IS 1888 : 1982Method of load test on soils for shallow foundations

IS 1888 specifies the method of load test on soils for shallow foundations — the in-situ Plate Load Test (PLT). A rigid steel plate is placed at the proposed founding level and loaded incrementally; settlement under each increment is measured. The result is the soil's bearing capacity and modulus of subgrade reaction at the test depth — direct empirical input to shallow foundation design.

Use IS 1888 PLT for: - Foundation design verification for important buildings (hospitals, schools, multi-storey > 4 storey) - Allowable bearing capacity estimation when SPT-based correlations are inadequate - Settlement estimation for sensitive structures (water tanks, machine foundations) - Subgrade modulus (k_s) for raft foundation analysis - Pavement design input — modulus of subgrade reaction for rigid pavement design (IRC:58:2015) - Pre-failure foundation forensics — when settlement is observed in completed structure - Heritage foundation assessment — non-destructive characterisation of existing footings

PLT is more accurate than SPT-based correlations for shallow founding strata (top 1-3 m below test plate); cheaper than full triaxial test programme; faster than long-term consolidation tests for granular soils. For deep foundations and pile design, use IS 2911 Part 4:1985 (pile load test) instead.

IS 1888 is referenced by: - IS 6403:1981 — bearing capacity calculation methods - IS 1080:1985 — design of shallow foundations - IS 8009 Part 1:1976 — settlement of shallow foundations - IRC:58:2015 — rigid pavement design (k_s input) - IRC:37:2018 — flexible pavement design (subgrade strength)

Equipment: - Test plate: square or circular, mild steel, 25 mm thick - Standard sizes: 300 × 300 mm, 450 × 450 mm, 600 × 600 mm, 750 × 750 mm - Larger plates more representative; smaller plates for thinner founding strata - Hydraulic jack with calibrated load cell or proving ring (capacity 100-500 kN typical) - Reaction system: kentledge dead-weight platform OR ground anchor reaction beam - Settlement gauges: 3-4 dial gauges with reference frame independent of load + reaction

Test pit: - Excavated to proposed founding depth - Pit dimensions: minimum 3-5 × plate dimension to ensure undisturbed soil under test - Square or rectangular shape; bottom carefully levelled - For deep tests (> 3 m): adequate shoring

Procedure: 1. Excavate to founding depth, level the bottom of the pit 2. Place test plate centrally 3. Apply gentle seating load (~1/10 of expected ultimate); record initial readings 4. Apply load increments — typically 1/10, 1/8, 1/6 of estimated ultimate, then doubling 5. Hold each load step until settlement rate < 0.02 mm per minute (granular) OR 0.02 mm per 5 min (cohesive) 6. Record cumulative settlement at end of each load step 7. Continue until either: - Settlement = 0.025 × plate width OR - Settlement-vs-load curve shows yielding (sudden settlement at constant load) - Test load reached limit of jack capacity 8. Unload in 4 increments, record rebound 9. Compute ultimate bearing capacity from break in load-settlement curve OR from settlement criterion

Test cycle (for cyclic test): - Load to ~50 % of expected ultimate, unload to 0 - Reload to ~75 %, unload to 0 - Reload to ultimate, unload to 0 - Yields elastic vs plastic settlement separately

Bearing capacity from PLT (Terzaghi-Peck):

• Ultimate q_u from break in load-settlement curve
• Allowable q_safe = q_u / FS, where FS = 2.5 to 3.0

Scale effect — plate to footing: - Granular soils (sand): q_u of large footing > q_u of small plate - q_u(footing) ≈ q_u(plate) × (B_footing / B_plate)^0.5 - Cohesive soils (clay): q_u practically independent of footing size - q_u(footing) ≈ q_u(plate)

Settlement scaling (granular): - s_footing / s_plate = (B_footing / B_plate × (B_plate + 0.3) / (B_footing + 0.3))^2 - Where s = settlement, B = footing width (m) - Implication: settlement of footing > settlement of plate at same pressure (footing is larger; mobilises deeper soil)

Subgrade modulus (k_s): - k_s = applied pressure / settlement (typical units: kN/m² per mm of settlement, or pci, or MN/m³) - For rigid pavement design (IRC:58:2015): k_s = 50-200 MN/m³ typical for compacted granular sub-base - For raft foundation Winkler model: k_s used directly

Test plate sizes — when to use which: - 300 × 300 mm: sand / silt; small structures; shallow founding (< 1 m) - 450 × 450 mm: granular soils; medium structures - 600 × 600 mm: cohesive soils; medium / large structures - 750 × 750 mm: deep founding; large structures; raft foundation design

Reaction loading: - Kentledge: dead weight ≥ 1.2 × max test load - Ground anchors: provide adequate reaction; spacing ≥ 3 × plate dimension from test plate

Settlement criterion (Terzaghi-Peck): - Ultimate at settlement = 0.025 × plate width (for granular) - Stricter for sensitive structures: settlement = 0.0125 × plate width

Test sample size: - 1 PLT per major founding stratum (minimum) - For project area > 1000 m²: 2-3 PLTs at characteristic locations - For variable soil: 4-5 PLTs at corners + centre

• IS 6403:1981 — bearing capacity (uses PLT-derived q_u or correlations).
• IS 1080:1985 — design of shallow foundations.
• IS 8009 Part 1:1976 — settlement of shallow foundations.
• IS 2131:1981 — Standard Penetration Test (cheaper alternative; correlations less accurate than direct PLT).
• IS 2720 Part 4:1985 — soil grain-size analysis.
• IS 2720 Part 7 / 8 — Proctor compaction.
• IS 2720 Part 10:1991 — UCS for cohesive soils.
• IS 2720 Part 13:1986 — direct shear (granular soils).
• IS 1498:1970 — soil classification.
• IS 1892:1979 — site investigation procedure.
• IS 2911 Parts 1-4 — pile foundations (deep alternative).
• IRC:58:2015 — rigid pavement design (k_s from PLT).
• IRC:37:2018 — flexible pavement design (subgrade strength).
• NBC 2016 Part 6 — structural design (foundation chapter).

1. Test plate too small for actual footing. 300 mm plate result for 3 m wide raft = 10× scale-up; correction factor highly uncertain. Use largest practical plate; ideally 600-750 mm. 2. Reaction system interferes with test plate. Kentledge support footings load adjacent ground; their settlement bias reading. Maintain spacing. 3. Pit bottom disturbed during excavation. Remoulded soil layer at bottom; PLT measures disturbed layer, not undisturbed. Carefully level + remove disturbed soil before plate placement. 4. Test below water table without dewatering. Water entering pit destabilises soil; PLT result unreliable. Dewater + maintain dry test conditions. 5. Loading rate too fast. Soil doesn't have time to fully respond to each increment; result over-estimates capacity. Hold each step until settlement rate < 0.02 mm / 5 min. 6. Settlement reference frame supports too close. Frame supports settle along with plate; gauges read low. Reference frame supported ≥ 3-5 plate diameters away. 7. Single test interpolated for entire site. Soil heterogeneity ignored; design for weakest stratum, not characteristic. Run multiple PLTs across site. 8. Scale effect not applied for granular soil footing design. Plate result used directly for large footing; under-estimates settlement. Apply scale-up formula. 9. PLT in clay where consolidation matters. Short-term PLT shows undrained behaviour; long-term consolidation settlement not captured. For clay, PLT must be supplemented with consolidation test. 10. Settlement criterion too lax. 0.025 × B may be acceptable for warehouse but excessive for hospital / equipment foundation. Use stricter criterion for sensitive structures. 11. Calibration of jack / load cell stale. Force measured wrong by 10-20 %. Use calibrated equipment; certificate within 6 months. 12. No test at multiple founding depths. PLT at 1 m depth may give different result than at 1.5 m if stratum changes. Test at planned founding depth + check + 0.5 m below.

Foundation investigation cascade for a typical building:

1. Reconnaissance — desk study, walk-over. 2. Boring + sampling (IS 1892) — boreholes at corners + centre. 3. Index tests on every stratum: - Moisture content, gradation (IS 2720 Part 4), Atterberg limits (Part 5) - Specific gravity (Part 3) - Classification (IS 1498) 4. Strength tests: - Cohesive: UCS (Part 10), triaxial UU - Granular: SPT (IS 2131) + direct shear (Part 13) 5. In-situ tests: - Plate Load Test (this code, IS 1888) at proposed founding level - SPT continuous in borehole - Pressuremeter (advanced; less common in India) 6. Synthesis: - Allowable bearing pressure (IS 6403:1981) - Settlement (IS 8009) - Foundation type recommendation (shallow / deep) 7. Foundation design (IS 1080:1985 for shallow; IS 2911 Parts 1-4 for piles).

For minor / small projects: - SPT only, with empirical correlations to bearing capacity - PLT optional

For important / sensitive projects: - SPT + PLT + lab tests + multiple boreholes - PLT mandatory at proposed founding level - Multiple PLTs for site characterisation

For pavement (rigid): - PLT as the source of subgrade modulus k_s - Used directly in IRC:58:2015 slab thickness calculation

Typical PLT cost (2026): - ₹50,000-1,50,000 per test (depending on depth, plate size, reaction system) - Significantly cheaper than full triaxial programme + lab testing - High value-for-money for foundation design verification