Presumed Allowable Bond or Friction Between Rock and Concrete or Grout for Piles
| Category of rock | Presumed allowable bond or friction between rock and concrete or grout for piles (kPa) | |
| as defined in Table 2.1 | Under compression or transient tension | Under permanent tension |
| 1(c) or better 1(d) or 2 | 700 300 | 350 150 |
Notes:
| 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:
Notes: 1. TCR of the designated grade = (a+c+d+f)/L.
The allowable bearing pressure, bond or friction of soils and rocks should be determined by one of the methods given in clauses 2.2.1 to 2.2.5.
Rational design method for calculating the ultimate capacity should be based on sound engineering approach and should include:
Normally, the allowable capacity is estimated by applying a factor of safety of 3 to the calculated ultimate bearing capacity. However, other factors of safety may be adopted having regard to the nature of the soil or rock, its variability over the site and the reliability of the design method.
In lieu of a rational design method, the allowable capacity for soils and rocks may also be taken as those presumed values derived from empirical correlation and as stipulated below provided that the following conditions are complied with:
The presumed values for rock are based on the assumption that slip of the rock will not occur. Therefore, where the rock profile is inclined at such an angle that the bearing capacity of the rock mass may be affected, the rock joints should be checked to ensure that there is no unfavourable joint orientation that could permit slip of the rock to occur.
The allowable vertical bearing pressure for foundations on horizontal ground may be estimated from Table 2.1 on the basis of the material description.
The allowable lateral bearing pressure for rock may be taken as one third of the allowable vertical bearing pressure provided that no adverse rock joints exist.
The allowable bond or friction between rock and concrete for piles may be estimated from Table 2.2.
A presumed allowable vertical bearing pressure of 100 kPa (if dry) or 50 kPa (if submerged) may be used for the design of footings on horizontal ground of minor temporary structures such as fencing and hoarding.
Foundations of any building or structure shall be designed and constructed to withstand safely all the dead, imposed and wind loads without impairing the stability or inducing excessive movement to the building or of any other building, street, land, slope or services.
The allowable capacity of the soil/rock under working loads where any foundation is founded shall be the lesser of:
The allowable capacity may be increased by 25% when such increase is solely due to wind effects.
In determining the said factor of safety against failure, due consideration shall be given to the form and depth of the foundation, loading characteristics, the general geological conditions of the ground and its surrounding including the presence of dissolution features, jointing conditions and any other relevant characteristics for rock.
In choosing the method for the determination of the ultimate capacity or for the estimation of settlement, care must be taken to ensure that the site investigation, testing, derivation of parameters, computations, method of construction and standards of acceptance are mutually compatible and consistent with such method.
The classification of soils and rocks used in this Code is set out in Table 2.1. Further definition and description can be obtained from GEOGUIDE 3.
For the purpose of this Code of Practice the following symbols apply:
| σ’ | = | effective vertical overburden pressure |
| σv‘ | = | mean vertical effective stress (kPa) |
| β | = | shaft resistance coefficient |
| γ | = | bulk unit weight of the soil |
| γ’ | = | submerged unit weight of the soil |
| γs‘ | = | effective unit weight of the soil |
| γw | = | unit weight of water |
| nh | = | constant of horizontal subgrade reaction |
| ζcs, ζγs, ζqs | = | influence factors for shape of foundation |
| ζci, ζγi, ζqi | = | influence factors for inclination of load |
| ζcg, ζγg, ζqg | = | influence factors for ground surface |
| ζct, ζγt, ζqt | = | influence factors for tilting of foundation base |
| af | = | inclination of the base of the footing |
| f′ | = | effective angle of shearing resistance |
| τs | = | ultimate shaft friction under transient tension |
| µ | = | friction factor |
| w | = | sloping inclination in front of the footing |
| n | = | Poisson’s ratio |
| A | = | cross-section area of the pile |
| B | = | width or diameter of test plate |
| Bf | = | least dimension of footing |
| Bf‘ | = | Bf – 2eB |
| c’ | = | effective cohesion of soil |
| cc | = | temporary compression of the hammer cushion |
| cp | = | temporary compression of pile |
| cq | = | temporary compression of ground at pile toe |
| dl | = | elemental length of the pile |
| D | = | least lateral dimension of the pile |
| Df | = | depth from ground surface to the base of shallow foundation |
| Dmin | = | minimum dead load |
| e | = | coefficient of restitution |
| eB | = | eccentricity of load along B direction |
| eL | = | eccentricity of load along L direction |
| E | = | Young’s modulus of the material of the pile |
| Eh | = | efficiency of hammer |
| Es | = | Young’s modulus of soil |
| fcu | = | characteristic strength of concrete |
| fy | = | characteristic strength of steel |
| Gk | = | characteristic dead load |
| h | = | hammer drop height |
| H | = | horizontal applied load |
| Ia | = | adverse imposed load including live and soil loads |
| kp | = | ground borne vibration coefficient |
| l | = | depth of the consolidation strata |
| L | = | length of the pile in mm (For piles with rock sockets, L should |
| be measured to the centre of the rock socket. For piles without | ||
| rock sockets, L may generally be measured to the pile toe.) | ||
| Lf | = | longer dimension of footing |
| Lf‘ | = | Lf – 2eL |
| N | = | SPT N-value |
| Nav | = | average SPT N-value along pile shaft but not exceeding 40 |
| Nc, Nγ, Nq | = | general bearing capacity factors which determine the capacity |
| of a long strip footing acting on the surface of a soil in a | ||
| homogenous half-space | ||
| Nq’ | = | bearing capacity factor which determines the capacity of |
| replacement piles embedded in granular soil | ||
| NSF | = | negative skin friction |
| p | = | perimeter of the pile, or perimeter of the circumscribed |
| rectangle in the case of H-pile | ||
| P | = | vertical applied load |
| Pc | = | allowable ground-bearing capacity of the piles without wind |
| Pcw | = | allowable ground-bearing capacity of the piles with wind |
| Pn | = | design pile capacity under working load without wind |
| Pnw | = | design pile capacity under working load with wind |
| Ps | = | structural strength of the pile without wind |
| Psw | = | structural strength of the pile with wind |
| Pu | = | ultimate capacity of pile |
| q | = | overburden pressure at base level of the foundation in the |
| ground adjacent to the foundation (see Figure 2.2(a) for sloping | ||
| ground) | ||
| qa | = | allowable vertical bearing pressure |
| qb | = | allowable bearing capacity of replacement piles embedded in |
| granular soil | ||
| qo | = | effective overburden pressure at the base of the foundation, i.e. |
| qo = γs‘ Df , whereγs‘ and Df are respectively the effective unit | ||
| weight and depth of the soil that originally exists above the | ||
| base of the foundation | ||
| qu | = | ultimate bearing capacity of the granular soil |
| qub | = | ultimate end bearing resistance of large diameter bored |
| piles/barrette piles | ||
| Qk | = | characteristic imposed load |
| Qu | = | ultimate resistance against bearing capacity failure |
| r | = | slope distance of recipient from pile toe (see clause 7.2.6) |
| Ra | = | allowable anchorage resistance of the pile (see clause 5.3.3) |
| Rbc | = | allowable bearing capacity for small diameter bored pile (see |
| clause 5.4.6) | ||
| Ru | = | ultimate anchorage resistance of the pile (see clauses 5.1.6 and |
| 5.3.3) | ||
| s | = | permanent set of pile. |
| S | = | settlement measured at test load W during loading test |
| Smax | = | maximum settlement measured in loading test |
| Ua | = | uplift due to the highest anticipated groundwater table |
| Up | = | uplift due to the highest possible groundwater table |
| vres | = | resultant ppv due to pile driving |
| W | = | design pile capacity under working load without wind in kN |
| W1‘ | = | effective weight of rock or soil cone |
| W2‘ | = | effective weight of soil column above rock or soil cone |
| We | = | nominal hammer energy |
| Wh | = | weight of hammer |
| Wk | = | adverse wind load |
| Wp | = | weight of pile |
| Wp‘ | = | effective self weight of the pile |
| Wr | = | weight of pile helmet |
| Wt | = | test load for plate load test (see clause 8.2(2)(b)) |
| x | = | distance of recipient from pile measured along the ground surface |
| y | = | depth of pile toe at the time of assessing vres |
For the purpose of this Code of Practice the following abbreviations apply: AP Authorized Person
CS1 Construction Standard 1
CS2 Construction Standard 2
ETWB Environmental, Transport and Works Bureau (a former government bureau)
GEO Geotechnical Engineering Office
GEOGUIDE 2 “Guide to Site Investigation” published by GEO GEOGUIDE 3 “Guide to Rock and Soil Descriptions” published by GEO HOKLAS Hong Kong Laboratory Accreditation Scheme
MQD Marble Quality Designation
NSF Negative Skin Friction
PNAP Practice Note for Authorized Persons, Registered Structural Engineers and Registered Geotechnical Engineers
PR Plan Public Relations Plan
RSE Registered Structural Engineer
RGE Registered Geotechnical Engineer
RSC Registered Specialist Contractor
RQD Rock Quality Designation
SPT Standard Penetration Test
TCR Total Core Recovery
UCS Uniaxial Compressive Strength
For the purpose of this Code of Practice the following glossary of terms applies:
Allowable bearing pressure. The maximum allowable bearing pressure that may be applied at the base of the foundation, taking into account the ultimate bearing capacity of the soil or rock, the magnitude and type of settlement expected and the ability of the structure to accommodate such settlement. [NOTE : The allowable bearing pressure is a combined function of the site conditions, including all construction in the vicinity, and the characteristics of the proposed foundation/structure.]
Allowable load. The maximum load that may be applied safely to a foundation after taking into account its ultimate bearing capacity, negative skin friction, pile spacing, overall bearing capacity of the ground below the foundation and allowable settlement.
Authorized Person. A person whose name is on the authorized persons’ register kept under section 3(1) of the Buildings Ordinance.
Bell-out. An enlargement of the base area of a pile, formed in situ by undercutting (under-reaming) the soil or rock at the base of a bored pile.
Designated Area. The Designated Area of Northshore Lantau as described in the GEO Technical Guidance Note No. 12 published by Geotechnical Engineering Office or PNAP APP-134.
Dry condition. For shallow foundations, dry condition means that the highest anticipated groundwater level is at a depth of not less than 1m or the width of the shallow foundation, whichever is the greater, below the base of the foundation. The width of the shallow foundation shall be the lesser dimension of a rectangular shallow foundation or the largest inscribed rectangle of an irregular shallow foundation (see Figure 2.4), or the diameter of a circular shallow foundation.
Final set. The penetration per blow of hammer at the founding level of a driven pile.
Foundation. That part of a building, building works, structure or street in direct contact with and transmitting loads to the ground.
Ground investigation. Any exploratory drilling, boring, excavating and probing of land for obtaining any information on ground conditions and includes the installation of instruments, sampling, field testing, any other site operation and laboratory testing of samples obtained from such operations.
Ground investigation field works. All site operations in ground investigation and exclude laboratory testing of samples and field density tests.
Highest anticipated groundwater level. (see definition given in clause 2.5.3)
Highest possible groundwater level. (see definition given in clause 2.5.3)
Meta-sedimentary rock. A sedimentary rock that shows evidence of having been subjected to metamorphism that differs from the conditions under which the sedimentary rock originated.
Negative skin friction. The downdrag skin friction resulted from the consolidation of compressible soil strata.
Permanent tension. Tension in a foundation element induced by a loading effect of permanent nature, such as soil loads and uplift due to groundwater, acting continuously throughout its service life.
Pile cap. A concrete structure built on the head of a pile or a group of piles for transmission of loads from the structure above to the pile or group of piles.
Pile spacing. The distance measured from centre to centre of adjacent piles.
Pre-boring. Removal of ground or underground obstacles by boring or other means prior to the installation of pile foundation. This operation shall be carried out for one of the following purposes: installation of socketed steel H-piles and mini-piles, removal of or penetration through underground obstructions for driven steel H-piles, or mitigation of the effect of vibration for driven steel H-piles (see clause 7.2.6).
Proof test. Test to be carried out on representative foundation units to ascertain the performance of foundation under load as required by regulation 30 of the Building (Construction) Regulations.
Qualified land surveyor. A person whose name is on the professional land surveyors’ register kept under section 11 of the Surveyors Registration Ordinance or the authorized land surveyors’ register kept under section 11 of the Land Survey Ordinance.
Raking pile. A pile installed at an inclination to the vertical.
Registered Geotechnical Engineer. A person whose name is on the geotechnical engineers’ register kept under section 3(3A) of the Buildings Ordinance.
Registered Specialist Contractor (Foundation Works). A contractor whose name is on the sub-register of the foundation works category in the register of specialist contractors maintained under section 8A of the Buildings Ordinance.
Registered Specialist Contractor (Ground Investigation Field Works). A contractor whose name is on the sub-register of the ground investigation field works category in the register of specialist contractors maintained under section 8A of the Buildings Ordinance.
Registered Structural Engineer. A person whose name is on the structural engineers’ register kept under section 3(3) of the Buildings Ordinance.
Rock socket. The penetration formed in rock for embedding a portion of a pile.
Rock socketed pile or Socketed pile. A pile with the toe portion embedded into a rock socket to derive load resistance through bearing, bond or friction with the rock.
Skin friction. The frictional resistance developed at the interface between a foundation member and the surrounding ground.
Site investigation. An investigation of the physical characteristics of the site and includes documentary studies, site survey and ground investigation.
SPT N-value. The uncorrected N-value obtained from standard penetration test.
Submerged condition. For shallow foundations, submerged condition means that the design groundwater level is at or above the base of the foundation.
Test driving/installation. Test driving or installation of one or more piles carried out to verify the design and/or other installation method.
Test pile. A pile to which a test is applied.
Transient tension. Tension induced to the foundation that is not categorised as permanent tension, such as wind load and load combination with wind.
Trial pile. A pile tested for the purpose of verifying the design of the piles, including the design parameters and the load carrying capacity, and such verification usually requires loading test.
Ultimate bearing capacity. The value of the loading intensity for a particular foundation at which the resistance of the bearing stratum becomes fully mobilized or undergoes substantial deformation.
Working load. The service load which the foundation is designed to carry.
This Code of Practice was prepared on the basis of being ‘deemed-to-satisfy’ the Building (Construction) Regulations as far as the design and construction of foundations are concerned. Departure from the requirements and recommendations of this Code of Practice or the use of other standards or codes of practice for design of foundations may require demonstration of the compliance with the provisions of the Building (Construction) Regulations.
This Code of Practice is intended for local use only. Methods of foundation design that are currently and commonly used in Hong Kong are included in this Code of Practice as far as possible. It should be noted that some methods of foundation design have been developed from practical considerations and experience and have been accepted on the basis that they have been demonstrated to have worked satisfactorily.
In addition to technical aspects, this Code of Practice also includes brief descriptions of local practices that could affect the design and construction of foundations. The descriptions cover mainly the purposes and objectives of the practices. Detailed procedural requirements are not included; reference should be made to the most current practice notes issued by the Buildings Department or other government departments or bureaux.
Design for seismic effect is not presently included in this Code of Practice. However, where seismic effect is considered in the design of the superstructure, it should also be considered in the design of the foundation.
The Buildings Department established the Technical Committee (TC) on the Code of Practice for Foundations for the purpose of collecting views and feedbacks on the use of the Code of Practice for Foundations published in 2004 (the 2004 Code) from the building industry and with a view to keeping the Code of Practice in pace with the advancement in design, analysis and construction practice.
This Code, Code of Practice for Foundations 2017 (the 2017 Code) is issued upon completion of the review by the TC, which has focused on four fronts: (a) the advancement in design and analysis; (b) the experience gained and the views and feedbacks received on the use of the 2004 Code; (c) the commonly adopted local practice on foundation construction; and (d) necessary updates consequent upon the publication of the relevant Codes of Practice, and the issue of relevant Practice Notes for Authorized Persons, Registered Structural Engineers and Registered Geotechnical Engineers.
The contributions and efforts given by the invited members of the TC in the preparation of the 2017 Code are greatly appreciated.
The 2017 Code will be reviewed regularly. The Buildings Department welcomes suggestions for improving the Code.
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