The Plate Load Test (PLT) is an in-situ field test that directly measures the bearing capacity and settlement characteristics of soil at the proposed foundation level. IS 1888:1982 governs the procedure in India. It is widely used for shallow foundation design, airport and highway subgrade evaluation, and rigid pavement design (k-value).
Purpose of the Plate Load Test
- Determine Safe Bearing Capacity (SBC) of soil
- Determine Modulus of Subgrade Reaction (k-value) for rigid pavement design
- Determine elastic/immediate settlement characteristics
- Verify soil treatment effectiveness (compacted fills, stabilized subgrade)
Test Equipment — IS 1888:1982
| Component | Specification |
|---|---|
| Loading plate | MS plates: 300, 450, 600, 750 mm diameter (circular) or 300 mm × 300 mm (square). Minimum thickness: 25 mm (600 mm dia); 50 mm (750 mm dia) |
| Reaction system | Kentledge (dead load on platform) or anchored reaction beam (anchor piles) |
| Loading mechanism | Hydraulic jack with pressure gauge (calibrated) |
| Settlement measurement | Three dial gauges (0.01 mm precision) arranged at 120° or 2 on diametrically opposite sides |
| Datum reference | Reference beam/frame independent of loading and test pit |
Test Setup Requirements
- Test pit: Depth = proposed foundation depth Df; minimum width = 5 × plate diameter
- Seating of plate: Clean bottom, lay 5–10 mm fine sand bed to ensure uniform contact
- Reaction beam clearance: ≥ 3.5 × plate diameter from plate centre to avoid influence zone overlap
- Plumb: Loading jack must be perfectly vertical over plate centre
Test Procedure — IS 1888
Load Increments
- Apply seating pressure: 7 kN/m² (to establish initial contact)
- Load in increments = 1/5th of estimated safe bearing capacity
- Alternatively: equal increments of 10–50 kN/m² depending on soil type
Settlement Readings
- Record settlement at 0, 1, 2.25, 4, 6.25, 9, 16, 25 min after each load increment
- Wait for settlement rate < 0.02 mm/hr before applying next increment (IS 1888 criterion)
- This is maintained load test (slow test, 1–3 days total)
- For quick test (NOT for SBC determination): use rate of loading method
Test Termination
Stop when any of the following occurs:
- Settlement = 25 mm (IS 1888 limit for load settlement curve)
- Load = 1.5 to 3 × expected SBC (failure conditions)
- Plate penetration rate accelerates without increase in load (shear failure)
Pressure-Settlement Curve and Interpretation
Characteristic Curve Shapes
- Dense sand / stiff clay: General shear failure — clear peak then loss of load, then residual strength
- Medium dense sand: Local shear failure — gradual curve, no clear peak
- Loose sand / soft clay: Punching shear — continuously increasing settlement with load, no failure point visible
Reading Ultimate Bearing Capacity (qf)
- From load-settlement curve: clear break point = qf (for general shear)
- Log-log plot: intersection of two straight lines = qf (for local/punching shear)
Safe Bearing Capacity (SBC)
SBC = q_allowable = minimum of:
- qf / FOS (with FOS = 3 by IS 1888)
- Load corresponding to settlement = 25 mm (for settlement control)
The lesser of these two governs the SBC.
Modulus of Subgrade Reaction (k-value)
k = p / s (kN/m³ or N/mm³)
where p = pressure (kN/m²) and s = settlement (mm) at a specified pressure.
For pavement design (IRC 58): k is determined at 1.25 mm settlement from the plate load test using 750 mm diameter plate.
Alternatively, k at 0.125 inch (3.175 mm) settlement is used in US practice (AASHTO).
Correction Factors and Scale Effect
Settlement Correction for Actual Footing
For clays: Sf = Sp (B / Bp) — settlement proportional to size
For sands: Sf = Sp × [B(Bp + 0.3) / Bp(B + 0.3)]²
- Sf = settlement of actual footing (m)
- Sp = settlement of plate (m)
- B = footing width (m)
- Bp = plate width (m)
Bearing Capacity Correction for Footing Size
For sands: qf (footing) = qf (plate) × (B / Bp)
For clays: qf (footing) = qf (plate) (independent of size)
k-Value Correction for Footing Size — Pavement Use
For rigid pavement, k from 750 mm plate corrected to equivalent plate size for larger areas:
k_{corrected} = k_{plate} × Correction factor from IRC 58 Table
DLC or compacted granular sub-base increases k; IRC 58 provides enhancement factor charts.
Limitations of the Plate Load Test
- Scale effect: Plate is much smaller than actual footing; stress bulb depth = 2B (plate) vs 2B (footing) — much shallower zone tested
- Layered profiles: If soft layer exists below 2B depth of plate, PLT will not detect it but actual footing may fail
- Time: Short-duration test; consolidation settlement of clays not fully captured
- Waterlogging: Test conducted at dry season may give optimistic results for wet season conditions
- Variability: Single-point test; multiple tests needed for variable sites
Comparison of Tests for SBC
| Test | Scale | Depth Explored | Best For |
|---|---|---|---|
| Plate Load Test (IS 1888) | 300–750 mm plate | 2–3 plate widths | Shallow stiff soils, pavements |
| SPT (IS 2131) | 50 mm split spoon | Borehole depth (any) | Sands, silts; SBC from N-value correlation |
| CPTU/SCPT (IS 4968) | 36–50 mm cone | Any depth | Soft clays, layered profiles |
| Triaxial/UCS (lab) | 38–100 mm sample | At sampling depth | Clays; undrained shear strength |
Frequently Asked Questions
Why use a 750 mm plate for rigid pavement k-value determination?
The larger plate reduces the scale effect and better represents the stress distribution under a concrete slab. IS 1888 specifies 750 mm diameter plate for pavement subgrade testing. Smaller plates give higher k-values due to scale effects, which would be unconservative for pavement design.
What is the difference between SBC and allowable bearing pressure?
SBC (Safe Bearing Capacity) from IS 1888 already includes a FOS of 3 on ultimate bearing capacity and the settlement criterion (25 mm). Allowable bearing pressure in a structural design context must also consider the structural settlement tolerance. For a rigid structure, allowable settlement may be less than 25 mm, further limiting the permissible pressure.
Can PLT be used for pile foundation design?
No. PLT is only for shallow foundations where the stress influence zone (2B) can be fully explored. For pile foundations, static load tests on trial piles (IS 2911 Part 4) or high-strain dynamic tests (PDA) are used.