1 Introduction

The Miocene rocks form the rocky substrate of much of the coast of the central Algarve and are the main lithological support of the coastal cliffs (Fig. 1). They comprise layers of fine calcarenites alternating with fossiliferous calcarenites, locally lumachelic, which generally correspond to rocks of low to very low resistance. Miocene rocks were affected by intense karst dissolution. The karst cavities are usually filled with red Plio-Pleistocene silty sands with clay intercalations, forming a discontinuous cover of the Miocene rocky substrate. The presence of a low strength rocky substrate, affected by abundant karst features, often hidden by a sandy cover with much lower strength than the substrate, produces relevant geotechnical conditioning for engineering projects in the region and the stability of the coastal cliffs. The intense and growing human occupation in the region, especially along the coast, gives relevance to those geotechnical conditions.

Fig. 1
figure 1

Localization and geological map of the Central Algarve (details in Rocha et al. 1981, 1983). Square marks indicate locations of sample collection. From west to east: Pintadinho beach, Altar Point; Caneiros beach; Alfanzina W; Carvalho beach; Benagil; Marinha beach; Algar Capitão-Caramujeira; Albandeira beach; Galé beach; Castelos point; S. Rafael beach

This study presents several campaigns of laboratory tests for geotechnical characterization of Miocene calcarenites, including identification and strength tests which may be a source of valuable data for future projects considering the almost inexistent record of geotechnical data published on these rocks.

2 Geological Setting

The Miocene rocks occupy a large extension of the central Algarve (Fig. 1) and correspond essentially to the Lagos-Portimão Formation (lower Miocene, Aquitanian–Burdigalian) (Rocha et al. 1981, 1983; Antunes and Pais 1993). It is mainly composed of alternating beds of fine-grained biocalcarenites and biocalcarenites with abundant remains of macrofossils. Bedding planes do not generally correspond to mechanical discontinuities but sharp transitions from layers of fine-grained calcarenites with few remains of macrofossils to layers with debris and moulds of macrofossils, surrounded by fine calcarenite matrix. Often, the coarsest beds correspond to lumachel. The upper part of the rock mass has suffered dissolution and precipitation of calcite, which led to the formation of calcretes, with frequently much higher strength than the underlying rocks (Marques 1994, 1997).

The unit thickness does not exceed 50 m west of Albufeira and increases southeast up to 85 m in the Balaia region. Near Vilamoura, the unit was detected under the Plio-Pleistocene cover, in a borehole, with a measured thickness exceeding 80 m (Silva 1988). The beds generally dip less than 6º to the east or south, except near Ponta dos Castelos (SW Portimão), with dip up to 15º–20º to the west, due to a major N-S fault, and near Ponta da Atalaia (SW Albufeira) with dips also up to 18º.

The Miocene rocks were affected by intense karstification with variable morphological characteristics. Westwards of Arade River and eastwards of Armação de Pêra, the karst morphology is characterized by a high frequency of sinkholes, with diameters of a few to tens of metres their bottom located below the current mean sea level, with less frequent karst galleries. Limestone precipitations usually harden the sinkhole walls with strength much higher than the surrounding rocks. They are filled with Plio-Pleistocene reddish sandy-clay deposits, with little resistance to erosion. On the coast, the rapid erosion of the karst filling motivates its exhumation, which produces a highly irregular coastline, with frequent sea stacks (see Marques 1997). In the sector between the mouth of Arade River and Armação de Pêra, along the coast, the karstification is dominated by galleries, sometimes with large sections (width of up to 30 m and visible heights of the order of 20 m) and the sinkholes present often correspond to the collapse of the gallery ceilings.

3 Methods, Results and Discussion

The laboratory tests were performed on block samples collected in the field in several locations (Fig. 1). The tests carried out included uniaxial compressive strength (σc); indirect tensile strength (Brazilian test) (σt); ultrasonic P wave velocity (vus); water content; calcium carbonate content; dry unit weight (γd). The strength tests were made considering the ISRM suggested methods (ISRM 2015) and adaptations imposed by the rocks’ low strength and friable character. Also, in some of the σc tests, some specimens with height less than 2.5 times the diameter were used due to specimen preparation difficulties. The specimen for strength tests was drilled to provide cores with diameters of 53.5 mm and 44.5 mm. The rocks observed in several outcrops and sampled were grouped into four lithological types, which include: 1) Lumachelic calcarenite (CL); 2) Fine-grained calcarenite with macrofossils (CFM); 3) Fine-grained calcarenite (CF); 4) Calcrete (CC). The tests were made in several fields and laboratory campaigns made over the past two decades. Due to the varied availability of test equipment, it was not possible to follow the same testing protocol, i.e. using the same tests for all the rock samples collected in the field. In consequence, test results relations and comparisons are made within the ones which were made according to the same testing protocol.

The summary of test data is in Table 1. The results obtained indicate that these rocks are mainly composed of calcium carbonate, mainly composed of bioclasts weakly cemented, with a quite variable macrofossil content, and correspond to calcarenites, despite other designations be found in the geological literature. As expected, test results show a very wide range of variations due to the large textural heterogeneity of these rocks. Dry unit weight varied widely, reflecting the variety of voids' density and volume in the matrix and the macrofossils moulds. The same happened with the ultrasonic P wave velocity, which corresponds mainly to a generally low strength rock. The σc values obtained for dry specimen were quite varied (Fig. 2) with more frequent results between 1 MPa and 7.5 MPa, with higher, less frequent values resulting from local stronger cementation. In contrast, the wet specimen provided much lower, more frequent values between 0.4 MPa and 4 MPa (Fig. 3).

Table 1 Summary of statistical data of the Miocene rocks testing
Fig. 2
figure 2

Unconfined compressive (top) and indirect tensile (bottom) plot for dry and wet specimen

Fig. 3
figure 3

σc (top left) and σt (top right) variation with water content. CaCO3 (bottom left) and vus (bottom right) variations with σc of the wet specimen. Grey marks indicate data points not used for correlations

The median uniaxial compression test values indicate a strength decay of about 35% for CFM and 85% for CL. The Brazilian test results also indicate a strength reduction by wetting, with more frequent dry values between 0.8 MPa and 1.6 MPa. The corresponding wet values between 0.08 MPa and 0.8 MPa, providing a strength reduction of about 77% for CFM and 48% for CL, in opposition to the σc test values. The compressive to tensile strength ratio is low for the context of rocks, with a general value of about 6 for CFM and less than 2 in CL. These values and relations with the opposite character for the same lithology are quite puzzling, requiring further analysis and may reflect significant variations in the strength parameters mainly due to variations in cement content and fissures size and density in the same lithology. However, a not neglectable number of tests provided σc values lower than 1 MPa, suggesting that particular care must be taken when analysing field conditions for specific projects. This aspect is relevant mainly because, in the more current engineering projects, strength data tends to be only derived from SPT, which tend to provide N values above 60 blows, which does not provide a reliable picture of its strength.

In many cases, the SPT tests are made with the rock dry or near dry due to the local climatic conditions and rock mass saturation in rainy periods implies a considerable rock strength reduction. Were found some relations between wet specimen uniaxial compressive strength and CaCO3 content and ultrasonic velocity (Fig. 3), with other relations, namely with dry unit weight, providing poor correlations, which are also due to the very heterogeneous character of these rocks.

4 Concluding Remarks

The Miocene calcarenites of the central Algarve are mainly soft to very soft rocks, which suffer a considerable strength reduction with wetting. The unconfined strength reduction depends on the rocks' fossil content, from circa 35% for CFM to 85% for CL, while in the indirect tensile tests, the rocks exhibited an opposite behaviour (less 77% for CFM and 48% for CL). Compressive to tensile strength ratios are lower than common values for most rock types, varying between 6 for CFM and less than 2 in CL. The results of the tests presented in this study may be a helpful reference and guidance for site investigation projects in the region. However, it is felt that further studies are required to get a more comprehensive knowledge of the geotechnical behaviour of the rock masses composed by these rocks. The low strength of the Miocene rocks coupled with the very frequent and extensive karst features indicate that the use of geophysical methods such as Ground Penetrating Radar (GPR) is highly recommendable, especially bearing in mind the low strength of these rocks, which may not be able to withstand heavy construction loads located near or over open karst voids.