Influenza del procedimento di cementazione dei tubi inclinometrici sulle misure

Si descrivono alcuni casi di impiego delle misure inclinometriche per lo studio dei cinematismi di frana in tre pendii naturali con diversa morfologia e caratterizzati da una copertura quasi sempre in moto su un substrato roccioso. Gli inclinometri analizzati sono stati cementati con procedimenti diversi in funzione delle caratteristiche dei terreni attraversati: metodo tradizionale di iniezione di boiacca di cemento in risalita dal fondo foro; riempimento dall’alto con ghiaia miscelata a cemento; iniezioni successive di boiacca di cemento e riempimento dall’alto con ghiaia. Per i tre pendii si dispone anche di rilievi topografici per cui è possibile valutare l’affidabilità delle misure inclinometriche confrontandole pure con gli spostamenti di superficie. Dall’analisi delle misure inclinometriche è possibile esprimere una valutazione sull’efficacia dei procedimenti di cementazione utilizzati ed individuare i procedimenti di cementazione più idonei in dipendenza delle specifiche situazioni geotecniche e finalità delle misure.

Three different applications of inclinometer measurements to monitor the response of natural slopes are described in the paper. In all cases, soil movements occur in the upper layers of the slope, where a relatively shallow cover of soils slides over a stronger bedrock. Thus, the base of all inclinometers is fixed in a stable deposit. In the paper, the attention is focused on the grouting procedures adopted to cement the inclinometer casings against the surrounding soil. In fact, the stiffness and strength of the annular grout should be as close as possible equal to those of the surrounding soil, to obtain measures that closely represent the response of the in situ soil. To this aim, the measurements from a total of seven inclinometers are examined and interpreted herein together with the installation procedures; these have been varied according to local experience and soil conditions. Some of the casings have been grouted using the classic water-cement mix. In this case the mix was pumped from the base of the hole upwards, until the space between the soil and the casing had been filled. Some other casings were grouted from the top downward. In this case the annular space was filled using a mixture of cement and gravel. The remaining casings were grouted in stages. At the beginning a water-cement grout was pumped upwards from the base and also from mid-half of the column. When the level of mix in the annular space did not raise, because the grout flowed in the fissures of the rock, fine gravel was poured from the top in order to thicken the mix and keep it around the tubes. It shall be noted that the base of the inclinometers reaches a rock layer or a gravel alluvium that are highly permeable; hence the loss of grout at the base is certainly possible. Examining the data collected in the seven casings, it can be recognized that the grouting procedure can have a significant impact on the quality and reliability of the measures. On one hand it is necessary to fill completely the annular space between the casing and the soil, making a continuous grout, with no voids. On the other hand, the stiffness of the grout should be essentially equal to that of the surrounding soils. If such requirements are fulfilled the response of the inclinometer will show no time lag. In essence, even small movements will be detected and it is not necessary to wait until enough displacements have accumulated to trace the position of the failure surface. The erratic data from inclinometers T12 and Arche 2 can be taken as an example of a discontinuous or partially completed grout, presumably due to leaching into the base strata. In this cases the inclinometer casings would not be fixed and could sustain some irregular displacements different from those actually occurring in the surrounding soil. On the other hand the more regular data from V12 can be interpreted as the effect of a continuous grout, that fills completely the annular space between the tubes and the surrounding soil. However, some doubts arise with respect to the stiffness of this grout, since two years of continuous monitoring seem insufficient to detect the sliding surface. In fact, the inclinometer T4I, where a mix of fine gravel and cement was used to grout the casing, showed negligible time lag, allowing to detect the sliding surface after a short period of observations. Finally, in the case of Arche 3, in which the grout is likely to be continuous, the interpretation of the measures processed with different procedures does not yield a unique trend. Therefore it is not possible to understand the failure mechanism, if present.