Abstract: With a deep foundation pit project of Nanning Metro Line 4, we studied the supporting characteristics of double-row piles in round gravel mudstone composite stratum in Nanning using ABAQUS finite element software and field monitoring data, and concluded that double-row piles are well applicable in the round gravel mudstone composite stratum. We further investigated into the influence of pit-in-pit excavation on double-row piles under the conditions of different soil strength weakening percentages and the variations of pit toe coefficient and depth ratio around thresholds. Our findings suggest that the weakening of soil strength has the greatest influence on the supporting effect of double-row piles, followed by pit toe coefficient and depth ratio in sequence. Meanwhile, we propose to take whether the maximum increase percentage of lateral displacement of double-row piles exceeds 20% as the criterion to assess the influence degree of inner pit excavation on the safety and stability of foundation pit. In addition, by analyzing the failure modes and causes of the foundation pit under corresponding working conditions of each parameter, we gave corresponding countermeasures.

Key words: double-row piles, pit-in-pit, round gravel-mudstone mixed strata, numerical simulation, safety factor