Table 2.1 Presumed Allowable Vertical Bearing Pressure under Foundations on Horizontal Ground/Bedrock
| Category | Description of rock or soil | Presumed allowable bearing pressure (kPa) |
| Rock (granite and volcanic): | ||
| 1(a) | Fresh to slightly decomposed strong to very strong granite or volcanic rock of material weathering grade II or better, with 100% TCR of the designated grade which has a minimum UCS of rock material not less than 75 MPa (or an equivalent point load index strength PLI50 not less than 3 MPa) | 10,000 |
| 1(b) | Fresh to slightly decomposed strong granite or volcanic rock of material weathering grade II or better, and with not less than 95% TCR of the designated grade, which has a minimum UCS of rock material not less than 50 MPa (or an equivalent point load index strength PLI50 not less than 2 MPa) | 7,500 |
| 1(c) | Slightly to moderately decomposed moderately strong granite or volcanic rock of material weathering grade III or better, and with not less than 85% TCR of the designated grade, which has a minimum UCS of rock material not less than 25 MPa (or an equivalent point load index strength PLI50 not less than 1 MPa) | 5,000 |
| 1(d) | Moderately decomposed, moderately strong to moderately weak granite or volcanic rock of material weathering grade III or better, and with not less than 50% TCR of the designated grade. | 3,000 |
| 2 | Meta-Sedimentary rock: Moderately decomposed, moderately strong to moderately weak meta-sedimentary rock of material weathering grade III or better, and with not less than 85% TCR of the designated grade. | 3,000 |
| 3 | Intermediate soil (decomposed granite and decomposed volcanic): Highly to completely decomposed, moderately weak to weak rock of material weathering grade V or better, with SPT N-value ³ 200 | 1,000 |
Table 2.1 Presumed Allowable Vertical Bearing Pressure under Foundations on Horizontal Ground/Bedrock (Continued)
| Category | Description of rock or soil | Presumed allowable bearing pressure (kPa) |
| Non-cohesive soil (sands and gravels): | Dry Submerged | |
| 4(a) | Very dense – SPT N-value >50 | 500 250 |
| 4(b) | Dense – SPT N-value 30-50; requires pick for excavation; 50 mm peg hard to drive | 300 150 |
| 4(c) | Medium dense – SPT N-value 10-30 | 100 50 |
| 4(d) | Loose – SPT N-value 4-10, can be excavated with spade; 50 mm peg easily driven | <100 <50 |
| Cohesive soil (clays and silts): | ||
| 5(a) | Very stiff or hard – Undrained shear strength >150 kPa; can be indented by thumbnail | 300 |
| 5(b) | Stiff – Undrained shear strength 75-150 kPa; can be indented by thumb | 150 |
| 5(c) | Firm – Undrained shear strength 40-75 kPa; can be moulded by strong finger pressure | 80 |
Notes:
- The presumed values for allowable bearing pressure given are for foundations with negligible lateral loads at bearing level.
- The self weight of the length of pile embedded in soil or rock does not need to be included into the calculation of bearing stresses.
- Minimum socket depth along the pile perimeter is 500 mm for categories 1(a) and 1(b), and 300 mm for categories 1(c), 1(d) and 2.
- TCR of the designated grade is defined in Figure 2.1.
- The TCR of the designated grade should be proved to a depth at least 5 m into the specified category of rock. This requirement is deemed to be complied with if the rock underneath the minimum socket depth as mentioned in note (3) above has a length of at least 5 m which can be divided into a number of segments (in consecutive manner) such that (a) each segment is 1 m; and (b) the calculated TCR in accordance with Figure 2.1 of each segment should satisfy the required percentage of TCR of the designated grade.
- The bearing surface of rock on which the foundation will be rested should be of the designated category and in an intact condition for a depth not less than 600 mm.
- Weathering grades are defined in GEOGUIDE 3.
- The point load index strength of rock quoted in the table is the equivalent value for 50 mm diameter cores.
- The definition of Dry Condition and Submerged Condition are given in clause 1.2.
- Where the ground is intermediate between dry and submerged, the presumed value may be obtained by linear interpolation.
- The use of presumptive values does not preclude the requirement for consideration of settlement of the structure.
Notes: 1. TCR of the designated grade = (a+c+d+f)/L.
- a, c, d and f are materials of the designated grade or better.
- b are materials inferior than the designated grade.
- e are materials washed away during drilling.
- The maximum continuous length of materials washed away/inferior to the designated grade, b+e, should not be greater than 300mm
- TCR of the designated grade should not be confused with TCR of the core run shown in the site investigation report, which is equal to (a+b+c+d+f)/L
hkbuildercodeofpractice 