CTR09132 Foundation Analysis and Design

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Consider the bearing capacity equation for strip foundations under drained conditions. Assume vertical loads, horizontal ground surface and no load eccentricity (i.e. equation only includes bearing capacity factors and ground water level effects).

What is the most sensitive parameter on the estimation of bearing capacity?

1. When analysing the sensitivity of each parameter/variable consider a reference case and introduce variations of ±20% on each value at a time. For example, to assess the sensitivity of the bearing capacity to the angle of shearing resistance (f’) consider that B = 2.0 m, D = 1.0, c’ = 0 kPa, and g = 20 kN/m3 , then take f’ref = 30  and calculate the bearing capacity within the range of 24<f’<36 for at least 5 different values of f’.

2. To quantify the sensitivity of each parameter, define normalising coefficients (R). For example, when assessing the sensitivity of the bearing capacity to the angle of shearing resistance (f’) consider R = q/qref where q is the calculated bearing capacity for a given f’ and qref is the bearing capacity calculated with f’ref = 30o

3. Another useful manner to quantify the sensitivity of each parameter is to estimate the percentage of error on the bearing capacity with an overestimate of a single unit of the variable being assessed.

For example, when assessing the sensitivity of the bearing capacity to the angle of shearing resistance (f’) consider the error on the bearing capacity by increasing f’ by 1o and estimate the error as e = [q/q(f’+1)]100% for each of the f’ values considered in hint (1).

Requirements:

a) Each of your sensitivity analyses needs to be supported by graphs and tables that clearly show the variations on parameters being considered.

b) Your answer needs to consider the effects of B, D, g, f’ as well as different ground water levels (i.e. at ground surface and at z = 2B)

c) Restrict your answer to a single definition of the bearing capacity factors Nc, Nq and Ng, but explicitly state which one is being used in your answer.

Consider that this borehole is the only available information as part of preliminary desk study for a pile foundation design. Assess the quantity/quality of the information available and plan a site investigation campaign that will enable you to acquire all geotechnical data to design a pile of 20 m depth and 0.6 m diameter. You are free to decide on pile characteristics (i.e. pile type, pile material, construction process, etc.) but all your decisions and assumptions need to be justified.

Consider two popular methods for the calculation of settlement of shallow foundations on coarse-grained soils (e.g. Schmertmann, 1970 and Burland & Burbridge, 1985) and discuss using your own words their advantages, disadvantages and differences. Do this within the context of a rectangular footing with B = 2 m, L = 3 m and founded at a depth of 1 m below the ground surface with no presence of ground water level.