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SBC of 200 kN/m²? Why Your Foundation Can Still Settle
An Indian engineer's guide to safe bearing capacity, foundation settlement, IS 6403, IS 8009 and the soil conditions encountered on construction sites.
Imagine a residential project in Ahmedabad. The soil investigation report arrives. The structural engineer reads:
Safe Bearing Capacity: 200 kN/m²
The proposed footing is designed for a net pressure of 180 kN/m².
The natural conclusion? "180 is less than 200. The foundation is safe."
But there is an important question:
Was the 200 kN/m² recommendation checked for settlement, or only for shear failure?
A foundation can have enough bearing resistance to support a building without shear failure and still experience settlement that affects serviceability.
This matters in Indian projects involving layered alluvial deposits, compressible clay, variable fill, expansive soil, groundwater and different footing sizes. Let's understand why with practical calculations and Indian Standards.
1. Safe Bearing Capacity and Settlement Are Not the Same Thing
Before discussing calculations, separate three terms.
Ultimate bearing capacity: The pressure associated with shear failure of the supporting soil.
Safe bearing capacity based on shear: The ultimate bearing capacity reduced using an appropriate factor of safety.
Allowable bearing pressure: The pressure that satisfies the relevant shear and settlement requirements.
A soil deposit can possess adequate shear strength but still deform excessively under sustained loading.
For a shallow foundation, allowable net bearing pressure should be governed by the more restrictive of the shear and settlement criteria:
Net allowable bearing pressure = Minimum (net safe bearing capacity based on shear, net pressure corresponding to permissible settlement).
This is consistent with considering shear and settlement in foundation design using IS 6403:1981, applicable amendments and relevant foundation standards.
Two reports showing similar SBC values may not give the designer the same information about settlement.
2. Practical Indian Example: A Residential Building in Ahmedabad
Consider a hypothetical G+5 residential building on a plot in Ahmedabad.
Parameter | Value |
|---|---|
Net ultimate bearing capacity | 750 kN/m² |
Assumed factor of safety against shear | 3.0 |
Net safe bearing capacity | 250 kN/m² |
Settlement-controlled net pressure | 160 kN/m² |
Proposed net footing pressure | 180 kN/m² |
Step 1: Check shear safety
Net safe bearing capacity = 750 ÷ 3 = 250 kN/m².
Proposed net footing pressure = 180 kN/m².
Since 180 is less than 250, the proposed pressure satisfies the assumed shear-safety criterion.
Step 2: Check settlement
The settlement analysis indicates the maximum acceptable net pressure for the assumed foundation geometry and permissible settlement is 160 kN/m².
But the footing applies 180 kN/m². Therefore, the settlement criterion is not satisfied.
Step 3: Determine the governing pressure
Net allowable bearing pressure = Minimum (250, 160) = 160 kN/m².
Design check | Result |
|---|---|
Shear bearing capacity | Pass |
Settlement criterion | Fail |
Overall assessment | Revision required |
Possible responses include changing the footing arrangement, reconsidering founding depth, evaluating ground improvement or adopting another suitable foundation system. The correct decision depends on actual ground and structural conditions.
Passing the shear bearing-capacity check does not mean passing the settlement check.
All values are hypothetical, not recommendations for any specific Ahmedabad plot.
3. Why Foundation Size Matters Even When SBC Is the Same
Two square footings carry different loads but produce the same uniform additional contact pressure of 200 kN/m².
Parameter | Footing A | Footing B |
|---|---|---|
Size | 2 m × 2 m | 4 m × 4 m |
Area | 4 m² | 16 m² |
Uniform pressure | 200 kN/m² | 200 kN/m² |
Corresponding load | 800 kN | 3,200 kN |
The pressure at foundation level is the same, but the distribution of additional stress below the footings differs.
One approximate approach is the 2:1 stress distribution method.
Formula
Δσᵥ = q × B × L / [(B + z)(L + z)]
Here, Δσᵥ is the approximate additional vertical stress at depth z (kN/m²); q is the uniformly applied additional pressure (kN/m²); B and L are footing width and length (m); and z is depth below footing base (m).
This approximate method is described in the Federal Highway Administration's Geotechnical Engineering Circular No. 6: Shallow Foundations.
Footing A: 2 m × 2 m
At 3 m below the footing base:
Δσᵥ = 200 × 2 × 2 / [(2 + 3)(2 + 3)]
= 800 / 25
Δσᵥ = 32.0 kN/m²
Footing B: 4 m × 4 m
At the same depth:
Δσᵥ = 200 × 4 × 4 / [(4 + 3)(4 + 3)]
= 3,200 / 49
Δσᵥ = 65.3 kN/m²
Comparison
Parameter | Footing A | Footing B |
|---|---|---|
Contact pressure | 200 kN/m² | 200 kN/m² |
Depth below footing | 3 m | 3 m |
Additional vertical stress | 32.0 kN/m² | 65.3 kN/m² |
At this depth, the larger footing produces approximately 2.04 times the additional vertical stress under this simplified method.
If a compressible clay layer exists 3 m below foundation level, its compression could be significant. Larger footings generally influence a greater volume of soil, although actual settlement depends on layer thickness, stiffness, compressibility and stress history.
Important: The 2:1 calculation estimates stress increase, not foundation settlement. An appropriate settlement analysis is still required.
4. Indian Ground Conditions: Why the Soil Profile Matters More Than One Number
India does not have uniform ground conditions. Even neighbouring plots can have different stratigraphy, groundwater and engineering behaviour.
A previous project's SBC should not simply be copied into the next report.
Ahmedabad and North Gujarat: Layered Alluvial Deposits
Parts of Ahmedabad and North Gujarat contain alluvial deposits with changing proportions of sand, silt and clay.
The Central Ground Water Board has documented complex aquifer systems in Ahmedabad district. The foundation engineer must evaluate the sequence, thickness and engineering behaviour of individual soil layers.
Consider a hypothetical borehole:
Depth below ground | Soil condition |
|---|---|
0–1.5 m | Fill / disturbed soil |
1.5–4.5 m | Medium dense sand |
4.5–7.5 m | Compressible silty clay |
7.5–12 m | Dense sand |
Imagine an isolated footing founded at 1.5 m depth.
The immediate bearing stratum might show adequate resistance, while the deeper silty clay contributes to settlement due to increased stress.
Adequate SPT resistance at founding level does not by itself establish acceptable settlement. Soil layers within the relevant stress-influence zone must be assessed.
IS 1892:2021 is relevant to planning subsurface investigation locations and depths.
Gujarat and Other Regions: Black-Cotton and Expansive Soil
Clay-rich expansive soil can shrink when its moisture reduces and swell when moisture increases.
Unlike conventional settlement from loading, expansive soil can also cause upward movement or heave.
Assess swelling and shrinkage alongside strength and compression under loading.
Relevant testing may include:
Liquid limit and plastic limit.
Free swell index.
Swelling pressure.
Consolidation and compressibility properties, when appropriate.
Indian testing standards include IS 2720 (Part 5), IS 2720 (Part 40) and IS 2720 (Part 41).
Depending on actual conditions, an engineer may evaluate deeper foundations, under-reamed piles, rafts, ground treatment or another solution. IS 2911 (Part 3):2021 addresses under-reamed piles.
No foundation type should be selected solely because the soil is described as black-cotton soil.
Mumbai and Coastal Construction: Compressible Deposits and Groundwater
Some coastal and reclaimed-ground projects require attention to soft compressible deposits, made ground and groundwater.
Long-term consolidation settlement may be a controlling concern. The existence of such deposits must be shown by investigation rather than assumed from a city's location.
Industrial Areas: Filled Ground and Heavy Equipment
Consider a warehouse in Sanand, Changodar or another Indian industrial corridor.
If the site has been filled or levelled before construction, inadequately compacted fill or compressible soil below it can cause settlement despite apparently acceptable near-surface resistance.
Warehouse floors and equipment foundations can be particularly sensitive to differential movement.
Designers should consider fill characteristics, preparation, loading, deformation tolerance and foundation arrangement.
A useful soil investigation identifies risks before construction, not after operational problems or floor cracking appear.
5. How Much Settlement Can Clay Produce? A Worked Example
Let's calculate primary consolidation settlement for a simplified normally consolidated clay layer.
Sᶜ = [Cᶜ × H / (1 + e₀)] × log₁₀[(σ′₀ + Δσ′) / σ′₀]
Symbol | Meaning |
|---|---|
Sᶜ | Primary consolidation settlement |
Cᶜ | Compression index |
H | Thickness of compressible clay |
e₀ | Initial void ratio |
σ′₀ | Initial effective vertical stress |
Δσ′ | Average increase in effective stress after consolidation |
Hypothetical parameters
Parameter | Value |
|---|---|
Clay layer thickness, H | 2.0 m |
Compression index, Cᶜ | 0.25 |
Initial void ratio, e₀ | 0.90 |
Initial effective stress, σ′₀ | 100 kN/m² |
Average effective stress increase, Δσ′ | 40 kN/m² |
Substitute:
Sᶜ = [0.25 × 2 / (1 + 0.90)] × log₁₀[(100 + 40) / 100]
Sᶜ = (0.50 / 1.90) × log₁₀(1.40)
Sᶜ = 0.03845 m
Primary consolidation settlement ≈ 38.5 mm
Under these assumptions, the 2 m clay layer alone contributes about 38.5 mm of primary consolidation settlement.
This is not total settlement of the foundation. Other layers can deform and immediate settlement, secondary compression or other movements may need evaluation.
The calculation assumes normally consolidated soil, one-dimensional primary consolidation and an appropriate representative effective stress increase.
Overconsolidated clay must be assessed using its preconsolidation stress and recompression/compression behaviour.
IS 8009 (Part 1):1976, with relevant amendments, is an Indian reference for shallow foundation settlement. Applicable laboratory consolidation testing is covered by IS 2720 (Part 15).
6. Total Settlement and Differential Settlement: Which Matters More?
Consider two columns supported by separate footings:
Foundation | Settlement |
|---|---|
Column A | 12 mm |
Column B | 30 mm |
Differential settlement = 30 − 12 = 18 mm.
But is 18 mm acceptable? Not enough information yet.
If the columns are 4 m apart, the approximate ratio of settlement difference to spacing is:
18 / 4,000 = 0.0045 ≈ 1/222.
This is a geometric indicator of relative movement, not an automatic code-compliance verdict.
Acceptance depends on structural arrangement, stiffness, loads, materials, finishes and serviceability requirements.
An apartment building, warehouse floor, machine foundation and water-retaining structure may require different settlement tolerances.
A single settlement limit should never be copied across all Indian projects without evaluating the structure.
7. Why Groundwater Must Not Be Ignored
Groundwater affects effective stress and can influence bearing capacity and settlement calculations.
In India, groundwater conditions can vary with seasons, rainfall, dewatering, drainage and nearby activity.
A groundwater depth observed in the dry season may not represent the highest credible level during the structure's service life.
Ask:
What groundwater level was recorded during drilling, on what date, and under what conditions?
Is seasonal groundwater variation relevant?
How do pore-water pressures, soil unit weights and effective stress affect calculations?
Will excavations require dewatering or additional excavation support?
One reading should not automatically be taken as the permanent design water table.
8. Gross vs Net Bearing Pressure: A Common Report-Review Mistake
Suppose the soil report recommends net allowable bearing pressure = 200 kN/m², but a designer compares that directly with gross foundation pressure.
This may not be a like-for-like comparison.
Under a simplified uniform soil profile:
Net foundation pressure = Gross foundation pressure − original overburden pressure removed at founding level.
Parameter | Value |
|---|---|
Foundation depth | 1.5 m |
Soil unit weight above foundation | 18 kN/m³ |
Original overburden pressure | 27 kN/m² |
Gross foundation pressure | 210 kN/m² |
Net pressure = 210 − (18 × 1.5)
= 210 − 27
= 183 kN/m²
Gross pressure is 210 kN/m², while net pressure is 183 kN/m² under the stated simplified assumptions.
Actual calculations must consistently account for footing and structural loads, excavation, removed overburden, backfill and groundwater.
Never compare a gross design pressure directly with a net allowable pressure without the correct conversion and assumptions.
9. Does a Higher SPT N-Value Mean Lower Settlement?
Not automatically.
SPT provides valuable resistance information and allows some engineering correlations, but an isolated N-value does not fully characterize the compressibility of all relevant soil layers.
Imagine two sites, each with a corrected SPT N-value of 25 at the proposed founding level.
At Site A, the deeper profile is predominantly dense sand. At Site B, the immediate bearing layer overlies thick, compressible clay.
Settlement behaviour can differ substantially despite similar near-surface SPT resistance.
Interpret SPT with soil classification, groundwater, stratum thickness, foundation geometry and appropriate laboratory testing.
IS 2131:2025 addresses the Standard Penetration Test of Soil.
Read our earlier article: SPT N-Value: Why 30 Blows Does Not Mean a Safe Foundation.
10. What Tests Help Engineers Assess Settlement?
Not all projects need every geotechnical test. Choose investigation methods based on expected ground conditions and actual engineering questions.
Investigation or test | What it helps establish |
|---|---|
Borehole logging | Strata sequence and thickness |
SPT | Penetration resistance and relevant correlations |
Undisturbed sampling | Specimens for appropriate strength and compressibility tests |
Consolidation test | Compression and consolidation characteristics |
Grain-size analysis | Particle-size distribution and soil classification |
Atterberg limits | Consistency and plasticity of fine-grained soil |
Shear strength tests | Strength parameters for applicable bearing checks |
Groundwater observations | Water conditions for effective stress and construction |
Plate load test | Site-specific load-settlement response at test scale |
A plate load test may help, but its settlement response should not be assumed to represent that of a much larger footing without considering scale, footing geometry and soil layering.
Good geotechnical engineering means selecting the right tests and interpreting the results properly, not simply performing the largest number of tests.
11. Checklist: Before Accepting an SBC Recommendation in an Indian Soil Report
Check 1 — Net or gross SBC? Identify the basis of the recommended bearing pressure.
Check 2 — Founding depth? Confirm the depth used for the recommendation.
Check 3 — Footing dimensions and loads? Confirm that the actual structural design matches geotechnical assumptions.
Check 4 — Settlement checked? Identify whether shear or settlement governs allowable pressure.
Check 5 — Borehole depth and location? Did the investigation capture the significant soil influence zone?
Check 6 — Compressible layers? Examine relevant deeper strata, not just the founding layer.
Check 7 — Groundwater? Review levels observed and design assumptions for variation.
Check 8 — Laboratory parameters? Confirm relevant inputs are credible and representative.
Check 9 — Total and differential settlement? Check criteria appropriate to the proposed structure.
Check 10 — Changes after investigation? Filling, excavation, dewatering or adjacent construction can require reconsideration.
A technically useful soil report lets the structural engineer understand the basis of recommendations rather than just copying an SBC number.
12. Indian Standards Worth Keeping as a Reference
Indian Standard | Subject |
|---|---|
IS 6403:1981 | Determination of bearing capacity of shallow foundations |
IS 8009 (Part 1):1976 | Calculation of settlements of shallow foundations under symmetrical static vertical loads |
IS 1904:2021 | General requirements for design and construction of foundations in soils |
IS 1892:2021 | Subsurface investigation for foundations |
IS 2131:2025 | Standard Penetration Test of Soil |
IS 2720 (Part 15):1986 | Determination of consolidation properties |
IS 1888:1982 | Method of load test on soils |
IS 2911 (Part 3):2021 | Design and construction of under-reamed piles |
For applicable amendments, editions and the current status of Indian Standards, consult the official Bureau of Indian Standards catalogue.
Additional technical references
Final Thought: SBC Is a Starting Point, Not the Entire Answer
On many Indian construction sites, the conversation can end with one number: 200, 250 or 300 kN/m².
But the allowable bearing pressure means little without the soil profile, foundation geometry and design assumptions behind it.
A proper assessment must answer two separate questions:
Can the soil support the applied foundation pressure without unacceptable shear failure?
Will settlement stay within appropriate limits for the proposed structure?
Both matter.
Next time you receive a soil investigation report, ask:
"Is this the allowable bearing pressure after considering both shear and settlement for our actual foundation?"
That single question may tell you whether the recommendation is ready to use or requires more assessment.
About GTSM Testing Labs LLP
GTSM Testing Labs LLP provides geotechnical investigation, structural assessment and construction material testing services based in Ahmedabad, Gujarat.
We believe reports should communicate findings and engineering significance clearly enough to help consultants and project teams make informed decisions.
Website: www.gtsmtestinglabs.in
Technical note: All worked examples are hypothetical and educational, not project-specific design recommendations. Actual foundation design requires suitable investigation, verified engineering parameters and professional engineering judgement.
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