6. DESIGN OF SINGLE PILES AND DEFORMATION OF PILES
6.11 PILE DESIGN IN KARST MARBLE
The design of piles founded in karst marble requires consideration of the karst morphology, loading intensity and layout of load bearing elements. The main problem affecting the design is the presence of overhangs and cavities, which may or may not be infilled. The stability of the piled foundation will depend on the particular geometry of such karst features, and the rock mass properties, particularly of the discontinuities.
McNicholl et al (1989b) reported the presence of a weak, structureless soil layer above the marble rock surface in the Tin Shui Wai area and suggested that this might have been affected by slumping and movement of fines into the underlying cavities. Mitchell (1985) reported similar findings in Malaysia. The significance of this weaker material on the pile design should be carefully considered.
Chan et al (1994) proposed a system for classifying the marble rock mass in Hong Kong. An index termed Marble Quality Designation (MQD) is put forward. This index is a combined measure of the degree of dissolution voids, and the physical and mechanical implications of fractures or a cavity-affected rock mass (Figure 6.22). The marble rock mass is classified in terms of MQD values. This marble rock mass classification system is used in the interpretation of the karst morphology, and offers a useful means for site zoning in terms of the degree of difficulties involved in the design and construction of foundations. A summary of the proposed classification system, together with comments on its engineering significance, is given in Table 6.7. An approach to the design of piles on karst marble in Hong Kong, which makes use of the classification system, is described by Ho et al (1994).
Foundations on karst marble in Yuen Long and Ma On Shan areas have successfully been constructed using bored piles, steel H-piles and small-diameter cast-in-place piles.
However, it must be stressed that no simple design rules exist which could overcome all the potential problems associated with karst formation.
Large-diameter bored piles are usually designed as end-bearing piles founded on sound marble that has not been or is only slightly affected by dissolution, such as rock mass with Marble Class I or II. The founding level of the piles and allowable bearing pressure of the marble beneath the pile base should be assessed taking into consideration the sizes and distribution of dissolution and the increase of stresses due to foundation load. The assessment of the allowable bearing pressure of volcaniclastic rocks should take into account any honeycomb structure as a result of preferential weathering of marble clasts.
The concept of 'angle of dispersion' is sometimes used to determine the founding level of end-bearing piles (Chan, 1996). This concept requires that there should be no major cavities within a zone below the pile base as defined by a cone of a given angle to the vertical, within which sensible increase in vertical stress would be confined. This approach is acceptable as an aid to judgement in pile design. Careful consideration should be given to the nature and extent of the adverse karst features and of their positions, in plan and elevation, in relation to nearby piles and to the foundation as a whole, together with the quality of the intervening rock.
Total Cavity Height (m)
Note : At the rockhead, where the top section is shorter than 5 m but longer than or equal to 3 m, the MQD is calculated for the actual length and designated as a full 5 m section. If the top section is shorter than 3 m, it is to be grouped into the section below. Likewise, the end section is grouped into the section above if it is shorter than 3m.
Figure 6.22 - Definition of Marble Quality Designation (MQD)
Marble Quality Designation, MQD (%)
0 1 2 3 4 5 10
0 25 50 75 100
90%
75%
50%
Average RQD = 25%
I
II
III
IV
V
Maximum possible length of cavities in 5 m core
RQD1
L2(mPD) L1(mPD)
l3
RQD2
RQD3
l2
l1
Average RQD = L2
Σ RQDi li L1
L1 – L2
Marble rock recovery ratio (MR)
L2
Σ li L1
L1 – L2
=
where L1-L2 usually = 5m MQD = Average RQD x MR
Marble Class
Zero marble rock core either cavity or decomposed non-marble rock
Table 6.7 – Classification of Marble (Chan, 1994a) 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 : (1) 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.
(2) 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 quality of adjacent sections and its proximity to the proposed foundations.
Domanski et al (2002) reported the use of shaft-grouted large-diameter bored piles socketed in a marble formation. The formation contains a series of small cavities with infilled materials, and is generally without significant voids. Grouting was carried out in two stages. The grouting at the pre-treatment stage was used to increase the strength of infill materials in the cavities. It also prevented the chances of excessive loss of bentonite during subsequent bored pile excavation. After casting the pile, post-grouting was applied in the second stage to enhance the shaft resistance. Results of pile loading tests indicated that the ultimate shaft resistance could reach 970 kPa, which is comparable to the shaft resistance measured in piles socketed in other types of rock.
For driven steel H-piles, they are commonly designed to be driven to sound marble, such as rock mass with Marble Class I or II. Despite the requirement of hard driving, there are chances that the driven piles can be affected by karst features beneath the pile toe or damaged during driving. A pile redundancy is provided for these uncertainties (GEO, 2005).
No definite guidelines can be given for the percentage of redundancy as this depends on the extent, nature and geological background of the karst features and the type of pile. Each site must be considered on its own merits. Some discussion on the consideration of redundancy factors (i.e. the factor by which the pile capacity is reduced) is given by Chan (1994a).
Where redundant piles are provided for possible load redistribution, the effect of this possible re-distribution should be considered in the design of the pile cap. Where the foundation consists of a number of pile caps rather than the usual single raft, it may be necessary to increase the redundancy, and to ensure adequate load transfer capacity between the pile caps by means of inter-connecting ground beams.
Pre-boring may be used if the piles have to penetrate overhangs or roofs and install at great depths. In such circumstances, the piles are less likely to be underlain by karst features and the pile redundancy can be adjusted accordingly.
The final set for driven piles on marble bedrock is usually limited to not greater than 10 mm in the last ten blows. Past experience indicated that such a hard driving criterion may result in pile damage. It is prudent to measure the driving stress when taking the final set of the piles. Li & Lam (2001) reported other termination criteria that had been used successfully for seating piles on a marble surface. These included 30 mm per 30 blows and 25 mm per 17 blows. Chan (1996) discussed the forms of blow count records that indicate possible damage of installed piles. Blow counts should be recorded for every 500 mm penetration when the driving is easy and every 100 mm penetration when the driving is hard (e.g. penetration rate smaller than 100 mm for every 10 blows).
Due to the uncertainty and variability of karst features in marble and the requirement of hard driving, non-destructive tests should be carried out to ensure the integrity of installed driven piles. The Code of Practice for Foundations (BD, 2004a) requires 10% of installed piles that are driven to bedrock to be checked by Pile Driving Analyzer (PDA). A higher percentage should be used on sites underlain by marble. Kwong et al (2000) reviewed some piling projects in the Ma On Shan area. The percentage of installed driven piles subject to PDA tests ranged between 12% and 28%. Piles might rebound from the hammer impact when they are driven hard against the marble bedrock. This could lead to extra settlement in static pile loading tests. In such case, re-tapping of the piles may be necessary to avoid the extra settlement.
For driven piles that are sitting on surface karst, it may be prudent to carry out re- strike test of the installed piles. This is to ensure that the marble supporting the installed piles does not collapse or become weakened due to the driving and setting of piles in the vicinity.
A performance review of foundation construction is usually required for piling works on sites underlain by marble (ETWB, 2004). This should include a review of the ground conditions experienced during pile driving, pile installation or foundation construction, and an assessment of pile driving or construction records. Blake et al (2000) described the design and construction problems encountered for driving piles at Ma On Shan and the mitigation measures taken after reviewing the piling records. In the performance review, pile caps were re-analysed using grillage models with the actual length of piles. Additional piles were installed to maintain the local redundancy where piles were found to be damaged. The verticality of driven piles was measured with inclinometers attached to the steel H-sections.
They observed that the majority of the piles were deflected from the vertical alignment on contact with marble surface. A minimum radius of curvature of 23 m was measured in one case. Despite the observed deflection, the load-carrying capacity of the pile was not adversely affected when it was load-tested.
Small-diameter cast-in-place piles 'floating' in the soil strata well above the top of marble surface have also been used. They are mostly for low-rise buildings such as school blocks, whose superstructure loads are comparatively smaller. There were a few occasions where such a foundation system was designed to support up to 15-storey high building (Wong & Tse, 2001). The design for a 'floating' foundation usually allows the spreading of foundation loads in the soil and limits the increase of vertical effective stress on the marble surface to a small value, so as to prevent the collapse of any cavities due to the imposition of foundation loads. Meigh (1991) suggested the allowable limit of increase in vertical effective stress in marble affected by different degree of dissolution features (Table 6.8). Alternatively, the allowable increase of vertical effective stress can be determined by a rational design
approach to demonstrate that the deformation of the marble rock and the infilled materials within cavities would not adversely affect the performance of the foundation.
Table 6.8 – Limits on Increase of Vertical Effective Stress on Marble Surface (Meigh, 1991) Site Classification(1) Limits on Increase of Vertical Effective
Stress at Marble Surface
A Design controlled by settlement in soil stratum
B 5 – 10 %
C 3 – 5 %
D < 3 %
Note : (1) Site classification is based on Chan (1994a).
Chan (1996) highlighted the difficulties in using numerical tools to predict the bearing capacity of rock mass over a dissolution feature or adjacent to a pinnacle or cliff because of the lack of understanding of the extent and conditions of the dissolution features and the degree of dissolution along the joint system. This remains the case despite recent advancement in the degree of sophistication of numerical modelling. A pragmatic approach using simple calculations, rules of good practice and engineering judgement remains the best available solution in designing pile foundations in marble.
For local areas with adverse karst features, it may be feasible to design a thickened pile cap to cantilever from or span across the problematic area, provided that the outline of the area is well defined by site investigation.