Costes de los eslabones del Canal Tradicional

ABSTRACT

Pile foundation is a significant structural element in many structures. Selection of suitable pile type for different ground conditions is the most fundamental decision to be made by foundation designers. It is no doubt that minimizing the pile numbers under a given design loading is the prime goal of an ideal foundation design in terms of economics. The reduction of pile numbers and quantities also shortens the construction time and enhances sustainability, as well as improves safety during construction, especially in the case of complex underground conditions with construction constraints in a well-developed dense populated city like Hong Kong.

To achieve an economical foundation design, it is important to explore and develop innovative ideas, which may involve modifications from traditional foundation types and construction techniques. Foundation design in Hong Kong is required to comply with the standard design codes or specifications set out by different government authorities. The most optimized foundation layout may therefore be different, even under the same loading condition, if design requirements by different authorities are to be satisfied. This paper will focus on the discussion on current bored pile foundation design criteria under different design codes and specifications of different government authorities for private and public housing developments such as Buildings Department (BD), Hong Kong Housing Authority (HKHA), Architectural Services Department (ASD) and Highways Department (HyD). Further discussion on the feasibility of bored pile foundation modification satisfying different design standard is presented.

the end-bearing capacity and bond friction between rock and concrete. Pile shaft capacity is achieved by reinforced concrete pile shaft with steel cage.

2.1 Buildings Department (BD)

In Hong Kong, private developments need to be approved by the Buildings Department and the associated bored pile foundation design should comply with “Code of Practice for Foundation 2004 (COP Foundation 2004)” in general.

Some of the major current design criteria specified in COP Foundation 2004 are listed below:

• Allowable end-bearing capacity achieved by rock socket diameter or enlarge base (bell-out) diameter. Size of bell-out cannot exceed 1.5 times the pile shaft diameter.

• Allowable bond or friction capacity between rock and concrete. Design bond length for frictional resistance cannot exceed 2 times pile shaft diameter or 6.0m whichever is smaller.

• Minimum clear horizontal spacing of 500mm between shaft surfaces or edge of bell-outs should be provided.

• Adequate horizontal restraints in at least 2 directions shall be provided to individual piles or pile cap. • Pile and pile cap should not be used together to resist lateral forces.

• Either rigid cap or flexible cap design assumption can be adopted specified in COP Concrete 2004. Some design criteria commonly adopted in practice but not specified in the standard very clearly are as follows:

• Minimum steel ratio for both shear links and vertical bars for pile shaft. • Compression steel can be adopted for enhanced the pile shaft capacity. 2.2 Hong Kong Housing Authority (HKHA)

Most public housing developments are approved by Housing Department and their Independent Checking Unit (ICU), the associated bored pile (Type 3) foundation design should comply with “General Specification (GS) HKHA” in general. Most of design criteria comply with BS8004 and BS8110, but “Code of Practice for Foundations” published by BD has been adopted as a common standard nowadays except for the aspects discussed below.

Some of the major design criteria specified in GS which are not specified in COP Foundation 2004 are listed below:

• Allow only one type of pile in each pile cap.

• Maximum design displacement of pile caps and piles is 25 mm

• If the center to center spacing of two adjacent Type 3 piles is less than 2.5 times the diameter of the larger pile base and the difference in founding levels of these two piles is greater than the clear distance between their bases on plan, justification is necessary.

Some design criteria commonly adopted in practice but not specified in GS clearly are as follows: • Bond strength in type 3 piles is not specified.

2.3 Architectural Services Department (ASD)

Most government developments are approved by ASD and the associated bored pile foundation design comply with “ASD General Specification for Building 2007 (GS)” in general.

Some of the major current design criteria specified in the GS are described as follows:

• Do not use piles with enlarged bases unless specified otherwise. Allowable end-bearing capacity achieved by rock socket diameter or permitted enlarged base (bell-out) diameter. Size of permitted bell-out cannot exceed 1.5 times the shaft diameter.

• Minimum center to center spacing shall be 750mm or 2.0 times the least width whichever is the greater.

• Combination of load elements supported by one large pile cap shall not be allowed. (i.e. single column or wall support by a single pile cap only.)

• Piles shall not be positioned directly under any wall opening. • Tension piles shall not be permitted.

• Maximum design concrete strength shall not exceed 7.5MPa. • Minimum pile length shall be 5.0m.

• Founding levels of adjacent piles shall not differ by more than the clear distance between the pile bases.

• 0.4% of cross sectional area of bored pile as the nominal steel for bored pile.

• Either rigid cap or flexible cap design assumption can be adopted specified in COP Concrete 2004. Some design criteria commonly adopted in practice but not specified in the GS clearly are as follows:

• Bond stress between concrete and rock can be adopted for allowable pile capacity. • Compression steel can be adopted for enhanced the pile shaft capacity.

• Pile and pile cap should not be used together to resist lateral forces. 2.4 Highways Department (HyD)

Most government developments in major highways, roads and bridges should be approved by HyD and the associated bored pile foundation design should comply with “Structures Design Manual for Highways and Railways (SDM)” in general. SDM has specified that the foundation design should comply with BS5400 and it also specifies that foundation design should comply with BS8004, and the associated reinforcement design should comply with BS8110. Since the mentioned BS standards just describe the design principles instead of very detailed design criteria as in documents issued BD, ASD and HKHA as mentioned above, therefore foundation design in HyD’s project are very flexible and very much rely on the discussion between foundation designers and authority.

3 DISCUSSION

Typical design criteria for bored pile foundation under different authorities has been briefly introduced in the above sections and the summary that may influence the piling layout are listed in Table 1.

According to Table 1, different numbers of piles may be required to satisfy different authority’s criteria even though the magnitude of loading and ground conditions are exactly the same. These criteria are the main factors which will influence the pile numbers.

Consider the minimum design pile center to center spacing, for a group of 3.0m diameter bored piles, minimum pile spacing is 3.50m under COP Foundation 2004, but 6.0m is required under ASD.

Allowable bearing capacity for a bored pile with 3.0m shaft diameter and a maximum 4.50m bell-out diameter resting on grade III/II rock (5.0MPa) is 79,521kN under COP Foundation 2004, but due to the limitation of no bell-out diameter is allowed under ASD, the allowable bearing capacity is just 35,342kN. This means that only 44% of BD’s criteria can be achieved. Allowable bond strength between rock and concrete per meter rock socket length for a 3.0m shaft diameter bored pile resting on grade III/II rock (5.0MPa) is 6,597kN under COP Foundation 2004, but NIL is allowed under ASD. Therefore, allowable design pile capacity by end bearing plus bond strength between rock and concrete for a 3.0m shaft diameter with maximum 4.50m bell-out diameter bored pile resting on grade III/II rock (5.0MPa) is 86,118kN under COP Foundation 2004, but due to the limitation under ASD, it is just 35,342kN, that is only 41% of BD’s criteria can be achieved.

Table 1: Major Design Criteria for Bored Pile Foundation Authority Pile Spacing Size of

Enlarged Bell- out

Allowance to Adopt Bond Strength between Rock & Concrete

Maximum Design Concrete Strength

Piling Layout Restraint

BD (COP Foundation 2004) 500mm between shaft surfaces or edge of bell- outs cannot exceed 1.5 times the shaft diameter 700kPa; bond length to be 2 pile shaft diameters or 6.0m whichever is smaller Not specified

at least 2 directions shall be provided to individual piles or pile cap ASD (General Specification for Building 2007) 750mm or 2 times the least

width whichever is the greater enlarged base is not allowed unless specified

Not specified shall not exceed 7.5MPa

a. Single column or wall support by single pile cap only

b. piles shall be positioned under wall opening c. Tension pile is not

allowed HKHA

(General Specification)

Same as BD Same as BD Not specified Same as BD a. Allow only one pile type in one pile cap b. Maximum horizontal

displacement of 25mm HyD

(SDM, BS8004)

Not specified Not specified Not specified Not specified

Not specified

Concrete strength for 45D concrete is 9.0MPa under COP Foundation 2004 that is 63,617kN in terms of pile shaft capacity for a 3.0m diameter bored pile. But the maximum concrete strength under general specification ASD is 7.5MPa only. This implies 53,014kN in a 3.0m diameter pile shaft capacity, and just 83% of BD’s criteria can be achieved.

Based on the explanation of above selective items extracted from Table 1, it seems that bored pile capacity cannot be fully utilized and it will affect the piling layout in terms of minimum pile spacing and pile numbers as well which are summarized as shown in Table 2.

Table 2: Comparison of Bored Pile Capacity Between ASD and BD Authority* Shaft Dia. / Bell-out Dia. (m) Max. Concrete Strength For 45D (MPa) Founding Criteria (MPa) Pile Shaft Capacity (kN) Bell-out Capacity (kN) Bond Strength between Rock & Concrete per meter (kN) Design Pile Capacity (Bearing + Friction) (kN) Min. Pile Spacing (m) ASD 3.0 / 3.0 7.50 5.0 53,014 35,342 0 35,342 6.00 BD 3.0 / 4.5 9.00 5.0 63,617 79,521 6,597 86,118 5.00

According to Table 2, it can be seen that the piling layout including pile numbers would be significant different under different requirements even when the magnitude loading of the superstructure are exactly the same. Apart from pile capacity, piling layout restraint as mentioned in Table 1 is also a factor that influences pile numbers, for instance SDM does not describe the foundation design very specifically, and therefore preparation of initial proposal for HyD’s projects may involve a lot of assumptions made by the designer instead of following very clear guidelines. For example at least 2 directions shall be provided to individual piles or pile cap is a design criteria under BD, but it seems not appropriate to apply under bridge structure since a certain length distance between bridge piers, and therefore long tie beam or strap beam between two piers is not effective, and to fulfill BD’s design criteria, 3 piles group must be adopted instead of 2 piles

group, and as a result leading to over design. According to the above discussion, it is highly desirable to reappraise piling design by unifying design standards to achieve the best economy.

4 RECOMMENDATION

Apart from unifying design standards as described above, practical operation and site considerations are also important factors that need to be taken into account. According to common practice in Hong Kong, due to mechanical restriction, maximum 3.0m shaft diameter of bored pile can be constructed normally. In order to enhance the bored pile capacity based on fundamental design principle, enlarging shaft diameter by rolling a larger steel casing is the most ideal direction, pile capacity will be increased about 7.0% by increasing every 100mm diameter in general. But a chain consideration from construction point of view is that a bigger lifting crane and RCD would require extra working area and headroom for the new machinery, and therefore should be a bigger concern in comparison with design considerations (see Plates 1, 2 & 3 below).

Plate 1: Steel casing for bored pile Plate 2: Reverse circulation drill rig (RCD)

Plate 3: Reverse circulation drill rig (RCD)

Maximum design bell-out diameter as 1.5 times bored pile shaft diameter is commonly adopted in Hong Kong practice, which limits the maximum diameter of the bell-out to 4.50m for a pile with 3.0m shaft diameter. But bell-out bit or trimming cutter currently used in Hong Kong can achieve more than 4.50m

diameter of bell-out, and the bearing capacity can be increased about 4.5% by increasing every 100mm diameter in general.

Allowable vertical bearing pressure could be higher than the presumed values suggested by COP Foundation 2004 by demonstration under a trial pile test. This approach has been adopted in a current project C801 Express Rail Line – Detailed Design for West Kowloon Terminus and the advantage is upgrading the allowable bearing capacity for grade III rock from 5.0MPa to 7.5MPa, representing an increase of allowable bearing capacity for bored pile by 50%.

5 CONCLUSIONS

The conclusion of this paper can be drawn as follows:

• Bored pile foundation is a common and effective foundation type in Hong Kong.

• Bored pile foundation design in Hong Kong requires to comply with standard design codes or specifications set out by different government authorities, and the most optimized foundation layout may therefore be different, even under the same loading condition.

• It is high time to reappraise the overall design approach for achieving the prime goal of an ideal foundation design in terms of economics by minimizing the amount of piles numbers, and reducing pile numbers through maximising the pile capacity.

• Recommend to unify various standards in order to prevent any unnecessary conservative design. • Innovative idea of enhancing bored pile capacity by rolling larger steel casing diameter (larger than

3.0m) should be considered.

• Innovative idea of relaxing the design requirement of limiting the maximum bell-out diameter to 1.5 times shaft diameter should be considered.

• Innovative idea of enhancing the allowable bearing pressure for rock category by trial pile test should be encouraged.

REFERENCES

Buildings Department 2004. Code of Practice for Foundations. Building Department, HKSAR Government, Hong Kong.

Architectural Services Department 2007. General Specification for Building 2007 Edition. Architectural Services Department, HKSAR Government, Hong Kong.

Highways Department, Structures Design Manual for Highways and Railways. Highways Department, Government of the Hong Kong Special Administrative Region.

Hong Kong Housing Authority 2008, General Specification 2008 Edition. Hong Kong Housing Authority, HKSAR Government, Hong Kong.

Civil Engineering and Building Structures Standards Committee. BS8004: 1986 Code of Practice For Foundations, Board of BSI

Civil Engineering and Building Structures Standards Committee. BS8110: 1997 Structural Use of Concrete, Board of BSI

Civil Engineering and Building Structures Standards Committee. BS5400: 1998 Steel, Concrete and Composite Bridges, Board of BSI

Chris Cheung, John Mennis, Keith Chong. Proposed Site Specific Allowable Vertical Bearing Pressure of Bedrock in project C801 Express Rail Line – Detailed Design for West Kowloon Terminus. AECOM

1 INTRODUCTION

The geology of Hong Kong Special Administrative Region (Hong Kong) is dominated by fill, marine deposit, colluvium and alluvium underlain by completely decomposed saprolites and solid granitic and volcanic bedrock. In some areas, an extra-ordinary deep bedrock poses a construction challenge to the conventional large diameter bored pile bearing on the rock and the pre-bored H-piles socketed into rock to support high-rise structures. The alternative is to use friction piles. However, in view of the limited shaft capacity of plain piles, the concept of shaft-grouting to enhance the shaft frictional capacity has been introduced to Hong Kong since 1990s and has been applied to friction bored piles and barrettes for site with weak saprolites. This technique has recently been extended to areas with the geology dominated by meta-siltstone/sandstone formation underlying by marble bedrock beyond 150m below ground. In recent years, the urban redevelopment has rapidly expanded in Hong Kong to satisfy the local demand and progressively more high-rise buildings are built within congested urban sites with works area and access constraints. In this regard, small diameter friction piles enhanced by the shaft-grouting become viable foundation options for sites characterized by extra-ordinary deep bedrocks.

2 A REVIEW OF SHAFT-GROUTING EXPERIENCE

The shaft-grouting technique was traditionally used for large diameter friction bored piles and friction barrettes for sites with weak saprolites and extra-ordinary deep bedrocks where the installation of the conventional driven piles and end bearing piles were found to be technically challenging. There were a few reported cases of using shaft-grouted piles in Hong Kong. The full-scale load testing programme for the KCRC West Rail Phase I reported by Plumbridge et al (2000) demonstrated that the shaft friction capacities measured for shaft-grouted barrettes and bored piles in completely decomposed granite (CDG) and volcanic (CDV) achieved up to a two or three fold increase in comparison with those without shaft-grouting. The use of shaft-grouted barrette in MTRC Kowloon Station Mega Tower Development reported by Chan et al (2004b) and the private residential developments at Tung Chung Designated Area reported by Sze et al (2007) also demonstrated the enhancement of shaft friction by shaft-grouting technique in sandy alluvium, completely decomposed rhyolite (CDR), CDG and diamict deposit (DD).

The shaft-grouting technique was also expanded to small diameter piles. In traditional design of small