The Mechanism of Grouting Action under the Base of Bored Pile-Paper

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Base grouting is used to reduce bored-pile settlement and improve performance by preloading the soil beneath the pile. Injection can consolidate or strengthen this soil and, in non-cohesive ground, may also enlarge the effective pile-base area. The induced force creates downward shaft friction; as the pile is subsequently loaded, this friction is first offset and then reversed, helping mobilize greater shaft resistance. The pile base also begins loading from an already stressed condition.<br /><br />The study compared instrumented, 1.5 m diameter piles at a bridge foundation in Wrocław: pile 62 was grouted, while pile 99 was not. Grouting involved 420 litres of grout at 1.5 MPa over 40 minutes and raised the pile head by 1.2 mm. Under a maximum test load of about 11,000 kN, the ungrouted pile settled approximately 31.5 mm, compared with 10–12 mm for the grouted pile. Settlement stabilization was also faster for the grouted pile.<br /><br />Measurements of axial force along the piles showed that the grouted pile transferred less load to its base—about 1,600 kN versus 3,600 kN—and more through shaft resistance. In the lower clay sections, measured shaft resistance was roughly three times higher for the grouted pile, though differences in surrounding soil may also have contributed. The permanent stress beneath the grouted pile base was estimated at about 700 kPa, around half the injection pressure.<br /><br />The results support the view that base grouting improves pile stiffness and settlement-based design capacity chiefly by enhancing shaft resistance, not solely by increasing base resistance. However, the findings come from a single comparative case; broader testing is needed before general design conclusions can be made.

Keywords

base grouting

bored pile settlement

pile foundation

shaft resistance

pile base area

soil consolidation

preloading

axial force

settlement reduction

bridge foundation