Paleosubsidence and active subsidence due to evaporite dissolution in the Zaragoza area (Huerva River valley, NE Spain): processes, spatial distribution and protection measures for transport routes

Engineering Geology 72 (2004) 309 – 329 www.elsevier.com/locate/enggeo Paleosubsidence and active subsidence due to evaporite dissolution in the Zaragoza area (Huerva River valley, NE Spain): processes, spatial distribution and protection measures for transport routes Jesús Guerrero *, Francisco Gutiérrez, Pedro Lucha Earth Sciences Department, Edificio Geológicas, Universidad de Zaragoza, C/. Pedro Cerbuna, 12, 50009 Saragossa, Spain Received 27 May 2003; accepted 27 October 2003 Abstract The lowest 17-km long reach of the Huerva River valley, down to its confluence with the Ebro River in Zaragoza city, flows across salt-bearing evaporites of the Ebro Tertiary Basin (NE Spain). Upstream, the horizontally lying Miocene evaporites are interfingered with non-soluble distal alluvial fan facies (shales and sandstones). The proportion of soluble facies in the Huerva River valley increases in a downstream direction towards the basin depocenter. On the basis of the type and magnitude of the paleosubsidence features, the valley has been divided into four reaches. Along reach I, undeformed terrace deposits less than 4 m thick rest on insoluble detrital bedrock. In reaches II and III, dissolution at the alluvium – bedrock boundary has generated local thickening, deformation and paleocollapse structures, which only affect the alluvial mantle. In reach IV, terrace deposits thicken to over 60 m resulting from a large-scale synsedimentary subsidence. In this sector, subsidence locally affects to both the alluvium and the underlying bedrock. This indicates that dissolution acts at the rockhead beneath the alluvial cover (alluvial karst) and within the evaporitic substratum (interstratal karst). The development of an intraevaporitic karst in reach IV is attributed to gypsum and salt dissolution. Irregular terrace gravel bodies (gravel pockets) embedded in a fine-grained matrix associated with paleocollapse structures have been interpreted as liquefaction – fluidization structures resulting from ground acceleration and suction induced by catastrophic collapses. Subsidence is currently active in the region affecting areas with a thin alluvial cover in reaches III and IV. The low subsidence activity in most of Zaragoza city is explained by the presence of thickened (around 50 m) and indurated alluvial deposits. In the surrounding area, numerous buildings in Cadrete and Santa Fe villages have been severely damaged by subsidence. Natural and human-induced subsidence favours the development of slope movements in the gypsum scarp overlooking Cadrete village. Several transport routes including the Imperial Canal (irrigation canal) and the recently completed Madrid – Barcelona high-speed railway are affected by human-induced sinkholes. The paleocollapse structures exposed in the trenches of this railway and a ring road under construction point to hazardous locations underlain by cavities and collapse structures where special protection measures should be applied. Rigid structures are recommended beneath the high-speed railway with sufficient strength to span the larger sinkholes with no deformation. Electronic monitoring devices linked to a warning system can detect subtle subsidence-induced deformations in carriageways or * Corresponding author. Facultad de Ciencias, Dpto. Ciencias de la Tierra, Universidad de Zaragoza, Pedro Cerbuna, 12, Saragossa 50009, Spain. Tel.: +34-976-761090; fax: +34-976-761106. E-mail addresses: jgiturbe@posta.unizar.es (J. Guerrero), fgutier@posta.unizar.es (F. Gutiérrez). 0013-7952/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.enggeo.2003.10.002 310 J. Guerrero et al. / Engineering Geology 72 (2004) 309–329 railways. This research demonstrates that the study of the paleokarst helps to understand the processes involved in the present-day subsidence phenomena and their general spatial distribution. D 2003 Elsevier B.V. All rights reserved. Keywords: Subsidence hazard; Sinkhole; Evaporite karst; Transport routes; High-speed railway; Spain 1. Introduction Natural and human-induced subsidence due to the dissolution of evaporites is a significant geohazard in some areas of Europe. Some of the areas where evaporite dissolution subsidence hinders development include the Permian and Triassic evaporitic terrain of England (Cooper, 1996, 2002; Cooper and Waltham, 1999), the towns of Biržai and Pasvalys in Lithuania (Paukštys and Narbutas, 1996), some regions in Germany (Reuter and Stoyan, 1993; Garleff et al., 1997) including Stuttgart, Eisleben and the Harz Mountains, the outskirts of Paris in France (Toulemont, 1984) and numerous Triassic and Tertiary evaporite areas in Spain (Gutiérrez et al., 2001). The most cost-effective way of planning in these areas lies in avoiding the existing sinkholes and the most subsidence prone areas (Paukštys et al., 1999). The application of this preventive philosophy requires the recognition of the areas affected by subsidence and the production of hazard maps. However these tasks are generally difficult, especially in extensively developed areas where evidence of recent subsidence activity are obliterated by human activities. In addition to the traditional geomorphological and geotechnical investigation approaches, the paleokarst features found in the geological record can help to understand the subsidence processes and locate problematic locations. Spain is probably the European country where subsidence due to evaporite dissolution has the largest impact on development and cultural heritage. The problems are essentially related to Triassic and Tertiary evaporites, covering respectively outcrops of 4500 and 30,500 km2. The areas affected by the worst subsidence problems correspond mainly to fluvial valleys developed on Tertiary evaporites made up of calcium sulphates or calcium –sodium sulphates and halite. The terrace deposits in these valleys show large thickenings (>100 m) and numerous deformations including paleocollapse structures generated by synsedimentary and post-sedimentary subsidence (Pinilla et al., 1995; Gutiérrez, 1996; Gutiérrez and Gutiérrez, 1998; Gutiérrez et al., 2001, 2002; Benito et al., 1998, 2000). Characteristics of the thickened terrace deposits reveal that synsedimentary subsidence controls the dynamics and sedimentary patterns of the fluvial systems. A review of the stratigraphical record and contemporaneous activity of dissolution subsidence in valleys crossing Tertiary evaporites may be found in Gutiérrez et al. (2001, 2002). The present-day subsidence affects extensive areas with alluvium-covered evaporites, dominantly in the lower alluvial levels where the greater part of the modern development takes place. The most severely damaged areas are Calatayud city (Gutiérrez, 1998a; Gutiérrez and Cooper, 2002) and the outskirts of Zaragoza city (Fig. 1), at the confluence of the Huerva and Gállego Rivers with the Ebro River. Subsidence hazards have been extensively studied in the Gállego and Ebro River valleys (Benito et al., 1995; Soriano and Simón, 1995). However, the efforts devoted to the Huerva River valley, one of the main urban growth areas of Zaragoza city, have been very scant. Traditionally, the majority of the investigations in these valleys on the Tertiary evaporites have attributed the subsidence phenomena to gypsum karstification. Recent studies based on borehole data and interpretations of the paleokarst features highlight the relevance of salt and sodium sulphate karstification in the subsidence phenomena (Benito et al., 1998; Gutiérrez and Cooper, 2002; Gutiérrez et al., 2002). This work analyses the past and recent subsidence in the Huerva River valley. The paleosubsidence features found in the stratigraphical record are used to understand the processes involved in the current subsidence phenomena and to identify hazardous J. Guerrero et al. / Engineering Geology 72 (2004) 309–329 311 Fig. 1. Catastrophic sinkhole formed inside a factory located in the Ebro River valley close to Zaragoza city (Europa industrial estate). A 60-cm long bag is located next to the edge of the sinkhole at the left. locations that could affect highly vulnerable transport routes. 2. Geological and geomorphological setting The study area corresponds to a small portion of the central sector of the Ebro Basin which constitutes the southern Tertiary foreland basin of the Pyrenees in the NE of Spain (Fig. 2). The basin fill is primarily composed of Paleogene and Neogene continental sediments deposited in alluvial fans that pass distally into lacustrine environments. The conglomeratic facies at the margins of the basin grade progressively into sandstones, clays, marls and evaporite – carbonate facies towards the sedimentary axis of the basin (Riba et al., 1983). Several evaporitic formations were deposited as the most subsiding areas migrated to the south of the basin (Riba et al., 1983; Ortı́, 1989, 1997). In general, the Paleogene formations are affected by folding structures while Neogene strata remain subhorizontal. From the lower Miocene, an extensional stress regime has operated in the central sector of the Ebro Basin generating three main joint families with WNW – ESE, N – S and E – W trends (Arlegui and Soriano, 1998; Arlegui and Simón, 2000). Once the basin became exorheic, presumably in the late Miocene –early Pliocene, a new drainage network started to dissect the endorheic infill. These alluvial systems have developed stepped sequences of mantle pediments and terraces that partially cover the Tertiary infill. The Ebro River longitudinally crosses the Ebro Depression following in its central sector which is the axis of a very gentle NW – SE trending synclinal structure (Quirantes, 1978). The Huerva River is a southern tributary of the Ebro River originating in the Iberian Range that drains transversally the Depression crossing the different facies of the Neogene fill (Fig. 2). The study area covers the lowest 30-km long reach of the Huerva River valley down to its confluence with the Ebro River in Zaragoza city. In this sector, the valley is excavated in horizontally lying Miocene sediments of the Longares, Zaragoza and Alcubierre Formations. Distal alluvial fan sandstones and clay facies of the Longares Formation grade into the evaporitic facies of the Zaragoza Formation between 23 and 17 km upstream of the Huerva River mouth. These interfingered formations are capped by the 70m-thick limestone sequence of the Alcubierre Formation. This resistant limestone unit forms La Plana and La Muela mesas at about 200 m above the valley bottom flanking the Huerva River valley to the east and west, respectively (Fig. 2). At outcrop, the Zaragoza Formation comprises 300 m of gypsum, marls and shales with an evident increase in the proportion of gypsum in a downstream direction (Esnaola et al., 1995). Borehole data 312 J. Guerrero et al. / Engineering Geology 72 (2004) 309–329 Fig. 2. (A) Lithostratigraphical and geomorphological map of the Huerva River valley. (B) Geographical location of Ebro Depression (shaded area), Zaragoza city and the Ebro River in the Iberian Peninsula. (C) Explanation to the map. (D) Distribution of the Miocene lithostratigraphic units in the Huerva River valley. J. Guerrero et al. / Engineering Geology 72 (2004) 309–329 form nearby locations indicate that in the subsurface this formation is composed of anhydrite/gypsum, halite, glauberite (Na2 Ca[SO 4 ] 2 ) and thenardite (Na2SO4) (Ortı́ and Salvany, 1997; Esnaola et al., 1995; Ortı́, 2000). In the Ebro Valley, a few kilometres upstream of Zaragoza, a 120-m-thick halite unit has been drilled at a depth of around 100 m below the elevation of the Ebro River floodplain (Torrescusa and Klimowitz, 1990). The subsurface lithostratigraphy of the Zaragoza Formation in the Huerva Valley is not well known due to the lack of deep borehole data. However, drillholes for the bridge of the Madrid –Barcelona high-speed railway, located 7 km upstream of the Huerva River mouth, have crossed halite bodies up to 4 m thick at 40 m below the floodplain. The presence of halite close to the surface has been detected in other locations along the Huerva Valley like Cadrete village. It seems probable that previously existing halite beds have been removed by dissolution from the outcropping evaporite sequence. The existence of halite and Nasulphates in the bedrock is a crucial factor for the development of dissolution-induced subsidence phenomena due to their high solubility. Whereas the solubility of gypsum at 25 jC is 2.4 g/l, halite, glauberite and thenardite solubilities reach 360, 118 and 519 g/l, respectively (Ford and Williams, 1989). The entrenchment and lateral migration of the Huerva River throughout its evolution has generated an asymmetric valley with stepped terraces on the western margin and a prominent gypsum escarpment on the east side (Fig. 2). The asymmetric configuration of the valley and the existence of a scarp are common features of fluvial systems that cross the Tertiary evaporitic formations in Spain (Gutiérrez et al., 1994, 2001). The Huerva River valley loses its asymmetry downstream of Cuarte village (8 km upstream of the river mouth) where the valley is excavated in thickened terrace alluvium (Fig. 2). An outstanding geomorphic feature is the 6-km long and E – W trending Valdespartera Depression located on the west margin of the Huerva Valley. This recently captured depression with a flat bottom mantled with a thin veneer of gypsiferous silts and marls has been interpreted as a karst polje developed in gypsum bedrock (Soriano, 1993) (Fig. 2). The study area is characterised by a semiarid climate. At Zaragoza meteorological station, the mean 313 annual precipitation and temperature are 314 mm and 14.6 jC, respectively. The Huerva River valley is an extensively developed area with a rapidly growing population. Cadrete, Santa Fe and Cuarte villages and several industrial estates are placed on alluviumcovered evaporite bedrock susceptible to subsidence. Traditionally crop fields occupied most of the valley bottom. In the last few years, residential areas have been built in a significant part of the floodplain, including sectors flooded in July 1990. A recent flood event in May 7th, 2003 has affected numerous recent buildings located in the floodplain. On the other hand, Zaragoza city with around 700,000 inhabitants is built on thickened alluvium overlying evaporites at the confluence of the Ebro River with its tributaries the Huerva and Gállego Rivers. The mean annual discharge of the Ebro and Huerva Rivers are respectively 230 and 3.5 m3/s. 3. Paleosubsidence and paleosinkholes recorded in the alluvial deposits Throughout its evolution, the Huerva River alluvial system has been affected by subsidence phenomena caused by the karstification of the evaporitic bedrock. Dissolution-induced subsidence (synsedimentary and postsedimentary) has given place to anomalous features in the alluvial levels. These include abrupt alluvium thickness variations, significant changes in the relative height of the terrace levels along the valley and numerous atectonic deformational structures including dramatic paleocollapses. Previous geomorphological research (Soriano, 1990) identified nine terrace levels in the Huerva River valley. Detailed mapping and thorough field survey of the studied area have allowed us to recognise a stepped sequence of 12 terrace levels (T12: 115 –110 m, T11: 105– 90 m, T10: 93 –75 m, T9: 75 – 62 m, T8: 62 –53 m, T7: 52 –48 m, T6: 42– 39 m, T5: 56 – 34 m, T4: 35 – 30 m, T3: 25– 18 m, T2: 17– 7 m, T1: 8 –2 m) and 7 covered pediment levels correlative to some of the terrace levels (P10, P9, P8, P5, P4, P3, P2) (Fig. 2). Based on the type and magnitude of the paleosubsidence features found in the terrace deposits, the studied part of the valley has been divided into four reaches. These reaches define areas of variable subsidence susceptibility affected by different types of 314 J. Guerrero et al. / Engineering Geology 72 (2004) 309–329 subsidence. They can be considered as a rough zonation of the subsidence hazard. 3.1. Reach I The upper reach (I) has its lower limit around Botorrita village, 23 km upstream of the Huerva River mouth (Fig. 2). In this valley section, the various terrace deposits show no evidence deformation and rest on insoluble detrital facies bedrock. The deposits of each terrace level are dominantly of a channel gravel facies with an even thickness of less than 4 m. 3.2. Reach II In the second reach (II), from Botorrita to Cadrete villages (17 km upstream of the river mouth), the bedrock changes from a clastic distal fan facies gradually into evaporitic sediments composed of gypsum with relatively thick clay and marl layers (Fig. 2). In this stretch, the terrace alluvium is of similar thickness to that in the upper reach, but shows some small-scale disturbances including poorly defined failure surfaces and reoriented clasts. The boundary between the alluvium mantle and the gypsiferous beds commonly shows an irregular geometry due to the suballuvial dissolution of the rockhead and the consequent differential settlement of the cover. 3.3. Reach III (Cadrete –Cuarte) In the third reach (III), between Cadrete and Cuarte villages (from 17 to 8 km upstream of the river mouth), the Tertiary bedrock shows a progressively higher proportion of gypsum and paleosubsidence features become more widespread (Fig. 2). Terrace Fig. 3. (a) Deposits of P8 pediment in the Madrid – Barcelona high-speed railway cutting showing a synformal structure generated by synsedimentary subsidence. This ductile gravitational deformation is affected by subsequent postsedimentary collapse failures. Note the irregular geometry of the bedrock – alluvium boundary and the dip of the gypsum beds towards the synform axis indicating both suballuvial and intraevaporitic dissolution. (b) Interpretative diagram of outcrop shown in (a). J. Guerrero et al. / Engineering Geology 72 (2004) 309–329 315 Fig. 4. Solutionally enlarged joint (grike) filled with gravelly gypsiferous silts derived from overlying P8 pediment deposits. The outcrop is located in a gypsum quarry at the east margin of the Huerva River valley, a few meters to the north of the high-speed railway. deposits are locally thickened due to synsedimentary subsidence. In Cadrete and Santa Fe villages, the T2 and T1 terrace levels sediments are over 15 m thick. On the west margin of the valley, to the west of Cuarte, the deposits of the T10 and T9 terrace levels reach up to 8 m in thickness and show two gentle synforms over 50 m long. In these structures, the dip of the strata decrease from the base to the top and the Fig. 5. Thickening of T8 terrace deposits showing a cumulative wedge out arrangement in the eastern side of the Huerva River valley (reach IV). The sharp increase in subsidence magnitude probably suggests the outer limit of highly soluble facies (salt, Na-sulfates) in the shallow subsurface. 316 J. Guerrero et al. / Engineering Geology 72 (2004) 309–329 beds thicken towards the core (cumulative wedge outs). These synformal structures are interpreted as the cross-section of basin structures resulting from the syndepositional bending of the alluvial mantle and the consequent development of subsidence depressions (synsedimentary subsidence). Abundant paleosubsidence features controlled by brittle deformational structures also affect terrace deposits. In several outcrops subvertical failure planes with reoriented fabrics clearly record the development of sinkholes up to several metres wide. The fact that these deformations are restricted to alluvial deposits overlying evaporites invalidates a neotectonic origin. Soft sediment deformations shown by gravel pockets are commonly found in terrace deposits associated to paleosinkholes. The gravel pockets are irregular gravel bodies embedded in a fine-grained sediment interpreted as liquefaction and fluidization structures resulting from the elutriation and plastic remoulding of loose sediments. The generation of these structures may be favoured by rapid deposition, immature sediment texture and the presence of low permeability layers (Postma, 1983; Johnson, 1986; Nocita, 1988). In the studied examples, the dynamic load and the downward suction produced by the sudden development of collapse structures are the most probable genetic causes (Gutiérrez, 1998a,b). Ground acceleration due to earthquakes seems to be an unlikely cause due to the low seismic activity of the area. These softsediment deformations in the detrital deposits overlying a karstified evaporitic substratum can be considered as probable indicators of past catastrophic subsidence activity. To the north of Santa Fe, T8 terrace is tilted dipping away from the valley. This postsedimentary deformation is attributed to differential subsidence caused by the karstification of the evaporites. railway, the pediment alluvium shows synforms 80 –120 m wide with cumulative wedge-outs recording subsidence phenomenon coeval with sedimentation (Fig. 3). Some of these synsedimentary structures are locally affected by normal and reverse fault planes. In some cases, this gravitational deformation also affects the underlying gypsum bedrock showing that dissolution also acts within the evaporitic sequence (Fig. 3). Exposures in some abandoned quarries located at the margin of the valley display the upper part of a dense network of vertical openings in the gypsum bedrock filled with sediments derived from the overlying P8 pediment cover. Most of these cavities, up to 5 m wide, have vertical and parallel boundaries and some of them taper downwards (Fig. 4). These filled fissures and 3.4. Reach IV (Cuarte– Huerva river mouth) In the lowermost reach (IV), up to the confluence with the Ebro River in Zaragoza city, the valley is excavated in thickened terrace alluvium and no bedrock exposures are found at the east margin (Fig. 2). Borehole data indicate that the deposits of the P8 pediment reach more than 30 m in thickness at the east valley margin. In the trench dug for the high-speed Madrid– Barcelona Fig. 6. Dissolution conduit more than 2 m in major axial length developed in the gypsum beds exposed in the trench of the highspeed Madrid – Barcelona railway at kilometer 328.5. The ellipsoidal section suggests a phreatic origin for this relict cavity. J. Guerrero et al. / Engineering Geology 72 (2004) 309–329 317 Fig. 7. The high-speed railway cutting exposing a cavity filled with alluvial deposits derived from an overlying and stripped mantled pediment deposit. pipes are interpreted as joint planes and intersections enlarged by dissolution acting beneath the detrital cover. Strike measurements indicate that these infilled grikes have dominantly E – W and N – S orientations coinciding with the regional joint trends. The characteristics of the gravely gypsiferous silts filling these cavities (matrix-supported texture, chaotic fabrics, massive or funnel-like structure) suggest that the pediment deposits moved downward as a water-saturated mixture probably with viscoplastic behaviour. It is likely that these openings were developed by groundwater flowing through the alluvial aquifer during the accumulation of the pediment deposits. Downstream of Cuarte, the deposits of terrace T8 thicken abruptly changing in 500 m from less than 4 Fig. 8. The high-speed railway cutting exposing showing the strongly weathered and brecciated gypsum bedrock. The gypsum blocks are associated with a karstic residue and fine-grained alluvium derived from a previously existing alluvial cap. 318 J. Guerrero et al. / Engineering Geology 72 (2004) 309–329 to more than 60 m in thickness. The alluvium – bedrock boundary, exposed in a recent cut at the east valley margin, plunges rapidly downstream and the overlying T8 terrace deposits show a progressive thickening in a cumulative wedge out arrangement (Fig. 5). In this sector, the subsequent terrace deposits rest on thickened alluvium rather than being inset into the bedrock. It has not been possible to elucidate whether the thickened alluvium belongs solely to terrace T8 or involves several terrace levels. The distribution of exposed terrace deposits and borehole data suggest that the sediments affected by synsedimentary subsidence fill a dissolution trough with an axis parallel to the valley and slightly displaced to the east margin. This trough may be composed of several basins (depocentres) as indicate the sharp and complex variations in the strike and dip of the terrace deposits. The clearly defined upstream limit of the thickened terraces could correspond to the edge of relatively thick salt deposits in the subsurface. In contrast to the upper reaches, the thickened terrace deposits have a high proportion of floodplain facies forming units up to 15 m thick. Grey and greenish marls deposited in swampy environments are relatively abundant and commonly occur in the Fig. 9. (a) Paleocollapse structure filled with insoluble residue and irregular terrace gravel bodies (gravel pockets) in a cutting for the high-speed railway situated at the west margin of the Huerva River valley. The Miocene substratum is bent and collapsed due to interstratal karstification. (b) Diagram of outcrop shown in (a). J. Guerrero et al. / Engineering Geology 72 (2004) 309–329 core of synforms that record the development of areas affected by differential subsidence. In the trench of the fourth Zaragoza ring-road excavated 50 m to the north of the high speed railway, the bottom of a large gravel channel several tens of meters coincides with the axis of a very gentle syndepositional synform. This coincidence suggests that the position of the channel has been controlled by the subsidence phenomenon. The recently dug trenches for the Madrid –Barcelona high-speed railway and a ring road in this reach of the valley show some excellent exposures of the alluvium – bedrock boundary. The fresh gypsum rock, locally with clay and marl beds up to several metres thick, displays dissolution conduits with subcircular sections up to 3 m in diameter (Fig. 6). Most of these pipes are filled by a detrital deposit derived from the overlying alluvial cover (Fig. 7). In some cases, the alluvial fill shows a concentric arrangement or a sheath fold-like structure. The boundary between the terrace deposits and the bedrock is generally very irregular. The fresh gypsum rock may grade into gypsum blocks with a variable proportion of a karstic residue (made up of grey marls with gypsum particles) and/or alluvium derived from overlying surficial deposits (Fig. 8). This gypsum breccia (packbreccia or floatbreccia) or the fresh gypsum passes into marl and silty material up to several meters thick frequently composed of a chaotic mass of angular fragments derived from the nonsoluble bedrock. This material has been inter- 319 preted as an insoluble residue, which represents the product derived from the dissolution of the evaporitic substratum at the alluvium – bedrock boundary. In addition to the large-scale structures generated by synsedimentary subsidence, the terrace deposits in this reach show numerous collapse structures reaching 70 m in width. Some of these structures also affect the adjacent Tertiary sedimentary rocks (Fig. 9). Commonly, the gypsum strata dip towards the collapse forming a synform that may be affected by subvertical failures surfaces (synthetic and antithetic) with several metres of structural throw (Fig. 9). In other cases, the gypsum is highly brecciated indicating upward migration (stoping) of dissolution cavities developed at some depth within the evaporitic bedrock (intrastratal karstification). In these cases, the disturbed Miocene sediments correspond to the upper part of a breccia pipe with an unknown vertical height. The development of this deep-seated karstification may be controlled by the presence of halite bodies at depth (Fig. 10). In several paleocollapse structures, gravel pockets of terrace alluvium up to several metres in size have been incorporated into the underlying karstic residue forming a chaotic jumble (Fig. 9). The mixture of both materials is related to liquefaction – fluidization processes triggered by the catastrophic collapse. All these observations indicate that these breakdown structures were formed during or soon after the generation of the alluvial levels when the alluvium was in a water-saturated state. Fig. 10. Interpretative diagram integrating the dissolution and paleosubsidence features found in the study area. Ductile and brittle deformation affecting the evaporitic Miocene bedrock is related to intraevaporitic (interstratal) karstification of soluble strata (probably halite). The subsidence recorded by alluvial Quaternary sediments overlying a karstic residue may be due to both suballuvial and interstratal dissolution. 320 J. Guerrero et al. / Engineering Geology 72 (2004) 309–329 4. Contemporaneous subsidence in the Huerva River valley Evidence of active subsidence have been identified in reaches III and IV of the Huerva River valley. In most cases, subsidence occurs on young alluvial levels and in areas with a relatively thin alluvial cover over the soluble bedrock. Examples of ground subsidence in a bare karst setting are very scarce. 4.1. Reach III (Cadrete – Cuarte) In this reach, the Zaragoza –Valencia motorway (N-330), buildings of Cadrete, Santa Fe and Cuarte villages and several industrial estates have been damaged by dissolution subsidence. Cadrete village is settled on T2 terrace level at the foot of a gypsum scarp. Borehole data indicate that the deposits of T2 terrace in Cadrete village are composed of an anomalously thick sequence (up to 18 m) of sand and silt sized deposits with intercalated gravel bodies. The bedrock is made up of gypsum and marls with interbedded halite lenses and layers up to 5 m thick occurring below a depth of 18 m. At the present time, many buildings in the old part of Cadrete are severely damaged by subsidence and others have been demolished (Fig. 11). In this village, the old water pipe network suffers from continuous breakage supplying additional water to the soluble bedrock. This extra water input accelerates the karstification and subsidence that in turn favours new water leakage acting as a self-accelerating process. Numerous buildings in Cadrete village built along the foot of the scarp are also threatened with slope movements. The 80-m high escarpment is made up of two clearly distinguishable units. The lower unit, 50 m thick, consists of an alternation of gypsum, clay and sand layers, with the insoluble beds being up to 5 m in thickness. The upper unit is composed of 30 m of gypsum beds with thin marl and clay partings. The regional jointing and stress release joints parallel to the scarp favour the separation of blocks. If it is considered that 220– 300 m of overburden has been removed, applying a density of 2 g/cm3, the overconsolidated scarp sediments have undergone an unloading of 44– 60 kg/ cm2 (4.4 –6 MPa). In the upper unit, small rock falls and topples are the main types of mass movements. These are especially abundant at the base of the unit due to natural undermining affecting the clay beds of the lower unit. The clay strata of the lower unit favour the development of curved failure surfaces giving rise to numerous rotational slides up to 2000 m3 in volume (Fig. 12). These rocks are Fig. 11. Severely damaged building due to dissolution subsidence next to a sinkhole in Cadrete village at the foot of the gypsum scarp. J. Guerrero et al. / Engineering Geology 72 (2004) 309–329 Fig. 12. Slump triggered by the excavation of a berm in an old rotational landslide in Cadrete gypsum scarp. There is a recently formed sinkhole next to a hammer in the foreground. also locally affected by cambering processes, which cause the downward bending of the strata towards the valley. The valleyward dip of the rocks favours lateral spreading and translational sliding of the brittle gypsum blocks resulting in rock falls and topples. The solutionally widened joints in the gypsum constitute preferential infiltration paths for the surface water. The downward flow through the karstified joints is interrupted at the top of the clay beds reducing their mechanical strength due to the increase in water content. This water input also increases the pore fluid pressure reducing the effective normal stresses and the shear strength of potential failure surfaces. In addition, gypsum and probably salt dissolution at the foot of the scarp further reduces the slope stability promoting the development of mass movements. This hypothesis is supported by the subsidence damage affecting 321 buildings next to the scarp and the generation of sinkholes (Figs. 11 and 12). Recently, a rotational slide has been reactivated due to the excavations carried out to build two water tanks at the toe and middle part of the scarp. The tanks are already damaged and leak an important quantity of water that favours dissolution and sliding. Buildings in the old part of Santa Fe village located on the T2 terrace have been damaged by ground subsidence. These include the Santa Fe convent built in the 14th century and reconstructed in the 18th century. This monument was abandoned due to stability problems and nowadays is almost in ruins. Although no borehole data have been obtained for this sector, the outcrops indicate that the thickened alluvial cover exceeds 15 m. Like in Cadrete village, it seems likely that subsidence in this part of Santa Fe is due to gypsum and probably salt dissolution. Other subsidence mechanisms also occur in the gypsiferous areas. Active subsidence at the opposite side of the valley is attributed to hydrocompaction of a 15-m-thick sequence of gypsiferous silts deposited in alluvial fans fed by lateral creeks (J. Torrijo, personal communication) The gypsiferous silts in the Ebro Depression are characterised by high porosity and loose packing with interstitial gypsum crystals that bound the skeletal grains (Salas et al., 1974; Artieda, 1993). Collapse tests carried out in the laboratory and in the field indicate that these sediments may undergo over 10% of compaction with the addition of water when subject to a load (Faraco, 1975; Jimenez-Salas et al., 1974). The rapid volume loss and the consequent ground settlement result from dissolution of the gypsum bonds and reorganisation of the skeletal grains with a tighter packing. At the present time, Santa Fe urban area is growing fast and new buildings have been constructed on these alluvial fan deposits without considering any subsidence mitigation measures. At the west margin, between Santa Fe and Cuarte, two circular sinkholes around 3 m in diameter and a meter deep have been found. Both collapse dolines damage N-330 motorway and have been formed in the beds of two lateral creeks that cross terrace T2. In addition, two human-induced sinkholes have formed next to two water tanks due to water leakage. All these observations agree with the past subsidence 322 J. Guerrero et al. / Engineering Geology 72 (2004) 309–329 activity recorded by the terrace deposits in this reach of the valley. 4.2. Reach IV (Cuarte– Huerva River mouth) At the southern bank of the Ebro River, the city of Zaragoza is built on T1, T2, T3 and T4 terrace levels of the Ebro and Huerva fluvial systems. The majority of the city lies on thickened and slightly cemented alluvial deposits. Boreholes drilled on the T2 terrace in the inner city found the evaporite bedrock at a depth of 55 m, several tens of meters below the current river channel. In Zaragoza city, the subsidence is uncommon and the hazard very low indicating that the thick and indurated alluvium minimise the development of sinkholes. Besides, the most soluble facies located close to the surface have been removed by dissolution during the deposition of the thickened alluvium. Most of the subsidence problems occur in peripheral areas of Zaragoza where there is a relatively thin alluvial mantle. The generation of sinkholes is especially frequent along the flanks of the Imperial Canal and particularly on the downgradient side (Fig. 2). According to Sastago (1976), the construction of this unlined irrigation canal was stopped a few kilometres downstream of Zaragoza city due to the continuous breakages produced by sinkholes. In 2002, failure of the canal and the consequent spill induced the catastrophic formation of two sinkholes close to the 3rd ring road. The largest one, 17 m deep and around 5 m in diameter with overhanging edges, occurred in February 16th on a crop field 60 m from the ring road. The smaller one, 5 m deep and 7 m across, formed in May 12th damaging a small construction (Fig. 13). In the coming years, serious problems may arise from subsidence affecting the new communication routes. During construction, several sinkholes have affected the trench dug for the Madrid – Barcelona high-speed railway located on pediment P4 to the west of the Huerva River valley. The trenches of the high-speed railway and the 4th ring road expose dissolution cavities in the gypsum bedrock and dramatic paleocollapse structures affecting alluvial deposits and bedrock (Figs. 3, 6– 9 and 14). These breakdown features warn about the presence of cavities at certain locations beneath the line of both communication routes. A clear spatial association between paleosubsidence features and active sinkholes related to paleokarst reactivation has been observed in the Jalón River valley (Ebro Basin) and the Alfambra River valley (Calatayud Neogene Graben) (Gutiérrez, 1998a,b). Several factors may favour the reactivation of the paleokarst exposed in the Fig. 13. Collapse sinkhole formed in May 12th, 2003 close to the third Zaragoza ring-road near the downstream side of the unlined Imperial Canal. J. Guerrero et al. / Engineering Geology 72 (2004) 309–329 323 Fig. 14. Paleosubsidence feature in a cutting for the high-speed railway at kilometer 328.5. The subsidence structures and the brecciated gypsum strata cause concern about the presence of cavities and unstable ground beneath the railway platform. trenches leading to the generation of new sinkholes: (1) the thickness of the sediments overlying the cavities has been reduced by excavation; (2) the loading and vibrations from heavy machinery reduce the mechanical strength of the highly brecciated sediments; and (3) the trenches act as new base levels that favour the circulation of groundwater beneath their platforms. Furthermore, in some stretches the highspeed railway runs next to irrigated crop fields. In spite of the obvious existence of an extensively developed endokarst, no protective engineering measures capable of spanning sinkholes have been applied during the construction of the transport routes. On March 1st, 2003, while the promotional trips on the high-speed railway were being performed, a sinkhole around 5 m in diameter with overhanging edges formed beneath the railway line (Fig. 15). The problem was dealt with by filling the doline with shuttered concrete. However, the sinkhole is the result of the upward migration by stoping or ravelling from a deeper dissolution cavity and these processes may still be active. Experience indicates that sinkholes in the area commonly suffer reactivations despite of being filled. During the review process of the article two new sinkholes have formed in the vicinity of the high-speed railway. A large number of recently active sinkholes has been reported in the Valdespartera Depression, at the west margin of the Huerva River valley (Fig. 2). This 6-km long elongated depression, interpreted as a karst polje (Soriano, 1993), has a flat bottom mantled by a thin veneer of gypsiferous silts and marls. The basin results from the erosional lowering of an E – W trending strip between the Ebro River pediment P4 and densely dissected gypsum outcrops, so it clearly postdates the pediment P4. According to the local farmers, the sudden occurrence of sinkholes is relatively frequent after irrigation. In addition, the bottom of this poorly drained basin has been flooded several times in the past few decades due to storm-derived high runoff. Fig. 15. Collapse sinkhole formed beneath the embankment of the high-speed railway in March 1st, 2003. This subsidence event occurred while the promotional trips prior to the inauguration of the railway were being performed. 324 J. Guerrero et al. / Engineering Geology 72 (2004) 309–329 The Valdespartera Depression has been captured by the headward erosion of an infilled valley at its northern edge. The bottom of this infilled valley has several sinkholes that disrupt drainage. Additionally, to the north of the depression, three surfaces of the T4 Ebro River terrace are tilted in different directions due to dissolution-induced subsidence. A new residential area is being built on the Valdespartera infilled valley. In the next years, the subsidence and flooding prone Valdespartera Depression will be the main urban growth zone for Zaragoza city. A large number of buildings (9687 flats), a museum, an aquatic park and a football stadium have been planned by the local administration for this controversial area. In this reach of the valley, the numerous paleocollapse structures found in the old alluvial deposits are in agreement with the sinkhole activity affecting areas mantled by a thin alluvial cover. However, it seems that the valley is not affected by the large-scale synsedimentary subsidence recorded by the thickened alluvium. Probably, this process is attenuated or cancelled out once the alluvium reaches a critical thickness and the most soluble facies located close to the surface have been removed by dissolution. 5. Construction of transport routes over sinkhole prone areas The prevention of subsidence hazards on transport routes such as roads or railways should start in the initial stages of the project avoiding as far as possible the subsidence features and the most hazardous areas. A well-conceived selection of the route requires the production of a detailed subsidence hazard map showing the ‘‘a priori’’ unstable areas (Fischer et al., 1993; Gutiérrez-Santolalla et al., submitted for publication). The first step for this purpose is the identification of surface subsidence features with aerial photographs. It is very useful to map with imagery from all the available dates, as sinkholes are frequently quickly masked or obliterated by human activities and natural processes. In the study area, numerous sinkholes are filled by farmers and developers soon after they form and many of them are hidden by buildings. Besides, the comparison of the different age maps may allow to recognise the most active areas and assess the frequency of sinkhole generation (van Zuidam, 1976; Gutiérrez-Santolalla et al., submitted for publication); an estimate of the hazard can be given as the number of sinkholes per year per unit area (Beck, 1991). The inexpensive and highly efficient aerial photograph interpretation needs to be complemented with a thorough field survey. Examination of the ground may help to pinpoint small subsidence features indistinguishable in the photographs. At this stage, it is generally very helpful to interview the local landowners and farmers in order to locate old (filled or degraded) and recent (post-aerial photographs) sinkholes. The obtained data allow us to produce maps showing the known locations affected by subsidence. The areas adjacent to the existing sinkholes and with a high density of dolines may be considered as highly hazardous areas (Cooper and Calow, 1998; Paukštys et al., 1999), especially in sectors where these features tend to form clusters. It is important to bear in mind that it is very difficult to evaluate the potential hazard of those areas with no surface subsidence features. Subsidence hazard maps should be considered as non-corroborated hypothesis with an unknown predictive capacity unless they are validated (Cendrero, 1997; Gutiérrez-Santolalla et al., submitted for publication). The next sinkhole could occur in any place. Once the line of the infrastructure has been selected, a subsurface exploration program is commonly conducted in order to detect cavities and areas affected by subsidence with no surface expression. This investigation generally includes drilling programs and geophysical surveys. Drillholes may easily miss subsurface cavities and stoping or ravelling structures unless an expensive program of deep and closely spaced drillholes is carried out. Geophysical methods have proved worthwhile to find hidden subsidence features and unstable areas in gypsum karst areas such as England (Patterson et al., 1995; Cooper, 1995, 1998) or China (Gongyu and Wanfang, 1999). Geophysical studies can help to locate anomalies for further focused drilling programs (Cooper and Calow, 1998). Along 1 km of the Madrid –Barcelona highspeed railway line, where several sinkholes occurred during the earthworks, georadar, microgravimetry and electric tomography surveys were performed. In order to test the accuracy of these methods in detecting underground cavities, the alluvial cover was excavated to show the actual distribution of shallow subsur- J. Guerrero et al. / Engineering Geology 72 (2004) 309–329 face voids. Electric tomography was the most faithful method although it failed in recording some cavities (Pérez, 2002, personal communication). In the studied alluvial karst setting, the cavities and dissolution conduits are generally difficult to detect since they are commonly filled by migrating detrital deposits derived from overlying surface sediments. In the stretches where the infrastructure runs in dug trenches, a very efficient way of recognising hazardous points lies in studying the paleokarst exposed in the excavated cuts. However, sectors flanked by undisturbed bedrock may be underlain by cavities and breakdown structures that propagate towards the surface. A very common mitigation measure in carbonate karst areas is to improve and strength the ground filling the voids by grouting (Sowers, 1996). This method may be insufficient for the construction of a road or railway in the studied evaporite karst area due to several reasons. The paleokarst features demonstrate that many sinkholes result from the upward propagation of deep-seated voids. Very likely a grouting program would only fill shallow voids leaving unstable cavities at a greater depth. Grouting may also block most of the flow paths concentrating underground flow along particular conduits favouring focused karstification. According to Cooper (1995, 1998) grouting could accelerate dissolution of the evaporite bedrock adjacent to the grouted area. If grouting is carried out, sulphate resistant cement should be used. Because it may not be possible to locate all the unstable areas, in some cases the safest and most cost-effective approach is to build the whole structure with protection measures. Conducting an exhaustive investigation looking for subsidence features that are difficult to remediate may be impractical (Cooper and Saunders, 2002). Roads can be reinforced incorporating tensile membranes or geogrids in the road sub-base and embankment. This structure allows the road to bend but prevents catastrophic collapse. Once subsidence has occurred its location will be obvious and some remedial measures can be undertaken. This protection measure has been applied to the Ripon bypass in northern England that crosses an area underlain by Permian gypsum formations and affected by catastrophic subsidence (Kempton et al., 1996; Cooper and Waltham, 1999; Cooper and Saunders, 2002). 325 Problematic sectors can be equipped with electric settlement monitoring devices linked to a warning system. Benchmarks and a network of inclinometers and extensometers have been installed in viaducts over gypsum karst areas around Paris (Arnould, 1970). A network of light transmitters and receptors capable of detecting small deformations and connected to an alarm system are used around Sachensburg village (Harz Mountains, Germany). The high speed railway requires special constructive protection measures as even a slight settlement may cause the derailment of the train running at 350 km/h. Bearing in mind the priority of public safety, the use of continuous rigid structures beneath the railway with a sufficient strength to span the larger sinkholes is recommended. This structure could be composed of reinforced concrete slabs supported by heavy-duty load-bearing beams (Braja, 1998). Obviously, the key design parameter would be the maximum diameter of the sinkhole at the time of formation. This information can be obtained from the geomorphic record, paleokarst and the numerous documented subsidence events. Since the structure will not allow the detection of sinkholes generating beneath the railway, an electronic monitoring system could be used to measure changes in the load bearing points of the structure. In addition to these measures, an added degree of security could be obtained by piling those areas that are undergoing active subsidence and treating the railway effectively as a bridge structure lying on the ground. Additional mitigation measures include the use of highly efficient drainage systems to avoid seepage of surface water, plus the lining of irrigation ditches and channels in the proximity of the line and the delineation of a protection zone preventing irrigation and water withdrawal. 6. Discussion and conclusions The morphostratigraphical evolution of the lower course of the Huerva River valley has been affected by subsidence phenomena caused by dissolution of salt and gypsum bearing evaporitic bedrock. The subsidence recorded in the alluvial deposits (terraces and pediments) has been controlled by the lithostratigraphical composition of the bedrock. Four reaches 326 J. Guerrero et al. / Engineering Geology 72 (2004) 309–329 have been differentiated on the basis of the type and amount of subsidence. The deposits of the terrace levels overlying detrital sediments in reach I are less than 4 m thick and remain undeformed. In reaches II and III, the alluvial deposits are affected by local thickening, gravitational deformation and paleocollapse structures generated by dissolution acting at the alluvium – bedrock boundary. Downstream of Cuarte village (reach IV), the terrace deposits are affected by large-scale synsedimentary subsidence reaching more than 60 m in thickness. This thickened alluvium fills a dissolution trough parallel to the valley and over 6-km long. The terrace and pediment sediments are also affected by numerous ductile and brittle gravitational deformations including spectacular subsidence structures (structural basins, synforms, fault-bounded collapses). In this reach, the subsidence generally affects to both alluvial cover and the underlying bedrock sediments indicating that dissolution acts both at the alluvium –bedrock boundary (alluvial karst) and within the evaporitic substratum (interstratal karst). Very likely, the development of an intraevaporitic karst results from the interstratal dissolution of salt layers. Suballuvial karstification is corroborated by the presence of a relatively thick insoluble residue between the alluvial deposits and the fresh bedrock. The rapid formation of some collapse structures has produced fluidization structures including irregular bodies of gravel embedded in a fine matrix, the so-called gravel pockets. These soft sediment deformation structures resulting from local ground acceleration induced by catastrophic sinkhole formation may be used as indicators of the subsidence kinematics. Evidence of contemporaneous subsidence has been found in reaches III and IV, usually in low alluvial levels with thin cover. Sinkholes in the bare evaporite outcrops are very uncommon. The scarcity of sinkholes in areas with indurated thickened alluvium, like Zaragoza city, is attributed to the greater mechanical strength of the cover and the absence of highly soluble rocks close to the surface because of their extensive previous dissolution. Active subsidence affects numerous buildings, important communication routes and arable land. In Santa Fe village, subsidence damage is also explained by hydrocompaction of highly collapsible gypsiferous silts. In many places, such as on the flanks of the Imperial Canal, the sinkhole hazard is clearly aggravated by changes in the hydrogeological conditions through leakage and the additional supply of groundwater. Some stretches of the recently built high-speed Madrid –Barcelona railway cross alluvial surfaces highly prone to sinkhole formation, as demonstrated by the geomorphic evidence and historical records. Clearly, a similar degree of activity may be expected in the future in these areas. In addition, the paleocollapse structures exposed in other sections of this railway warn of the presence of underground cavities. In addition, human activity in these areas could reactivate the paleokarst and lead to dangerous sinkholes. Since it is not feasible to stop the subsidence, correction measures should be preferentially focused on the protection of infrastructure, with special construction design and the detection with monitoring systems. Even slight deformation could cause the derailment of a train running at 350 km/h. Wenhui (1990), in a compilation of damage to Chinese railways related to geological processes, indicates that karst ground collapse accounts for 55% of yearly traffic suspensions. Two of the 60 dissolution subsidence occurrences caused train crashes. The incorporation of tensile membranes (geogrids) in the embankments of the roads is suggested. Settlement in problematic locations could be monitored with electronic sensors linked to a warning system. Continuous rigid structures with a sufficient strength to span the largest sinkholes are recommended for the high-speed railway because even The generation of sinkholes beneath the structures could be detected by installing electronic load monitoring devices in the supporting structure. Additional mitigation measures include high efficient runoff drainage, the lining of irrigation ditches and channels and the delineation of irrigation and water abstraction exclusion zones at both sides of the line. This study reveals that, with the exception of the areas with thickened alluvium, there is a good agreement between the paleosubsidence recorded in the alluvial deposits and the current sinkhole activity. This shows that the study of the paleokarst can be used as a complementary prediction tool for assessing subsidence hazard and locating critical points. The paleosubsidence structures supply crucial data for understanding the present-day subsidence phenomena. J. Guerrero et al. / Engineering Geology 72 (2004) 309–329 Acknowledgements The authors would like to thank to Dr. Anthony H. Cooper (British Geological Survey) and Dr. Kenneth S. Johnson (Oklahoma Geological Survey) for the thorough review of the paper, and to Javier Torrijo, Octavio Plumed and Miguel Pérez for providing information about subsidence activity and salt deposits in the area. We are also grateful to Dr. Concha Arenas for helping in the interpretation of the Neogene sediments and insoluble residues. 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