Question #$1$

A residential development is proposed in the eastern part of New Territories, Hong Kong. The development consists of $5$ tower buildings, each having a $3-$story podium and $25$ stories of residential floors. Borings showed that below a surface layer of fill materials $(4$m$)$ was completely decomposed granite $(15m)$ followed by moderately$/$slightly decomposed granite $($bed rock$).$ Each of the buildings is to be supported by a raft foundation having a width of $30m,$ a length of $33m$ and a thickness of $3m$

and the raft is to be founded at $4m$ below the ground level.

The first batch of ground investigation works contained $9$ drillholes. Field tests included standard penetration tests, constant head permeability tests, falling head permeability tests, and in$-$situ density tests. Undisturbed triple tube retractable core barrel $($Mazier$)$ samples were also collected in the CDG layer for laboratory tests. The second batch of ground investigation works contained $5$ additional drillholes. The main purpose of the additional drillholes was to justify the adequacy of using shallow

foundation; there were only SPT tests involved and no samples collected for laboratory tests. The SPT $N-$values for the founding CDG layer varied from about $80$ to $200,$ with a characteristic value of being greater than $100.$ The observed groundwater table was about $10m$ below the ground level.

Based on the site investigation and laboratory testing, the soil parameters adopted are as follows $($the natural unit weight and the saturated unit weight are assumed to be the same for simplicity$)$:

$(1)$ Use the Meyerhof method to calculate the ultimate bearing capacity of the raft foundation and the allowable bearing capacity $(FS=3).$

$(2)$ Assuming the strength parameters of the $CDG$ are $c=0$ and $=36,$ re $-$calculate the ultimate and allowable bearing capacities of the raft, and compare the calculated values with that from $(1).$

$(3)$ Assuming the strength parameters of the $CDG$ are $c=4$kPa and $=38,$ re $-$calculate the ultimate and

allowable bearing capacities of the raft, and compare the calculated values with that from $(1).$

$(4)$ Compare the calculated values in $(1)$ with the presumed allowable bearing pressure in the "Code of Practice for Foundations", and state your design considerations $($e$.$g$.,$ what value you would take in your design and why; any additional tests you would ask for to support your decision and how$).$

$(5)$ Estimate the settlement of the raft foundation under the calculated working load.

$(6)$ Consider a worse scenario that the water table is located at the level of the base of the raft. Re$-$calculate the ultimate bearing capacity of the raft foundation using the Meyerhof method.