Tunnel boring machine collision with an ancient boulder beach during the excavation of the Barcelona city subway L10 line: a case of adverse geology and resulting engineering solutions

Abstract

The existence of a layer of hard boulders up to 1 m in size within the soft sediments of the Holocene Llobregat delta in the SW of Barcelona city caused the damage and stoppage of the EPB-type tunnel boring machine that excavated the subway L10 line. This layer constitutes a detrital deposit of exceptionally large grain size developed in the base of the delta. It originated as an alluvial fan in the northern margin of the delta during the last fall of the Mediterranean Sea level, at the end of the Pleistocene period. The subsequent sea level rise (Holocene transgression) caused the flooding of this alluvial fan and the reworking of its materials by sea waves. Thereby a retrogradant boulder beach developed, in which the largest rock fragments of the former alluvial fan remained densely packed. The tunnel boring machine precisely collided with the main boulder concentration area of this ancient beach. The cutter tools and the structure of the cutterhead were seriously affected. The repair tasks required the excavation of a mine in the tunnel face under atmospheric pressure and a sealing and stable environment. To get these conditions a jet grouting block from the surface was performed to consolidate the deltaic sediments around the tunnel face. Additional TAM injections were carried out to improve some deficiencies in the sealing of the block. Subsequently the shield of the tunnel boring machine was embedded into the block and the water table was depressed by pumping wells. After 13 months of complicated works, the tunnel boring machine resumed the excavation encountering further problems. Although the layer of boulders was early identified by the geological and geotechnical studies of the L10 constructive project, its precise geological characteristics and potential adverse influence on tunnelling was then not valued in depth. A better knowledge of the geology of this layer would have been helpful in order to foresee the incident and propose alternatives.