Description
The schematic map of deep trenches, with main and possibly secondary branches, and typical cross sections are shown in Figs. 1a,b,c,d (Pun & Urciuoli, 2008). Essentially only the technology to excavate the trench is different from that used for shallow trenches. Deep trenches can reach the maximum depth of 30 m and are excavated by means of grab shells (Fig. 2). The sides of the trenches, being vertical, should be supported by means of slurry, e.g. polymeric mud (Fig. 3), therefore costs increase very much respect to shallow trenches.
c) Plan, d) Longitudinal section.(from Urciuoli, 2008)
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Because of the high depth, it is difficult to provide the good laying of the discharge pipe; moreover the volume of materials necessary to fill trenches is very large. Therefore new technologies are continuously advancing, for example drainage cage may be dropped directly inside the trenches. Two new types of technologies adopted conveniently for deep trenches are described below:
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Narrow trench fitted with a draining geocomposite with a high capacity collection surface, buried with slightly compacted excavated soil. This is a geocomposite consisting of a draining core combined with two geotextile filters with a socket at the base for fitting the drainage tube. The features of this system are: excellent filtering, constant hydraulic efficiency, good excavation volume and no soil to dispose of, total or drastic reduction of inert materials, higher output and extra safety in the yard. All these features make draining with this system an innovative technique compared to traditional systems. Vertically continuous draining is possible for deep trenches by combining this system with suitable draining composites by securing them and superimposing them by means of suitable, simple measures (Figs. 3a, b).
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Deep trenches can be carried out as panels constituted by ‘’aerated concrete’’: gravel with high permeability
(10-1 m/s) and cement with a good compression strength (Fig. 4). The technology used is that used for diaphragms, therefore any depth can be reached. The panels usually have the plan dimensions: 0,8-1m x 2,5-3m; first the odd-numbered ones are constructed. This system characterized by ‘aerated concrete’ can be realized as secant piles as well (Fig. 4), but the previous technique is faster.
About the maintenance and monitoring, the same consideration for the shallow drain system can be made.
Design methods
The design criteria of deep trenches are the same as adopted for the shallow trenches. Numerical analyses can be carried out or, more easily, design charts can be used (see fact sheet 4.1).
Functional suitability criteria
Type of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Fall | 0 | Deep drainage is used to stabilize translational slides of large extension or rotational slides characterized by a deep slip surface. |
| Topple | 0 | |
| Slide | 8 | |
| Spread | 2 | |
| Flow | 5 | |
Material type |
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| Descriptor | Rating | Notes |
|---|---|---|
| Earth | 9 | Translational slides occur typically in fine-grained soils strongly altered and characterized by a permeability much higher than that of the underlying layer. |
| Debris | 7 | |
| Rock | 3 | |
Depth of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Surficial (< 0.5 m) | 6 | The maximum depth for deep drainage system is 20-25m; therefore the best efficiency value is calculated at a depth less than that. As a consequence the depth of slip surface should not be more than 15-20m. |
| Shallow (0.5 to 3 m) | 7 | |
| Medium (3 to 8 m) | 7 | |
| Deep (8 to 15 m) | 5 | |
| Very deep (> 15 m) | 2 | |
Rate of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Moderate to fast | 4 | The steady-state condition is attained some time after drainage construction (i.e. at the long term) in fact after drain installation, a transient phenomenon of equalization of pore pressures occurs. The drains are completely effective after such a delay and they represent the suitable mitigation method for very slow landslides. |
| Slow | 7 | |
| Very slow | 8 | |
| Extremely slow | 9 | |
Ground water conditions |
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| Descriptor | Rating | Notes |
|---|---|---|
| Artesian | 4 | This system is suitable for lower shallow freatic water table. |
| High | 8 | |
| Low | 4 | |
| Absent | 0 | |
Surface water |
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| Descriptor | Rating | Notes |
|---|---|---|
| Rain | 7 | The methods of analyzing the stabilization effect of drains commonly available in literature assume the presence of a film of water at ground surface. In areas where the weather is not very rainy, such as in Southern Europe, this assumption underestimates the effects of drains on slope stability. However the rain –water infiltration influences the system performance less than in the case in which shallow trenches are adopted. |
| Snowmelt | 5 | |
| Localized | 1 | |
| Stream | 1 | |
| Torrent | 0 | |
| River | 0 | |
Reliability and feasibility criteria
| Criteria | Rating | Notes |
|---|---|---|
| Reliability | 7 | The good working of drains depends strongly on the maintenance, possibly flushing of the perforated pipe. However the service is enough long. |
| Feasibility and Manageability | 8 | Technique and design process are well established and widely used in suitable conditions. |
Urgency and consequence suitability
| Criteria | Rating | Notes |
|---|---|---|
| Timeliness of implementation | 6 | Some uncertainties about good construction of the system can exist because of the high depth to reach and the large spil volume involved. |
| Environmental suitability | 4 | will be updated |
| Economic suitability (cost) | 6 | Deep drains are more costly than the surface drains because of the deep excavation and the large soil volume involved. |
References
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BS: 6031 1981. Code of Practice for Earthworks. British Standard Institution.
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Carder D.R., Watts G.R.A, Campton L, Motley S, (2008). “Drainage of earthworks slopes”. Published project report PPR341, TRL.
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D’Acunto B., Urciuoli G. (2006). “Groundwater regime in a slope stabilised by drain trenches. Mathematical and Computer Modelling. Pergamon-Elsevier Science LTD, 43(7-8). 754-765.
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D’Acunto B., Parente F., Urciuoli G. (2007). “Numerical models for 2D free boundary analysis of groundwater in slopes stabilized by drain trenches”. Computers & Mathematics with applications, Vol. 53(10), 1615-1626.
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D’Esposito N. (2007). “Analisi 3D Dell’efficienza di trincee Drenanti Utilizzate per la Stabilizzazione dei Pendii. Graduate thesis, University of Naples Federico II.
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Desideri A., Miliziano S., Rampello S. (1997). “Drenaggi a Gravità per la Stabilizzazione dei Pendii”. Hevelius Edizioni, Benevento.
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Di Maio C., Santagata P., Viggiani C. (1986). “Analisi del processo di consolidazione indotto da un sistema di trincee drenanti”. In: Atti XVI Conv. Nazionale di Geotecnica, Bologna, 3, 283-289.
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Hutchinson, J.N. (1977). “Assessment of the effectiveness of corrective measures in relation to geological conditions and types of slope movement (General Report)”. Bulletin of the Int. Association of Engineering Geology Vol. 16, 131-155.
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Italdreni, Divisione di Greenvision ambiente S.p.A. Trincee drenanti con il sistema “Twindrain” Prodotti geosintetici per il drenaggio dei terreni, Documentazione tecnica.
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Masciarelli G.(2006). “Sistemi drenanti superficiali per la stabilizzazione di un versante”. A.Di.S.Convegno: L’innovazione tecnologica nella difesa del suoloPotenza –16 Marzo 2006 .
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Pun W.K., Urciuoli G. (2008). “Keynote paper: Soil nailing and subsurface drainage for slope stabilization”, In: 10th International symposium on landslides and engineered slopes, June 30 - July 4, 2008, Xi’an, China.
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Stanic B. (1984). Influence of drainage trenches on slope stability. ASCE, Journal of Geotechnical Engineering, Vol. 110 (11), 1624-1635.
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Ziccarelli M. (2010). “Stabilizzazione dei Pendii mediante drenaggi”. Rende, 26 May 2010.