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 in) was completely decomposed granite (-15 m) followed by moderately/slightly decomposed granite (bed rock). Each of the buildings is to be supported by a raft foundation having a width of 30 m, a length of 33 m and a thickness of 3 m and the raft is to be founded at 4 m below the ground level.
The first batch of ground investigation works contained 6 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 3 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 SLIT 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 10 m 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 simp icity):
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From lab tests
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For design purposes
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c (kPa)
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4) (degree)
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c (kPa)
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$ (degree)
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7 (kN/m3)
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Fill
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3.72
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34.1
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1.5
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33
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19
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CDG
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4.15
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36.8
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4
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36
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19
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(I) 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 4)=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 (I) 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 working load.
(6) Consider a worse scenario that the water table is located at the level of the base of the raft. Recalculate the ultimate bearing capacity of the raft foundation using the Meyerhof method.