Evaluating rock mass groutability at shallow depths: a novel approach based on geo-structural surveys and permeability tests

Abstract

The study describes the geological and geo-structural setting of an unstable sea arch in a very attractive landscape of Southern Italy. It evaluates the applicability of some reinforcement work designs consisting of rock mass grouting at shallow depths along the arch vault and abutments. To this aim, the hydraulic conductivity of the rock mass and its groutability were evaluated in detail, integrating field geo-mechanical surveys, boreholes, Lugeon tests, and grouting tests with different grout compositions. Results from site tests were compared with grout take values calculated by applying classical empirical approaches available in the literature, based on rock mass permeability, or by using a novel deterministic approach based on geo-mechanical field surveys. Considering the shallow depths of the tests, the results suggest that the former gave high values, inconsistent with grout tests. On the contrary, the novel deterministic approach provided grout take values consistent with the results of grouting tests, resulting in the most reliable approach for the arch’s thin, shallow, and highly fractured structure.The use of different grout compositions in the site tests proved that extensive use of highly dense and viscous slurries is required (bentonite-cement grout) due to the high-grade weathering and karstification of joint surfaces, which deeply increases their hydraulic conductivity. However, the rock mass grouting must be considered complementary to other reinforcement measures such as bolting, especially along the arch vault.