Gypsum/anhydrite - Springer Link

Gypsum/anhydrite: some engineering problems Is¸ık Yılmaz

Abstract The engineering properties of gypsum/ anhydrite change as a consequence of groundwater, pressure and heat. Gypsum and anhydrite may experience dissolution in the near-surface zone and along the discontinuities, creating karst terrain to depths related to present and/or past groundwater levels. This paper reviews the problems of constructing on gypsum/anhydrite and their engineering significance and provides short case histories to illustrate some of the problems encountered. Résumé Les caractéristiques mécaniques du gypse et de l’anhydrite varient en fonction de la présence d’eau, de la pression et de la chaleur. Le gypse et l’anhydrite peuvent être soumis à des processus de dissolution à proximité de la surface et le long de discontinuités, permettant ainsi le développement de réseaux karstiques en profondeur, dans des zones en rapport avec les niveaux piézométriques actuels ou anciens. L’article analyse les difficultés techniques liées à la construction sur les formations de gypse ou d’anhydrite et présente quelques courtes études de cas pour illustrer certains des problèmes rencontrés. Key words Gypsum 7 Anhydrite 7 Dissolution 7 Karst 7 Heave Mots clés Gypse 7 Anhydrite 7 Dissolution 7 Karst 7 Gonflement

Introduction As gypsum is susceptible to rapid dissolution wherever there is active circulation of groundwater that is undersaturated with respect to calcium sulphate, substantial underground voids and cave systems may develop. The

Received: 19 February 2000 7 Accepted: 13 May 2000 I. Yılmaz (Y) Cumhuriyet University, Faculty of Engineering, Department of Geology, 58140, Sivas, Turkey

spontaneous collapse of individual caverns allows upward migration of voids, either by gradual caving of thinly bedded strata or (occasionally) by the sudden failure of more competent, thickly bedded rocks, leading ultimately to subsidence of the overlying ground surface (Thompson et al. 1998). As a foundation material, gypsum differs from other rocks in that voids may be found at almost any depth within the rock mass. They may result directly from solution weathering near the surface and along discontinuities, or as specific cave systems at depths related to present or past groundwater levels. Problems caused by the presence of the gypsum/anhydrite rock and their engineering significance are outlined in this paper. The two basic problems that occur are subsidence due to the solubility characteristics of gypsum and heave due to the hydration of anhydrite (to form gypsum). Calcium sulphate is found throughout the world and hence the problems discussed in this paper are relevant to most countries. This paper uses short case histories to illustrate some of the problems that may be encountered during construction in areas of gypsum/anhydrite bedrock.

Engineering problems associated with gypsum and anhydrite Dissolution As noted above, gypsum is susceptible to rapid dissolution wherever there is active circulation of groundwater that is undersaturated with respect to calcium sulphate. Gypsum is more soluble than limestone: 2.5 kg of gypsum can be dissolved in 1 m 3 by water according to the following equation: CaSO472H2OcH2O ] Ca c2cSO P2 4 c3H2O Chemical solution tends to occur along existing discontinuities, e.g. joints and faults. Thus, until a late stage of development, karstic gypsum terrains usually have cavity patterns related to the discontinuity geometry. In a closed system the water quickly becomes saturated, hence both dissolution and precipitation are possible. Karst features in the gypsum usually consist of a network of small-diameter passages (Culshaw and Waltham 1987). If the gypsiferous rock is massive and jointed, large openings are created along preferential flow paths. Culshaw and

Bull Eng Geol Env (2001) 59 : 227–230 7 Q Springer-Verlag

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Waltham (1987) describe the gypsum beds in the Podolie region of the Ukraine (Russia) which include hundreds of kilometres of joint-controlled caves/passages greater than 5 m in height and width. Karacan and Yılmaz (1997) also reported similar networks of joint-controlled caves/ passages in gypsum in the south-west Sivas area of Turkey. Collapse passages are a characteristic feature peculiar to gypsum karst. Cover rocks start to collapse slowly, as a result of dissolution of the rocks (Bögli 1980; White 1988). According to White (1988), collapse passages and are developed longitudinally may be 100–1000 m in width and 1–15 km long. Massive gypsum covers a large area around Sivas, Zara and I˙mranlı which is extensively karstified with numerous sink holes and depressions. Hafik Lake to the east of Sivas, Tödürge Lake to the west of Zara and Ulas¸ Lake to the south of Sivas now occupy the karst depressions. Most of the sink holes were formed along fault traces and groundwater discharge to the river can be observed in some places (Alagöz 1976; Karacan and Yılmaz 1997). In addition, dolines and collapse passages play an important role in the groundwater flow system (Karacan and Yılmaz 1997). Subsidence can be regarded as the vertical component of ground movement and clearly has serious effects on buildings, services and communications. It may take place gradually, almost imperceptibly, or it may occur quite suddenly, affecting areas as small as a few square metres or as large as many square kilometres (Lundgren 1999). Gypsum is more soluble than limestone: 2100 mg/l of gypsum can be dissolved in non-saline waters compared with 400 mg/l of limestone; hence sinkholes and caverns can develop in thick beds of gypsum more rapidly than in limestone terrain. Being weaker than limestone, gypsum does not have the same arching potential and hence collapses more readily. Once passageways have developed in massive gypsum, seepage flow rates increase such that the dissolution proceeds in a rapidly accelerating manner (Bell 1993). Even in areas where active dissolution of gypsum is taking place, the nature and the time scale of any associated subsidence will vary according to both the size of the individual cavities and the thickness and geotechnical properties of overlying strata. Cooper (1988), however, has shown that in the Ripon area of the UK, cavities from the relatively shallow gypsum deposits can eventually migrate to the surface if the thickness of the overlying non-soluble material is not sufficient to choke the migrating void. If the foundations of a dam contain soluble minerals, water seeping through the gypsiferous rocks may create voids and any previously dry fissured material under substantial hydraulic gradients will suffer dissolution and increasing seepage losses. Conglomerates cemented by soluble material will clearly also suffer significant loss of strength such that the integrity of the foundations is compromised (James and Kirkpatrick 1980). The St. Francis Dam, for example, was constructed in Los Angeles in 1926 but by 1928 had been destroyed due to the dissolution of the conglomerate with gypsum cement in 228


the left abutment. The damage and associated costs were considerable, but, more importantly, 400 people died as the result of this disaster (Ransome 1929). The oldest dam known to be affected by gypsum karst is the McMillan Dam in New Mexico. Seepage and collapses occurred on the left side of the dam, following which springs with a high discharge developed downstream of the dam. Despite considerable efforts in 1909, the seepage in the left abutment could not be stopped (Brune 1965). In England, Ripon (North Yorkshire) suffers the worst subsidence caused by gypsum dissolution. Here, at least 30 major collapses have occurred in the past 150 years and further collapses are likely. The subsidence hollows are commonly 10–30 m in diameter and reach 20 m in depth; east of the city they attain dimensions of up to 80 m diameter and may be 30 m deep. Numerous sags and small collapses on farmland go undetected, but careful analysis of historical levelling data suggests some areas of general ground lowering. Many of the subsidence features at Ripon have a reticulate pattern related to the intersections of the joint systems in the rock. In the urban area of Ripon, the catastrophic subsidence has caused about ^1,000,000 worth of damage in the last 10 years and has generated problems for both planners and engineers (Cooper 1998). The Marchalico Viicas landslide is a marl debris flow carrying numerous large gypsum blocks. The flow, 3 km in length, is traversed by the motorway and reaches Rio Aguas, where groundwater springs and resurgences emerge from the base of the gypsum escarpment. These egresses were the water source for the now abandoned hamlet of Marchalico Vinicas (Eyers et al. 1998). Heave problems Calcium sulphate occurs naturally in two different forms: as anhydrite (CaSO4) and as gypsum (CaSO4.2H2O). The stability of both is a function of temperature, the amount of water available and pressure. Only anhydrite is stable above 58 7C at a pressure of about 100 kPa. Generally, only gypsum is stable below 38 7C, although when water is not present, anhydrite may still occur but in a metastable form at these lower temperatures. When water can react with anhydrite, the difference in specific volume between the initial anhydrite and the subsequent gypsum is as shown in the equation below (Wittke 1990): ANHYDRITE ] GYPSUM CaSO4 c 2H2O ] CaSO42H2O (46 cm 3) (36 cm 3)

(74 cm 3)

A comparison of the specific volumes indicates that the specific volume of crystallised gypsum is greater than that of anhydrite by DVp[(74P46/46)!100]p61%. In contrast, as shown by the above equation, anhydrite with two molecules of water possesses a volume 11% greater than that of crystallised gypsum. The anhydrite is metastable especially at shallow depths and tends to rehydrite to gypsum as it comes into contact with circulating groundwater within the near-surface zone (Murray 1964; Holliday 1970; Mossop and Shearman 1973). This involves a volume increase of between 30 and 58%

Gypsum/anhydrite: some engineering problems

(Blat et al. 1980) which exerts pressures which have been variously estimated at between 2 and 70 MPa. It is likely that such hydration can take place relatively quickly. When it occurs at shallow depths it causes expansion, but this process is gradual and is usually accompanied by the removal of gypsum in solution. At greater depths anhydrite is effectively confined during the process. This results in a gradual buildup of pressure and the stress is finally liberated in an explosive manner (Bell 1993). Transition between gypsum and anhydrite in both directions of the chemical reaction causes changes in volume between the original and resultant systems on both sides of the reaction. Because this transition results in changes in the crystalline structure of gypsum and anhydrite, calculation of volume changes in solid phases should be based on the molar volumes of gypsum, anhydrite and water. The volume change due to the hydration of anhydrite is c62.6% for open systems, whereas it is P9.0% for closed systems. Similarly, the volume change due to dehydration of gypsum is P38.5% for open systems and c9.9% for closed systems (Zanbak and Arthur 1986). A closed system is defined as an environment where water is trapped with calcium sulphate minerals before and after the transition and an open system as an environment where free water may enter into the hydration process or be released during dehydration and leave the system. Actual volume changes are controlled by the porosities before and after the transitions (Zanbak and Arthur 1986). Before the 1960s, structures built on anhydrite were often oversized. A classic example is the Czernitz tunnel (Poland) which was excavated in an anhydrite formation in 1858 (Yüzer 1982). Redfield (1963) cites the case of the Vobarno tunnel in Italy through anhydrite and gypsum formations. Completed in 1931, it gave no trouble until 1940 when it suddenly began to crack, fracture and progressively heave, such that the concrete lining disintegrated to rubble. Brune (1965) reports that one night in June 1948 a loud boom was heard in the small town of Paint Rock, Texas. Soon afterwards it was discovered that uplifting of the rock had occurred on the Smith Brothers’ ranch in the nearby Kickapoo Creek. The uplift extended along 300 m of the stream channel and reached a maximum height of 3 m above the previous channel bottom. In 1954 a similar “explosion” took place 11 km north of Moran, Texas. A farmer ploughing heard an explosion and saw a cloud of dust and debris at a distance. He later found that about 300 m of stream channel along Deep Creek had risen by as much as 6 m, scattering rock fragments over the surrounding countryside (Brune 1965).

Other problems Serious structural damage can be attributed to heaving and settlement of soils containing anhydrous calcium sulphate, which is exacerbated when these types of soil are periodically and/or differentially exposed to wetting. Conversely,

even without a rise in the level of the groundwater table, expansive clay minerals in the vicinity of dehydrating gypsum can take the liberated water of crystallisation and expand to lift existing structures (Azam 1997). Dehydration of gypsum is associated with a volume decrease of up to 38%, which may lead to excessive settlement of the overlying structures. Furthermore, shrinkage in the gypsum layer and the pore pressure effects of released water from the crystal structure of gypsum, change the state of stress within the sediment, which may cause significant deformation and fracturing (Ko et al. 1995). The occurrence of expansive clay minerals within evaporites also exacerbates the potential problems. Montmorillonite, illite and corensite are the most typical clay minerals associated with evaporites. Many tunnels in Switzerland and Germany which were excavated in the Keuper Series exhibited deformations of up to 50–60 mm. Initially, anhydrite was thought to be responsible for these deformations. However, subsequent research and observations since 1960 have demonstrated that in addition to anhydrite, swelling clay minerals such as corrensite and montmorillonite may also have played a part (Grob 1972; Götz 1978; Fecker 1980; Yüzer 1982). In gypsum karst terrain, the probability of encountering small or large voids during tunnel construction is likely to be high. These hollows may cause instability and there may be water discharge related to them. Where such hollows occur, there is likely to be a considerable increase in grout take in order to infill these voids. Hawkins (1979) describes the presence of such voids and the extra grout needed in the Keuper Series of the Bristol area (UK). Where tunnels pass through gypsiferous horizons and the lining is not impermeable, dissolution may occur outside of the lining such that the integrity of the tunnel may be compromised after completion. Gypsum can cause serious hazards when it acts as a cementing agent as dissolution of the cement can result in the breakdown of the soil structure, the leaching of the fine fraction and the development of soil pipes (Abduljauwad and Al-Amoudi 1995). Anhydrite dissolves even more catastrophically; even small fissures are enlarged, swiftly producing a rapidly deteriorating situation (James and Lupton 1978). The formation of cavities due to leaching of gypsum and anhydrite in the subsoil may trigger the collapse of light structures without adequate warning (Azam et al. 1998). For the reasons discussed above, it is essential to investigate the presence of any gypsum lenses that occur in the near-surface zone where roads are to be constructed (Livneh et al. 1998).

Conclusions Problems associated with gypsum/anhydrite exist throughout the world. The two main types of problems related to these forms of calcium carbonate which are of particular significance for engineering construction are


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dissolution and expansion, with or without the presence of expansive clay minerals. Dissolution is the more common. At both design and construction stages, it is important to make allowance for the possible change in nature of the minerals as a consequence of groundwater, pressure and heat. Adequate design of structures on gypsum/anhydrite should involve a comprehensive and detailed investigation of any karstic features and a full appraisal of the most appropriate foundation designs in the light of these. Successful designs include reinforced waffle/rigid concrete slabs and reinforced concrete piers with beam support.

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