Shallow foundation design in geotechnical engineering requires two parallel checks to be satisfied simultaneously: the soil beneath the foundation must be strong enough to carry the applied load without shearing — the bearing capacity check — and the resulting deformation of the soil must not cause settlement that exceeds the structural or operational tolerance of the building — the settlement check. Satisfying one without the other is not sufficient. A foundation may be safe against shear failure yet cause unacceptable differential settlement; conversely, a very stiff soil may develop shear failure before settlement becomes critical.

This guide covers the complete shallow foundation design workflow used in geotechnical engineering practice: foundation types, ultimate and allowable bearing capacity theory, the four principal calculation methods (Terzaghi, Meyerhof, Hansen, Vesic), bearing capacity factors and correction factors, the effect of groundwater and eccentric loading, settlement analysis methods (immediate and consolidation), and how specialist software automates the full analysis for multi-layer soil profiles.

Foundation types and selection criteria #

The first step in any foundation design is selecting the appropriate foundation type for the site conditions, load magnitude, and structural requirements. Foundations are broadly classified as shallow or deep:

Foundation type	Typical depth	When selected
Spread footing (isolated)	0.5–3.0 m	Individual columns; competent soil within 2–3 m of surface
Strip footing	0.5–3.0 m	Load-bearing walls; continuous linear loads
Raft / mat foundation	0.5–3.0 m	Heavily loaded structures; weak soils; to reduce differential settlement
Pile foundation	5–50+ m	Surface soils inadequate; heavy loads; large settlements in shallow methods
Drilled shaft (caisson)	5–30+ m	Large individual column loads; bearing on rock or very dense stratum

This guide focuses on shallow foundation design — spread footings, strip footings, and raft foundations — where the depth of embedment D_f is less than approximately 2.5 times the foundation width B. Deep foundation design (piles, drilled shafts) is outside the scope of this guide.

The foundation selection criterion for shallow foundations is straightforward: if competent bearing soil exists within a reasonable depth from the surface, and if the estimated settlement under shallow foundation loading is acceptable, a shallow foundation is preferred on cost grounds. Where these conditions are not met, deep foundations transfer the load to a deeper competent stratum.

Shear failure modes #

Bearing capacity theory is built on the concept that when the applied foundation pressure exceeds the soil’s shear strength along a failure surface, a shear failure occurs. Three failure modes are recognised depending on soil density and compressibility:

General shear failure #

Occurs in dense sands and stiff clays. A well-defined failure surface develops from the edge of the foundation to the ground surface. The load-settlement curve shows a clear peak (ultimate bearing capacity) followed by a post-failure decrease. This is the failure mode assumed by Terzaghi’s bearing capacity theory and is the basis of most design methods.

Local shear failure #

Occurs in loose to medium-dense sands and soft to firm clays. The failure surface does not fully develop to the ground surface. The load-settlement curve shows no distinct peak — the foundation continues to sink with increasing load. Terzaghi accounted for local shear failure by reducing the shear strength parameters: c* = (2/3)c and φ* = arctan[(2/3)tanφ], and then applying the general shear failure equations with the reduced parameters.

Punching shear failure #

Occurs in very loose sands, compressible silts, and organic soils, particularly for deep foundations at high embedment ratios. The foundation punches vertically into the soil with no development of lateral failure surfaces. It is not well captured by the classical bearing capacity equations and requires separate treatment.

Ultimate bearing capacity — general equation #

The general bearing capacity equation, in its modern form following Meyerhof (1963) and subsequent modifications, provides the theoretical ultimate bearing capacity q_u of a shallow foundation on a c-φ soil:

q_u = c · N_c · F_cs · F_cd · F_ci + q · N_q · F_qs · F_qd · F_qi + 0.5 · γ · B · N_γ · F_γs · F_γd · F_γi

where:

c = effective cohesion of the soil (kPa)
N_c, N_q, N_γ = dimensionless bearing capacity factors (function of friction angle φ’)
q = effective overburden pressure at foundation level = γ · D_f (kPa)
γ = unit weight of soil below foundation level (kN/m³)
B = foundation width (m)
F_cs, F_qs, F_γs = shape factors
F_cd, F_qd, F_γd = depth factors
F_ci, F_qi, F_γi = inclination factors

The three terms in the equation represent three distinct mechanisms contributing to bearing capacity: the cohesion term (c · N_c), the surcharge term (q · N_q) from the weight of soil above foundation level, and the self-weight term (0.5γBN_γ) from the wedge of soil beneath the foundation. Each method described below uses this general framework but applies different expressions for the bearing capacity factors and correction factors.

Terzaghi bearing capacity method #

Karl Terzaghi’s 1943 bearing capacity equation was the first rigorous solution to the shallow foundation bearing capacity problem and remains a fundamental reference in geotechnical engineering. Terzaghi assumed general shear failure, a rough foundation base (full mobilisation of base friction), and ignored the shear resistance of soil above foundation level (treating the overburden as a surcharge only).

For a continuous (strip) foundation — Terzaghi’s original formulation — the equation is:

q_u = c · N_c + q · N_q + 0.5 · γ · B · N_γ

For square and circular footings, Terzaghi provided empirical modifications:

Foundation shape	Terzaghi ultimate bearing capacity equation
Strip (continuous)	q_u = c·N_c + q·N_q + 0.5·γ·B·N_γ
Square	q_u = 1.3·c·N_c + q·N_q + 0.4·γ·B·N_γ
Circular	q_u = 1.3·c·N_c + q·N_q + 0.3·γ·B·N_γ

Terzaghi’s method has important limitations: it does not include depth factors, inclination factors, or ground slope factors, and it was not originally formulated for rectangular foundations. For inclined loads, eccentric loads, or embedded foundations where the shear resistance of the overburden soil is significant, the Meyerhof, Hansen, or Vesic methods are more appropriate.

Meyerhof bearing capacity method #

G.G. Meyerhof (1963) extended Terzaghi’s approach to include the shear resistance of soil above the foundation level, introduced depth factors to account for the embedment depth of the foundation, and added inclination factors for loads that are not applied vertically. Meyerhof’s method is the most widely used bearing capacity approach in North American practice and forms the basis of many national design standards.

Meyerhof introduced separate shape, depth, and inclination factors applied to each of the three bearing capacity terms:

Factor type	Symbols	Effect on result
Shape factors	s_c, s_q, s_γ	Increase bearing capacity for square and circular footings relative to strip; account for three-dimensional effects of the failure mechanism
Depth factors	d_c, d_q, d_γ	Increase bearing capacity with increasing embedment depth D_f, accounting for the shear resistance of soil above foundation level
Inclination factors	i_c, i_q, i_γ	Reduce bearing capacity when the resultant load is inclined to the vertical — inclined loads reduce the horizontal resistance of the failure mechanism

Meyerhof’s bearing capacity factors (N_c, N_q, N_γ) are slightly different from Terzaghi’s — in particular, his N_q is higher because he accounts for overburden shear resistance. For φ = 0 (undrained loading of saturated clay), Meyerhof gives N_c = 5.14, N_q = 1.0, N_γ = 0, consistent with Skempton’s (1951) results.

Hansen bearing capacity method #

Brinch Hansen (1970) further extended the bearing capacity framework to include base inclination factors (for foundations on a sloped base) and ground inclination factors (for foundations on a sloping ground surface). Hansen’s method is the standard approach in European geotechnical practice and forms the conceptual basis of the bearing capacity provisions in Eurocode 7 (EN 1997-1).

Hansen’s inclination factors are more conservative than Meyerhof’s for heavily inclined loads, and his depth factors use a different formulation that produces smaller depth corrections than Meyerhof for shallow embedment ratios (D_f/B < 1). For vertical, concentric loading on level ground, Hansen and Meyerhof typically give similar results.

Hansen’s additional factors beyond Meyerhof:

Factor type	Symbols	Accounts for
Base inclination	b_c, b_q, b_γ	Foundation base tilted at angle α from horizontal (e.g. foundations on sloped bedrock)
Ground inclination	g_c, g_q, g_γ	Ground surface sloping away from the foundation at angle β (e.g. foundations near an embankment or slope crest)

Vesic bearing capacity method #

A.S. Vesic (1973, 1975) proposed modifications to the bearing capacity factor N_γ and several of the correction factors, particularly for shape and depth, producing results that are generally slightly higher than Hansen for the same input parameters. Vesic’s N_γ = 2(N_q + 1)tanφ is the most commonly used modern expression and has largely replaced Terzaghi’s original tabulated N_γ values.

Vesic’s method is widely referenced in US geotechnical practice, particularly in AASHTO bridge design and US Army Corps of Engineers guidance. It is the preferred method when both shape and depth effects are significant and when loads are vertical and concentric.

Method comparison — when to use each #

Method	Best suited for	Primary use regions
Terzaghi	Preliminary estimates; concentric vertical loads; strip, square, and circular footings only	Reference method globally
Meyerhof	General shallow foundation design including depth effects and inclined loads	North America, Middle East, South Asia
Hansen	Inclined loads, sloped ground or foundation base, Eurocode 7 compliance	Europe, Scandinavia, international projects
Vesic	Vertical concentric loads; AASHTO and US Army Corps of Engineers applications	United States

Running multiple methods simultaneously and comparing results is good engineering practice — it identifies cases where the chosen method’s assumptions may be poorly suited to the site conditions, and provides a range of estimates that helps calibrate the factor of safety selection.

Bearing capacity factors — N_c, N_q, N_γ #

The bearing capacity factors N_c, N_q, and N_γ are dimensionless functions of the effective friction angle φ’ that reflect the geometry of the failure mechanism beneath the foundation. They are the core of all bearing capacity calculations and their values have been refined progressively since Terzaghi’s original tabulation.

The most widely accepted modern expressions are:

N_q = e^π·tanφ’ · tan²(45 + φ’/2)

N_c = (N_q − 1) · cot(φ’) [for φ’ > 0; N_c = 5.14 for φ’ = 0 by L’Hôpital’s rule]

N_γ = 2(N_q + 1) · tan(φ’) [Vesic expression, most widely used]

Reference values for common friction angles:

φ’ (degrees)	N_c	N_q	N_γ (Vesic)	Typical soil
0	5.14	1.00	0.00	Soft to medium saturated clay (undrained)
5	6.49	1.57	0.45	Very soft silt
10	8.35	2.47	1.22	Soft silt or very loose sand
15	10.98	3.94	2.65	Loose sand, soft clay
20	14.83	6.40	5.39	Medium-dense sand
25	20.72	10.66	10.88	Medium-dense to dense sand
30	30.14	18.40	22.40	Dense sand, gravel
35	46.12	33.30	48.03	Very dense sand, gravel
40	75.31	64.20	109.41	Dense gravel

The rapid increase in N_q and N_γ with friction angle explains why even modest improvements in φ’ — from improved compaction or densification of granular fill — can produce significant increases in allowable bearing capacity. Note that N_γ remains sensitive to which expression is used (Terzaghi, Meyerhof, Hansen, and Vesic all give slightly different N_γ values), which is one reason bearing capacity results differ between methods.

Shape, depth, inclination, and ground factors #

Correction factors modify the basic strip foundation bearing capacity equation for the actual foundation geometry and loading conditions. The following summarises the most commonly used expressions from Meyerhof and Hansen/Vesic:

Shape factors #

Foundation shape	s_c	s_q	s_γ
Strip	1.0	1.0	1.0
Rectangle (B × L)	1 + (B/L)(N_q/N_c)	1 + (B/L)·tanφ’	1 − 0.4(B/L)
Square (B = L)	1 + N_q/N_c	1 + tanφ’	0.6
Circular (B = D)	1 + N_q/N_c	1 + tanφ’	0.6

Depth factors (Meyerhof) #

For D_f/B ≤ 1:

d_c = 1 + 0.4 · (D_f/B)
d_q = 1 + 2·tanφ’·(1−sinφ’)² · (D_f/B)
d_γ = 1.0 (depth factor for N_γ term is unity in all methods)

Inclination factors (Meyerhof) #

For a load inclined at angle β from the vertical:

i_c = i_q = (1 − β°/90°)²
i_γ = (1 − β°/φ°)²

Inclination factors reduce bearing capacity significantly for inclined loads. For β = 30° on a φ’ = 30° soil, the inclination factors alone reduce bearing capacity by approximately 45% compared to a vertical load.

Allowable bearing capacity and factor of safety #

The ultimate bearing capacity q_u is the theoretical maximum pressure the soil can sustain before shear failure. In design, the allowable bearing capacity q_a is the safe working pressure to be applied, incorporating a factor of safety against shear failure:

q_a = q_net,u / FS + q

where q_net,u = q_u − q is the net ultimate bearing capacity (ultimate capacity above the existing overburden), FS is the factor of safety, and q is the overburden pressure at foundation level (added back to give gross allowable pressure).

Factor of safety selection #

Condition	Typical factor of safety
Thorough site investigation, well-characterised soil, static loading	2.5
Routine investigation, moderate uncertainty, static loading	3.0
Limited investigation, high uncertainty, or dynamic/seismic loading	3.0–4.0
Temporary structures, short-duration loads	2.0–2.5

In practice, the allowable bearing capacity is often further constrained by settlement rather than by shear failure — particularly for wide foundations on compressible sands and for any foundation on clay. The governing design criterion is whichever of the two checks (shear failure or settlement) produces the lower allowable pressure. Both checks must always be performed.

Groundwater effects on bearing capacity #

The presence of a water table within the zone of influence beneath the foundation reduces effective stress in the soil, which in turn reduces the bearing capacity. The effect is significant — groundwater at foundation level can reduce bearing capacity by up to 50% compared to a dry soil of the same unit weight. Three cases are typically considered:

Case	GWT position	Effect on calculation
Case 1	GWT at or above ground surface	Use submerged unit weight γ’ = γ_sat − γ_w throughout. Overburden q = γ’·D_f. The N_γ term uses γ = γ’.
Case 2	GWT at foundation level	Overburden q = γ·D_f (moist unit weight above GWT). The N_γ term uses γ = γ’ (submerged unit weight for soil below foundation).
Case 3	GWT at depth d below foundation level (d ≤ B)	Interpolate linearly between Case 2 (d = 0) and no correction (d = B). For d ≥ B, the groundwater table has no significant effect on bearing capacity.

The rule of thumb is: when the groundwater table is deeper than B below the foundation base, its effect on bearing capacity can be ignored. Above this depth, the correction must be applied. Seasonal fluctuations in groundwater level should use the highest anticipated level for bearing capacity calculations.

Eccentric and inclined loads #

Eccentric loading occurs when the resultant of all applied forces does not act through the centroid of the foundation — a common situation in practice due to moments from wind, seismic forces, retaining wall thrust, or structural frame action. Eccentric loads are handled using the effective area method (Meyerhof, 1963):

The eccentricities in each direction are:

e_L = M_L / P and e_B = M_B / P

where M_L and M_B are the bending moments about the length and width axes respectively, and P is the total vertical load. The effective foundation dimensions are then:

B’ = B − 2e_B and L’ = L − 2e_L

The bearing capacity is calculated using the effective dimensions B’ and L’ in place of B and L. The effective area A’ = B’ × L’ is also used to compute the foundation contact pressure. Eccentricity must satisfy e < B/6 (and L/6) to avoid tensile stress at the foundation edge — the “middle third” rule — otherwise the foundation design must be revised.

For inclined loads — where the applied resultant makes an angle β with the vertical — inclination factors i_c, i_q, and i_γ reduce the bearing capacity to account for the reduced horizontal resistance of the failure wedge. All three methods (Meyerhof, Hansen, Vesic) provide inclination factor expressions, with Hansen’s being most conservative for large inclinations.

Settlement analysis — immediate, consolidation, and secondary #

A shallow foundation on compressible soil will settle under load. The total settlement is composed of three components that occur at different rates and by different mechanisms:

Immediate (elastic) settlement #

Immediate settlement occurs during or shortly after load application, before significant drainage of pore water can occur. It is caused by shear deformation of the soil skeleton at constant volume (in saturated soils) or by compression of the soil skeleton and air voids (in partially saturated soils). The magnitude depends on the elastic modulus of the soil and the stress distribution beneath the foundation.

Calculation methods supported in Dartis Foundation:

Theory of elasticity — Boussinesq stress distribution with integrated elastic settlement; requires elastic modulus E_s and Poisson’s ratio ν
Janbu (1956) method — accounts for variation of E_s with depth using influence factors; widely used in Scandinavian practice
Schmertmann (1978) method — uses CPT-derived modulus profile with a strain influence factor I_z diagram; the preferred method when CPT data is available (see the CPT guide)
Leonards (1974) 3D correction — corrects one-dimensional settlement calculations for three-dimensional stress conditions beneath finite-area footings

Primary consolidation settlement #

Primary consolidation settlement is the time-dependent compression that occurs in saturated fine-grained soils as excess pore water pressure, generated by the applied foundation load, dissipates through drainage. It can continue for months to decades in thick, low-permeability clay layers.

Settlement is calculated using oedometer test parameters:

For normally consolidated clay (σ’₀ ≥ σ’_p):

S_c = [C_c / (1 + e₀)] · H · log(σ’_f / σ’₀)

For overconsolidated clay where σ’_f < σ’_p (within the preconsolidated stress range):

S_c = [C_r / (1 + e₀)] · H · log(σ’_f / σ’₀)

For overconsolidated clay where σ’_f > σ’_p (spanning the NC/OC boundary):

S_c = [C_r / (1 + e₀)] · H · log(σ’_p / σ’₀) + [C_c / (1 + e₀)] · H · log(σ’_f / σ’_p)

where H is the thickness of the compressible clay layer, e₀ is the initial void ratio, σ’₀ is the initial effective vertical stress, and σ’_f is the final effective vertical stress after load application.

Time rate of consolidation #

The time required for a specified percentage of primary consolidation to occur is governed by the coefficient of consolidation c_v and the drainage path length H_dr:

t = T_v · H_dr² / c_v

where T_v is the dimensionless time factor corresponding to the desired degree of consolidation (T_v = 0.197 for U = 50%; T_v = 0.848 for U = 90%). H_dr = H/2 for a doubly drained layer (drainage at top and bottom); H_dr = H for a singly drained layer.

Secondary consolidation settlement #

Secondary consolidation (creep) occurs after primary consolidation is essentially complete, as the soil skeleton continues to rearrange at constant effective stress. It is characterised by the secondary compression index C_α = Δe / Δlog(t) and is generally negligible for inorganic clays and sands, but can be significant for organic soils, peats, and highly plastic clays.

S_s = C_α / (1 + e_p) · H · log(t₂ / t₁)

where e_p is the void ratio at the end of primary consolidation, and t₁ and t₂ are the start and end of the secondary compression period of interest.

Settlement tolerance limits #

The permissible total and differential settlement depends on the structural system, cladding type, and sensitivity of the supported equipment or function. Indicative limits:

Structure type	Maximum total settlement	Maximum differential settlement (δ/L)
Steel frame buildings	50–100 mm	1/300
Concrete frame buildings	25–50 mm	1/500
Load-bearing masonry walls	25–50 mm	1/600–1/1000
Machine foundations	5–25 mm	1/750–1/1500
Embankments (flexible)	300+ mm	Not critical

Multi-layer soil profiles #

Real sites rarely consist of a single uniform soil layer. Most geotechnical investigations reveal a stratigraphy of multiple layers with different unit weights, friction angles, cohesion values, and compressibility parameters. Applying a homogeneous analysis to a layered profile can significantly overestimate or underestimate bearing capacity and settlement.

For bearing capacity in layered soils, the critical consideration is the depth of the failure mechanism relative to layer boundaries. If the stronger layer extends below the failure zone depth (approximately 2B below the foundation), the calculation may treat the upper layer as representative. Where a weaker layer exists within the failure zone, the bearing capacity must be calculated considering the weakest relevant layer, or using an appropriate punching shear correction method.

For settlement in layered profiles, each compressible layer is analysed separately using its own compressibility parameters (C_c, C_r, e₀, c_v) and the stress increment at its mid-depth. Settlements from each layer are summed to give total settlement. This is the primary advantage of good-quality oedometer testing on samples from each distinct layer — it provides the layer-specific parameters needed for accurate settlement prediction.

Dartis Foundation supports unlimited soil layers, allowing engineers to define each layer individually with its own strength and compressibility parameters. The software applies the appropriate stress distribution method to compute the vertical stress increment at the mid-depth of each layer and calculates settlement contributions from each layer separately before summing to total settlement.

How DartiGeo and Dartis Foundation handle foundation design #

DartisTech offers shallow foundation bearing capacity and settlement analysis in two products: as the Bearing Capacity and Settlement module within the comprehensive DartiGeo platform, and as the standalone Dartis Foundation application for dedicated foundation design workflows.

Bearing capacity methods supported #

Method	Supported in DartiGeo	Supported in Dartis Foundation
Meyerhof	✔	✔
Hansen	✔	✔
Vesic	✔	✔
SPT-based methods (Terzaghi & Peck, Meyerhof SPT, Teng, Bowles, Burland & Burbidge, Anagnostopoulos)	✔	—
CPT-based methods (Robertson & Campanella)	✔	—

Settlement methods supported #

Method	Supported in DartiGeo	Supported in Dartis Foundation
Theory of elasticity (immediate)	✔	✔
Schmertmann (1978) — from CPT	✔	✔
Janbu (1956)	✔	✔
Leonards (1974) 3D correction	✔	✔
Primary consolidation settlement	✔	✔
Secondary consolidation settlement	✔	✔

Key features of both products #

Multi-layer soil profiles: Unlimited soil layers, each with independent strength and compressibility parameters, enabling realistic analysis of complex stratigraphies.
Groundwater depth as a direct input: Both Case 1, 2, and 3 groundwater corrections are applied automatically based on the input groundwater depth.
Eccentric and inclined loading: Load eccentricity and inclination are direct inputs; effective foundation dimensions B’ and L’ and the appropriate inclination factors are computed automatically.
Multiple methods run simultaneously: All supported bearing capacity methods are calculated for the same inputs, allowing direct comparison and method selection based on site conditions.
Load-settlement curves: Bearing capacity and settlement are calculated across a range of applied pressures, producing graphical load-settlement relationships and allowable pressure curves for different footing sizes.
Fine calculations report: Every step of the calculation — bearing capacity factors, all correction factors with values, intermediate results, and final allowable pressure — is shown explicitly in a formatted PDF or Word report, with the published equation referenced for each factor. This level of transparency is essential for peer review and regulatory submission.

In DartiGeo, foundation design inputs can draw directly on SPT N-values from the borehole logging module and CPT data from the CPT interpretation module — there is no re-entry of soil data between modules, and the full site investigation dataset feeds into the foundation analysis in a single integrated project file.

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Frequently asked questions #

What is the difference between ultimate and allowable bearing capacity? #

Ultimate bearing capacity (q_u) is the theoretical maximum pressure the soil can sustain before shear failure occurs along a well-defined failure surface. Allowable bearing capacity (q_a) is the safe working pressure applied in design — equal to the net ultimate bearing capacity divided by a factor of safety (typically 2.5–3.0 for static loading), plus the existing overburden pressure at foundation level. The allowable bearing capacity may be further reduced below this value if settlement calculations show that the shear-failure-limited pressure would cause unacceptable deformation. The governing design criterion is always the lower of the two checks.

Which bearing capacity method is most accurate — Terzaghi, Meyerhof, Hansen, or Vesic? #

No single method is universally most accurate. Each was calibrated to different theoretical frameworks and experimental databases. For vertical, concentric loading on level ground, all four methods give similar results within 10–20% of each other. Differences become more significant for eccentric loads (where Meyerhof’s effective area method is essential), inclined loads (where Hansen’s inclination factors are more conservative and generally preferred), and foundations on sloped ground (where Hansen’s ground inclination factors are uniquely available). The recommended practice is to run all available methods, compare results, and apply engineering judgement in selecting the design value — which is exactly what both Dartis Foundation and DartiGeo facilitate by running multiple methods simultaneously.

What causes differential settlement and how is it controlled? #

Differential settlement — the difference in settlement between two adjacent foundation elements — is caused by variation in soil stiffness or layer thickness across the building footprint, non-uniform structural loading, different foundation depths or sizes, or changes in groundwater level during or after construction. It is generally more damaging than uniform settlement because it induces bending moments and shear forces in the structure. Control measures include: increasing foundation width to reduce contact pressure; choosing a raft foundation to distribute load uniformly; preloading compressible layers before construction; installing vertical drains to accelerate consolidation; and designing the structure to tolerate the predicted differential settlement without damage.

How does groundwater depth affect bearing capacity? #

Groundwater reduces the effective stress in the soil, which reduces the confining pressure on the failure mechanism and therefore reduces bearing capacity. The magnitude of the effect depends on the depth of the water table relative to the foundation and the foundation width. When the water table is at foundation level, the N_γ term (which uses the soil unit weight below foundation) must use the submerged unit weight γ’ = γ_sat − 9.81 kN/m³ instead of the moist unit weight — a reduction of typically 8–12 kN/m³. This can reduce the N_γ contribution by 50% or more. When the water table is deeper than one foundation width (B) below foundation level, the correction can be ignored.

What geotechnical data do I need for bearing capacity and settlement calculations? #

For bearing capacity, the minimum data required is: soil unit weight (γ), effective cohesion (c’) and friction angle (φ’) from direct shear or triaxial testing, foundation geometry (B, L, D_f), and groundwater depth. For immediate settlement on sand, Young’s modulus (E_s) from CPT correlations or pressuremeter testing is needed. For primary consolidation settlement on clay, oedometer test parameters are required: compression index (C_c), recompression index (C_r), initial void ratio (e₀), preconsolidation pressure (σ’_p), and coefficient of consolidation (c_v). All of these can be obtained from the laboratory test program described in the geotechnical laboratory testing guide.

What is the minimum depth of investigation for a shallow foundation? #

The borehole or CPT depth for shallow foundation design should extend to at least 1.5 times the width of the largest loaded area below the foundation level, or to the depth at which the stress increment from the applied load decreases to less than 10% of the effective overburden stress at that depth — whichever is greater. For a 5 m wide isolated footing at 1.5 m depth, this means investigating to approximately 9–10 m below ground level. For a raft foundation 20 m wide, investigation to 30+ m may be required to fully characterise all compressible layers within the zone of influence. The borehole logging guide covers investigation depth planning in more detail.

Foundation Design — Bearing Capacity & Settlement Guide for Geotechnical Engineers