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Chapter 2.13 Foundations

Any structure, from domestic to multi-storey, must be supported by the ground beneath.The interaction of geotechnics and structure cannot be avoided. Although modern construction involves geotechnical engineers almost universally, structural engineers must have a good understanding of geotechnics. This chapter reviews simple pad and strip foundations, piles, rafts and finishes with some methods of assessing retaining walls.

Figure 2.13.2 Wall Strip Footing

 

Foundations are the unseen face of structural engineering; hidden from view; but nevertheless an important part of structural engineering.

Foundations and substructures also provide interesting and difficult challenges, not only in selecting the ‘right’ solution but in the construction sequences, which can have major influence on project costs.

Figure 2.13.5 Pile Capacity Calculation Parameters

 

 

Pile in Clay Soil Capacity Assessment

Ultimate Pile Capacity

Pu = Ab.Sub.Nc + Ap.Sua.α ……where:

Ab = Area of base
Ap = Surface area of shaft

Sua = Shear strength of clay at shaft (this can vary, and can be summed in discrete lengths based on site investigation data)

Sub = undrained shear strength of clay at base of shaft

Nc = bearing coefficient (commonly 9.0)

α = adhesion factor (in the absence of data 0.45 may be used)

Safety Factor

A safety factor of 2.0 may be used to find capacity to compare with unfactored (working) loads

Typical Shear Strength Values for Soil

Hard Soil > 150 kPa (kN/mm²)
Stiff Soil = 75-150 kPa (kN/mm²)
Firm Soil = 40-75 kPa (kN/mm²)

Retaining Wall Base – Eccentricity Outside Middle Third

Figure 2.13.15 Flooded Retaining Wall – Eccentricity Outside Middle Third
If the retaining wall in figure 2.13.11 was involved in a flash flood, for a short period, with water saturating the soil behind to a maximum level of 0.5m below ground level, there would be a change in the bearing pressure under the base – superimposed on the soil lateral pressure and surcharge – assessed in the calculations for sliding and overturning on the previous pages.

Sliding resistance and overturning resistance would need to be re-evaluated, taking into account the increased lateral pressure, as illustrated in figure 2.13.10.

In the following pages, ignoring the sliding resistance, the increased bearing pressure is used to calculate conservatively steel reinforcement needed in the base.

Retaining Wall Base Example
Eccentricity Outside Middle Third

From figure 2.13.15….

Water Pressure =10kN/m3 x 2.5²/2
= 31.2kN/m,
and offset from base = 2.5m/3 = 0.83m

Therefore ‘water’ overturning moment
= 31×0.83 = 26kNm/m (adding soil and surcharge moments from previous pages)

Total overturning moment=37.5+26=64kNm/m

Effective Eccentricity, E = M/V- 0.15
= 64/112.2 – 0.15 = 0.57-0.15 = 0.42m > 0.33

i.e. OUTSIDE MIDDLE THIRD

Maximum pressure (from figure 2.13.14 & 16)

P1 (Max.) = 2V/3B (L/2-e)
= 2×112.2/(3x1m)/(2/2-0.42)
= 129 kPa (N/mm²) < 130 (see previous page)

Length of pressure block = 3(L/2-e)
=3x(2/2-0.42) = 1.98m (~ 2m)