Secant Piles Shafts and Directional Drilling (SPPD) for a Modular Drainage System-Paper

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The paper presents a modular deep-drainage system using secant-pile shafts and directional drilling (SPDD) to reduce groundwater-driven landslide movement and protect nearby infrastructure. The case study concerns the slope near Lagaro in central Italy, where monitoring linked ground movement and viaduct-foundation instability to rainfall and groundwater fluctuations. A deep shear surface was identified at depths of approximately 15–40 m.<br /><br />The system uses large, accessible main shafts connected by drilled conduits to gravel-filled drainage piles, other shafts, and discharge points. Directional drilling enables long, accurately placed connections without requiring intermediate shafts. At Lagaro, 11 shafts, up to 5.6 m in internal diameter, served three slope areas. The design included nearly 800 gravel piles and thousands of metres of lateral and bottom drainage conduits.<br /><br />The approach addresses limitations of older shaft-based systems, including confined and hazardous working conditions, difficulty drilling in hard or heterogeneous ground, limited connection distances, and challenging maintenance. The larger shafts provide safer access and accommodate directional-drilling equipment, while secant-pile construction allows excavation with reduced use of drilling fluids—an advantage in unstable ground.<br /><br />Construction encountered highly variable geology, including hard rock blocks within a soft, chaotic matrix, requiring different drilling tools and methods. During the works, no surface effects or increase in landslide movement were observed. Monitoring after completion showed that discharged water volumes responded to rainfall and that seasonal groundwater-level fluctuations decreased. The authors conclude that SPDD can be cost-effective for medium- to long-distance drainage connections, particularly where conventional solutions would require additional shafts or extensive excavation.

Keywords

secant-pile shafts

directional drilling

deep drainage system

landslide mitigation

groundwater fluctuations

slope stabilization

drainage piles

viaduct foundation

Lagaro Italy

modular drainage