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.
1.1.1 RECLAIMED LAND WITH CONSOLIDATION SUBSTANTIALLY COMPLETED
Substantial consolidation will have occurred in land that had been reclaimed a long time ago and the design rules specified in clause 2.4.1 therefore need not be followed unless the building superstructure is particularly sensitive to movement or there is evidence showing noticeable on-going ground settlement. For practical design purpose, the effect of consolidation may be ignored when the ground has undergone a minimum of 95% of primary consolidation settlement. In the absence of a detailed consolidation assessment, the number of years after reclamation required to achieve the 95% degree of consolidation for marine clay of an aggregate thickness H could be taken as follows:
Thickness of clayey deposits without interbedding sand/silt layers, H
Number of years
H ≤ 5m
10
5m < H ≤ 10m
20
10m < H ≤ 15m
30
The above table does not apply to situation where site formation works (except for minor filling up of ground e.g. for the construction of on-grade floor slabs) will be carried out and/or extensive shallow foundation with high soil bearing pressure will be placed to further consolidate the reclaimed lands.
Where the design of structures requires long-term monitoring and/or maintenance, the designer, through the AP, should alert the developer of such requirements and their implications and advise him to inform any prospective buyers who may have to bear the costs for such requirements.
Where it is intended not to follow the design rules given in clause 2.4.1, the problem of differential and total settlement should be fully considered. The time-related total and differential settlement (including predicted time-settlement curves) should be assessed based on site-specific ground investigation, and measures to overcome or accommodate the problem should be provided.
To ensure the reliability of the time-settlement relationship estimated at the design stage, continuous settlement monitoring (through instrumentation) throughout the construction period should be carried out and the assessment of the settlement should be reviewed from time to time.
In the settlement assessment, reference may be made to the settlement measurements collected during the reclamation period and any previous settlement assessments made for the reclamation. However, such data should only be used as reference to the historical settlement characteristics of the site or as supplementary information to the site-specific assessment unless their accuracy can be guaranteed. The historical settlement record and settlement assessment for government reclamation, if available,
can be obtained from the government department who undertook the reclamation projects, usually the Civil Engineering and Development Department.
Unless recommended otherwise, the lowest floor slabs of a building should not be designed as on-grade slabs.
Floor slabs directly above a raft-type pile cap may be designed as on-grade.
The following structures may also be designed as on-grade structures provided that they can be readily repaired or replaced if damaged by settlement:
fence walls, landscaping structures and lightweight covered walkway; and
floor slabs used for car parking, loading and unloading, vehicular ramp or pedestrian pavement.
For structures such as transformer rooms and pump houses, the foundations should be carried down through the reclaimed materials to a firm stratum with the lowest floor slabs designed as suspended.
Underground utilities and drainage underneath a building should be supported by suspended floor slabs or pile caps. The pipe connection at the interface between the structurally supported portion and the on-grade portion of pipes should be designed to accommodate differential settlement due to the subsidence of the latter.
Where significant settlement due to long-term consolidation of the ground is anticipated, measures should be provided in the pile cap design to mitigate the migration of soil into any void that may be formed underneath the pile cap due to consolidation of the ground below.
The effect of negative skin friction on pile elements should be duly assessed.
Reclaimed land is liable to significant subsidence due to long-term consolidation of underlying compressible material. All structures and foundations (including floor slabs, partitions, fence walls, ancillary structures, underground utilities and drainage) built on reclaimed land must be designed with due consideration of the effect of such significant subsidence.
Acceptance of estimated settlement and rotation of foundations should be considered on a case-by-case basis, as different structures will have different tolerance in accommodating movements of their foundations. The acceptable settlement and rotation for foundations should therefore be determined for each individual case with respect to integrity, stability and functionality of the supported structure.
Where differential settlement is anticipated, it should be assessed accurately or conservatively, and its effect on the supported structure should be checked to ensure that it is acceptable in respect of strength and serviceability.
(2) Reference Criteria
For buildings or structures not particularly sensitive to movement, the following movement criteria, evaluated at the base of a shallow foundation or
in case of a deep foundation, the base of pile cap, may be used as a reference for developing case specific criteria:
The maximum total settlement should not exceed 30 mm;
The differential settlement between columns / vertical elements should be limited to 1:500; and
The maximum angular rotation should not exceed 1:500 due to wind or other transient loads.
The above criteria should be assessed based on working loads. For criteria (a) and (b), the full dead loads should be considered, and the imposed loads may be reduced in accordance with the Code of Practice for Dead and Imposed Loads.
In general, criterion (a) could be deemed to be satisfied if the foundation rests directly on categories 1(a), 1(b), 1(c), 1(d) and 2 rock or if the foundation elements are driven to sound bearing strata with SPT N values ≥ 200.
Differential settlement should be considered in situations where its evaluation is considered necessary, for example, mixed foundation systems, piles with significant difference in lengths, substantial variation in the properties or depths of compressible strata under the foundations. The differential settlements should be properly controlled or appropriately catered for in the design of superstructure.
(3) Individual Case
Where the anticipated movement of the foundation is in excess of the reference criteria specified in (2) above, an assessment should be carried out to demonstrate that its effect:
will not cause or induce any overstress in the building or structures supported by the foundations, or in any nearby buildings, structures, or surrounding ground; and
will not cause any strength or serviceability problem either in the connections of services or utilities, or in the connections with the surrounding structures, pavements, streets or roads.
Prediction of settlement is an important part of foundation design to ensure the future stability and serviceability of the structure supported by the foundation. The prediction of settlement comprising immediate settlement, primary consolidation settlement and secondary consolidation settlement should be:
based on the results of a proper site investigation and appropriate laboratory or field tests identifying the conditions of the groundwater and the ground that contribute to the settlement of the foundation;
based on the principles of mechanics or established empirical methods proven with adequate correlation; and
applicable to Hong Kong soils and in conformity with case histories.
(2) Foundations on Granular Soils
Methods for computing immediate settlements of foundations on granular soils are based on theory of elasticity, empirical correlations or full-scale loading
tests. Empirical correlations between foundation settlement and results of insitu tests such as standard penetration tests generally provide an acceptable solution for estimating the settlement of a shallow foundation on granular soils. Based on the theory of elasticity, the settlement of a shallow foundation can be calculated using an equation of the following form:
S = qnet Bf ‘ Fo e E
s
Where
Se = immediate settlement
q net = mean net foundation bearing pressure (the net foundation bearing pressure is the total foundation bearing pressure less effective overburden pressure at the base of the foundation)
Bf’ = effective width of the foundation Es = Young’s modulus of soil
Fo = a coefficient whose value depends on the shape and dimensions of the foundation, the variation of soil stiffness with depth, the thickness of compressible strata, Poisson’s ratio, the distribution of ground bearing pressure and the point at which the settlement is calculated. Reference should be made to GEO Publication No. 1/2006 for determination.
(3) Foundations on Fine-Grained Soils
For fine-grained soils, immediate settlement may be estimated using the same equation for granular soils. In addition to the immediate settlement, consolidation settlement should also be considered. An estimate of the consolidation settlement can be made using the settlement-time curve obtained from oedometer tests or other sources of reference that suit the conditions of the site. Consolidation settlement may be considered to consist of primary consolidation and secondary consolidation stage.
The primary consolidation settlement of a soil layer due to an applied loading depends on the relative magnitudes of the initial vertical effective stress acting on the soil and the effective preconsolidation pressure, and can be estimated as follows:
For s
‘ = s
‘< s
‘ + Ds
S = H (CR log sn0 ‘ + Dsn )
n0 p n0 n p s
sn0 ‘
For s
‘ < s
‘ < s
‘ + Ds
S = H (CR log sn0 ‘ + Dsn + RR log
sp ‘ )
n0 p n0 n p s
sp ‘ sn0 ‘
For s
‘< s ‘ + Ds < s ‘ S = H (RR log sn0 ‘ + Dsn )
n0 n0 n p p s
sn0 ‘
Where
σv0′
=
initial vertical effective stress in the soil layer
σp’
=
effective preconsolidation pressure, which is the maximum vertical effective stress that has acted on the soil layer in the past and can be determined from laboratory oedometer tests
Δσv
=
change in vertical effective stress due to the fill and future imposed load on the soil layer to be considered
Sp
=
ultimate primary concerned
consolidation
settlement
of
the
layer
Hs
=
thickness of the soil layer to be considered
CR
=
compression ratio, equal to the slope of the virgin compression portion of the ε-logσ ‘ plot as shown in Figure 2.5
=
Cc 1 + e0
RR
=
recompression ratio, equal to the average slope of the recompression portion of the ε-logσ ‘ plot as shown in Figure 2.5
=
Cr 1 + e0
Cc
=
compression index which can be estimated from laboratory
oedometer tests
Cr
=
recompression index which can be estimated from laboratory
Allowable Vertical Bearing Pressure of Shallow Foundation founded on Soil
The allowable vertical bearing pressure of foundations founded on soils derived by bearing capacity equation may be taken as:
q = qu – qo + q
a F o
where qa = allowable vertical bearing pressure
qu = ultimate bearing capacity of the granular soil, which should be limited to 3,000kPa
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
F = factor of safety not less than 3
The ultimate bearing capacity of the soil for shallow foundation may be estimated by the following equation:
u
q = Qu
= c’ N ζ ζ ζ ζ
+ 0.5 B ‘ γ ‘ N ζ
ζ ζ ζ
q N ζ ζ ζ ζ
Bf ‘ Lf ‘
c cs ci ct cg
f s γ γs
γi gt γg
q qs qi qt qg
where Nc, Nγ, Nq = general bearing capacity factors which determine the capacity of a long strip footing
Qu = ultimate resistance against bearing capacity failure c’ = effective cohesion of soil
γs‘ = effective unit weight of soil
q = overburden pressure in the ground adjacent to the foundation and at same level as the base of the foundation (see Figure 2.2(a) for sloping ground)
Bf = least dimension of footing
Lf = longer dimension of footing
Bf‘ = Bf – 2eB
Lf‘ = Lf – 2eL
eL = eccentricity of load along L direction
eB = eccentricity of load along B direction 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 Notes:
A shallow foundation is taken as one in which the depth to the bottom of foundation is less than or equal to 3m.
q should not include any overburden pressure that may be temporarily or permanently removed during the design life of the foundation. In its derivation, the maximum effective overburden depth of subsoil should not be greater than Bf and suitable adjustments should be made to discount any voids that may be allowed for underground utilities.
Figure 2.2 shows the generalised loading and geometric parameters for the design of a shallow foundation and the bearing capacity factors are given in Table 2.3.
Any weak geological features present in the ground may affect the validity of the bearing capacity equation. Therefore the geological characteristics of the ground should be considered in the evaluation of the bearing capacity.
For shallow foundations on or near the crest of a slope, the ultimate bearing capacity may be obtained by linear interpolation between the value for the foundation resting at the edge of the slope and that at a distance of four times the foundation width from the crest. The latter may be assumed to be equal to that of a foundation placed on flat ground. Figure 2.3 summarizes the procedures for the linear interpolation. The effect of the foundation works on the overall stability of the slope should also be checked.
The bearing capacity equation is applicable to rectangular shaped shallow foundations. For shallow foundation of an irregular shape, the calculation may be based on the largest inscribed rectangle as shown in Figure 2.4.
The effective unit weight of the soilγs’ may be taken as follows:
Dry condition (see clause 1.2 for definition):
γs‘ = γ
whereγis the bulk unit weight of the soil
Submerged condition (see clause 1.2 for definition):
For static groundwater:
γs‘ = γ’
whereγ’is the submerged unit weight of the soil
For groundwater flows under an upward hydraulic gradient:
γs‘ = γ – γw (1 + ί)
where ί is the upward hydraulic gradient; and γw is the unit weight of water.
For intermediate groundwater levels, γ s‘ may be interpolated between the above limits.
The allowable capacity for soils and rocks may also be estimated by appropriate load testing of the foundation on site. The following should be considered when using this method:
the variation at founding conditions between the location of the testing foundation and locations of the actual foundations;
the duration of load application in the test as compared to the working life of the foundation; and
the scale effect of the test relative to the full size of the foundation.
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