Actual and Nominal Pore Water Pressure Distribution in Earth Dams

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1 Actual and Nominal Pore Water Pressure Distribution in Earth Dams Enzo Fontanella University of Rome La Sapienza, Dipartimento di Ingegneria Strutturale e Geotecnica, Rome, Via Eudossiana 18, , enzo.fontanella@uniroma1.it Luca Pagano University of Naples Federico II, Dipartimento di Ingegneria Idraulica, Geotecnica e Ambientale, Naples, Via Claudio 21, , luca.pagano@unina.it Augusto Desideri University of Rome La Sapienza, Dipartimento di Ingegneria Strutturale e Geotecnica, Rome, Via Eudossiana 18, , augusto.desideri@uniroma1.it ABSTRACT In assessing dam safety with respect to global instability, nominal pore water pressure distributions are often adopted for each stage of a dam's life, referring to a hypothetical expected performance of the different dam components. However, seepage phenomena taking place within the dam may substantially modify pore water pressure distributions. In this context, the present paper represents and interprets the singular pore water pressure distributions measured within an Italian earth dam, pertaining to the type of zoned earth dam with an internal clay core. Pore water pressure interpretation is supported by the representation of the evolution of the measured seepage flows. The work shows that pore water pressure may differ strongly from the nominal distribution during the first stages of the dam's life, when, if nearly undrained conditions take place, pore water pressures are discontinuous as a result of their dependency on total stresses. Pore water pressure may also assume other than expected distributions after some decades of operation, this time as a result of suffusion phenomena induced by seepage processes within the dam embankment. Measured pore water pressure distribution, suitably interpreted as piezometric head contours, clearly show that part of the downstream shell contributes to embankment water-tightness. KEYWORDS: Case history, Earth dam, Measurement, Seepage, Pore water pressure INTRODUCTION In several cases safety conditions of slopes are strongly affected by pore water pressure distribution within the system (Rojas et al., 2008; Pagano et al., 2008; Pagano et al., 2010a; Toll et al., 2011) and the slopes of earth dams are no exception to this phenomenon. Changes in pore water pressure distribution within the dam body and foundation soils may also provide

2 Vol. 17 [2012], Bund. S 2486 information with regard to safety related to other feared mechanisms, such as erosion of watertight zones. Reliable monitoring and an appropriate interpretation of pore water pressures are thus fundamental activities through which dam safety conditions are checked over time and possible issues concerning slope stability and erosion phenomena may be brought to light (Jappelli, 2003, Amorosi et al. 2008). Evaluation of embankment safety factors is often carried out by computing nominal pore water pressure distributions at different stages of the dam's life. Computations provide a hypothetical expected (or nominal) dam performance obtained in the arbitrary hypothesis that each dam component be homogeneous in terms of mechanical and hydraulic properties (e.g., Fanelli et al., 1998; Pagano et al., 1998, 2001, 2009; Burghignoli et al., 2002; Fontanella & Desiseri, 2004; Sica et al., 2008; Sica & Pagano, 2009). However, there are inhomogeneities within each dam component, such that to some extent actual pore water pressure distributions depart from nominal distributions (Pagano et al., 2006). When such inconsistencies are significant they typically lead to the mistaken belief that measurements are anomalous and/or unreliable (Pagano et al., 2010b). Inhomogeneities may be due to differences in intrinsic or compaction-induced properties arising during material placement or may progressively develop during the operational stages, due to erosion phenomena caused by seepage processes. The seepage forces acting within the dam may enhance phenomena such as suffusion within the embankment soils and the foundation soils, erosion along the contact between the embankment and a conduit spillway, drain or other appurtenance, new fractures, widening of existing fractures, or erosion of soil particles along the wall of the cracks (Sherard, 1986; Gould & Lacy, 1993; Torbla and Rikartsen, 1997; Johansen and Eikevik, 1997; Riemer et al., 1997; Charles, 1997). When not prevented by appropriately designed filters, such mechanisms may result in a new configuration of the hydraulic properties, associated with increases in leakages and modification in pore water pressure distribution. Erosion phenomena induce changes in the permeability distribution, which in turn alters pore water pressure distribution. Appropriate monitoring and interpretation of such alteration may thus allow the progress of erosion phenomena to be detected over time. The objective of this paper is to show that different mechanisms acting within the dam body may produce very different pore water pressure distributions from those expected. A further aim is to demonstrate that, during the operational stages, an appropriate interpretation of pore water pressure distribution allows water-tightness problems to be identified. The topic is dealt with by interpreting the monitoring data from an Italian case, the Polverina dam. Below the case history is first introduced. Subsequently, monitoring data are presented and interpreted. MATERIALS AND METHODS The case study adopted to accomplish the aim of the paper is that of the Polverina Dam, a zoned earth dam 27.5 m high, built in central Italy between March 1964 and November The dam s cross section is shown in Fig. 1. The core is a clayey sandy silt (PI = 12-18%) compacted around the optimum of the Standard Proctor Test. The permeability coefficient k is less than m/s. The downstream shell is made of sandy gravel (30% sand). The upstream shell is made of sandy gravel near the core and of a coarser material in the outer zone. Around the center of the valley, foundation soils consist of a twenty-meter deep fluvial deposit which lies on a marly bedrock. A lacustrine clayey silt deposit (Plasticity Index PI = 20-26%; permeability k m/s; compressibility index Cc = ) is included within the sandy gravel layer. It is around 8 m thick at the valley center, but then thins out and disappears towards the two abutments. A half-meter thick concrete cut-off wall provides water-tightness inside the foundation soils. The wall runs from the foundation level of the core down to the marls crossing the sandy gravel and

Piezometric heads refer to three points located in the lower part of the core. Fig.

3 Vol. 17 [2012], Bund. S 2487 clayey sandy silt. Figure 1: RESULTS Fig. 2a plots time histories of impounding levels and piezometric heads during the first five years of operation. Piezometric heads refer to three points located in the lower part of the core. Fig. 2b also plots piezometric head distribution along these three measurement points at ten different times ranging between the construction stages and the first operational stages.

2 over a much longer time period of 34 years. Fig.

4 Vol. 17 [2012], Bund. S 2488 Figure 2: Fig. 3 adds the seepage flow evolution to that of all quantities presented in Fig. 2 over a much longer time period of 34 years. Fig. 4 draws the evolution of piezometric heads at two points located outside the core in the downstream foundation soils, at a cross section very close to that containing the points considered in Fig. 2.

5 Vol. 17 [2012], Bund. S 2489 Figure 3: Figure 4:

the reference period of Figure 3, when steady state conditions take place within the dam.

hypothesis that shell materials are infinitely pervious.

6 plots piezometric head contours after several decades of the dam's operation (October 2000) using the observations

6 Vol. 17 [2012], Bund. S 2490 Figure 5: Figure 5 compares computed and observed piezometric head ditributions within the core at the end of the reference period of Figure 3, when steady state conditions take place within the dam. Computation has been carried out by solving Richards' equation within the clay core and foundation layers, in the hypothesis that shell materials are infinitely pervious. Other hypotheses are plane seepage and homogeneity in permeability within the core. Fig. 6 plots piezometric head contours after several decades of the dam's operation (October 2000) using the observations detected at all measurement points previously considered along with additional measurement points available in the core and in the downstream shell. Figure 6: DISCUSSION Peaks in seepage flow measurements (Fig. 3b) are associated with the occurrence of rainfall infiltrating the dam's downstream shell. The mean trend of seepage flow shows rapid growth

7 Vol. 17 [2012], Bund. S 2491 between 1967 and 1972, caused by the first complete impounding of the dam and the associated seepage through the core. Seepage is then characterized by a stable trend for a long period (until 1991), with fluctuations which arise consistently with the oscillation of the impounding level. Seepage flows began to grow from Interpretation of this increase is not straightforward, since it could occur as a result of a water-tightness problem within the clay core or due to the evolution of the transient seepage process within the core after the growth of the impounding level (beginning at K indicated in Fig. 3). The measurement points of piezometric heads in the core of the Polverina dam (Fig. 2) are located in the dam's core just above the top of the diaphragm. During the construction stages the dead load soon produces core saturation, and excess pore water pressure later develops under nearly undrained conditions. Piezometric head distribution (Fig. 2) differs from that expected, typically symmetrical with the maximum at the core axis. Distribution, instead, arises initially with the maximum located upstream (Fig. 2, curve 1) and subsequently with the maximum located downstream (Fig. 2, curve 3). After construction, piezometric heads follow a decreasing trend due to core consolidation, still with maximum located at the downstream piezometer (Fig. 2). During the construction stages and the subsequent start of trial fills the low permeability and high deformability of the core material produced conditions in the water phase which were not in equilibrium with the hydraulic boundary conditions. In nearly undrained conditions pore water pressure values give rise to distributions which are very difficult to interpret since they are strongly affected by static conditions. Indeed, pore water pressure distribution depends on total stress distributions, which may be discontinuous since they are sensitive to mechanical nonhomogeneities in stiffness randomly distributed in the core material (Pagano et al., 2006). The anomalous pore water pressure distribution during construction described above may hence be explained by invoking a dependency of pore water pressure field on a complex total stress distribution. The first complete impounding of the Polverina dam produces significant increments in core piezometric heads (Fig. 2a point 6) resulting from the reservoir pressure on the upstream boundary of the core. Piezometric head increments are higher downstream (Fig. 2, curves 6,7) and this inconsistency may be once again explained as an effect of undrained conditions taking place within the core that make pore water pressure distribution once again sensitive to static conditions and to stiffness inhomogeneity. Such increments are followed (Fig. 2, Fig. 3) by consolidation trends which last differently at each measurement point depending on location. Consistently with what is expected, the closer the point to the upstream core boundary the shorter the consolidation time due to pore water pressure build-up near values associated with steady state conditions. Piezometric heads become progressively higher upstream, consistently with what is expected (Fig. 2, curve 8, 9 and 10) in the presence of a seepage process governed by the presence of the reservoir and directed from upstream through to downstream. The updated rational distribution can be explained on the basis of re-equilibrium processes occurred within the core that make values insensitive to total stress distribution and dependent only on hydraulic properties and boundary conditions. For the same reasons, after consolidation, changes in piezometric heads are consistent with the change in the impounding level. As expected, the time lag is higher downstream and sensitivity to impounding changes is enhanced upstream. At the end of the analyzed period when the impounding level follows oscillating up to a nearly constant maximum value, stable piezometric heads seem to indicate the reaching of nearly steady state conditions, to be understood with respect to an average value of the impounding level. The comparison between computed and observed piezometric heads in steady state conditions (Figure 5) clearly show that predicted piezometric heads are lower than those measured at the

8 Vol. 17 [2012], Bund. S 2492 central and downstream piezometers. Inconsistency between predicted and observed behavior can be explained in terms of a suffusion process that is believed, to some extent, to have translated the water-tightness section downstream by reducing downstream permeability or extending the seepage domain. Piezometric contours (Fig. 6) clearly show that a significant part of the saturation line is contained in the downstream shell, i.e. that the shell is accomplishing a water-tightness task along with the core. This behavior is likely to be due to a reduction in shell permeability associated to migration of fine particles from the core. Measurement points located downstream in the foundation soils outside the dam body (Fig. 4) show very similar trends to those of seepage flow measurements, supporting the hypothesis that the increase in seepage is associated with dam hydraulic properties which are changing. CONCLUSIONS The paper showed that pore water pressures measured within a zoned earth dam may draw a picture which significantly differs from that expected. Two different mechanisms may lead to pore water pressure distributions diverging from the nominal distribution. First, the presence of nearly undrained conditions, which make pore water pressures an indicator of total stress acting at the measurement point and, as such, subject to the effects of random mechanical inhomogeneities. Second, erosion phenomena, which change the permeability distribution within the system. The assessment of safety conditions with respect to slope instability and erosion processes of earth dams in operation therefore has to be carried out with pore water pressure distributions obtained from careful interpretation of pore water pressure measurements, rather than from distribution from predictions based on ideal behavior of the different dam components. The paper also showed that pore water pressures are useful for detecting water-tightness problems when significant re-equilibrium processes take place, making them sensitive to change in hydraulic properties and insensitive to static conditions. Effectiveness is only with respect to that problems involving changes in permeability distribution. If such conditions are met different representations (time histories, spatial distributions, contours) can be effective in catching changes in the hydraulic behavior and typically require many measurement points along the seepage paths. In this case, pore pressures may indicate hydraulic changes associated to cracks or erosion processes even when seepage flows do not clearly indicate if a trouble really exists. REFERENCES 1. Amorosi A., Bilotta E., Callisto L., Croce P., Elia G., Fargnoli P., Fontanella E., Modoni G., Pagano L., Rampello S., Sica S., Vinale F.. Valutazione della sicurezza delle dighe in terra e degli argini fluviali. Rivista Italiana di Geotecnica, Vol. 4, pp Bilotta E., Pagano L., Sica S. (2010) Effect of ground-motion asynchronism on the equivalent acceleration of earth dams Soil Dynamics and Earthquake Engineering, Vol.30, N.7 pp , doi: /j.soildyn Burghignoli A., Desideri A., Tamagnini R. (2002) Numerical modelling of dam using the modified Cam Clay extended to the unsaturated condition Third International Conference on Unsaturated Soils. Recife (Brasile), vol. 1, pp Charles J.A. (1997) Special Problems Associated with Earthfill Dams Nineteenth Congress on Large Dams (General Reporter), CIGB ICOLD, Florence, Italy May 1997, pp

9 Vol. 17 [2012], Bund. S Fanelli M., Giuseppetti G., Mazza G., De Marco S., La Barbera G., Palumbo P., Ruggeri G., Ribacchi R., Pagano L., Silvestri F., Vinale F., Sembenelli (1998) La Potenzialità della Modellazione Matematica e delle Procedure di Calcolo Automatico nel Progetto e nella Valutazione della Sicurezza delle Dighe L'Energia Elettrica, Vol.75, N. 4, pp Fontanella E., Desideri A. (2004) Condizioni di sicurezza delle dighe in terra con riferimento a problemi di fratturazione idraulica Associazione Geotecnica Italiana. Valutazione delle condizioni di sicurezza e adeguamento delle opere esistenti. Palermo, settembre 2004, pp Gould J.P. & Lacy H.S. (1993) Seepage Control in Dam Rehabilitation Geotechnical Practice in Dam Rehabilitation, ASCE Geotechnical Special Publication No. 35, North Carolina State University, pp Jappelli R. (2003) Le Costruzioni Geotecniche per le Grandi Dighe in Italia Rivista Italiana di Geotecnica (Italian Geotechnical Journal), V. 37, N. 2, pp (in Italian) 9. Johansen P.M. & Eikevik (1997) Internal Erosion and Rehabilitation of Jukla Rockfill Dams. Nineteenth Congress on Large Dams, CIGB ICOLD, Florence, Italy May 1997, pp Pagano L., Sica S., Coico P. (2009). A study to evaluate the seismic performance of road embankments Soils and Foundations, Vol. 49, N. 6, p , doi: /sandf Pagano L., Desideri A., Vinale F. (1998) Interpreting the Settlement Profiles of Earth Dams Journal of Geotechnical and Geoenviromental Engineering, vol. 124, N.10, pp Pagano, L., Picarelli, L., Rianna, G., Urciuoli, G. (2010a) A simple numerical procedure for timely prediction of precipitation-induced landslides in unsaturated pyroclastic soils Landslide Vol.7, N. 3,pp , doi: /s x 13. Pagano L., Silvestri F., Vinale F. (2001) A back-analysis of Beliche Dam-Discussion to the paper by Naylor D.J., Maranha J.R., Maranha das Neves E., Veiga Pinto A.A. (1997). Geotechnique, vol. 51, N. 4, p Pagano L., Fontanella E., Sica S., Desideri A. (2010b) Effectiveness of pore water pressure measurements in the interpretation of the hydraulic behavior of two earth dams Soils and Foundations, vol. 50, N.2, pp , doi: /sandf Pagano L., Sica S., Desideri A. (2006) Representativeness of Measurements in the Interpretation of Earth Dam Behavior Canadian Geotechnical Journal, Vol.43, N.1, pp , doi: /t Pagano, L., Zingariello, M.C., Vinale, F. (2008). A large physical model to simulate flowslides in pyroclastic soils. Proceedings of the 1st European Conference on Unsaturated Soils, E-UNSAT 2008: Unsaturated soils- advances in geo-engineering, Durham, UK, 2-4 July 2008, pp Riemer W., Gavard M., Soubrier G., and Turfan M. (1997) The Seepage at the Ataturk Fill Dam Nineteenth Congress on Large Dams, CIGB ICOLD, Florence, Italy May 1997, pp Rojas J.C., Pagano L., Zingariello M.C., Mancuso C., Giordano G., Passeggio G. (2008) A new high capacity tensiometer: first results Proceedings of the 1st European Conference on Unsaturated Soils, E-UNSAT 2008: Unsaturated soils- advances in geo-

10 Vol. 17 [2012], Bund. S 2494 engineering, Durham, UK, 2-4 July 2008, pp , London:CRC Press - Taylor & Francis Group 19. Sherard J. L. (1986) Hydraulic Fracturing in Embankment Dams, Journal of Geotechnical Engineering, ASCE, 110 (10), pp Sica S., Pagano L. (2009) Performance-based analysis of earth dams: procedures and application to a sample case Soils and Foundations, Vol. 49, n. 6, pp , doi: /sandf Sica S., Pagano L., Modaressi A. (2008) Influence of past loading history on the seismic response of earth dams Computers & Geotechnics, Vol. 35, N. 1, pp 61-85, doi: /j.compgeo Torbla I. & Rikartsen C. (1997) Songa, Sudden Variation of the Leakage in a 35 Years Old Rockfill Dam Nineteenth Congress on Large Dams, CIGB ICOLD, Florence, Italy May 1997, pp Toll D.G., Lorenco S.D.N., Mendes J., Gallipoli D., Evans F.D., Augarde C.E., Cui Y.J., Tang A.M., Rojas Vidovic J.C., Pagano L., Mancuso C., Zigariello C., Tarantino A. (2011) Soil Suction Monitoring for Landslides and Slopes Quarterly Journal of Engineering Geology and Hydrogeology, vol. 44, N.1., pp , doi: / / ejge

Procedia Earth and Planetary Science 9 ( 2014 ) The Third Italian Workshop on Landslides

Available online at www.sciencedirect.com ScienceDirect Procedia Earth and Planetary Science 9 ( 2014 ) 214 221 The Third Italian Workshop on Landslides Prediction of suction evolution of silty pyroclastic