Bearing Pressure Increase
Under Earthquake Loads
Why does IS 1893 allow you to use higher bearing pressures during seismic conditions โ and what does Table 1 actually mean in practice?
The Logic Behind Increased Bearing Pressure
Understanding the engineering rationale before diving into the numbers
Under static loading, a foundation carries permanent dead loads and sustained imposed loads day after day, year after year. Foundations are designed conservatively because the load is always there โ any excess settlement or failure will accumulate over time.
Under earthquake loading, the seismic force is transient and short-lived โ it lasts only for a few seconds to a couple of minutes. The probability that the full seismic force acts simultaneously with the full static load is extremely low. Two key reasons justify an increase in allowable bearing pressure during earthquake events:
Transient Nature of Load
Seismic forces act for a very short duration. Soil does not respond to short-duration loads the same way as sustained loads. Under brief loading, many soils exhibit higher resistance than under long-term static loading.
Reduced Settlement Risk
Settlement is a cumulative phenomenon. A transient load spike does not produce the same settlement as a sustained load. Especially in granular soils, short-duration dynamic loading may not produce significant permanent deformation.
Full FOS required
Settlement over time
IS 1893 allows increase
Transient, not sustained
Percentage Increase in Net Bearing Pressure & Skin Friction
Clause 6.3.5.2 โ the three-row table that governs seismic foundation design
| Sl No. | Soil Type | Classification | Allowable Increase |
|---|---|---|---|
| i) | Type A Rock or Hard Soils |
Well-graded gravels, well-graded sands, gravelโsand mixtures, or poorly-graded sand / clayey sand having N > 30 | + 50% |
| ii) | Type B Medium or Stiff Soils |
Poorly-graded sands or sandy gravels with little or no fines, or stiff to medium stiff fine-grained soils (ML / CL), having N between 10 and 30 | + 25% |
| iii) | Type C Soft Soils |
All soft soils other than SP with N < 10. Includes MI, MH, CI, CH, ML-CL-CH, MI-CI, MH-CH type soils | 0% (No increase) |
Also applies equally to skin friction for pile foundations in corresponding soil types.
๐ Rule 1 โ No Double-Dipping
If any increase in net bearing pressure has already been permitted for forces other than seismic (e.g., wind loads), the total combined increase when seismic force is also included must not exceed the Table 1 limits.
๐ Rule 2 โ Corrected N Only
Bearing pressure must be determined in accordance with IS 6403 or IS 1888. Only corrected SPT N-values (Nโ corrected for overburden) shall be used for soil classification.
Soil Classification for Table 1
How to identify which “Type” your site belongs to โ the critical first step
Type A โ Rock or Hard Soils
Well-graded gravel (GW) or well-graded sand (SW) with <5% passing 75 ยตm sieve. GWโSW mixtures, poorly-graded sand (SP) or clayey sand (SC) with N > 30
Type B โ Medium or Stiff Soils
Poorly-graded sands or SP with little/no fines having N = 10โ30. Also stiff to medium stiff fine-grained soils: ML or CL having N = 10โ30
Type C โ Soft Soils
All soft soils with N < 10. This includes MI, MH, CI, CH, MLโCLโCH, MIโCI, MHโCH (silts and clays of intermediate to high compressibility)
Type D โ Unstable / Liquefiable
Unstable, collapsible, or liquefiable soils. Requires site-specific geotechnical study and special ground treatment. Cannot be used for routine design โ see Clause 6.3.5.3
| USCS Symbol | Soil Description | N Range | Table 1 Type | % Increase |
|---|---|---|---|---|
GW, SW |
Well-graded gravel / sand | Any (low fines) | Type A | +50% |
SP, SC |
Poorly-graded sand / clayey sand | N > 30 | Type A | +50% |
SP |
Poorly-graded sand | 10 โค N โค 30 | Type B | +25% |
ML, CL |
Low compressibility silt / clay | 10 โค N โค 30 | Type B | +25% |
MI, MH, CI, CH |
Intermediate/high compressibility silt/clay | N < 10 | Type C | 0% |
Any |
Liquefiable / unstable soils | โ | Type D | Special |
Corrected N-Value Formula (Cโ Correction)
Why raw SPT values are not directly used โ and how to correct for overburden
In the Standard Penetration Test (SPT), the measured blow count depends not just on soil density, but also on the effective overburden pressure at the test depth. Sand at 15 m depth will give a higher raw N-value than the same sand at 1 m depth, simply because of the confining pressure โ not because the soil is actually denser.
IS 1893 mandates using corrected N-values (Nโ) that normalise all readings to a reference atmospheric pressure. The correction factor is:
๐ Which N to Use for Which Foundation Type
Note 3 to Table 1 specifies how to compute the representative N-value:
| Foundation Type | How to Compute Weighted Average N |
|---|---|
| Isolated Footing | Weighted average from depth of founding to depth of founding + significant influence depth |
| Raft Foundation | Weighted average of N from depth of founding downward |
| Single Pile | Weighted average from bottom tip of pile to bottom tip + 2ร pile diameter |
| Group Pile | Weighted average from bottom tip of pile group to bottom tip + 2ร width of pile group |
| Well Foundation | Weighted average from bottom tip of well to bottom tip + 2ร width of well |
Minimum Required N-Values by Seismic Zone
If site N-values fall below these thresholds, ground improvement or deep piles are required
| Seismic Zone | Depth Below Founding Level (m) | Minimum Corrected N | Remark |
|---|---|---|---|
| III, IV & V (High seismicity) |
โค 5 m | N โฅ 15 | For depths between 5 m and 10 m, use linear interpolation |
| Zone II (Low seismicity) |
โค 5 m | N โฅ 10 | Interpolate linearly for depths 5โ10 m |
Clause 6.3.5.3 โ Liquefaction-Prone Sites
In soil deposits consisting of submerged loose sand and soils falling under classification ML, liquefaction is a concern when:
- Corrected N < 15 in Seismic Zones III, IV and V
- Corrected N < 10 in Seismic Zone II
Such sites should be avoided preferably for new structures of importance. If unavoidable, settlements must be investigated and ground improvement or deep pile foundations adopted. A simplified method for liquefaction evaluation is given in Annex F of IS 1893.
All Five Notes โ Explained Simply
Click each note to expand. These notes are legally binding parts of the standard.
Example: If wind permits a 15% increase on Type B soil, the remaining seismic increase is limited to 25% โ 15% = 10% (not an additional 25%).
Zone III, IV, V: N โฅ 15 at depths โค 5 m below founding level
Zone II: N โฅ 10 at depths โค 5 m below founding level
For depths between 5 m and 10 m, linear interpolation is recommended. If lower N values are encountered, ground improvement (vibro-compaction, dynamic compaction, stone columns, etc.) shall be adopted to meet these values, or deep piles anchored in competent strata shall be used.
N_corrected = Cโ ร N_field where Cโ = โ(Pโ / ฯ'แตฅ)Here Pโ = atmospheric pressure (โ100 kPa), and ฯ’แตฅ = effective vertical overburden pressure at the test depth.
Refer to IS 1498 : 1970 and IS 2131 : 1981 for soil notation and SPT test method. The weighted average N for the relevant zone below the foundation shall be computed as described in the Table 1 note (isolated footing, raft, pile, group pile, well foundation โ each has a specific averaging zone).
How to Apply Table 1 in Practice
A systematic workflow for checking and adjusting bearing pressure under seismic conditions
Determine Static Bearing Capacity
Using IS 6403 or IS 1888 (load test), calculate the net allowable bearing capacity under static loads (dead + live). Call this q_static (kPa or kN/mยฒ).
Conduct SPT and Correct N-Values
Perform SPT at the site per IS 2131. Collect field N-values at multiple depths. Apply the Cโ correction for effective overburden: Nโ = Cโ ร N_field, where Cโ = โ(100/ฯ’แตฅ).
Compute Weighted Average N for Foundation Zone
Based on foundation type, compute the weighted average Nโ for the relevant zone (see Note 3 to Table 1). For isolated footings, this is below the founding level.
Classify Soil (Table 2 of IS 1893)
Using the weighted average corrected N and the USCS soil symbol (IS 1498), classify the soil as Type A, B, C, or D as per Table 2 of IS 1893.
Apply Table 1 Percentage Increase
Look up Table 1: Type A โ 50%, Type B โ 25%, Type C โ 0%. Compute the seismic bearing capacity:
q_seismic = q_static ร (1 + %/100)
Check Minimum N Requirement (Note 3)
Verify that the corrected N meets the minimum values for the seismic zone. If not, mandate ground improvement or redesign with pile foundations.
Combine with Load Combinations (Clause 6.3.5.1)
Use unfactored loads when assessing bearing pressure. Earthquake force is added to dead load + imposed load as per Clause 6.3 of IS 1893. The seismic bearing capacity from Step 5 is the limit for the combined load case.
IS 1893 Bearing Pressure Calculator
Compute seismic bearing capacity, CN correction, and minimum N-value compliance
๐งฎ Seismic Bearing Pressure Checker
Enter your project parameters to compute seismic allowable bearing pressure per IS 1893 Table 1
