Global View of Engineering Geology and the Environment

This paper explains a procedure for the choice of ballast modules used for the design of direct continuous foundation in karst terrain. The presence of dangerous cavities is introduced in this procedure thereby evaluating risk failure. It also provides pertinent guidelines to direct the geotechnical survey of the terrain. Chimneys: both the low points in the folds as well as the softened clay capsules are points of weakness where, in a selective manner and at certain times the karstic process is reactivated creating chimneys, which eventually reach the surface. These chimneys are later filled with low consistency materials. The diagrams in figure 1show the different problems regularly appearing in the East Madrid area known as “Ensanche de Vallecas”, where the rocky karstified layer is often found beneath the clayey covering with a depth varying between 15 and 30 m. A geometric parameter of the softened clay capsules is also shown which is the dimension plan of the collapses (L) This parameter is the total minimum gap/space or free span, being bridged by the upper layers. In the case of the chimneys one can talk about a diameter (D) which defines the size of the channel, in its maximum dimension. Also, in the softened clay bags the geometrical parameter (H) is important, as it represents the minimum height of the horizontal clay layers which bridge the weakened area. 3 DANGER OF THE KARST PROCESS The karst process which appears in the East and Southeast area of Madrid entails a clear danger for future urban development. In presently built-up areas, such as Santa Eugenia, there are several pathologies directly related to the subsidence phenomena. However, the phenomenon has not been classified as catastrophic due in part to the fact that the karstic dynamic in the area is slow because:  The climatic conditions are dry and rainfall low.  The chalk material is of low permeability  There is clayey material in upper layers in the Tosco-Peñuela contact All these factors make the rainwater which infiltrates and circulates through the chalk massif not to be capable of reproducing or activating past karst dynamics. This is true only when human action does not alter the conditions of balance of the surroundings. In this sense, there are two risk factors introduced by manmade action:  Earth excavation which uncovers the chalk material stripping it of the protective impermeable covering.  Drainage leaks or other water ducts placed in ground where karst dynamics have acted beforehand thus producing the washing of low compact materials which fill cavities, galleries and chimneys. Thus, the karstic process is a danger for future urban development with two clearly differentiated aspects which affect different moments of the development. On one hand, the levelling and urbanization works should take into account the possibility that these actions may reactivate the karstic dynamics and should, therefore, utilize construction procedures which guarantee the stability of the terrain during works and the life time of the urbanization. In a second instance there is the construction of structures for residential, commercial and garage use. On this occasion the chief risk lies in the nondetection of the problems left by the karstic dynamics so that foundations are laid on material of insufficient bearing capacity. 4 CONTINUOUS FOUNDATIONS The types of foundations normally used for building structures in the new areas of development in southeast Madrid are:  Direct foundations with isolated elements such as footing or well  Direct foundations with continuous elements such as slabs or strap footings  Deep foundations with piles These three types of foundations mentioned are suitable for these areas but technical justification is needed which would take into account the peculiarities mentioned above. Below the procedure for calculation of the slabs and strap footings taking into account the risk of the karstification is explained. 4.1 Direct foundations with continous elements. Foundation slabs or strap footings are structures which produce the deformation of a volume of terrain with dimensions of the same magnitude as the width of same. In the area of Ensanche de Vallecas where the position of the rocky layer can be found at depths varying from 15 to 30 metres, the layer of clays situated on the rock is within the active zone of the slab deformation. Slab foundations have an advantage over other direct foundations which cover wider range of terrain, in such a way that certain rigidity differences are averaged out over all the active area and their deformation is more homogenous. They also have a greater capacity for bridging the karstic singularities and their softened areas. At present the most widely used calculation procedure for structure design of continuous foundation elements is based on the coefficient of ballast method. This is based on the hypothesis that for the range of work pressure, the terrain responds with settlement directly in proportion with the pressure at each point. The coefficient of proportionality is precisely the ballast module. Calculation programmes normally allow for variable ballast modules from one point to another of the slab. This is precisely the manner in which, by using several ballast modules placed in different positions each one corresponding to a calculation hypothesis the presence of softened areas under the continuous foundation can be modelized. The problem, therefore, is to determine which ballast should be used and how they should be distributed. At present there is some controversy when choosing the ballast module which should be used in the design of continuous foundations, independently from whether they are in karst terrain or not. The criteria followed in this article consists of obtaining a ballast module from the best estimate available of the total settlement of the slab. Thus, the best ballast module is that which reproduces most faithfully in the structure the settlement obtained by geotechnical analysis. In addition, in order to facilitate their practical use in the design of these elements, the representative ballast modules of each slab area should be constant. That is to say, the slab calculation should be carried out with two values of ballast module which would only vary by areas in the different calculation hypotheses. The estimate of the total settlement of the slab is based on considering the natural terrain as an elastoplastic semi-space, limited at a certain depth by an non-deformable rocky layer whose position corresponds with the beginning of the chalk layer, while the deformation area corresponds to the overlying clays. The slab settlement estimate is carried out in the first stage by the method of finite elements which would analyse the effect of the presence of softened areas. This method is often out of reach of the foundation recommendations of a building geotechnical report, so that, based on the observations in this first model, a second model is proposed by which they are obtained more rapidly and easily with the guarantee that they are within the correct order of magnitude. 4.2 Maximum settlement estimate by the finite elements method. By using this method, different geometric situations have been modelised in which the uniform charge is situated on a clay layer which is at the same time supported by a non-deformable layer. A softened area could exist in the core of the clays. The softened area is represented as a semi-circle of L diameter with H cover. The L and H values have been modified in the different models to obtain a graph representing the variation of the settlements according to these two parameters. The calculation mesh used is of similar form to that shown in the following (figure 2) Figure 2: Modeling by finite elements of a load in a clay layer located on a non-deformable stratum. The clays may contain a softened area. Other parameters necessary to develop the model are shown on the following table 1. Table 1: Calculation parameters