Barrettes (diaphragm walls)

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Description

Barretts (diaphragm wall elements) used for mechanical stabilization of landslides are typically 800 to 1200 mm in thickness and 2000 to 3000 mm in length, matching the size of the equipment used (Leoni and Manassero, 2003). If necessary, multiple panels can be excavated and cast jointly, to form special shapes, such as Tee, or to make longer panels typically up to almost three times the standard panel length, although. They can be placed in earth and debris slopes, typically at a maximum centre to centre spacing of twice the thickness, with the longitudinal axis aligned with the direction of movement, to form specific alignements across the direction of movement at strategic positions within the landslide (Ito et al., 1982; Hassiotis and Chameau, 1984; Soric and Kleiner, 1986; Popescu, 1991; Reese et al., 1992; Polysou et al., 1998; Poulos, 1999)

Construction of the barrettes involves three main stages:

1. Formation of guide walls defining the proposed shape and location of the barrette;

2. Excavation, typically by means of  rope or kelly operated clam shells grabs or by hydromills, depending on the nature of the ground to be excavated; a suitable drilling fluid, typically bentonitic mud or similar, is used to support the sides of the excavation; the drilling fluid is also essential to transport the cuttings in reverse circulation when using hydromills.

3. Backfilling with reinforced concrete; after cleaning the hole, for example by forced circulation of the drilling fluid with a high pressure, high capacity pump, the reinforcement cage is installed and concreting proceeds from the base upwords using a tremie pipe, to displace the drilling fluid, which is recovered to temporary storage for reuse in the next barrette.

The advantages of this technique may be summarized as follows:

• applicable in a variety of topographical conditions, subject to access constraints;

• applicable in relatively deep landslides (up to 15÷20 m deep) where other techniques may prove inadequate;

• conventional equipment may overcome thin layers of rock; hydromills can be used to cut into rock;

The heads of the barrettes are usually completed by a capping beam  to allow:

• redistribution of horizontal loads between barrettes;

• the installation of anchors, where required to improve the overall resistance of the structure;

• the installation of sub-horizontal drains, where required.

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Design methods

The design load on the barrettes may be determined in 2D limit equilibrium analyses by calculating the reaction on the vertical section corresponding to the barrettes which is necessary to guarantee, with the appropriate factor of safety, the stability of the portion of the slide located upslope of the barrettes in the absence of the downslope portion; in any case, the load on the barrettes cannot exceed passive soil pressure

The contribution of the downslope portion can be considered only  if this portion remains stable with an appropriate factor of safety once the driving force from the upper portion is removed; even in this case, it may be prudent to consider this mass only as confinement for the stable soil below, since even very small deformation such as shrinkage in a dry season may be sufficient to reduce or completely remove downslope support to the barrettes.

The design loads and the stability of the downslope portion in seismic conditions are normally determined from pseudostatic limit equilibrium analyses, taking into account the excess pore pressures that may develop in the slope, where applicable.

Once the net actions imposed by the landslide on the barrettes are known, a suitable soil-structure interaction analysis is carried out by an appropriate method to determine both the reactions in the stable soil into which the barrettes are anchored and the effects of actions on them.   

The spacing between barrettes must be determined balancing:

• economy and the need to avoid interference between adjacent piles during construction and/or with natural drainage;

• the need to ensure that soil arching develops between adjacent barrettes and that the soil does not “flow” between them.

The check that soil arching develops between adjacent barrettes and that the soil does not “flow” through them can be done by means of  analytical (simplified) tools (see for example Ito and Matsui, 1975) or 3D numerical analysis.

Provided soil arching is guaranteed, plain strain 2D soil-structure interaction analysis is representative of actual conditions, with the effects of actions on each barrette being those derived from the 2D analyses, multiplied by the centre to centre spacing of the barrettes. The same analysis may be used to determine their optimal length and the benefit of additional anchors, if used.

The calculation of the barrettes capacity in relation to the soil/structure interaction may be carried out according to several approaches and simplified methods (De Beer, 1977; Viggiani, 1981; Hassiotis and Chameau, 1984; Cantoni et al, 1989; Pearlman and Withiam, 1992).

Finite elemnt methods may be used instead to provide a simultaneous and consistent estimate of the soil-structure interaction both with the sliding mass and with the underlying stable soil. Finite element analyses in the time domain can also be used to refine the evaluation of the performance of the structure under seismic conditions.

The mechanical charateristics of the barrettes must be adequate to sustain the actions and the effects of actions on them. The structural checks must satisfy all applicable codes and standards on the subject.

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Functional suitability criteria

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Reliability and feasibility criteria

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Urgency and consequence suitability

References

• De Beer E.E. (1977). ”State-of-the-Art report: Piles subjected to static lateral loading”. Proc. of IX ICSMFE, Tokyo, Special Session 10, 547-553.

• Hassiotis S., Chameau J.L. (1984). ”Stabilization of slopes using piles: Final Report, Joint Highway Project, Indiana Department of Highways & U.S. Department of Transportation Federal Highway Administration & Purdue University, Purdue University, Interim Report.

• Hassiotis S., Chameau J.L., Gunaratne M. (1984). ”Design method for stabilization of slopes with piles”. Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol. 123, n° 4, 314-323.

• Ito T., Matsui T. (1975). ”Method to estimate lateral force acting on stabilizing piles”. Soils and Foundations, vol.15, n° 4, 43-59.

• Ito T., Matsui T., Hong W.P. (1982). ”Extended design method for multi-row stabilizing piles against landslide”. Soils and Foundations, vol.22, n° 1, 11-13.

• Pearlman S., Withiam J.L. (1992). ”Slope stabilization using in situ earth reinforcements” ASCE Conference, Berkeley.

• Polysou N.C., Coulter T.S., Sobkowicz J.C. (1998). ”Design, construction and performance of a pile wall stabilizing a landslide”. Proc. of 51th Canadian Geotechnical Conference.

• Popescu M.E. (1991). ”Landslide control by means of a row of piles”. Slope Stability Engineering, Thomas Telford, 389-394.

• Poulos H.G. (1999). ”Design of slope stabilizing piles”. The University of Sydney, Department of Civil Engineering – Centre of Geotechnical Research, May.

• Reese L.C., Wang S.T., Fouse J.L. (1992). ”Use of drilled shafts in stabilizing a slope”. Proc. of a Specialty Conference on Stability and Performance of Slopes and Embankments, Berkeley, California, R.B. Seed and Boulanger R.W. editors, Geotechnical Special Publication 31, ASCE,.1318-1332.

• Soric I., Kleiner I. (1986). ”Stabilization of a landslide with anchored bored piles”. Proc. of 8th Danube-European Conf. on SMFE, 341-348.

• Viggiani C. (1981). ”Ultimate lateral load on piles to stabilize landslides”. Proc. of X ICSMFE, Stockholm, 3, 555-560.

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