Shallow trenches filled with free-draining material

Category: MODIFYING THE GROUNDWATER REGIME – Deep drainage

Description

Drain trenches are used to stabilize translational or rotational slides which occur typically in highly weathered fine-grained soils, characterized by permeability higher than that of the underlying layer. Tipical layouts of shallow trenches, with main and possibly secondary branches, and typical cross sections are shown in Figs. 1a, b, c, d (Urciuoli, 2008).

Figure 1: Shallow trenches with only main branches: a) Plan, b) Cross section. Shallow trenches, with main and secondary branches: c) Plan, d) Longitudinal section (from Urciuoli, 2008).
Figure 1: Shallow trenches with only main branches: a) Plan, b) Cross section. Shallow trenches, with main and secondary branches: c) Plan, d) Longitudinal section (from Urciuoli, 2008).


Trenches should be excavated deep enough to intercept the regions of positive pore pressures. Shallow trenches can be excavated by means of an excavator up to a depth of approximately 5 m from the ground surface (Fig. 2). The width of the trench is dependent on the type of excavator being used and may vary from 0.5 to 1.0 m. In open areas, trenches can have sloping sides, the gradient of which is based on stability consideration (Fig. 3). Where there is not enough space, trench sides have to be formed to vertical and should be properly supported (Fig. 4). Guidelines on the design of lateral support to excavation are given in many publications, e.g. BS 6031:1981 (BSI 1981). However problems of trench instability can be reduced by opening up trenches in short lengths and backfilling the trench within a short time after excavation. (Urciuoli, 2008). Trenches need to have a high discharge capacity to avoid the saturation of the backfilling material or of the lower portion of it. This can be achieved providing a drainage layer of gravel materials or installing at the bottom of the trench a perforated pipe (with slots on the upper part). The perforated pipe should be wrapped with a geotextile to prevent the clogging of the slots by fine soil particles (Fig. 5). A compacted clay cover should be placed on the top of the trench to prevent ingress of surface water, which should be drained by means of a system of surface drainage network. The impermeable cover should have a minimum thickness of 0.5 m and should be compacted in layers (Fig. 5).

Figure 2: Excavator used for trenches up to a depth of approximately 5 m from the ground surface
Figure 2: Excavator used for trenches up to a depth of approximately 5 m from the ground surface
 Figure 3: Excavation of trenches with sloping sides
 Figure 3: Excavation of trenches with sloping sides
Figure 4: Excavation with vertical sides and supports
Figure 4: Excavation with vertical sides and supports
Figure 5: Scheme of a shallow trench (from Urciuoli, 2008)
Figure 5: Scheme of a shallow trench (from Urciuoli, 2008)

Trenches should be constructed starting from the lowest point in the area to be drained, so that they can drain water during construction. Inspection wells that intercept the trenches should be installed to allow:

  • monitoring of the working condition of the drainage system, possibly by measuring the flow;

  • maintenance, possibly flushing of the perforated pipe.

The reduction of pore water pressure varies along the slope longitudinal section, the maximum decreasing occurs at a distance from the head trench equal to 3-4 times the space along the cross section. Therefore the length of trenches is usually extended 3-4 time the wheelbase outside the slide area.

Figure 6: Examples of shallow trenches. The upper  part of the system is covered by stones in order  to lower the environmental impact
Figure 6: Examples of shallow trenches. The upper  part of the system is covered by stones in order  to lower the environmental impact
Figure 7: Phases of  the construction of a drainage trench using geocomposites (assembly of components).
Caption

 



Design methods

The first step in the design of a drainage system is the determination of the pore pressure change that is required to increase the factor of safety of the slope to the design value. The next step is to design the geometric configuration of drains that will result in the required pore pressure change.

The design of drain trenches can be carried out by using numerical analyses or easily by adopting design charts.

In the first case drainage works is analysed by means of numerical codes (DEM or FEM) and the problem may be solved by taking soil stratigraphy and heterogeneity into account and by assuming climate conditions acting at the upper boundary. The pore pressures calculated along the critical sliding surface should be used in slope stability analysis.

In the second case, non-dimensional charts obtained for homogeneous soil and very simple geometric schemes are used to estimate pore pressure, lowered by drains. Design charts are a general tool: they cannot consider hydraulic conditions at ground surface according to a seasonal trend, which necessarily depends on typical climatic features of the region being considered.

In fact methods of analyzing the stabilization effect of drain trenches commonly available in the literature (e.g., Hutchinson 1977, Desideri et al. 1997) model the groundwater regime as a steady-state phenomenon (seepage), and assume the presence of a film of water at the 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 (Urciuoli, 2008).

The majority of design charts are used to obtain the geometric configuration from the global efficiency of the drainage system, determined as a function of the pore water distribution that guarantees the safety factor chosen by the designer.

The design charts proposed by Urciuoli (2008) are based on steady-state analysis carried out for drains operating in 3D conditions, assuming a film of water fixed at ground surface. For more details about the boundary condition and the domain analysed see D’Acunto & Urciuoli, 2006. The results pointed out are that the lowering of the water table caused by drains is not homogeneous with depth in the drained domain: it depends upon the distance of the examined point from the drain boundaries and especially from the ground surface.

The drainage effect is weaker in the deepest zone of the slope. Because a simplification is required to handle the problem more manageably and to make the design charts, the model of infinite slope (1D) is adopted. According to that, the Author schematized the 3D pore pressure distribution resulting from the action of drains as a 1D distribution (equivalent to 3D distribution as regards its influence on slope stability). Therefore the effect of drainage is evaluated by means of the average efficiency along the sliding surface G, which expresses the difference between the initial and current value of mean pore pressure (at a generic time t), normalized to the initial value: 

 

 

Finally, for the steady-state solution (attained at long term), the function can be used:

 

 

The function plays a key role in designing slope stabilization by drains, because it considers the final distribution of pore pressure, used in the calculation to obtain the desired improvement in slope stability; the effectiveness of drains is correctly analyzed by considering the groundwater regime as a steady-state phenomenon.

In practice,  is calculated, after determining  from slope stability analysis, as the pore pressure that guarantees the safety factor chosen by the designer. From, by means of non-dimensional charts, the designer can determine the geometric characteristics of the drain system.

By using the pore water pressure distribution obtained by numerical analysis and adapting them to equivalent 1-D domain,  the value of  has been calculated for trenches with secondary branches and represented in design charts as a function of the following parameters:

 

H = depth of analysed volume W,

H0 = depth of drain,

D = depth of the plane on which efficiency is evaluated (correspondent to sliding surface),

Ly = longitudinal length of the analysed volume W (in the case of trenches it is the spacing between principal branches of drain trenches),

S = spacing between secondary branches of drain trenches,

i = spacing between horizontal drains,

l2 = length of secondary branches of drain trenches,

l1 = Ly-l2.

 

Four design charts, one for each plane on which the efficiency is evaluated, are reported below.

 



Functional suitability criteria

Type of movement

Descriptor Rating Notes
Fall 0 Drain trenches are often used to stabilize shallow translational slides of large extension.
Topple 0
Slide 7
Spread 1
Flow 4

Material type

Descriptor Rating Notes
Earth 9 Translational slides occur typically in fine-grained soils strongly altered and characterized by permeability much higher than that of the underlying layer.
Debris 7
Rock 1

Depth of movement

Descriptor Rating Notes
Surficial (< 0.5 m) 8 The maximum depth for the shallow drainage system is 5-6 m therefore the best efficiency value is calculated at a depth equal or less than 5-6 m. As a consequence shallow drain trenches are suitable when the depth of slip surface is not deeper than 5-6 m.
Shallow (0.5 to 3 m) 8
Medium (3 to 8 m) 4
Deep (8 to 15 m) 0
Very deep (> 15 m) 0

Rate of movement

Descriptor Rating Notes
Moderate to fast 2 The steady-state condition is attained some time after drainage construction (i.e. in the long term) in fact after drain installation, a transient phenomenon of equalization of pore pressures occurs. Drains are completely effective after a delay; therefore they represent a suitable mitigation method for very slow landslides.
Slow 8
Very slow 8
Extremely slow 8

Ground water conditions

Descriptor Rating Notes
Artesian 3 This system is suitable for shallow freatic water- table.
High 6
Low 3
Absent 0

Surface water

Descriptor Rating Notes
Rain 9 The methods of analyzing the stabilization effect of drains commonly available in the literature assume the presence of a film of water at the ground surface. However in areas where the weather is not very rainy, such as in southern Europe, this assumption underestimates the effects of drains on slope stability. The seasonal variation of rain-infiltration may be taken into account, as they influence the system performance.
Snowmelt 8
Localized 0
Stream 0
Torrent 0
River 0

Reliability and feasibility criteria

Criteria Rating Notes
Reliability 7 The good working depends strongly on the maintenance, possibly by flushing the perforated pipe. However the life-service is long enough.
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 7 Technologies used for excavation are well-kown and long-established and uncertainties are low.
Environmental suitability 4 will be updated
Economic suitability (cost) 7 Lless costly than other types of stabilization works and suitable for a large number of cases, even when structural measures are not effective.

References

  • BSI (1981). “BS: 6031 1981. Code of Practice for Earthworks”. British Standard Institution.

  • Burghignoli A., Desideri A. (1983). “Analisi dei moti di filtrazione indotti dall’esecuzione di scavi e trincee”. Atti XV Conv. Nazionale di Geotecnica, Spoleto, 2 51-56.

  • Burghignoli A., Desideri A. (1986). “Efficienza dei drenaggi”. Atti XVI Conv.Nazionale di Geotecnica, Bologna, 3, 293-298.

  • Burghignoli A., Desideri A. (1987). “On the effectiveness of tubolar drains”. Proc.IX ECSMFE,Dublin, 1, 121-124.

  • Carder D.R., Watts G.R.A., Campton L, Motley S (2008). “Drainage of earthworks slopes”. Published project report PPR341, TRL.

  • 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.

  • 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, 53(10): 1615-1626.

  • D’Esposito N. (2007). “Analisi 3D Dell’efficienza diTtrincee Drenanti Utilizzate per la Stabilizzazione dei Pendii”. Graduate thesis, University of Naples Federico II.

  • Desideri A., Miliziano S., Rampello S. (1997). “Drenaggi a Gravità per la Stabilizzazione dei Pendii”. Hevelius Edizioni, Benevento.

  • Di Maio C., Santagata P., Viggiani C. (1986). “Analisi del processo di consolidazione indotto da un sistema di trincee drenanti”. Atti XVI Conv. Nazionale di Geotecnica, Bologna, 3, 283-289.

  • Di Maio C., Viggiani C. (1987). “Influence of intermittent rainfall on effectiveness of trench drains”. Proc.IX ECSMFE,Dublin, 1, 149-152.

  • 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 16: 131-155.

  • Pellegrino A., Ramondini M., Urciuoli G. (2004). “Regime delle pressioni neutre in un pendio in Argille Varicolori stabilizzato con trincee drenanti”. International Workshop: “Living with landslides: effects on structures and urban settlements. Strategies for risk reduction”, 141-149. Anacapri.

  • Popescu M.E. (2002). “Landslide causal factors and landslide remedial options”. Keynote lecture, in Proceedings of the Third International Conference on Landslides, Slope Stability and Safety of Infra-Structures, Singapore, 61–81.

  • Pun W.K., Urciuoli G. (2008). “Soil nailing and subsurface drainage for slope stabilization”. Keynote paper in 10th International Symposium on Landslides and Engineering Slopes, June 30 ~ July 4, 2008, Xi’an, China.

  • Stanic B. (1984). “Influence of drainage trenches on slope stability”. ASCE, Journal of Geotechnical Engineering, 110, No. 11, 1624-1635.

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