Behavior of a 31 m high excavation supported by anchoring and nailing in residual soil of gneiss

• A 31-m excavation supported by anchoring and nailing was monitored.
• The excavation was performed in residual soil of gneiss with high NSPT.
• The area is located in a fault region that leads to a particular type of soil formation.
• Results demonstrate the importance of reliquiae soil layers on the observed behavior.

A 31 m high excavation located at Niterói/RJ in residual soil from gneissmigmatite with high NSPT (> 40) was performed for a building construction. Two levels of anchored walls were considered in order to guarantee stability. When the construction of the upper wall was almost completed, a crack parallel to the direction of the wall formed in the hilltop. It was decided that the construction works would cease in order to perform additional studies. The presence of reasonably continuous soil layers of different colors (gray, white, and red), with thicknesses varying from a few millimeters to some centimeters, clearly observed in the face of the cut slope. The inclination of these soil layers varied along the length of the excavation, and they were in an orientation that was unfavorable for the stability of the cut. Laboratory tests were performed and the presence of high activity clay mineral and low shear resistance parameters was determined for the gray clayey sand soil. Those weakness planes, i.e., the thin layers of gray clayey sand, had not been properly detected in the previous studies that supported the design of those walls. Considering these new findings, the original project was reviewed, and it was decided that soil nailing would be used for accomplish the excavation to the original planned level. The upper wall was also reinforced with additional anchors. Load measurements in the nails indicated that the stress state in the soil mass corresponded to the active condition at the final level of excavation. Significant lateral movements were measured and varied according to the direction and inclination of the cited weakness planes. It was observed that the reliquiae planes were fundamentally important to the behavior of the excavation.