Geosynthetics applications for the mitigation of natural disasters and for environmental protection

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<ul><li><p>8/10/2019 Geosynthetics applications for the mitigation of natural disasters and for environmental protection</p><p> 1/51</p><p>Geosynthetics applications for the mitigation ofnatural disasters and for environmental protection</p><p>H. Brandl</p><p>Emeritus Professor; Institute for Geotechnical Engineering, University of Technology, 1040 Vienna,</p><p>Austria. Telephone: +431 58801 22101, Telefax: +431 58801 22199, E-mail: heinz.brandl@tuwien.ac.at</p><p>Received 7 July 2011, revised 17 August 2011, accepted 17 August 2011</p><p>ABSTRACT: The paper first describes the versatile application of geosynthetics for the mitigation</p><p>of floods, landslides, rockfalls, debris flows and avalanches. It focuses on dykes or flood protection</p><p>dams, on geosynthetic-reinforced stabilising fills (up to 130 m height) and barrier dams.</p><p>Geosynthetic-reinforced floating embankments (up to 70 m height) in creeping slopes and seismic</p><p>areas show clear advantages over rigid structures (e.g. bridges) not only from a geotechnical point</p><p>of view but also regarding economy, maintenance and environmental aspects. Environmentalprotection is predominantly considered by gaining renewable energy from the ground via energy-</p><p>geosynthetics. Several other applications are also mentioned. Compaction optimisation and control</p><p>of geosynthetic soil structures is recommended by roller-integrated CCC (continuous compaction</p><p>control), thus improving their behaviour significantly.</p><p>KEYWORDS: Geosynthetics, Natural disaster mitigation, Flood protection, Rockfall protection,</p><p>Landslide protection, Environmental protection, Floating embankments, Energy-geocomposites</p><p>REFERENCE: Brandl, H. (2011). Geosynthetics applications for the mitigation of natural disasters and</p><p>for environmental protection. Geosynthetics International, 18, No. 6, 340390. [http://dx.doi.org/</p><p>10.1680/gein.2011.18.6.340]</p><p>1. FLOOD PROTECTION</p><p>1.1. General</p><p>Climate change requires a close cooperation between</p><p>hydrology, hydro engineering and geotechnical engineer-</p><p>ing. The increasing frequency and magnitude of floods</p><p>indicate that the previous prognoses of such events should</p><p>be revised for many regions. Consequently, the improve-</p><p>ment of old dykes or flood protection dams and the</p><p>construction of new ones has become essential since the</p><p>1990s. Environmentally optimal solutions are achieved by</p><p>(re-)creating retention areas, combined with dams that can</p><p>be overflowed, thus flattening the flood wave. Besides</p><p>these permanent flood protection measures, temporary</p><p>ones are increasingly required for emergency situations.</p><p>Geosynthetics have also proved very suitable for mobile</p><p>systems.</p><p>The topic of coastal protection should also be men-</p><p>tioned in the list of possible applications of geosynthetics</p><p>in the field of natural disaster mitigation. However, it will</p><p>not be discussed in this 2010 Giroud Lecture, because it</p><p>has already been dealt with in the 2006 Giroud Lecture</p><p>delivered by C. Lawson (Lawson 2008).Quality assessment of existing structures, design aspects</p><p>of new ones, and the required safety factors depend on</p><p>purpose, risk potential and monitoring intensity. Therefore</p><p>dykes (permanently loaded by water) and flood protection</p><p>dams (only temporarily loaded by water) should be distin-</p><p>guished. Most geotechnical aspects, however, are relevant</p><p>for both.</p><p>1.2. Future trends and residual risks</p><p>Floods have affected millions of people worldwide in</p><p>recent decades. In several regions the magnitude and</p><p>frequency of flood waves have increased dramaticallysince long-term measurements and historical reports have</p><p>existed. Figure1 shows an example from Austria, where a</p><p>2000 to 10 000-year flood event was back-calculated from</p><p>the flood disaster in the year 2002. Such hitherto singular</p><p>values cannot be taken as design values for flood protec-</p><p>tion dams, but they underline the need for local overflow</p><p>crests or spillway sections. Moreover, they clearly demon-</p><p>strate that a residual risk is inevitable despite most</p><p>costly protective measures. The so called absolute safety</p><p>as frequently demanded by the public, by politicians, by</p><p>the media or by jurists cannot be achieved in reality.</p><p>1.3. Failure modes of dykes and damsKnowledge of possible failure modes is an essential</p><p>prerequisite both for a reliable quality assessment of</p><p>Geosynthetics International, 2011, 18, No. 6</p><p>3401072-6349 # 2011 Thomas Telford Ltd</p></li><li><p>8/10/2019 Geosynthetics applications for the mitigation of natural disasters and for environmental protection</p><p> 2/51</p><p>existing dykes and dams and for an optimised design of</p><p>new ones, in connection with the application of geosyn-</p><p>thetics. Figure2 summarises the dominating failure modes</p><p>for typical ground conditions along rivers (near-surface,</p><p>low-permeability sandy to clayey silts underlain by high</p><p>permeability sand or gravel)</p><p> slope failure due to excessive pore-water pressures,</p><p>seepage or inner erosion</p><p> overtopping of the dyke/dam crest</p><p> slope failure due to a rapid drop of the flood water</p><p>level</p><p> hydraulic fracture</p><p> surface erosion and failure of the water-side slope</p><p>due to wave action</p><p> piping due to animal activities, especially frombeavers and rats</p><p> unsuitable planting of dykes (especially trees with</p><p>flat roots).</p><p>Hydraulic failure is frequently underestimated, but may</p><p>occur in different forms (e.g. Eurocode 7; CEN 2004).</p><p> By uplift (buoyancy). The pore-water pressure under</p><p>the low-permeability soil layer exceeds the over-</p><p>burden pressure.</p><p> By heave. Upward seepage forces act against the</p><p>weight of the soil, reducing the vertical effectivestress to zero; soil particles are then lifted away by</p><p>the vertical water flow. This boiling dominates in</p><p>silty-sandy soil, and is combined with internal</p><p>erosion (Figures3a,3b and4).</p><p> By internal erosion. Soil particles are transported</p><p>within a soil stratum or at the interface of soil strata</p><p>(Figures 5 and 6). This may finally result in</p><p>regressive erosion, leading to ground failure of the</p><p>dyke or dam.</p><p> By piping. Failure by piping is a particular form of</p><p>internal erosion, where erosion begins at the surface,</p><p>and then regresses until a pipe-shaped discharge</p><p>tunnel is formed (Figure7). Failure occurs as soon asthe water-side end of the eroded tunnel reaches the</p><p>river bed or bottom of the reservoir. Frequently,</p><p>several tunnels develop (Figure8). This process may</p><p>be induced or significantly promoted by animal</p><p>activities, as field observations over many years have</p><p>revealed (Figure2).</p><p>Hydraulic failure may reach several tens of metres away</p><p>from dykes or dams, as experience has shown. This couldbe observed even for low flood protection embankments</p><p>with a relatively small hydraulic gradient.</p><p>1 10 100 1000 10 000x-year flood event</p><p>Maximum</p><p>discharge:m</p><p>/s3</p><p>0</p><p>100</p><p>200</p><p>300</p><p>400</p><p>500</p><p>Flood 2002: ca. 420 m /sBack calculated: 2000 to 10000-year event</p><p>3</p><p>Hitherto observed event</p><p>GEV-LM Extrapolation</p><p>Figure 1. Statistics for the annual maximum discharge values</p><p>of the river Kamp in Austria (Gutknechtet al. 2002)</p><p>Water outflow</p><p>Eroded material</p><p>Cracks</p><p>Low-permeability soil layer</p><p>Low-permeability soil layer</p><p>Low-permeability soil layer</p><p>Low-permeability soil layer</p><p>Low-permeability soil layer</p><p>Low-permeability soil layer</p><p>Low-permeability soil layer</p><p>Low-permeability soil layer</p><p>Sand</p><p>Sand</p><p>Sand</p><p>Sand</p><p>Sand</p><p>Sand</p><p>Sand</p><p>Sand</p><p>Figure 2. Failure modes of dykes and dams: near-surface</p><p>stratum of low permeability (possibly with local windows).</p><p>AfterVDZ (2002)</p><p>Geosynthetics applications for the mitigation of natural disasters and for environmental protection 341</p><p>Geosynthetics International, 2011, 18, No. 6</p></li><li><p>8/10/2019 Geosynthetics applications for the mitigation of natural disasters and for environmental protection</p><p> 3/51</p><p>Eurocode 7 (CEN 2004) states that in situations where</p><p>the pore-water pressure is hydrostatic (negligible hydrau-lic gradient) it is not necessary to check other than for</p><p>failure by uplift. In the case of danger of material</p><p>transport by internal erosion, filter criteria should be</p><p>used. If the filter criteria are not satisfied, it should be</p><p>verified that the critical hydraulic gradient is well below</p><p>the design value of the gradient at which soil particles</p><p>begin to move.</p><p>Experience has shown that the magnitude of the critical</p><p>hydraulic gradient where internal erosion begins is fre-</p><p>quently overestimated. Figure 9 summarises the critical</p><p>values on the basis of field observations, geotechnical</p><p>measurements, literature and long-term experience for</p><p>different soils. For comparison, the conventional criterion</p><p>(icrit 9/w), Lanes criterion, and the critical zones afterEurocode 7 (CEN 2004) or Chugaev (1965) respectively</p><p>are also plotted in the diagram.</p><p>Filter protection is generally provided by the use of</p><p>non-cohesive granular material (natural soil) that fulfils</p><p>adequate design criteria for filter materials. Filter geotex-</p><p>tiles have been used increasingly since the early 1970s.</p><p>Common filter criteria for soils are from Terzaghi and</p><p>Sherard, and for geotextiles fromGiroud (2003,2010) and</p><p>Heibaum et al. (2006). All criteria have particular limit-</p><p>ations, whereby non-cohesive and cohesive soils have to</p><p>be distinguished. Whereas two criteria are sufficient forgranular filters (the permeability criterion and the reten-</p><p>tion criterion), four criteria are required for geotextile</p><p>(a)</p><p>(b)</p><p>Figure 3. Boiling behind dykes indicates inner erosion and</p><p>beginning of hydraulic failure: (a) randomly distributed</p><p>multiple boiling; (b) detail of an erosion spot</p><p>Figure 4. Boiling volcanos after the flood</p><p>Phase 1 Phase 1Phase 2 Phase 2</p><p>(a) (b) (c)</p><p>Figure 5. Hydraulic fracture of dykes or flood protection</p><p>dams due to seepage through or beneath the dyke or dam</p><p>(Ziems 1967): (a) suffusion (fine particles move into pore</p><p>voids of coarse grain fractions); (b) contact erosion at the</p><p>interface of soil strata; (c) internal erosion in steady-state</p><p>flow condition</p><p>Figure 6. Contact erosion perpendicular or parallel to an</p><p>interface of soil strata</p><p>342 Brandl</p><p>Geosynthetics International, 2011, 18, No. 6</p></li><li><p>8/10/2019 Geosynthetics applications for the mitigation of natural disasters and for environmental protection</p><p> 4/51</p><p>filters (Giroud 2010): the porosity criterion and the</p><p>thickness criterion also have to be considered.</p><p>1.4. Quality assessment of dykes and dams</p><p>Comprehensive in situ investigations have disclosed that</p><p>the quality of dykes and dams with a height, H, 15 m is</p><p>mostly lower than that of higher earth structures. There-</p><p>fore low but long flood protection dams may involve</p><p>rather high risk potential. Dams of H&gt; 15 m, however,</p><p>have already been constructed for decades according to</p><p>the strict regulations of ICOLD.</p><p>When assessing the quality of dykes or flood protection</p><p>dams, the purpose and risk potential of the structureshould be taken into consideration. Dykes as fill dams</p><p>directly along reservoirs, rivers or canals are continuously</p><p>1</p><p>6</p><p>3</p><p>4</p><p>1 Free water table2 Piezometric level in permeable subsoil3 Low-permeability soil</p><p>4 Permeable subsoil5 Possible well; starting point for pipe6 Possible pipe</p><p>2</p><p>1</p><p>5</p><p>Figure 7. Example of conditions that may cause piping (Eurocode 7;CEN 2004)</p><p>Figure 8. Piping through a railway embankment during an</p><p>already sinking flood</p><p>Medium dense stiff</p><p>i cr</p><p>it</p><p>Fine Fine FineMedium Medium MediumCoarse Coarse CoarseClay</p><p>Silt Sand GravelCobbles Boulders</p><p>1.2</p><p>1.1</p><p>1.0</p><p>0.9</p><p>0.8</p><p>0.7</p><p>0.6</p><p>0.5</p><p>0.4</p><p>0.3</p><p>0.2</p><p>0.1</p><p>0</p><p>G,dst</p><p>G,dst</p><p>D</p><p>AE</p><p>Hydraulic fracture EN1997-1</p><p>icrit </p><p>W</p><p>0.001 0.002 0.006 0.02 0.063 0.2 0.63 2 6.3 20 64 100 200</p><p>Grain size (mm)</p><p>BC</p><p>Figure 9. Critical hydraulic gradients for hydraulic fracture (internal erosion) (Brandl and Hofmann 2006); icrit: depends not</p><p>only on grain size distribution and density/stiffness but also on flow pressure; G,dst partial safety factor for permanent</p><p>unfavourable effects</p><p>Geosynthetics applications for the mitigation of natural disasters and for environmental protection 343</p><p>Geosynthetics International, 2011, 18, No. 6</p></li><li><p>8/10/2019 Geosynthetics applications for the mitigation of natural disasters and for environmental protection</p><p> 5/51</p><p>monitored, whereas flood protection dams are loaded by</p><p>water only during floods. The latter are therefore more</p><p>critical, as experience has shown.</p><p>Besides levelling of the crest and control of the</p><p>geometric data, the following (geotechnical) investigations</p><p>have proven successful</p><p> seasonal field observations, especially during floods documentation of water discharge and localisation of</p><p>wet slope spots and zones during floods, depending</p><p>on the magnitude and duration of the flood wave</p><p> aerial photographs, especially during and after floods</p><p>(and in normal periods for comparison)</p><p> spot-checking by soundings, borings or exploratory</p><p>pits</p><p> field tests and laboratory tests</p><p> geophysical investigations</p><p> tracer testing</p><p> infrared photographs for localisation of seepage</p><p> roller-integrated continuous compaction control</p><p>(CCC).</p><p>Investigations in steps provide the best results, and are</p><p>the most cost-effective. Conventional spot-checking gives</p><p>only random data, whereas investigations covering the</p><p>entire area provide more detailed information. Conse-</p><p>quently, geophysical methods have been increasingly used</p><p>over the last 10 years. However, experience has shown that</p><p>a reliable interpretation is possible only if at least two or</p><p>three different methods are applied (e.g. geoelectrics,</p><p>micro-gravimetry, combined surface wave/refraction seis-</p><p>mic). Geophysical methods and electric tracers are also</p><p>used to check the effectiveness of sealing elements (sur-</p><p>face sealings, core sealings, cut-off walls). Multi-sensor</p><p>survey systems together with spatially targeted electrical</p><p>tracers make it possible to localise leakage areas, and</p><p>hence leaks. Moreover, they have proved suitable for</p><p>continuous leakage monitoring of sealing systems (liners,cut-off walls).</p><p>A complete quality assessment of the dyke or dam crest</p><p>is made possible by roller-integrated continuous compac-</p><p>tion control (CCC). This method registers the interaction</p><p>between roller and ground (for details see Section 5). The</p><p>measuring depth reaches 2 to 2.5 m, and thus covers the</p><p>most critical zone of dykes and dams. Weak points are</p><p>easily localised, which is especially important for struc-</p><p>tures with crests that can be overflowed (spillway section).</p><p>However, if the crest has a top cover of concrete, asphalt</p><p>or riprap, or if it is very uneven, CCC cannot be used.</p><p>The advantage of CCC is not only full control but also</p><p>a compaction of weak zones. Figure 10 shows some</p><p>results along an old dyke that was severely attacked by</p><p>the previous flood. The weak points and zones were not</p><p>visible along the crest, but could be clearly localised by</p><p>CCC during the first roller pass. After six roller passes</p><p>the quality of the top 2 m of the dam had improved</p><p>significantly, although relatively weak spots still existed.</p><p>However, the control values exceeded the lower limit</p><p>value (30 for this particular fill material) with only one</p><p>exception (23).</p><p>1. Roller pass</p><p>6. Roller pass</p><p>120</p><p>120</p><p>100</p><p>100</p><p>80</p><p>80</p><p>60</p><p>60</p><p>40</p><p>40</p><p>20</p><p>20</p><p>0</p><p>0</p><p>110/100</p><p>110/100</p><p>110/150</p><p>110/150</p><p>Cursor: 110/190Required limit</p><p>Cursor: 110190Required limit</p><p>15....</p></li></ul>