Apport de la géophysique à la stabilité des ouvrages

Abstract

The analysis of the stability of structures has been a challenge for researchers and engineers since the 17th century with Hooke. In this work, the contribution of geophysics to the stability of structures (bridges, tunnels, slopes, foundations etc.) is presented. For this purpose, we focus on structures such as slopes and foundations. In order to carry out this study, the nature of the discontinuities, the spacing of the discontinuities, the groundwater condition, the rock quality designation (RQD), the rock mass rating (RMR), the slope mass rating (SMR), the factor of safety (F.S), the Q-index, the allowable strength, the allowable stress, the Menard modulus and the limit pressure were taken into account. The slopes in Central Africa that are on the corridors connecting countries such as Cameroon to Chad, Gabon, Central Africa and Equatorial Guinea were studied by calculating the RMR, SMR Q and F.S of the ten slopes. The slopes of very important roads cut in gneiss, linking some Central African countries without maritime space and subject to particle fall were studied to verify the factor on which engineers base the embankments on these roads. The results show that, their angles vary between 25° and 82°, showing that they are geometrically acute. The RMR varies from 41 to 63 characterising that, these embankments are made in a rock in a diptych state. The SMR values from 29 to 54 show that the slopes are unstable. The F.S. calculated by different methods are all lower than the unit characterising their instability except for slopes number 5 and 9 for which the F.S. is higher than the unit, translating their relative stability. The number of joints per cubic meter (Jv) found varies from 3.7 to 12.6 m-3 and also shows that the slopes are unstable, which confirms the falls of particles and boulders that are often observed there. The different instability results show a probability of 60% for the first six slopes and 40% for the last four. To make them more stable, they can be plastered, sliced or cut at angles less than 10°. From the penetrometric and pressure data, it can be seen that the soil structure in these areas is generally of low bearing capacity (<0.1 MPa) with good allowable stresses (average stress around 2.00MPa). Therefore, it is advisable to base the structure beyond these low bearing capacity zones at a depth of around 8 m, i.e. deep foundations. All these results show that, although the slopes of Central Africa dug in the gneiss are made in geometrically acceptable conditions, they are generally unstable and, as a result, the foundations of the slopes are not very stable. They remain generally unstable and that, the geometric factor is the one that is mainly taken into account contrary to the RMR, SMR, and F.S.