Application of the electrical tomography technique in the design of a soil-nailing reinforcement for a slope in karstic limestone and sandy colluvial

This case studies the implementation of a large slope in the newly built A8 motorway, located close to Buelna (Asturias), where a mesh of electrical tomography profiles has been used to detect the pres- ence of karstified limestone ridges under sandy colluvium. The results collected were used to design a steeper slope and to undertake the necessary improvements of the colluvial area by using soil-nailing techniques. This reinforcement proves to be a more suitable solution, when necessary, than other more expensive options. soil-nailing (suelo claveteado). Este refuerzo demuestra ser una solución más adecuada, cuando es necesario, que otras opciones más costosas.

I. Pérez-Santisteban & J. Dorronsoro Pérez

Dept. of Geophysics, Orbis Terrarum, Madrid, Spain

F. Puell Marín

Technical Department, Orbis Terrarum, Madrid, Spain

1. INTRODUCTION

Excavations for the new A-8 road lying on colluvial deposits involve the creation of slopes with potential instability problems. The particular case of the slope studied in this article is located near Buelna in Astu- rias, so instability problems are compounded by fre- quent rainfall typical of the region.

The first excavations already showed that the sloping designed 3H:2V and 20 m high with no ad- ditional support were unstable. The option to lower them to 2H:1V would have involved the occupation of a large number of properties, as well as increasing both constructing difficulties and costs. Therefore, it was necessary to reinforce the slope with a tech- nique that exploited the possibility to know the posi- tion of bedrock.

Through a number of boreholes and some nearby outcrops, it was known that beneath the colluvium there was limestone bedrock. However, the topogra- phy of that bedrock was unknown. The limestones in this area are well karstified, and have an irregular and discontinuous distribution that is filled by colluvial material. Yet, due to the high gradient of the slope that would have required the creation of complicated road access, it was difficult to investi- gate the position of bedrock from boreholes (Fig. 1). To avoid this problem, and in order to know the distribution of the limestone bedrock, a geophysical campaign using electrical tomography technique was arranged. With the results, the slope reinforcement design would be as efficient as possible.

Electrical tomography is a noninvasive survey that discloses the electrical resistivity of the subsurface materials providing an image depth of layers having different electrical behavior.

Figure 1. Slope before reinforcement action. Some limestone ridges are visible outcropping at the bottom of the slope.

2. PROJECT DESCRIPTION

New A-8 road is located in the Cantabrian area of the Iberian Massif. Specifically, the slope investigat- ed is situated in the municipality of El Peral (Asturi- as) and is mainly formed by colluvial deposits under which the Calizas de Montaña Formation appears. Quaternary colluvial deposits are sometimes very thick, overlying Paleozoic materials (quartzite) in the higher areas and limestone in the lower areas. Colluvium deposits are caused by debris on the slopes so that they are composed of cobbles and blocks of the constituent materials of the slope (quartzites, sandstones, limestones) with a variable content of sandy clay matrix. Underlying the Calizas de Montaña formation is located. These limestones have been observed in nearby outcrops and are characterized by a high de- gree of karstification generally regarded as discon- tinuous outcrops and ridges morphology. Thus, the slope is formed by thick colluviums overlying the limestone karst. The excavation for the A-8 road on these colluvial deposits involves slopes of great instability for their lithological nature; also instability is worsened by frequent rainfall occurring in the area.

Containment measures originally designed con- sisted of 37° slope. This design involved the occupa- tion of a large number of properties in addition to being difficult to build and very expensive. Therefore, knowing the distribution of the lime- stone bedrock allow a more efficient slope rein- forcement design, that takes advantage of the possi- ble occurrence of limestones. To do this a geophysical campaign was planned through the elec- trical tomography technique, setting out quickly and noninvasively the distribution of different lithologies. The aim of this geophysical campaign was to ob- tain resistivity values of existing materials in depth, in order to determine the contact of Quaternary colluvial materials and Calizas de Montaña For- mation.

3. ELECTRICAL RESISTIVITY TOMOGRAPHY

Esta técnica mide los cambios en la resistividad eléctrica para proporcionar una imagen de las capas del terreno. Es ideal para determinar el contacto entre coluvión y caliza debido al alto contraste de resistividad esperado.

La campaña utilizó un equipo Terrameter de baja frecuencia (modelo ABEM SAS-4000) con un dispositivo Wenner. Se diseñaron 9 perfiles transversales (80-100 m de longitud, hasta 15 m de profundidad) y un perfil longitudinal mediante la técnica Roll-along de 180 m de longitud y 20 m de resolución en profundidad.

The apparent resistivity

Donde:

V, diferencia potencial entre dos electrodos (llamados M and N).

I, Intensidad de la corriente introducida en el terreno entre dos electrodos. (llamados A and B).

K, es una constante geométrica que depende de las distancias entre los cuatro electrodos a, B, M y N.

Las representaciones de la distribución de la resistividad aparente en el subsuelo se denominan pseudo-secciones, mientras que la resistividad real ($\rho$) es una propiedad intrínseca de las rocas y depende de la litología, la fábrica y los fluidos contenidos.

La tomografía eléctrica requiere el uso de instrumentación específica capaz de realizar un gran número de mediciones de forma rápida y fiable. El equipo utilizado para esta campaña es un Terrameter de CC de baja frecuencia, fabricado por la empresa ABEM, modelo SAS-4000, con el cual se ha empleado un dispositivo Wenner de dos cables. Este dispositivo se considera ideal para el terreno estudiado

Figura 2. Posición de los perfiles eléctricos (líneas verdes) para la campaña geofísica mediante tomografía eléctrica

Figura 3. Interpretación del Perfil 01 sobre la excavación del talud

Para estudiar este talud se diseñó una malla de 10 perfiles eléctricos: 9 transversales al talud y uno longitudinal (Fig. 2). Las longitudes de las secciones transversales oscilan entre 80 y 100 m, alcanzando una profundidad de 12,50 m la más corta y de 15 m la más larga. El perfil longitudinal se realizó mediante la técnica Roll-along, utilizando 63 electrodos y alcanzando una longitud de 180 m con una resolución de 20 m de profundidad

4. RESULTADOS Y MEDIDAS ADOPTADAS

La interpretación de los perfiles de tomografía eléctrica muestra la presencia de crestas calizas en todas las secciones transversales, aunque la cota de coronación es muy variable debido a la geometría irregular de la formación.

Los depósitos de ladera o coluviales presentan valores de resistividad muy variables, ya que están compuestos por una mezcla heterogénea de arcilla y bolos de caliza o cuarcita.

When the resistivity values exceed 500 Ω·m, they are interpreted as limestone bedrock. These limestones are karstified and highly tectonized, so depending on their degree of meteorización, los valores de resistividad varían entre 500 y 5000\ \Omega \ m

Figure 4. Longitudinal profile interpretation across the existing hillside, with the designed slope superimposed.

Figure 03 shows the interpretation of an electrical tomography profile over the current slope, with the projected slope superimposed.The shallower blue colors represent the conductive colluvial deposits, while the green and red colors correspond to the bedrock (resistivity values of the limestone 500 Ω/mLikewise, this profile shows that the limestone presents a ridge-shaped morphology..

In the longitudinal profile (Fig. 04), the irregular morphology of the limestone bedrock can be observed, with two limestone ridges detected 10 m apart from each other..

From the interpretation of the electrical tomography, a 3D model was also developed showing the irregular topography of the contact between the colluvium and the limestone bedrock (Fig. 5).

Figure 5. Design of soil-nailing based on the presence of limestone detected by electrical tomography.

Through the geology interpreted by means of electrical tomography, the slope stability has been modeled considering the points where the limestones appear. These limestones were detected from the base to the middle zone of each profile. Thus, the need for a wall was ruled out and replaced by a reinforcement system using soil-nailingwhich allows certain flexibility to continue or stop the reinforcement depending on the appearance of the rocky substrate.

Figure 6.

The new slope reinforcement design consists of (Fig. 6):

• Benches 1.60 m high.
• Mesh of soil-nailing Steel bars AE25 .
• Horizontal drains and weep holes.

The Soil Nailing technique soil-nailing t is used to reinforce soils with inadequate stability, such as in the case of Qc colluvium.

The fundamental concept of the soil-nailing consists of reinforcing the ground through the installation of passive anchors in which cement grout is injected into a slope or excavation, following a top-down construction process. This process prevents ground decompression; as the slope descends, the bars gradually take on the load (Fig. 7).

Figure 7. Image of the slope after the placement of the soil-nailing in the upper half.

5. CONCLUSIONS

The Electrical Resistivity Tomography (ERT) has proven to be a useful tool for accurately determining the position of the bedrock in slopes where boreholes are unfeasible due to the steep gradient and access constraints.

The geophysical survey allowed for the adjustment of the interface between deposits, considerably reducing the reinforcement measures. The presence of limestone at the base of the slope eliminated the need for a riprap wall, allowing for a steeper slope design, reducing costs, and avoiding further land occupation.

6. ACKNOWLEDGMENTS

The authors wish to express their sincere gratitude to Elena del Soto and Carlos Quintanal, from FCC, for providing the drawings and images, as well as for their valuable comments regarding the progress of the works.