Crystal Growth under Bridge Foundations

A high-speed railway viaduct experienced a sustained heave at high rates after its construction. The pillars of the bridge are founded on an Eocene hard anhydritic-gypsiferous claystone by means of massive foundations. Field investigations identified an active layer below the piles’ tips where expansions occur as a result of gypsum crystal growth in discontinuities from supersaturated aqueous solutions. The construction of the bridge is supposed to have triggered the swelling mechanism. An embankment, partially filling the valley, was built to slow the heave rate. The presence of soluble gypsum and anhydrite and inert materials is considered in the model developed to reproduce the observed expansive mechanism. The formulation describes the kinetics of dissolution and precipitation of minerals and the induced swelling strains by crystal growth. The calculated heave is compared with the field long-term vertical displacements measured before and after the construction of the embankment. A sensitivity study is also reported. 1. Heave of Pont de Candí viaduct Pont de Candí viaduct belongs to the high-speed railway Madrid-Barcelona line in Spain. The construction of the bridge finished in July 2002. The viaduct is located between Camp Magré tunnel and Lilla tunnel, which experience also swelling damage. The bridge deck has a length of 413 m divided into 10 spans (35 m; 8 x 43 m and 34 m). The continuous deck has a box cross section with a thickness of 3.5 m. The deck is supported by 9 pillars (P1 to P9) with a maximum height of 56 m in the centre of the valley. Figure 1 shows a general view of the bridge in December 2007. Each pillar is founded by means of a massive foundation consisting of a group of 3 x 3 bored piles, 1.65 m in diameter and 20 m long on average, capped by a rigid slab as shown in Figure 2. The thickness of the pile cap in pillars P3 to P8 is 3.5 m. Systematic levelling of the railway tracks, carried out by the railway administration since the end of the construction, revealed that the four central pillars of the bridge, especially pillars P5 and P6, were experiencing a sustained heave. The topographic levelling of the structure showed that in the period from September 2002 to May 2007 a maximum vertical displacement of 20 cm was reached at deck level between pillars P5 and P6. The heave measured at the structure evolved at high rates ranging from 5 to 10 mm/month. The valley where Pont de Candí viaduct is placed is located in the eastern boundary of the Ebro river depression. The valley is the result of intense tectonic activity, which resulted in faults and folded strata. Sediments belong to old Tertiary formations. The valley is covered in its central part by a 5 meters thick deposit of soft alluvium and colluvium soils. These Quaternary deposits overly a 15 m thick layer of brown clays and gypsum levels of Eocene age. Below, there is a harder red claystone formation with variable contents of anhydrite and gypsum, also of Eocene age, which was judged strong enough to support the large diameter piles. A continuous horizontal gypsum layer (0.6-2 m thick) divides the red claystone unit into an upper and a lower level. The upper claystone level presents high gypsum content. Anhydrite was not detected in this upper level. In sharp contrast, the lower claystone unit below the thin continuous gypsum layer has high anhydrite content and an increased strength. The sustained heave rates measured at the structure were incompatible with the expected circulation of trains at high speeds in excess of 300 km/h. A monitoring campaign of the structure and the subsoil was launched to understand the reasons of the vertical displacements observed in the bridge. Figure 1. General view of Pont de Candí Bridge in Dec. 2007 2. Field investigations Twelve boreholes with continuous recovery of cores where drilled along the axis of piles of the foundations of the four central pillars to study the state and the integrity of the foundations. The boreholes penetrated a few meters into the natural ground below the tip of the piles. No