Piles

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Description

Piles can be placed in earth and debris slopes, either at regular 2D spacing over the whole slide or portion thereof, to act as isolated dowels, or, more commonly, at close spacing along one or more specific alignments to form piled walls across the direction of movement (Ito et al., 1982; Hassiotis and Chameau, 1984; Soric and Kleiner, 1986;  Popescu, 1991; Reese et al., 1992; Polysou et al., 1998; Poulos, 1999) - Figure 1. 

Typically, large diameter bored cast-in-situ piles are used, with diameter 800 to 2000 (most frequently 1200) mm and spacing 1.2 to 2 times the pile diameter. The advantages of this technique may be summarized as follows:

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

• casings limit hole instability during construction and damage to green concrete in piles formed in moving slides;

• conventional equipment may overcome thin layers of rock.

Where access is difficult and/or the depth of sliding is modest, micropiles (200 to 300 mm diameter) are also used, normally reinforced by steel pipes to maximize bending and shear resistance of the micropiles.

Pile heads are usually completed by a capping beam  to allow:

• redistribution of horizontal loads between piles;

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

• the installation of sub-horizontal drains, where required to reduce the thrust on the wall.

Examples of applications are provided by Wilson (1970), Palladino and Peck (1972), Nethero (1982), Oackland and Chameau (1984), Isenhower et al. (1989), Rollins and Rollins (1992), Reese et al. (1992), Leoni and  Manassero (2003).

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Design methods

The design load on the pile wall may be determined in 2D limit equilibrium analyses by calculating the reaction on the vertical section corresponding to the piled wall which is necessary to guarantee, with the appropriate factor of safety, the stability of the portion of the slide located upslope of the wall in the absence of the downslope portion; in any case, the load on the wall 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 wall.

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 pile wall 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 piles are anchored and the effects of actions on the piles.

The spacing between the piles must be determined balancing:

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

• ensuring that soil arching develops between adjacent piles and that the soil does not “flow” between the piles.

The check that soil arching develops between adjacent piles and that the soil does not “flow” through the piles 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 pile being those derived from the 2D analyses, multiplied by the pile centre to centre spacing. The same analysis may be used to determine the optimal length of the piles and the benefit of anchors.

The calculation of the pile 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 piles must be adequate to sustain the actions and the effects of actions on the piles. 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

• Anagnostopoulos C., Georgiadis K. (2004). ”Stabilization of a highway with piles in a landslide area”. In: Lacerda W.A., Ehrlich M., Fontoura S.A.B., Sayao A.S.F. (eds.) ”Landslides: Evaluation and Stabilization”,  Taylor & Francis Group.

• Cantoni R., Collotta T., Ghionna V.N., Moretti P.C. (1989). ”A design method for reticulated micropiles structures in sliding slopes”. Ground Engineering, May.

• Conte E., Troncone A. (2004). ”An analysis of piles used to stabilize slopes”. In: Lacerda W.A., Ehrlich M., Fontoura S.A.B., Sayao A.S.F. (eds.) ”Landslides: Evaluation and Stabilization”,  Taylor & Francis Group.

• 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.

• Evangelista A., Lirer S., Pellegrino A., Ramondini M., Urciuoli G. (2004). ”Interpretation of field measurements for slope stabilizing piles” In: Lacerda W.A., Ehrlich M., Fontoura S.A.B., Sayao A.S.F. (eds.) ”Landslides: Evaluation and Stabilization”,  Taylor & Francis Group.

• 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.

• Hong W.P., Han J.G., SongT.S., Shin D.S. (2004). ” Reinforcement effect of stabilizing piles in large-scale cut slope”. In: Lacerda W.A., Ehrlich M., Fontoura S.A.B., Sayao A.S.F. (eds.) ”Landslides: Evaluation and Stabilization”,  Taylor & Francis Group

• Hong W.P., Han J.G. (1996). ”The behavior of stabilizing piles installed in slopes, In Landslides Glissements de terrain, Kaare Senneset editor, Balkema, Rotterdam

• Isenhower W.M., Wright S.G., Kayyal M.K. (1989). ”Design procedures for slide suppressor walls”. In: Transportation Research Record 1242, TRB, National Research Council, Washington D.C., 15-21

• 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.

• Leoni F., Manassero V. (2003). ”Consolidamento e rinforzo dei pendii in terra”. Proceedings of the 19th Conference of Geotechnics of Turin (CGT), Turin (Italy), (in Italian).

• Nethero M.F. (1982). ”Slide controlled by drilled pier walls”. In: Application of walls to landslide control problems, Proc. of two sessions, ASCE National Convention, Las Vegas, R.B. Reeves editor, 61-76

• Oackland M.W. and Chameau J.L.A. (1984). ”Finite element analysis of drilled piers used for slope stabilization”. In: Laterally loaded deep foundations: analysis and performance, Special Technical Publication 835, ASTM, Philadelphia, PA, 182-193

• Palladino D.J., Peck R.B. (1972). ”Slope failures in an overconsolidated clay, Seattle, Washington”. Geotechnique 22, 4, 563-595

• 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.

• Rollins K.M., Rollins R.L. (1992). ”Case histories of landslide stabilization using drilled shaft walls”. In: Transportation Research Record 1343, TRB, National Research Council, Washington D.C., 114-122 

• Shmuelyan A. (1996). ”Piled stabilization of slopes”. In Landslides – Glissements de terrain, Kaare Senneset editor, Balkema, Rotterdam

• 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, vol. 3, 555-560.

• Wilson S.D. (1970). ”Observational data on ground movements related to slope instability”. 6th Terzaghi Lecture, Journal of the Soil Mechanics and Foundations Division, ASCE, 96, SM5, 1521-1544

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