Plate Load Test

Plate Load Test (PLT) is a field test for safe bearing capacity of soil. Per IS 1888:1982, a calibrated steel plate (300, 450, 600, or 750 mm dia) is placed at the founding level and loaded incrementally; settlement is measured at each load increment until failure or required factor of safety is achieved. Used to verify the safe bearing capacity (SBC) at the actual site for design purposes — distinguished from laboratory triaxial / CPT correlations.

Procedure: (1) Excavate test pit to founding level. (2) Place plate centred on the test area. (3) Apply load via hydraulic jack reacting against a counter-weight or anchored frame. (4) Increase load in increments (typically 25-50 kN); maintain until settlement stabilises. (5) Continue until failure (sudden settlement increase) or 1.5× design load. (6) Compute SBC = ultimate bearing capacity ÷ FoS (2.5-3.0 typical).

Limitations: (a) Plate size effect — small plate behaviour differs from large foundation; correction factor needed (e.g., Bowles, Terzaghi). (b) Saturation — actual foundation may be at different moisture; testing at design saturation important. (c) Depth — plate test typically at shallow depth; deeper soil layers not characterised. (d) Cost — typical Indian PLT costs ₹50,000-150,000 per test; major projects do 1-3 tests at critical locations. The most-overlooked aspect of Indian PLT use: scaling. SBC at small plate (300 mm) does not directly apply to large foundation (3 m); apply Bowles or Terzaghi scaling factor.