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Code of Practice for Foundations – MARBLE ROCK MASS CLASSIFICATION

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2.8.2.1 MARBLE ROCK MASS CLASSIFICATION

The karst morphology of a marble rock mass may be interpreted using the marble rock mass classification system given in Table 2.5. Under this system, the marble rock mass is classified in terms of MQD, which is an index devised to measure the degree of dissolution voids and the physical and mechanical implications of fractures or cavity-affected rock mass. MQD should be calculated for each drill hole according to the definition and illustration given in Figure 2.6.

Table 2.5 Classification of Marble Rock Mass

Marble Class	MQD Range (%)	Rock Mass Quality	Features
I	75 < MQD ≤ 100	Very Good	Rock with widely spaced fractures and unaffected by dissolution
II	50 < MQD ≤ 75	Good	Rock slightly affected by dissolution, or slightly fractured rock essentially unaffected by dissolution
III	25 < MQD ≤ 50	Fair	Fractured rock or rock moderately affected by dissolution
IV	10 < MQD ≤ 25	Poor	Very fractured rock or rock seriously affected by dissolution
V	MQD ≤ 10	Very Poor	Rock similar to Class IV marble except that cavities can be very large and continuous

Notes :

In this system, Class I and Class II rock masses are considered to be a good bearing stratum for foundation purposes, and Class IV and Class V rock masses are generally unsuitable.
- Class III rock mass is of marginal rock quality. At one extreme, the Class III rating may purely be the result of close joint spacings in which case the rock may be able to withstand the usual range of imposed stresses. At the other extreme, the Class III rating may be the result of moderately large cavities in a widely- jointed rock mass. The significance of Class III rock mass would need to be considered in relation to the adjacent drill hole sections in the context of a 3 dimensional model.
- Table 2.5 is not applicable to the marble clast-bearing volcaniclastic rocks.

Code of Practice for Foundations – FOUNDATION PLANS

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1.1 FOUNDATION PLANS

Foundation plans should consist of adequate and relevant information so as to demonstrate the entire physical and conceptual designs. A typical foundation plan should include the following two parts:

(1) A Foundation Plan :

a block plan showing the location of the site;

details showing the characteristic features of the site and environments including locations of ground investigation boreholes, adjacent and nearby buildings and structures with foundations, lands, streets, utility services, slopes, nullahs, retaining walls and the like;
- layout arrangement, identification, expected depths and founding levels, structural details and material specifications of the foundations;
- for piled foundations, item (c) should include size, shape, cut-off level and structural details of the pile element, as well as details of pile shoe, pile head, splices and the pile to pile cap connections;
- layout arrangement of the pile caps if applicable;
- bearing capacity of foundations and method of verification on site;
- specification of structural materials;
- magnitude of characteristic dead, imposed and wind loads and their critical combinations acting on the foundations (for piled foundations, this should be given for each pile or each group of piles);
- installation specifications, including the founding criteria, method of installation, etc.;
- for piled foundations, item (i) should include method of controlling and monitoring the verticality, inclination and alignment of the piles during installation, the maximum number of piling rigs allowed to be concurrently driving piles at any one time for percussive piling in vibration sensitive sites, method of overcoming underground obstruction, etc.;
- where dynamic pile driving formula is used, the parameters for the assessment of the ultimate pile capacity, such as the hammer efficiency, efficiency of blow and penetration of pile per hammer blow; and
- proposals of precautionary and protective measures, monitoring plan and contingency plans to be implemented before and during the course of the construction works (see clause 7.2).

(2) Supporting Documents:

site investigation report with results of ground investigation, field and laboratory tests and photographs of all the soil samples and rock cores taken;
- design calculations based on recognized foundation engineering principles;

assessment on the effects of the proposed foundation works on adjacent and nearby buildings, structures, lands, streets, utility services, slopes, nullahs and retaining walls etc.; and
- for foundation works that may induce vibration or ground movement, a PR Plan (see clause 7.2.7).

Code of Practice for Foundations – MARINE FOUNDATIONS

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1.1.1 MARINE FOUNDATIONS

Corrosion protection of marine foundations should be provided in accordance with acceptable standards or codes of practice for design of maritime structures, such as “Port Works Design Manual Part 1” published by Civil Engineering and Development Department. The following should be considered as general guidelines:

(1) Concrete

Concrete should be of high density and low permeability. It should not be inferior to grade C45 and the water/cement ratio should not exceed 0.38. Condensed silica fume should be added to reduce the permeability of concrete. The cementitious content should be 380 to 450 kg/m³, of which the dry mass of condensed silica fume should be within 5 to 10% range by mass of the cementitious content.

Nominal concrete cover to all reinforcement in all exposure zone should be 75 mm.

Crack widths of concrete within tidal and splash zone should not exceed

0.1 mm under typical average long term loading conditions, which may be increased by a factor of 1.25 for flexural crack width design and control purpose.

Correct use of pulverized fuel ash in the concrete mix may increase resistance of concrete against sulphate attack.

(2) Steel

All structural steelwork above seabed level, whether fully immersed, within the tidal or splash zones, or generally above the splash zone, should be fully protected against corrosion for the design working life of the structure. Below seabed level, an allowance for corrosion loss of 0.05 mm per year on the outside face of steel is considered reasonable if no corrosion protection is carried out within this zone.

Stainless steel for use in marine environment should be of a grade which is absolutely free of any chloride. Common grade of stainless steel with the presence of chloride should not be used for marine works.

Steel embedded in concrete and steel in seawater in the same foundation should be isolated since the former is cathodic relative to the latter.

Code of Practice for Foundations – STEEL PILES

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1.1.1 STEEL PILES

Provisions for corrosion protection of steel piles should be given in the foundation plans where:

sulphate, chloride, aggressive chemical or other agents causing deterioration is present in the ground;
- the piles are placed at the splash and tidal zones of the sea;
- the piles are in contact with other metals;
- stray direct electric current is present; or
- damage by abrasion may occur.

Code of Practice for Foundations – CONCRETE FOUNDATIONS

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1.1.1 CONCRETE FOUNDATIONS

Provisions for corrosion protection of concrete foundations should be given in the foundation plans where:

sulphate, chloride, aggressive chemical or other agents causing deterioration is present in the ground;
- alkalis are present in the concrete and a high moisture content environment exists;
- the foundations are constructed on a landfill site; or
- damage by abrasion may occur.

To avoid the alkali-aggregate reaction occurring in reinforced concrete structures, the reactive alkali of concrete expressed as the equivalent sodium oxide per cubic metre of concrete should not exceed 3.0 kg.

Code of Practice for Foundations – CORROSION PROTECTION OF FOUNDATIONS

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1.1 CORROSION PROTECTION OF FOUNDATIONS

1.1.1 GENERAL

Foundations should be provided with adequate protections against corrosion, or alternatively, they should be suitably designed to allow for the effect of corrosion which may take place during their designed working life.

To ensure effective and economical designs for protection against corrosion, information on the presence of any corrosive material in the ground and the range of fluctuation of ground water table should be obtained.

Code of Practice for Foundations – MATERIALS AND STRESSES

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1.1.1 MATERIALS AND STRESSES

General

Materials and stresses shall comply with the requirements of the Building (Construction) Regulations and the relevant codes of practice.

Where the permissible stress method is used in the structural design of foundation members, the working stress may be increased by not more than 25% where such increase is solely due to wind loads.

(2) Concrete

The concrete used for foundation elements shall comply with the Building (Construction) Regulations and the relevant codes of practice. Subject to the provisions of this Code, the design of the reinforced concrete elements of a foundation should be carried out in accordance with the Code of Practice for Structural Use of Concrete.

For cast-in-place concrete foundations, the concrete strength should be reduced by 20% where groundwater is likely to be encountered during concreting or where concrete is placed underwater.

The axial compressive stress on a driven precast concrete pile under working loads should not exceed 0.2f_cu.

For marine foundations, concrete should not be inferior to grade C45 as required in clause 2.6.4. All concrete should be cast in dry condition as far as

possible. Where the concrete is placed under water, the concrete should be assumed as grade C25 for design purpose.

(3) Grout

The requirements for concrete given in this Code shall equally apply to grout.

(4) Steel

For driven steel bearing piles with a design safety factor on driving resistance of 2, the axial stress in the steel at working load should not exceed 30% of the yield stress.

For steel bearing piles installed in pre-bored holes or jacked to the required depth, in which no peak stresses due to impact are set up, the axial stress in the steel at working loads may be increased to 50% of the yield stress.

Structural element design may be carried out in accordance with the Code of Practice for the Structural Use of Steel, provided that the condition under any possible load test is considered.

For steel piles, the allowable bond stress between steel and grout (with a minimum characteristic strength of 30 MPa) may be taken as 400 kPa (or 320 kPa when grouting under water).

Shear studs designed in accordance with the Code of Practice for the Structural Use of Steel may be used to enhance the allowable bond stress provided that the overall allowable bond stress does not exceed 600 kPa (or 480 kPa when grouting under water).

Steel sections or other means as substitute for shear studs may also be considered.

The surface area for calculation of allowable steel/grout bond stress should be the total external surface area of the steel section.

For steel piles relying on the bond between steel and grout to resist tension or compression loads, the pile surface should be clean and free from loose mill scale, loose rust or any substance that may reduce the bond.

For corrosion protection of marine foundations, the guidelines as given in clause 2.6.4 should be followed.

Code of Practice for Foundations – RESISTANCE TO SLIDING, UPLIFT AND OVERTURNING

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1.1.1 RESISTANCE TO SLIDING, UPLIFT AND OVERTURNING

General

The foundations shall be so designed and constructed to fulfil the requirements given in this clause.

(2) Design Based on Highest Anticipated Groundwater Table

Where the design is based on the highest anticipated groundwater table, a building or structure shall be so designed and constructed such that:

the resistance to the sliding force acting thereon shall be at least 1.5 times the sliding force due to any loads;
- the resistance to the uplift force acting thereon shall be at least 1.5 times the uplift force due to any loads; and
- the resistance to the overturning moment acting thereon shall be at least 1.5 times the overturning moment due to wind loads, 1.5 times the overturning moment due to groundwater and 2 times the overturning moment due to loads other than wind loads and groundwater.

(3) Design Based on Highest Possible Groundwater Table

Where the design is based on the highest possible groundwater table, a building or structure shall be so designed and constructed such that:

the resistance to the sliding force acting thereon shall be at least equal to the sum of 1.1 times the sliding force due to groundwater and 1.5 times the sliding force due to other loads;
- the resistance to the uplift force acting thereon shall be at least equal to the sum of 1.1 times the uplift force due to groundwater and 1.5 times the uplift force due to other loads; and
- the resistance to the overturning moment acting thereon shall be at least equal to the sum of 1.5 times the overturning moment due to wind loads, 1.1 times the overturning moment due to groundwater and 2 times the overturning moment due to loads other than wind loads and groundwater.

(4) Resistance

The resistance to the sliding force shall be calculated as the sum of the sliding resistance due to the minimum dead loads plus that due to any permitted sliding resistance.

The resistance to the uplift force shall be calculated as the sum of the downward force due to the minimum dead loads plus that due to any permitted anchorage resistance.

The resistance to the overturning moment shall be calculated as the sum of the stabilizing moment due to the minimum dead loads plus that due to any permitted anchorage resistance.

The minimum dead loads should be taken as the weight of the structural elements plus the weight of any permanent finishes and backfill. In the dead load calculations, conservatively assumed values or the actual thickness and densities of the finishes and the backfill should be used. Finishes and backfill that could be removed should be ignored in the calculations.

(5) Special Considerations for Marine Structures

Marine structures should also be designed and constructed such that the resistance to sliding, uplift and overturning satisfies the requirements of acceptable standards or codes of practice for design of maritime structures, such as “Port Works Design Manual” published by Civil Engineering and Development Department.

Code of Practice for Foundations – UNDERGROUND WATER

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1.1.1 UNDERGROUND WATER

The lateral or uplift/buoyancy force due to underground water acting on a structure or its foundation may be calculated based on either the highest anticipated groundwater level or the highest possible groundwater level, which are defined below.

(1) Highest Anticipated Groundwater Level

The highest anticipated groundwater level shall be the level derived from reliable data. In determining the highest anticipated groundwater level, the following conditions should be taken into consideration:

the current and projected tidal variations;
- the design free surface water levels due to storm, wind surge and pounding;
- the design groundwater level taken into account the influences of rainfall, surface water run-off and groundwater movement;
- the damping of seawater tide influence by intervening ground;
- dewatering;
- the long term rise in sea level; and
- ground permeability.

The prediction of the highest anticipated groundwater level should be based on measurements of groundwater for a sufficiently long period that covers at least a wet season.

(2) Highest Possible Groundwater Level

The highest possible groundwater level shall be the level above which the groundwater would not rise under all possible extreme events such as severe rainfall, flooding and bursting of water mains. In the absence of reliable data to prove otherwise and except for low lying areas, the highest possible groundwater level may generally be taken as the ground surface of a building,

street, building works or street works. However, in low-lying areas such as reclamation, it may rise even above the ground surface.

Code of Practice for Foundations – STRUCTURAL REQUIREMENTS

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1.1 STRUCTURAL REQUIREMENTS

1.1.1 GENERAL

The structural design of foundations should comply with the provisions of the Building (Construction) Regulations.

1.1.2 DESIGN LOADS

The foundation of a building shall be designed to carry the working load with adequate factor of safety. Dead load, imposed load and wind load should be assessed

in accordance with the Code of Practice for Dead and Imposed Loads, Code of Practice on Wind Effects in Hong Kong and other relevant codes of practice. The imposed load should include buoyancy force and earth pressure. Buoyancy force should be assessed in accordance with clause 2.5.3. Earth pressure should be assessed by using recognized geotechnical engineering methods.

Where it is necessary to carry out foundation design based on a set of assumed loads, a detailed schedule of the assumed loads should be prepared and, before the commencement of the construction of the superstructure, it is necessary to demonstrate that the loads from detailed calculations of the superstructure do not exceed the assumed loads used in the foundation design.