Shallow Foundations

Working Platform Design - TWf2024:03

Subgrade Soil Type

Subgrade Soil:

Working Pressure and Loaded Area

Uniform pressure on ground surface, q (kPa):

Width of loaded area, B (m):

Length of loaded area, L (m):

Partial Factors

Load factor for live load, γ_Q:

Load factor for dead load, γ_G:

Material factor for friction angle, γ_φ:

Material factor for cohesion, γ_c:

Subgrade Properties

Friction angle of subgrade, φ_sk (°):

Cohesion of subgrade, c_uk (kPa):

Unit weight of subgrade, γ_s (kN/m³):

Platform Properties

Friction angle of platform material, φ_p (°):

Unit weight of platform material, γ_p (kN/m³):

Geometry

Water depth below ground surface, z_γ (m):

Design platform thickness, D (m):

Governing Equations:

General bearing capacity - Section 4.2.4

$$ q_u = c N_c s_c i_c d_c + 0.5 \gamma B N_\gamma s_\gamma i_\gamma d_\gamma + q_0 N_q s_q i_q d_q $$

Design factors - Section 4.6

EC7 and CIRIA SP123 are both limit state approaches with various partial factors for actions and material strength. The UK annex for EC7 directs the use of Design Approach 1, which uses two distinct sets of partial factors, Combination 1 and Combination 2 (see Table 3 for comparison).

In practice, the factor of 1.25 on tanφ results in factors of 2.3 to 2.9 on N_γ. From this it can be seen that, for working platforms, Combination 2 will always be the controlling set of factors for bearing capacity calculations. It is, therefore, only considered necessary to apply Combination 2 for the design of granular working platforms.

	EC7		SP123
	comb 1	comb 2
permanent action (γ_G)	1.35	1.00	1.00
variable action (γ_Q)	1.50	1.30	1.00
cohesion (γ_c)	1.00	1.40	1.25
shear angle (γ_f)	1.00	1.25	1.25
resistance (γ_R)	1.00	1.00	n/a

Useful charts

Figure 25 - Graphs for pressure beneath the centre of a foundation (after Janbu, Bjerrum and Kjaernsli, 1956)

Figure 26 - δ/φ for cohesive formation (after Hanna and Meyerhof, 1980)

Figure 27 - δ/φ for granular formation (after Hanna, 1981)