IDENTIFICATION AND MITIGATION OF SINKHOLE HAZARDS IN AN EVAPORITE KARST AREA (PERDIGUERA, SPAIN)

Sinkhole risks are becoming particularly severe in urban areas that lack careful planning and where karst depressions are frequently filled and developed. Sinkholes frequently have a higher probability of occurrence and a bigger genetic diversity in evaporite rocks than in carbonate rocks. This is because evaporites rocks (halite, gypsum, etc) have a higher solubility. Subsidence damage resulting from this dissolution generates considerable losses at the world. To contract with these risks, is needed the identification, investigation, prediction, and mitigation of sinkholes. Corrective measures might be applied to reduce the subsidence processes. A more practical solution for safe development is to reduce the vulnerability of the structures by using subsidence-proof designs. Therefore, this case study is located in the town of Perdiguera (Zaragoza, Spain), within the Ebro Basin. This town is affected by subsidence problems, which are associated with the dissolution of gypsiferous silts that generate sinking. These sinkholes are affecting the buildings threatening its structural integrity.


Sinkholes: a natural risk
The term Sinkhole is a Slavic word used by geologists to designate closed depressions that present karst ground, those in which dissolution processes play a decisive role in the configuration of geomorphology. Sinkholes can be related to the dissolution of carbonated rocks (limestones and dolomites) or evaporitic rocks o soils (gypsums and salts), being the above mentioned much more soluble.
According to their genesis the sinkholes can be of two types (Gutiérrez et al., 2005(Gutiérrez et al., -2006: (1) Solution sinkholes, when they are due to differential karstification on the surface of soluble rock or soils. Solution sinkholes are commonly shallow depressions that may reach up to several meters across (Figura 1A). In Figure 1B is showed a schema of solution sinkhole.
(2) Collapse sinkholes, in which subsurface karstification of soluble sediments causes gravitational deformation of the suprayacent sediments and the gradual or abrupt subsidence of the topographical surface.
Sinkholes, in addition to having a great interest from a geological point of view, in certain areas have a very relevant economic and social impact because of the risks and impacts that can arise from them.
According to Gutiérrez et al. (2008) and Galve et al. (2009), Spain is quite possibly the European country where the subsidence hazard related to evaporite karstification has the greatest economic impact, primarily because this type of Thus, various authors have studied the impact of karstic subsidence on the evolution of fluvial systems (i.e. Gutiérrez and Arauzo, 1997;Benito et al., 1998;Guerrero et al., 2008aGuerrero et al., , 2008bGalve et al., 2009Galve et al., , 2015Guerrero, Gutiérrez, 2017).

The town of Pediguera
The town of Pediguera is located in the Ebro Basin, about 25 km northeast of Zaragoza, the capital of Aragon. Specifically, the area known as Los Monegros district ( Figure 2). Its population is about 600 inhabitants and has an area of 110 km 2 . This district (Los Monegros) is characterized by a landscape of desert character (Figure 3). This is a consequence of aridity caused by low precipitations, wind, erosion and endorreism.
Wide surfaces are occupied by areas of natural vegetation that interspersed with the crop fields, highlighting the steppes, the flat-top reliefs ("muelas"), gullies and salt-lakes ("saladas"). Earthen architecture is traditional in Los Monegros. Also, it seems to show clearly that the adobe has been commonly used throughout the region constituting one of the possible materials (next to masonry and mud walls) with which to build the buildings of popular architecture ( Figure 4).
In this context, when Perdiguera is visited, we can observe a great number of buiding ruins in the town and surroundings ( Figure 5). And besides, solution sinkholes are affecting the buildings threatening its structural integrity. This is of great social significance because it prevents, and has prevented, a suitable development of the economy. It is also a source of problems, both for the people living in this town and for the municipal authorities.

GEOLOGICAL-GEOTECHNICAL CHARACTERIZATION OF AREA
From the geological point of view, Perdiguera is situated in the central sector of the Ebro Tertiary Basin, formed by the sedimentary contributions from the three mountain ranges that frame it (Pyrenees, Cordillera Ibérica and Coastal-Catalan Chains). It is filled by various materials in the Miocene, sedimented in marine and continental environments ( Figure 6).
The latter deposits include from facies of alluvial fans in the margins of the basin (sandstones and claystones) to beach-lake facies in the center of the basin (with carbonated, gypsiferous and halitic deposits). The Zaragoza Gypsum Formation (Quirantes, 1978) is the most important gypsiferous formation in this sector of Depression. In this sector, it presents massive, concretional and noduly gypsums. There are small intercalations of shales, marls and marly limestones. Its colors are white and white-grayish. Small levels of anhydrite, epsomite and halite are also common in them (Mandado, 1987). According to Klimowitz (1990), this formation locally reaches 800 m in thickness. In the subsurface the formation is primarily composed of various evaporitic minerals: i.e. halite, gypsum/anhydrite and glauberite (Salvany et al., 2007;Guerrero et al., 2008aGuerrero et al., , 2008b. Since the beginning of the Quaternary, the installation and structuring of the fluvial network has occurred. This produced the erosion of tertiary materials and a very important alluvial sedimentation. On the one hand directly linked to the Ebro river and its tributaries (alluvial terraces); and on the other hand, controlled by the surrounding tertiary reliefs and connecting these with the river courses (glacis).
The logs of boreholes realized next to Perdiguera have been gathered. Three geotechnical units have been differentiated: (1) Glacis deposits. This unit consists of brown gypsiferous silts and clays. Its thickness in the area reaches 15 m ( Figure 7A).
(2) Clays with gypsum. It corresponds to a dark gray karstic residue developed at the top of the bedrock. Its thickness varies from 3 to more than 10 m ( Figure 7B). (3) Gypsum bedrock. Constituted for gypsum beds with interbedded marls. It is fresh bedrock and it has a asymmetrical geometry ( Figure 7C).
On these units, anthropic waste and natural sinkhole fill deposits have been located in some points. Its thickness is variable.

THE IMPACT OF HUMAN ACTIVITY
Perdiguera is affected by subsidence problems, which are associated with the dissolution of gypsiferous silts (glacis deposits) that generate sinking.
The equilibrium solubilities of halite (NaCl) and gypsum (CaSO4.2H2O) explains why in evaporite karst cavities form and develop very quickly. Besides, these minerals have a appreciably lower mechanical strength and a more ductile rheology than the majority of soluble rocks. Additionally, these rock masses may deteriorate substantially at a human time-scale by quick dissolution guided by discontinuities. According to Gutiérrez et al. (2014), these circumstances explain why sinkholes in evaporite karst areas may be formed by a wider diversity of subsidence phenomenon.
Often, changes in the karst environment (naturals or anthropogenics) could accelerate the generation of sinkholes too, favoring or activating their incidence or revival.
In this sense, the foundations of most of the houses of Perdiguera, are based on glacis deposits (gypsiferous silts). These silts, in water-free conditions, have medium-strength characteristics (Torrijo and Cortés, 2008). However, at the moment of interaction with it, two processes are generated: (1) A redistribution of its structure (which at first is very open and supported by sulphated point cements, gypsum).
These two processes generate a significant decrease in the resistance of these deposits and generate important collapse processes by the appearance of hollows (dissolved areas). There is also another detonating cause of this subsidence process due to human action: The existence of leaks and breaks in the water and sewer network (Figure 8) causes the collapse of the gypsiferous silts, whose visible effect is the cracks and subsidence detected in the houses and on the streets.
These collapse processes produce much pathologies in Perdiguera buildings, as shown in Figures 9 and 10. Thus, finally, to explain in detail the process that occurs on the ground, it must be remembered that the silts present an open structure, in the form of "house of cards". This process is typical in saline sedimentary environments, where silt and clay particles form aggregates (small flocculates) and settle together in a random pattern ( Figure 10A). Then, when the reorientation occurs due to the inflow of water (similar to freshwater sedimentary environment), these particles tend to accumulate in a dispersed structure with a parallel orientation ( Figure 10B).
This house of cards structure, together with its gypsum component (highly soluble), makes them respond to an entrance of water to the ground diminishing its volume for reorganization of its structure (phenomenon of subsidence-collapse).

Sinkhole risk identification
According to Gutiérrez et al. (2014), the most important step in sinkhole hazard analysis is the construction of a comprehensive cartographic sinkhole inventory.
Depending on the information available, according to Galve et al. (2009b) and Gutiérrez et al. (2014), two types of models can be produced to predict the occurrence of future sinkholes: susceptibility models and hazard models.

Sinkhole risk mitigation
Several strategies (preventive measures) could take to reduce or eliminate the risks (economics and socials) related to sinkhole activity. According to Paukstys et al. (1999) and Buttrick et al. (2001), these measures could apply limiting development (or prohibiting) in the most hazardous areas through land use planning and regulations based on sinkhole susceptibility and hazard maps. In this sense, Gutiérrez et al. (2014) say: In order to minimize the impacts and hazards on these vulnerable and complex areas, man is head for learning how to "live with karst".
In the specific case of Perdiguera, in its land use planning and regulations, we can propose the next corrective measures: (1) Controlling the water table.
(6) Using grouting to fill cavities in the ground.
(7) Increasing the strength and bearing capacity of the soils by compaction grouting.
And In the specific case of the houses with pathologies, the following corrective measurements are proposed: (1) Review of the existing water and sewer network and replacement, if necessary, by pipes built with flexible materials. This will prevent the abrupt breakage of the pipes by a specific failure in the ground.
(2) Underpinning of existing foundations. In this sense, the solutions could go through the execution of micropiles, consolidating injections, etc.
(3) Future topographical control of the vertical and horizontal movements of the building, to confirm that the treatment is being effective.

CONCLUSIONS
The conservation of architectural, cultural and social heritage is a fundamental task for the preservation of our history and culture. However, nowadays, as in the past, the risk of natural disasters, such as subsidences, threatens this heritage constantly. Within Spain architectural heritage, earthen architecture is one of the main building traditions and is often threatened by natural hazards.
In this sense, Perdiguera is affected by subsidence problems, which are associated with the dissolution of gypsiferous silts (glacis deposits) that generate sinking. These sinkholes are producing much pathologies in Perdiguera buildings.
Thus, through the study of the case of Perdiguera, this research wants to provide a detailed analysis of a natural hazard called sinkholes, and also identifying conservation strategies. So, some corrective measures have been proposed in land use planning and regulations of Perdiguera.
Finally, the definition of these strategies has a specific and general nature in order to be applied not only to the case study, but also to similar cases.