Introduction

A proposed shopping complex include a two$-$storey building located close to one

boundary of the site. A section of the foundation plan shown in the figure below

consists of series of square footings supporting columns. The design load of the

footings varies and fall in the range of $1,100$ to $1,600kN(Q_{\text{a}\text{l}\text{l}(gross)}).$

Figure $2$: Proposed foundation plan $($not to scale$)$

Initial geotechnical investigations on the site revealed the presence of a silty sand

layer extending from the ground surface to a depth of $20$ m $.$ Properties of silty sand

were found to be c $=0,\frac{O}{?}={32}^0,=19\mathrm{.}5k\frac{N}{m^3},{}_{\text{s}\text{a}\text{t}}=20\mathrm{.}5k\frac{N}{m^3}$ based on

preliminary test results.

Foundations will be placed $1\mathrm{.}4$ m below the ground surface $(D_f).$ For most part of

the year, water table is located at $3\mathrm{.}5$ m from the ground surface. However, the

groundwater table can rise up to a level $1$ m below the bottom of the foundation

during the rainy season.

The clearance distance $(c$ in Figure $2)$ from the centre of the footing to the boundary

will be $0\mathrm{.}6$ m $.$ This limits the size of the foundation to $1$ m on the direction

perpendicular to the boundary.

Your task:

Assuming a factor of safety of $3,$ design a suitable foundation for a column carrying

a total allowable gross load of $Q_{\text{a}\text{l}\text{l}(gross)}=1300kN,$

$($i$)$ using Terzaghi's equation for ultimate bearing capacity, and

$($ii$)$ using general bearing capacity equation.

Compare the two designs and discuss the efficiency of the footings.

An alternative to the above design is to use pile foundations. It is proposed to drive

concrete piles with $0\mathrm{.}5$ m diameter and a $15$ m length. Determine if this is a suitable

alternative. Assume a factor of safety of $3.$ Extended questions:

A Standard Penetration Test $($SPT$)$ was carried out to estimate the friction angle of

the silty sand. SPT blow count corrected for hammer efficiency, borehole diameter,

use of liner and short rod length are given below $(N_{60}).$

Considering the effect of the foundation loading is felt down to $2B+D_f,$ determine

the bearing capacity of the ground for the square footing designed in Q $2$ above.

Use Terzaghi's bearing capacity equation. See Note $2.$

How can Standard Penetration Test $($SPT$)$ and Cone Penetration Test $($CPT$)$ be

utilised to estimate pile capacity?

About one fifth of Australia's land mass is covered by reactive $($expansive$/$highly

plastic$)$ soils. Explain how the sensitivity of these soils to moisture affect shallow

foundations.

Note $2$:

$N_{60}$ values given above need to be corrected for overburden pressure. An average

corrected SPT number, $(N_1{)}_{60},$ across the depth $2B+D_f$ should be obtained in order

to determine the friction angle and bearing capacity factors.

$(N_1{)}_{60}=C_N{N}_{60}$

Overburden correction factor $C_N$ is given by$,$

$C_N=(\frac{{}_{v,\text{r}\text{e}\text{f}}^{\text{'}}}{{}_{vo}^{\text{'}}}{)}^n$

Where ${}_{v,\text{r}\text{e}\text{f}}^{\text{'}}$ is the reference vertical overburden stress level which can be taken as

$100$kPa,$n$ is the exponent which can be assumed to be $0\mathrm{.}5$ and ${}_v^{\text{'}}$ is vertical

overburden stress at the depth considered.

Refer to Soil Mechanics and Foundation Engineering: Fundamentals and

Applications $1^{st}$ Edition $($Chapter $11)$ by Nagaratnam Sivakugan for more

information.

Report Format

Report should not exceed $15$ A$4$ pages including figures and appendices. Show

specimen calculations where necessary. Include a cover page, a brief introduction,

conclusions and references.