Laying building foundations on karstified limestone with micropiles

Pose de fondations de bâtiment sur des pierres à chaux karstifiées avec des micropieux

J. Pérez-Romero, J. M. Rodríguez-Guerrero, F. Ureña & I. Villegas Cemosa

Marco A. Rebollo, Badajoz Public Estate Agency

ABSTRACT

This work describes the research work carried out for laying the foundations of two blocks of flats in Badajoz(Spain) situated on a highly karstified Cambrian limestone massif. Geological and geotechnical surveys werecarried out in two phases. The first of these estimated the presence and position of cavities following ageological surface survey, and from data obtained in drillings and a geophysical survey using georadar andmicrogravimetry. The second phase involved inspecting the construction of micropiles, a solution adopted toreinforce the soil, and which has made it possible to contrast the cavity model initially assumed with the onefinally recorded. This methodology allows an initial estimate of the investment cost. The contrast between the assumed model and the recorded model ensures the solution is effective and has made it possible to optimisethe length of micropiles constructed at each point.

RÉSUMÉ

Ce travail décrit la recherche réalisée pour les fondations de deux immeubles de logements à Badajoz (Espag-ne) situés sur un massif rocheux de pierres à chaux cambriennes, touchées par un processus avancé de karsti-fication. L’analyse géologique et géotechnique a été réalisée en deux phases. La première phase a consisté à estimer la présence et l’emplacement de cavités à partir d’une reconnaissance géologique de surface et desdonnées obtenues à l’aide de sondages et de prospection géophysique par géoradar et microgravimétrie. Ladeuxième phase a consisté en la réalisation d’un contrôle de l’exécution des micropieux, solution qui a étéadoptée pour renforcer le terrain, qui a permis de comparer le modèle de cavités supposé initialement et lemodèle finalement observé. Cette méthodologie permet une estimation initiale des coûts de l’investissement. La comparaison entre le modèle supposé et le modèle observé offre des garanties quant à la validité de la so-lution et a permis d’optimiser la longueur des micropieux exécutés à chaque point.

Keywords:

Karst, micropiles, soil reinforcement, site investigation

1 INTRODUCTION

The area studied is a plot of land situated in Fuerte de San Cristóbal in the town of Badajoz (Spain), in which two blocks of flats were recently built with a basement floor, ground floor and four storeys. The total amount of flats built is 154 and the land consists of two rectangular sites, longitudinally arranged next to one other. The surface area of the first one (plot R3) is around 1000 m2 (50m x 20m) and the second one (plot R2) is approximately 700 m2 (50m x 14m).

The land is mostly made up of a limestone formation on which Miocene deposits appear on the eastern side of the plot.

The geotechnical survey took place in several successive phases. Several survey soil pits were constructed in the first place, in which no anomalies

were detected, and surface foundations were initially proposed using isolated footings.

Once the land was excavated as far as the foundation level, metric and decimetric sized cavities were observed, which were related to karstification processes in the limestone. In order to characterise the spatial distribution of cavities, an initial geophysical survey was carried out using georadar and gravimetry, and whose results are shown in Figure 1. Eight rotation drillings were also made and five dynamic penetration tests, cavities of up to 1m thick being detected.

The first improvement treatment consisted in depositing 130 m3 of concrete and mortar in the biggest cavities. A second survey, however, with 40 destruction drillings and a geophysical survey made it possible to confirm that the process used had not been successful since it had not managed to fill in the cavities.

LEGEND

Figure 1. Site investigation

Due to the difficulty presented by the terrain, the Badajoz Public Estate Agency asked Cemosa to carry out a geotechnical study and to design supplementary treatment to ensure correct foundations for the buildings. Field studies carried out by Cemosa involved 16 rotary boreholes with core sample retrieval at 200m. Geological maps of the area were also prepared, which are shown in Figure 2, and a geo-mechanical description of the rock mass.

2 GEOLOGY

2.1 Tectonics The site studied is set in the Ossa-Morena Area of the Iberian massif, specifically in the Alconera Formation of the Lower Cambrian. A feature of the Ossa-Morena Areas structure is the existence of great recumbent folds verging to the SW, and the existence of fan surfaces, also verging to the SW (Figure 3). The folds have depressed sides and the hinges are enlarged. As a consequence of this

folding, parallel and perpendicular fracturing has occurred, related to the axial plane of the fold, which is more intense in the hinge area of the latter.

Along the Upper Carboniferous (see Figure 3.c) there is an increase in dissolvent material which has caused contact metamorphism on calcareous materials, mottling them and increasing fracturing in certain directions. This increase in hot material causes infiltrated water from the surface and at a certain depth to heat up, rising and hydrothermally altering Cambrian materials with the help of certain fissures.

Following the intrusion of magmatic materials of a granodiorite nature and the opening up of large continental basins, the latter have been filled with Miocene era sandy clays and the positioning of the current water course network on these, with the consequent appearance of flood deposits (channel facies, flood plains and flood terraces). In a parallel fashion to these processes, a series of deposits of colluvial origin have been formed at the foot of the surrounding Paleozoic reliefs, as a consequence of the progressive degradation of the latter (Figure 3.d).

PLOT R2

PLOT R3

DOLINE

LEGEND

COLLUVIAL

MIOCENE SANDY CLAY

CAMBRIAN DOLOMITE LIMESTONE

DISCORDANT CONTACT

JOINT, FISSURE

ANTICLINE FOLD

TREND AND PLUNGE

GEOLOGICAL PROFILE

OF FOLDING

Figure 2. Geological setting

2.2 Local geological setting There are quite monotonous dolomite limestone

and marmoreal limestones outcrops in the area studied, highly verticalized and tectonized. The site studied is located on the southern flank of a thick saddle fold (Figure 2), a hinge area having been located on the actual site, and the great deformations that have occurred have subjected the massif to very high stress. The general dip of materials in the area represents an inclination of around 60º - 65º to the NE or SW, depending on the fold flanks considered. It has been confirmed that there is a related system of joints and perpendicular fractures which clearly favour karstification processes.

In discordant contact with the calcareous materials described is a formation formed by clays with sandy and carbonated clay sections that have a marked brown shade attributable to the Miocene era. The thickness of this series in the area studied is not great, not exceeding 8 m and increasing nearer the centre of the basin.

Magmatic rock intrusion, through the original Dolomite limestones, has occurred in the eastern corner of plot R3, which has caused a system of fractures arranged in a bunch shape and cemented by a series of fluids of a hydrothermal origin, mainly consisting of carbonates and with a marked whitish tone. The Dolomite limestones have been altered as a consequence of the above, becoming high consistence marly limestones and calcareous marls.

3 DESCRIPTION OF THE GROUND PROFILE

A great deal of office work has been done in order to determine the geological factors that determine the areas in which karstification processes are most intense. Data provided by the geological surface survey have been used, as well as results from boreholes and research using georadar and microgravimetry. The presence of dolines and areas of weakness on the surface, shown by small thalwegs or projections in the soil, are clearly related to the presence of joints, and it is therefore

foreseeable that the same thing has happened at great depths. The particular families of joints identified on the land are outlined in Table 1.

A)S0 NE

B)

ALCONERA FORMATION

C)

D)RIVER GUADIANA

LEYENDA

Figure 2. Tectonics Table 1. Joints and fractures observed in outcrops

Type of discontinuity

Family Joint direction

Dip angle

Joint J1 0 65 Joint J3 (J1 combined) 185 80 Joint J2 280 80 Joint J4 (J2 combined) 120 75 Joint J5 60 45 Fault F1 280 65 Fault F2 F1 combined) 175 75

3.1 Plot R3 No karstification processes were detected in the

south-western half of the R3 area, which is a consequence of protection against water afforded by the presence of the Miocene age deposits.

A series of anomalies related to karstification were detected in the north-eastern half, which were

grouped together into four isolated areas located very close to the limit with plot R2. It was possible to determine that the four altered areas were of limited size, partly filled with decalcified clay and with no great development at depth, making it possible to use localized soil reinforcement treatment.

3.2 Plot R2 There are a great many fractures in this area and

the presence of faults that have favoured karstification processes. A great number of cavities of up to 8 m wide have been identified, generally filled with decalcified clay situated throughout the R2 area and up to 10 m deep.

From all the data available, four longitudinal geological cross-sections were produced of the soil in plot R2 (Figure 4), apparent fans in fissures being indicated, healthy or altered limestone areas and the possible extension of cavities.

As can be seen, the advanced development of karstification processes required a reinforcement treatment of the whole area to improve the mechanical properties of the existing soil and to limit the subsequent advance of karst processes caused by the dissolution of carbonates.

4 FOUNDATION DESIGN

4.1 Foundations in Plot R3 The soil present in Plot R3 basically consists of a limestone mass, with a generally healthy and rigid presence in localized areas, and karst cavities of limited extension. These limestones lie, in discordant contact, on Miocene deposits whose deformation module was estimated at 30MPa. In order to avoid problems related to differential settlings, surface foundations using heavy slabs were chosen. In those areas in which cavities were located, supplementary treatment using micropiles injected with the IRS system was applied. The construction of micropiles made it possible to join the cavities together with the micropiles acting as locking pins. The injection system caused the fissures to seal and a stabilization of the decalcification clays, which achieved a clear improvement in the area treated. Finally, the rigidity of the slab was considered sufficient to admit the presence of smaller cavities that had not been detected.

4.2 Foundations in Plot R2 In this case, a general treatment was chosen consisting of pillars on footings as foundations, the latter resting on tubular section steel micropiles.

LEGEND

ALTERED LIMESTONE

JOINTS AND FISSURES

HEALTHY LIMESTONE

CLAY

MICROPILE IN ALTERED ROCK AREAS

MICROPILE DRILLED IN CLAY

CAVITIES

Figure 4. Profiles assumed by site investigation Figure 5. Profiles estimated after drilling of micropiles

This solution has been described by Uriel et al

(1990) and consists, in the first place, in rejecting the shaft contribution in decalcification clay sections due to the fact that it is much more deformable than limestone. Base resistance is also rejected, since it is not possible to rule out the presence of cavities beneath the range of the micropile.

As for shaft resistance in rock sections, the above mentioned work has been taken into account, as well as the recommendations outlined in French standards (AFNOR 1992) and those proposed by Oteo (2003). In this way, final shaft resistance of 0.5MPa was considered and a security coefficient of 2.5 was chosen, which is suitable for the present circumstances in the area.

The minimum length to be covered by each micropile was thereby assessed, taking into account the geological profiles assumed for the R2 area (Figure 4).

4.3 Construction inspection and results During construction of the foundations in the R2 area, a detailed inspection was carried out of the micropiles. The sections bored in healthy rock, in altered rock and in cavities were all checked. Boring of each micropile was concluded when the length of the sections passing through rock reached the minimum level previously assessed. Once the boring had finished, injection was carried out using the IRS system. Low mobility injections were initially carried out in the large cavities. The acceptance of grout during the following injection phases made it possible to check that continuous sealing in the micropile area was occurring, thereby preventing corrosion phenomena or the onset of karstification.

5 CONVERGENCE OF THE MODEL

The data obtained during the boring of approximately 200 micropiles has made it possible to review the geological profiles initially assumed (Figure 4). This review is shown in Figure 5, the following conclusions being reached:

- In general, research prior to the construction of the micropiles detected most of the karst cavities, being quite accurate with respect to the positions of these cavities.

- The degree of karstification in the massif seems higher than assumed following the geotechnical investigation. Part of this difference is perhaps due to the fact that during the construction of the micropiles, some sections consisting of altered rock were considered to be filled cavities.

- Geophysical surveys, geo-radar and microgravimetry, have not detected some of the cavities that were subsequently crossed by the micropiles. It is particularly difficult to identify cavities below 10 m.

6 CONCLUSIONS

The research carried out on the land has made it possible to lay the foundations for two buildings on a limestone massif with intense karstification processes.

Research was carried out in two distinct phases: an initial estimate of the positions of the cavities was initially made and, subsequently, the construction of micropiles has made it possible to

check the places where the cavities were actually located.

The methodology used has made it possible, firstly, to estimate the cost of the work with a fair level of accuracy. Secondly, the convergence of the model assumed with the one finally deduced entails a guarantee that the geotechnical design is correct and that the solution is valid.

The resulting length of the micropiles has therefore been optimised, thereby harmonising safety and financial investment requirements in the foundations for the two buildings.

7 ACKNOWLEDGMENTS

The authors of this article would like to thank the collaboration given by Ferrovial and by Kronsa during the course of the work described in this document.

8 REFERENCES

AFNOR 1992. Norme Francaise P 11-212 Fundations profondes pour le bâtiment.

IGME 1992. Hoja 775 Badajoz del Mapa Geológico de Es-paña. E. 1:50.000.

C. Oteo C. 2003. Criterios para el diseño de micropilotes. Jornadas Técnicas SEMSIG-AETESS, 3ª sesión - micro-pilotes. CEDEX, Madrid.

Uriel A., Ortuño L. & Puebla F.J. 1990. Cimentaciones me-diante micropilotes en zonas kársticas. Ingeniería Civil 74, pp 17-22.