• Bearing Capacity Calculator: Terzaghi’s Method
• Professional Excel Spreadsheets (GEOtExcel)
• Ultimate Bearing Capacity Calculation – Academic Education Focus
• Ultimate Bearing Capacity Calculation (Design) as per IS 6403 Code
• Beyond IS 6403: Technical Critique & Recommendations for Bearing Capacity Estimation by GEOtExcel

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CODE OF PRACTICE FOR DETERMINATION OF BEARING CAPACITY OF SHALLOW FOUNDATIONS

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IS6403 Professional Excel Spreadsheets (GEOtExcel)

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1. SCOPE

1.1 This standard covers the procedure for determining the ultimate bearing capacity and allowable bearing pressure of shallow foundations based on shear and allowable settlement criteria.

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2. TERMINOLOGY

2.0 For the purpose of this standard, the following definitions shall apply:

2.1 Terms Relating to Bearing Capacity

2.1.1 Net Loading Intensity — The net loading intensity on the foundation is the gross intensity of loading minus the weight of displaced soil above the foundation base.

2.1.2 Ultimate Bearing Capacity — The intensity of loading at the base of the foundation which would cause shear failure of the soil support.

2.1.3 Safe Bearing Capacity — Maximum intensity of loading that the foundation will safely carry without the risk of shear failure of soil irrespective of any settlement that may occur.

2.1.4 Safe Bearing Pressure or Net Soil Pressure for Specified Settlement — The intensity of loading that will cause a permissible settlement or specified settlement of the structure.

2.1.5 Allowable Bearing Capacity — The net intensity of loading which the foundation will carry without undergoing settlement in excess of the permissible value for the structure under consideration but not exceeding net safe bearing capacity.

2.2 General Terms

2.2.1 Density Index (Relative Density) — The ratio of the difference between the void ratios of cohesionless soil in the loosest state and any given state to the difference between its void ratios at the loosest and densest states.

2.2.2 Effective Surcharge at the Base Level of Foundation — The intensity of vertical pressure at the base level of foundation, computed assuming total unit weight for the portion of soil above the water table and submerged unit weight for the portion below the water table.

2.2.3 Footing — A structure constructed in brickwork, masonry or concrete under the base of a wall or column for the purpose of distributing the load over a larger area.

2.2.4 Foundation — That part of a structure which is in direct contact with soil and transmits loads into it.

2.2.5 Shallow Foundation — A foundation whose width is greater than its depth. The shearing resistance of the soil in the sides of the foundation is generally neglected.

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3. SYMBOLS

3.1  For the purpose of this code and unless otherwise defined in the text, the following letter symbols shall have the meaning indicated against each:

نماد (Symbol)	(Meaning)	(Unit)
$A$	Area of footing	${\text{cm}}^2$
$A^{\mathrm{\prime }}$	Effective area of footing	${\text{cm}}^2$
$B$	Width of strip footing, width of footing, side of square footing, diameter of circular footing	$\text{cm}$
$B^{\mathrm{\prime }}$	Effective width of footing	$\text{cm}$
$b$	Half of $B$B	$\text{cm}$
$c$	Cohesion	${\text{kgf/cm}}^2$
${\epsilon }_1$	Undrained cohesion of the top layer	${\text{kgf/cm}}^2$
${\epsilon }_2$	Undrained cohesion of the lower clay layer	${\text{kgf/cm}}^2$
$D_f$​	Depth of foundation	$\text{cm}$
$D_w$​	Depth to water table	$\text{cm}$
$d$d	Depth of top clay layer with undrained cohesion ${\epsilon }_1$ε1​	$\text{cm}$
$d_c,d_q,d_{\gamma }$​	Depth factors
$e$	Eccentricity of loading	$\text{cm}$
$e_B$​	Eccentricity of loading along the width	$\text{cm}$
$e_L$​	Eccentricity of loading along the length	$\text{cm}$
$H$	Horizontal component of loading	$\text{kgf}$
$i_c,i_q,i_{\gamma }$​	Inclination factors
$K_d$​	Depth factor (varies linearly from 1 for depth $D_f=0$Df​=0 to 1.33 for depth $D_f=B$Df​=B)
$L$	Length of footing	$\text{cm}$
$L^{\mathrm{\prime }}$	Effective length of footing	$\text{cm}$
$N$	Corrected standard penetration value
$N_c,N_q,N_{\gamma },N_c^{\mathrm{\prime }},N_q^{\mathrm{\prime }},N_{\gamma }^{\mathrm{\prime }}$	Bearing capacity factors
$N_{\varphi }$​	${\tan }^2(\pi \mathrm{/}4+\varphi \mathrm{/}2)$tan2(π/4+ϕ/2)
$q$	Effective surcharge at the base level of foundation	${\text{kgf/cm}}^2$
$q_a$​	Net soil pressure for a specified settlement of 25 mm	${\text{kgf/cm}}^2$
$q_c$​	Static cone penetration resistance	${\text{kgf/cm}}^2$
$q_d$​	Net ultimate bearing capacity based on general shear failure	${\text{kgf/cm}}^2$
$q_d^{\mathrm{\prime }}$	Net ultimate bearing capacity based on local shear failure	${\text{kgf/cm}}^2$
Dr	Relative density of soil
$W^{\mathrm{\prime }}$	Correction factor for location of water table
$s_c,s_q,s_{\gamma }$​	Shape factors
$\alpha$	Inclination of the load to the vertical	degrees
$\varphi$	Angle of shearing resistance of soil	degrees
$\gamma$	Bulk unit weight of foundation soil	${\text{kgf/cm}}^3$

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4. GENERAL

4.1 Sufficient number of undisturbed samples, about 40 to 100 mm in diameter or more or block samples should be obtained, where possible. These samples are for the determination of field density of soil and conducting tests for determining the relevant shear and consolidation parameters of the soil. Tests on soils should be conducted in accordance with relevant parts of IS : 2720*.

4.2 Position and fluctuation of water table should be ascertained. Reference may be made to IS : 1892-1979† and IS : 2132-1972‡ for guidance regarding investigations and collection of data.

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5. ULTIMATE NET BEARING CAPACITY

5.0 GENERAL

Three types of failure of soil support beneath the foundations have been recognized, depending upon the deformations associated with the load and the extent of development of failure surface.

They are: a) general shear failure, b) local shear failure and c) punching shear. The choice of which method of analysis is best suited in a given situation is difficult to make, because only limited test data are available on full sized foundations to verify the reliability of the computed bearing capacity. However, guidelines given in relevant clauses may be used for guidance. Wherever possible bearing capacity calculations shall be made on the basis of shear strength parameters ϕ and c obtained from appropriate shear tests [see IS: 2720 (Parts XI and XIII)*] or from plate load test results as given in IS: 1888-1981 or from static cone penetration resistance qc obtained from static cone penetration test as given in IS: 4968 (Part III)-1976.

General Shear Failure

Local Shear Failure

Calculation for Cohesive-Frictional, Cohesionless and Cohesive Soils

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5.0.1 Effect of Eccentricity

a) Single Eccentricity — If the load has an eccentricity $e$, with respect to the centroid of the foundation in only one direction, then the dimension of the footing in the direction of eccentricity shall be reduced by a length equal to $2e$. The modified dimension shall be used in the bearing capacity equation and in determining the effective area of the footing in resisting the load.

b) Double Eccentricity — If the load has double eccentricity with respect to the centroid of the footing then the effective dimensions of the footing to be used in determining the bearing capacity as well as in computing the effective area of the footing in resisting the load shall be determined as given below: ​ A′=L′×B′

$$L^{\mathrm{\prime }}=L-2e_L$$​$$B^{\mathrm{\prime }}=B-2e_B$$

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5.1 Soils with Cohesion and Angle of Shearing Resistance:

5.1.1 Strip Footings:

The following formulae shall be used for calculating ultimate net bearing capacity in the case of strip footings:

a) In case of general shear failure$$q_d=c{N}_c+\gamma D_f(N_q-1)+0.5\gamma B{N}_{\gamma }$$​

b) In case of local shear failure$$q_d^{\mathrm{\prime }}=\frac{2}{3}c{N}_c^{\mathrm{\prime }}+\gamma D_f(N_q^{\mathrm{\prime }}-1)+0.5\gamma B{N}_{\gamma }^{\mathrm{\prime }}$$

The values of $N_c,N_c^{\mathrm{\prime }},N_q,N_q^{\mathrm{\prime }},N_{\gamma }$ and $N_{\gamma }^{\mathrm{\prime }}$​ may be obtained from Table 1.

TABLE 1: BEARING CAPACITY FACTORS

$\varphi$ (Degrees)​	${\mathrm{<br\; class="Apple-interchange-newline">N}}_c$ ​	$N_q$​	​$N_{\gamma }$​
0	5.14	1.00	0.00
5	6.49	1.57	0.45
10	8.35	2.47	1.22
15	10.98	3.94	2.65
20	14.83	6.40	5.39
25	20.72	10.66	10.88
30	30.14	18.40	22.40
35	46.12	33.30	48.03
40	75.31	64.20	109.41
45	138.88	134.88	271.76
50	266.89	319.07	762.89

Notes:

• For obtaining values of $N_c$​, $N_q$​ and $N_{\gamma }$​, calculate ${\varphi }^{\mathrm{\prime }}={\tan }^{-1}(0.67\tan \varphi )$.
• Read $N_c^{\mathrm{\prime }}$​, $N_q^{\mathrm{\prime }}$​, and $N_{\gamma }^{\mathrm{\prime }}$​ from the Table corresponding to the value of ${\varphi }^{\mathrm{\prime }}$ instead of $\varphi$ which are values of $N_c$​, $N_q$​, $N_{\gamma }$​ respectively.

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5.1.2 Modified Ultimate Net Bearing Capacity Formula:

The ultimate net bearing capacity obtained in 5.1.1 for strip footing shall be modified to take into account, the shape of the footing, inclination of loading, depth of embedment and effect of water table.

The modified bearing capacity formulae are given as under:

a) In case of general shear failure $q_d$

​$$q_d=c{N}_c{s}_c{d}_c{i}_c+\gamma D_f(N_q-1)s_q{d}_q{i}_q+0.5\gamma B{N}_{\gamma }{s}_{\gamma }{d}_{\gamma }{i}_{\gamma }{W}^{\mathrm{\prime }}$$

b) In case of local shear failure $q_d^{\mathrm{\prime }}$

​$$q_d^{\mathrm{\prime }}=c{N}_c^{\mathrm{\prime }}{s}_c{d}_c{i}_c+\gamma D_f(N_q^{\mathrm{\prime }}-1)s_q{d}_q{i}_q+0.5\gamma B{N}_{\gamma }^{\mathrm{\prime }}{s}_{\gamma }{d}_{\gamma }{i}_{\gamma }{W}^{\mathrm{\prime }}$$

5.1.2.1 The shape factors shall be as given in Table 2.

TABLE 2: SHAPE FACTORS

Shape of Base	$s_c$​	$s_q$​	$s_{\gamma }$​
i) Continuous strip	1.00	1.00	1.00
ii) Rectangle	$1+0.2B\mathrm{/}L$	$1+0.2B\mathrm{/}L$	$1-0.4B\mathrm{/}L$
iii) Square	1.3	1.2	0.8
iv) Circle	1.3	1.2	0.6

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5.1.2.2 The depth factors shall be as under:

$$d_c=1+0.2\frac{D_f}{B}\sqrt{N_{\varphi }}$$​​$$d_q=d_{\gamma }=1\text{for}\varphi <{10}^{\circ }$$$$d_q=d_{\gamma }=1+0.1\frac{D_f}{B}\sqrt{N_{\varphi }}\text{for}\varphi >{10}^{\circ }$$

Note — The correction is to be applied only when back filling is done with proper compaction.

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5.1.2.3 The inclination factor shall be as under:

$$i_c=i_q={(1-\frac{\alpha }{{90}^{\circ }})}^2$$$$i_{\gamma }={(1-\frac{\alpha }{\varphi })}^2$$

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GEOtExcel: Shape, Depth and Inclination FACTORS: Continuous Strip

GEOtExcel: Shape, Depth and Inclination FACTORS: Circle

GEOtExcel: Shape, Depth and Inclination FACTORS: Rectangle

GEOtExcel: Shape, Depth and Inclination FACTORS: Square

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5.1.2.4 Effect of water table

a) If the water table is likely to permanently remain at or below a depth of $(D_f+B)$ beneath the ground level surrounding the footing then $W^{\mathrm{\prime }}=1$.

b) If the water table is located at a depth $D_f$​ or likely to rise to the base of the footing or above then the value of $W^{\mathrm{\prime }}$ shall be taken as 0.5.

c) If the water table is likely to permanently get located at depth $D_f<D_w<(D_f+B)$, then the value of $W^{\mathrm{\prime }}$ be obtained by linear interpolation.

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5.2 Cohesionless Soil ( c=0 )

The ultimate net bearing capacity shall be calculated as given in 5.2.1 and 5.2.2.

5.2.1 Based on Relative Density

The formulae given in 5.1.1 and 5.1.2 shall be used, together with relevant shear strength parameter.

5.2.1.1 The relative density as given in Table 3 shall be used as a guide to determine the method of analysis.

TABLE 3 METHOD OF ANALYSIS BASED ON RELATIVE DENSITY

i)	Greater than 70 percent	Less than 0.55	Dense	General shear
ii)	Less than 20 percent	Greater than 0.75	Loose	Local shear (as well as punching shear)
iii)	20 to 70 percent	0.55 to 0.75	Medium	Interpolate between i) and ii)

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5.2.2 Based on Standard Penetration Resistance Value

The standard penetration resistance shall be determined as per IS: 2131-1981* at a number of selected points at intervals of 75 cm in the vertical direction or change of strata if it takes place earlier and the average value beneath each point shall be determined between the level of the base of the footing and the depth equal to 1.5 to 2 times the width of foundation. In computing the value, any individual value more than 50 percent of the average calculated shall be neglected, and average re-calculated (the values for all loose seams shall however be included).

5.2.2.1 The ultimate net bearing capacity shall be calculated from the following formula (covering effect of other factors as mentioned in 5.1.2):

$$q_d=q(N_q-1)\cdot s_q{d}_q{i}_q+\frac{1}{2}\gamma B{N}_{\gamma }{s}_{\gamma }{d}_{\gamma }{i}_{\gamma }{W}^{\mathrm{\prime }}$$

Where $q$q may be read from Fig. 1, $N_{\gamma },N_q$​ may be read from Table 1, $s_q,d_q,i_q,s_{\gamma },d_{\gamma },i_{\gamma },$​and $W^{\mathrm{\prime }}$may be obtained as in 5.1.

SPT-Derived ϕ

The methods in this section follow IS 6403:1981, but for modern limitations and advanced correlations, refer to the GEOtExcel Technical Critique & Recommendations section at the end of this post.

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5.2.3 Method Based on Cone Penetration Test (CPT) for Cohesionless Soil

The static cone point resistance ‘qc​’ shall be determined as per IS: 4968 (Part III)-1976† at number of selected points at intervals of 10 to 15 cm. The observed values shall be corrected for the dead weight of sounding rods. Then the average value at each one of the location shall be determined between the level of the base of the footing and the depth equal to BB to 2 times the width of the footing. The average of the static cone point resistance values shall be determined for each one of the location and minimum of the average values shall be used in the design. The ultimate bearing capacity of shallow strip footings on cohesionless soil deposits shall be determined from Fig. 2.

The methods in this section follow IS 6403:1981, but for modern limitations and advanced correlations, refer to the GEOtExcel Technical Critique & Recommendations section at the end of this post.

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5.3 Cohesive Soil (when $\varphi =0$)

5.3.1 Homogeneous Layer

5.3.1.1 The net ultimate bearing capacity immediately after construction on fairly saturated homogeneous cohesive soils shall be calculated from following formula:

$$q_d=c{N}_c{s}_c{i}_c$$ where$$N_c=5.14$$

The value of $c$ shall be obtained from unconfined compressive strength test. Alternatively, it can also be derived from static cone test (see 5.3.1.2). The values of $s_c$​, $d_c$​ and $i_c$​ may be obtained as in 5.1. If the shear strength for a depth of $B$ beneath the foundation does not depart from the average by more than 50 percent, the average may be used in the calculation.

5.3.1.2 Alternately, cohesion $c$ shall be determined from the static cone point resistance $q_c$​ using the empirical relationship shown below:

Soil Type	Point Resistance Values ($q_c$qc​) kgf/cm²	Range of Undrained Cohesion (kgf/cm²)
Normally consolidated clay	qc​<20	$q_c\mathrm{/}18$ to $q_c\mathrm{/}15$
Over consolidated clays	$q_c>20$	$q_c\mathrm{/}26$ to $q_c\mathrm{/}22$

The methods in this section follow IS 6403:1981, but for modern limitations and advanced correlations, refer to the GEOtExcel Technical Critique & Recommendations section at the end of this post.

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5.3.2 Two Layered System

In the case of two layered cohesive soil system which do not exhibit marked anisotropy the ultimate net bearing capacity of a strip footing can be calculated by using the formula given below:$$q_d=c_1{N}_c$$ ​

where $N_c$​ may be obtained from Fig. 3.

The methods in this section follow IS 6403:1981, but for modern limitations and advanced correlations, refer to the GEOtExcel Technical Critique & Recommendations section at the end of this post.

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5.3.3 Desiccated Soil

In the case of desiccated cohesive soils, the undrained cohesion is likely to decrease along with depth and is likely to get stabilized at some depth below ground level, around 3.5 m, if other factors do not influence. If a plot of undrained cohesion values as shown in Fig. 4 is obtained, and where the pressure bulb falls within the desiccated top soil, the ultimate net bearing capacity shall be obtained with the assumption of cylindrical failure surface from Table 4.

The methods in this section follow IS 6403:1981, but for modern limitations and advanced correlations, refer to the GEOtExcel Technical Critique & Recommendations section at the end of this post.

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6. ALLOWABLE BEARING CAPACITY

6.1 The allowable bearing capacity shall be taken as either of the following, whichever is less:

a) Net ultimate bearing capacity divided by a suitable factor of safety (net safe bearing capacity).

b) The net soil pressure that can be imposed on the base without the settlement exceeding the permissible values as given in IS: 1904-1978 (safe bearing pressure).

6.1.1 Safe Bearing Pressure

The permissible settlements for different types of soil formations are specified in IS: 1904-1978*. The methods for calculations of settlements for assumed pressure from standard penetration resistance are specified in IS: 8009 (Part I)-1976†; by calculating the settlements for two or three probable soil pressures and interpolating, the net soil pressure for permissible settlement may be estimated. This safe bearing pressure can also be calculated based on plate load test (see IS: 1888-1982).

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Beyond IS 6403: Technical Critique & Recommendations for Bearing Capacity Estimation by GEOtExcel

GEOtExcel Advisory Note: While IS 6403:1981 is a long-standing standard for shallow foundation design, its empirical correlations were developed several decades ago. Modern geotechnical practice, including Eurocode 7 and advanced literature by Das and Bowles, offers more refined methods. Users are advised to consider the following critiques when using the IS 6403:

Comparison Table: IS 6403 vs. Modern Geotechnical Practice

Feature / Clause	IS 6403:1981 Approach	Recommendation
SPT in Cohesionless Soil (Cl. 5.2.2)	Uses N values for ϕ from a simple chart (Fig. 1).	Requires explicit N60​ (energy) and N1(60)​ (overburden) corrections.
CPT in Sands (Cl. 5.2.3)	Uses raw average qc​ with Figure 2 curves.	Requires normalizing qc​ for effective stress or correlating to ϕ for stress-dependent analysis.
CPT in Clays (Cl. 5.3.1.2)	Simple ratios like qc​/15 or qc​/18 to estimate cu​.	Uses cu​=(qc​−σv0​)/Nkt​, accounting for total overburden stress (σv0​).
Layered Soils (Cl. 5.3.2)	Relies on static charts (Fig. 3) for Nc​ factors.	Recommends Punching Shear analysis to model stress distribution through stiff-over-soft layers.
Desiccated Soils (Cl. 5.3.3)	Assumes a fixed linear reduction of cohesion with depth.	Uses Unsaturated Soil Mechanics; warns of strength loss (collapse) upon saturation.

1. Internal Friction Angle (ϕ) from SPT (Fig. 1)

• IS 6403 Limitation: The code provides a direct relationship between the N value and ϕ in Fig. 1. Although it mentions “Corrected N value”, it lacks explicit instructions for essential modern corrections like N60​ (energy correction) and N1(60)​ (overburden pressure correction).
• Modern Recommendation: Following Das (2019) design engineers should first apply energy and stress-level corrections to the field N value before estimating ϕ. Relying on uncorrected or partially corrected values may lead to overestimating the soil’s shear strength.

$$N_{60}\approx N\cdot \frac{\text{actual hammer efficiency}}{60}$$

$$N_{60}=\frac{N\cdot {\eta }_H\cdot {\eta }_B\cdot {\eta }_S\cdot {\eta }_R}{60}$$ ​​$$C_N=\sqrt{\frac{P_a}{{\sigma }_{v0}^{\mathrm{\prime }}}}$$

$$(N_1{)}_{60}=N_{60}\cdot C_N$$​

$P_a$​ = 100 kPa

Recommended Correlations:

Reference	Applicability	Formula
Wolff (1989)	Sands & non-plastic silts	${\varphi }^{\mathrm{\prime }}=27.1+0.3N_{60}-0.00054[N_{60}{]}^2$
Hatanaka & Uchida (1996)	Clean sands	${\varphi }^{\mathrm{\prime }}=\sqrt{20\cdot N_{1(60)}}+{20}^{\circ }$
Kulhawy & Mayne (1990)	Sands & gravelly sands	$${\varphi }^{\mathrm{\prime }}={\tan }^{-1}{\left[\frac{N_{60}}{12.2+20.3\left(\frac{{\sigma }_o^{\mathrm{\prime }}}{p_a}\right)}\right]}^{0.34}$$

2. Cohesionless Soils based on CPT (Clause 5.2.3 & Fig. 2)

• IS 6403 Limitation: The standard calculates bearing capacity directly from the average qc​ value using the empirical curves in Figure 2.

• Modern Recommendation: Modern practice emphasizes that qc​ is highly dependent on effective overburden pressure. It is recommended to normalize qc​ values for stress levels or use them to derive the friction angle (ϕ) for a more robust analysis. Modern Formula (Kulhawy & Mayne): Provides a stress-dependent effective friction angle:

$${\varphi }^{\mathrm{\prime }}=17.6+11\cdot {\log }_{10}\left[\frac{q_c\mathrm{/}{P}_a}{{\sigma }_{v0}^{\mathrm{\prime }}\mathrm{/}{P}_a}\right]$$

Method / Reference	Formula / Procedure	Parameters & Notes
Kulhawy & Mayne (1990) – Effective Friction Angle	${\varphi }^{\mathrm{\prime }}=17.6+11\cdot {\log }_{10}\left[\frac{q_c\mathrm{/}{P}_a}{{\sigma }_{v0}^{\mathrm{\prime }}\mathrm{/}{P}_a}\right]$	$P_a=100\text{kPa}$ ${\sigma }_{v0}^{\mathrm{\prime }}$= vertical effective stress at test depth
Eslami-Fellenius (CPTu) – Direct Approach	Step 1: $q_e=q_t-u_2$ Step 2: $q_{Eg}=\sqrt[n]{q_{e,1}\times q_{e,2}\times \cdots \times q_{e,n}}$ ​​ Step 3: $q_b=C_{te}\times q_{Eg}$​	$q_t$ = corrected cone resistance $u_2$​ = pore pressure behind cone Influence zone = $2B$
Eurocode 7 – Pore Pressure Correction	$q_t=q_c+u_2\cdot (1-a)$	$a$a = net cone area ratio (0.7 to 0.85)

• Furthermore, settlement—rather than shear—often governs design in sands, which requires more advanced CPT-based settlement analysis (e.g., Schmertmann’s method).

3. Undrained Cohesion (cu​) from CPT (Clause 5.3.1.2)

• IS 6403 Limitation: The code uses simple ratios (e.g., qc​/15 or qc​/18) to estimate cohesion from cone resistance. This approach ignores the total overburden stress at the test depth.
• Modern Recommendation: Eurocode 7 suggests the use of the Nkt​ factor (Cone Factor) where cu​=(qc​−σv0​)/Nkt​. This method accounts for the in-situ stress state (σv0​), providing a far more accurate representation of soil strength, especially at greater depths:

$$c_u=\frac{q_c-{\sigma }_{v0}}{N_{kt}}$$

(Where σᵥ₀ is the total overburden stress and Nkt is typically 10–20.)

Typical $N_{kt}$ Values by Soil Type

Soil Type / Condition	Typical $N_{kt}$​ Range
Highly sensitive clays (e.g., quick clays)	8 – 12
Soft, normal clays	12 – 16
Firm to stiff clays	16 – 20
Eurocode 7 (default, no site calibration)	15
Overconsolidated clays (OCR > 2)	18 – 20

4. Two-Layered Cohesive Soil Systems (Fig. 3)

• IS 6403 Limitation: The bearing capacity factors (Nc​) in Fig. 3 are based on simplified charts. These do not fully capture the complex “Punching Shear” failure mechanism that occurs when a thin stiff crust overlies a soft clay layer.

• Modern Recommendation: Approaches suggested by Das and Bowles analyze the distribution of stress through the top layer and the shearing resistance along the failure perimeter. For critical projects, numerical analysis or the punching shear theory is recommended over the static charts of IS 6403.

• Modern Formula (Meyerhof & Hanna): For a stiff layer over a soft layer:

$$q_u=q_{u(b)}+\frac{2c_{a}H}{B}+{\gamma }_1{H}^2(1+\frac{2D_f}{H})\frac{K_s\tan {\varphi }_1}{B}-{\gamma }_{1}H\le q_{u(t)}$$

Term	Physical meaning	Unit
$q_u$​	Ultimate bearing capacity of the two‑layer system	kPa
$q_{u(b)}$​	Ultimate bearing capacity of the underlying soft layer alone	kPa
$\frac{2c_{a}H}{B}$​	Shearing resistance along the two vertical sides of the punched zone (adhesion)	kPa
${\gamma }_1{H}^2(1+\frac{2D_f}{H})\frac{K_s\tan {\varphi }_1}{B}$	Frictional resistance along the inclined failure surface within the stiff layer	kPa
$-{\gamma }_{1}H$	Reduction due to the weight of the stiff layer pushing down on the soft layer	kPa
$q_{u(t)}$	Ultimate bearing capacity of the top (stiff) layer alone (upper bound)	kPa

5. Desiccated & Fissured Soils (Clause 5.3.3)

• IS 6403 Limitation: The standard assumes a linear reduction of cohesion with depth (Table 4 and Fig. 4). This is a highly empirical and simplified model for complex desiccated profiles.
• Modern Recommendation: Modern practice utilizes Unsaturated Soil Mechanics. The strength of desiccated soils is primarily governed by Matric Suction, which varies seasonally with moisture content. Design engineers should be cautious of the “collapse” or “swelling” potential of these soils upon wetting, which the linear model in IS 6403 does not address.

• Semi-Empirical Model (Vanapalli & Mohamed, 2013):

$$q_{u(unsat)}=[c^{\mathrm{\prime }}+(u_a-u_w{)}_{AVR}(S^{\psi })\tan {\varphi }^{\mathrm{\prime }}]N_c{\zeta }_c+q^{\mathrm{\prime }}{N}_q{\zeta }_q+0.5\gamma B{N}_{\gamma }{\zeta }_{\gamma }$$

Term	Physical meaning	Unit
$q_{u(unsat)}$	Ultimate bearing capacity of unsaturated soil	kPa
$c^{\mathrm{\prime }}$	Effective cohesion (saturated condition)	kPa
$(u_a-u_w{)}_{AVR}$​	Average matric suction over the influence zone	kPa
$S$	Degree of saturation	– (0 to 1)
$\psi$	Empirical pore size distribution parameter (typically 1–4)	–
${\varphi }^{\mathrm{\prime }}$	Effective friction angle	degree
$N_c,N_q,N_{\gamma }$​	Bearing capacity factors (function of ${\varphi }^{\mathrm{\prime }}$)	–
${\zeta }_c,{\zeta }_q,{\zeta }_{\gamma }$​	Combined correction factors (shape, depth, inclination, ground slope)	–
$q^{\mathrm{\prime }}$	Effective overburden pressure at footing base ($=\gamma \times D_f$​)	kPa
$\gamma$	Unit weight of soil	kN/m³
$B$	Footing width	m

• Unified Effective Stress Approach:

$$q_u=[c^{\mathrm{\prime }}+\chi s\tan {\varphi }^{\mathrm{\prime }}]N_c{d}_c+q{N}_q{d}_q+0.5\gamma B{N}_{\gamma }{d}_{\gamma }$$

Term	Physical meaning	Unit
$q_u$​	Ultimate bearing capacity (unsaturated/saturated)	kPa
$c^{\mathrm{\prime }}$	Effective cohesion	kPa
$\chi$	Bishop’s parameter (depends on degree of saturation, $0\le \chi \le 1$)	–
$s$	Matric suction ($=u_a-u_w$​)	kPa
${\varphi }^{\mathrm{\prime }}$	Effective friction angle	degree
$N_c,N_q,N_{\gamma }$​	Bearing capacity factors (function of ${\varphi }^{\mathrm{\prime }}$)	–
$d_c,d_q,d_{\gamma }$​	Depth correction factors (embedment effect)	–
$q$	Effective overburden pressure at footing base	kPa
$\gamma$	Unit weight of soil	kN/m³
$B$	Footing width	m

Final Conclusion:

Users are encouraged to use GEOtExcel as a powerful tool for IS 6403 compliance but should supplement their designs with modern correlations for enhanced reliability and safety in complex soil conditions.

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✳️ Bearing Capacity Calculator
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✅ Terzaghi’s Method

1️⃣ Ultimate Bearing Capacity Calculation – Academic Education Focus
2️⃣ Ultimate Bearing Capacity Calculation (Design) as per IS 6403 Code

☑️ [GEO-2026-IS6401-Design] & [GEO-2026-IS6401-Cocept]
📊 “Developed by GEOtExcel Co., Copyrighted.”

1️⃣ Effect of Water Table on the Bearing Capacity
2️⃣ Effect of Relative Density on the Bearing Capacity of Cohesion-less Soils
3️⃣ Effect of Eccentric Loading on the Bearing Capacity

🟥 YouTube Channel: https://www.youtube.com/@Dr.Fahmi.GEOtExcel
☑️ academy.dr.fahmi@gmail.com

Dr. Ahmad Fahmi
Associated Professor,
The University of Bonab
& CEO of GEOtExcel Company (KAPA)

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