of the treated area, blending in with the natural rock; even if masonry is used, it can be coloured as the colour of the natural rock.
Disadvantages
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Labour demanding;
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If adequate drainage along the facing is not provided, pore water pressures can build up by inducing several instability problems;
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Not suitable if a high band of hard and heavy rock needs to be supported above the treatment.
Design methods
When the weak band is defined, a good rock that shows evidence of much greater resistance to erosion underneath should be excavated for placing the foundation. Dressed stones and cement in a mixture of sand 1:4 or 1:3 are used to build the cement masonry wall following the line of the slope. For sections less than 2 m high, dry stone masonry and well-dressed stones are preferred. In cement-bound masonry, the weep holes should be at least 75 mm in diameter sloping downwards, and they should be installed every 500 mm (either horizontally or vertically); they must be included also in the lowest level of masonry. The bound masonry should not be more than 500 mm thick, with no cavities between the weak rock band and the masonry facing it, to avoid any collapse or preferential way for water flow. A grout pipe may be provided for subsequent grouting to ensure good contact between the overhang and the supporting facing (GEO, 2003). If a support thicker than 500 mm is required, it can be provided by careful dry stone packing behind the masonry wall. When the lower surface of the material to be supported is reached it is important to pack in the stones and mortar very tightly: a solid and tight packing is needed at the last millimetres to avoid that the entire measure will be useless.
When possible, niches, ledges and concavities should be created along the wall to facilitate plants establishment. The use of natural stone is always preferred, and the use of undisguised concrete beams should be avoided. When no natural stones are available, the use of stones with at least a pigment like the colour of the natural stone should be preferred (McMillan et al., 2011). Typical details of dentition works are provided by GEO (2003) (Figure 2).
Functional suitability criteria
Type of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Fall | 9 | Typical measure used when instabilities such as topples, or falls can occur on hard rocks that are placed on to a softer rock band subjected to weathering process |
| Topple | 7 | |
| Slide | 0 | |
| Spread | 0 | |
| Flow | 0 | |
Material type |
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| Descriptor | Rating | Notes |
|---|---|---|
| Earth | 0 | Only suitable for rock slopes |
| Debris | 0 | |
| Rock | 9 | |
Depth of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Surficial (< 0.5 m) | 9 | The masonry wall provides only a protection of the shallowest layers, without any deep support of the rock slope |
| Shallow (0.5 to 3 m) | 6 | |
| Medium (3 to 8 m) | 1 | |
| Deep (8 to 15 m) | 0 | |
| Very deep (> 15 m) | 0 | |
Rate of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Moderate to fast | 2 | If the measure is well constructed, it will protect the rock from shallow instabilities of any rate of movement |
| Slow | 2 | |
| Very slow | 3 | |
| Extremely slow | 4 | |
Ground water conditions |
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| Descriptor | Rating | Notes |
|---|---|---|
| Artesian | 2 | The measure is suitable for any type of ground water conditions; a drainage system is provided for avoiding the pore pressure build up |
| High | 3 | |
| Low | 4 | |
| Absent | 6 | |
Surface water |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Rain | 2 | Not suitable along river banks |
| Snowmelt | 2 | |
| Localized | 2 | |
| Stream | 0 | |
| Torrent | 0 | |
| River | 0 | |
Reliability and feasibility criteria
| Criteria | Rating | Notes |
|---|---|---|
| Reliability | 8 | If construction requests are respected, the measure is reliable for rock protection also in the long term. |
| Feasibility and Manageability | 10 | Simple technique. Potential benefits and limits of applicability are well established |
Urgency and consequence suitability
| Criteria | Rating | Notes |
|---|---|---|
| Timeliness of implementation | 8 | Easily implemented when natural stones are available |
| Environmental suitability | 6 | Low impact to the environment when natural stones combined with vegetation are implemented along the rock slope. The negative impact of the measure with the surrounding environment increases when masonry is preferred to natural stones. |
| Economic suitability (cost) | 6 | Moderate labour cost and equipment. |
References
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Fookes, P. G., & Sweeney, M. (1976). Stabilization and control of local rock falls and degrading rock slopes. Quarterly Journal of Engineering Geology, 9(1), 37-55.
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Hearn, G. J. (Ed.). (2011). Slope engineering for mountain roads. Geological Society of London.
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GEO (Geotechnical Engineering Office), (2003). Guidelines on the use of prescriptive measures for rock cut slopes. GEO REPORT No. 161
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Howell, J. (1999). Roadside bio-engineering. Site handbook. Department of Roads, His Majesty's Government of Nepal.
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McMillan, P., Harber, A. J., & Nettleton, I. M. (2011). Rock engineering guides to good practice: rock slope remedial and maintenance works (No. PPR 555).