Foundations in Limestone Areas of Peninsular Malaysia
Foundations in Limestone Areas of Peninsular Malaysia
Foundations in Limestone Areas of Peninsular Malaysia
ABSTRACT
GEOLOGY
Limestone formation is widespread in Peninsular Malaysia and the world. Sowers (1976)
reveals that it comprises more than 10% of the earth crust by volume. In Peninsular Malaysia,
the major limestone areas are Langkawi, Kedah-Perlis, Kuala Lumpur, Kinta Valley and Gua
Musang. In Kuala Lumpur (capital of Malaysia), about one third of the area is on limestone
formation. The limestone formation in Peninsular Malaysia is of Ordovician to Triassic age.
In Peninsular Malaysia, almost all limestone are re-crystallised and technically referred to as
marble. Since their formation, the limestone has been subjected to high pressure and temperature
accompanying the regional metamorphism and granite intrusion. Generally, the limestone is
white, pale grey or slightly yellowish, fine to coarse re-crystallised rock. In some places, it is
dark to almost black because of carbonaceous or argillaceous impurity. However, the limestone
in Peninsular Malaysia is remarkably pure.
Karst refers to a characteristic topographic feature or landscape which can be developed by rock
undergoing dissolution by downward percolating meteoric water (Jakucs 1977). Several rock
types under such natural “weathering/solution” environment can develop karstic topography.
However, the most common are those developed by carbonates (calcite, CaCO3 and dolomite,
[Mg,Ca]CO3).
In Peninsular Malaysia, under tropical humid conditions, calcite and dolomite limestones or
their metamorphised equivalents develop tropical features which show spectacular tall steep-
sided hills (tower karst or mogote) (Jennings 1982) and solution features such as pinnacles,
sinkholes and cavities.
The treacherous and almost unpredictable karstic bedrock associated with extremely variable
overburden soil properties is a typical feature of limestone (Yeap 1985), which lead to a variety
of geotechnical problems and hazards. Some of these common engineering problems are
discussed below:-
Pinnacles
Pinnacles are columns or cones of limestone or marbles left by dissolution of the surrounding
rock. Pinnacles with soft or loose overburden immediately above them pose tremendous
challenge to engineers to ensure proper seating of piles on the rock particularly, driven concrete
piles.
Subsidence
Sinkhole
This incident is a common phenomenon in the karst areas, which is covered by loose and
non-cohesive sands over limestone bedrock. It is commonly known that limestone can be
dissolved by acidic solution from rain or polluted groundwater. After certain period, flow or
penetration of groundwater, through weak zone in limestone will develop channels and voids.
These channels will act as passages for water together with the loose sand to flow into voids and
cavities in the limestone. The movement of sand into existing cavities or voids will then develop
empty spaces in the sand layer where arching occurred. This continuous process will reach a
critical stage where the roof of the space will no longer support the weight of the overburden.
This will result in the occurrence of sinkholes or cave-in. Sinkholes are hazards to both shallow
and deep foundations as their formation or emergence is much more sudden and catastrophic.
Slump zone
Zones of weakness often occur immediately above the bedrock of limestone. The slump zone
above limestone formation is usually identified by the very low SPT ‘N’ values or low cone
resistance where SPT ’N’ values of zero are often detected. The formation is either due to
subsurface erosion as a result of overburden material slumping into cavities in the limestone or
residual of weathering of ancient karst features with their resolution channels (Tan & Ch’ng
1986).
Caverns/Cavities
Cavities are voids form by dissolution of the rock in the limestone which will pose problems
if the roof of cavities are not of sufficient strength to support the foundation resting on them,
especially for empty or partially filled cavities.
GEOTECHNICAL INVESTIGATION
FOUNDATION
Bored piles and barrettes are generally used for highrise buildings. The size of bored piles
ranges between 600 mm to 1500mm diameter although 2200mm have been installed in Malaysia.
The maximum depth of installed bored pile is 82m for 1500mm diameter. Barrettes are
sometimes used in thick overburden where bored pile machines have difficulty in reaching its
required depth.
The following types of foundations are normally used for low-rise buildings:-
- Footing on raft directly on limestone
- Footing with piles such as precast concrete, steel, bored and barrette as well as
micropiles
- Raft with and without piles
Footings directly on limestone are usually selected for locations with shallow bedrock.
Cavity probing to check the presence of cavities and their effects on the foundation must be
analysed to ensure safety and stability of the foundation. Very often treatment to localised
depression on bedrock by concreting or micropiles are needed.
Different types of piles including precast reinforced and prestressed concrete piles have been
used in limestone formation. However, the geotechnical load carrying capacity is very much
lower than the structural capacity of the pile because of the uncertainly on the bedrock profile
and presence of cavities, solution channels, overhangs etc. In areas where the overburden soils
are soft or loose, Oslo point rock shoe is required to prevent pile deflection during installation on
contact with inclined rock surface as shown in Fig 1a. The hardness of the harden steel used for
Oslo point rock shoe as shown in Fig. 1b should be larger than 300 (Brinell Hardness) and the
yield strength of rock shoe should not be less than 760 MPa. The main purpose of the shoe is to
reduce the bending stress generated during driving when the pile toe is in contact with inclined
rock surface. The rock shoe shall be designed to take the full required load at the contact and
extra care should be taken in the construction to prevent altering its property, in particular, by
welding.
DESIGN
The allowable bearing pressure on limestone imposed by the footing at shallow depth is
based on the strength of the rock and its discontinuity. The design curves of BS 8004, which
limit the maximum allowable bearing pressure of up to 10 MN/m2 can be used. Alternatively, a
simple approach recommended by Canadian Geotechnical Society (1992) could also be used.
The strength of the limestone is generally ranges between 30MPa to 100MPa and mostly
between 40MPa and 60MPa. Settlement check is also required especially between two column
loads of large difference.
The influence of the roof thickness of cavities and their dimensions on the footing should be
investigated and analysed to prevent induced collapse of the roof. The allowable geotechnical
capacity of driven piles is limited usually by the allowable capacity of the Oslo rock shoe which
is slightly socketed into the limestone. For precast concrete piles, they normally should not
exceed 75% of the allowable structural capacity.
The skin friction along the pile shaft could be considered to reduce the end bearing pressure
on piles. Smaller pile should be preferred as it allows higher redundancy. In areas where doubt
of potential collapse of roofs of cavities, further redundancy is needed by having deep ground
beams to distribute the loads when one failed, otherwise the cavities should be treated.
The bored piles, barrettes and micropiles are designed mainly based on friction in soils and
rock socket. The base resistance should be ignored for the following reasons:-
- The base is practically very difficult or impossible to clean or remove because of
the erratic nature of the limestone bedrock and often loose sand overburden as
illustrated in Fig. 2
- To reduce base pressure on potential undetected thin roof of cavities
The easier way to design the above piles is based on the commonly used SPT (Standard
Penetration Test) ‘N’-value and the detailed classification of the soil. The friction resistance
between the concrete and the soil varies from 1.5N (kN/m2) to 2.5N depending on the SPT N-
values and types of materials. For N-values exceeding 100, a further reduction on skin friction
should be imposed.
The shaft friction between pile and limestone is generally based on the rock quality in terms
of unconfined compression strength (qu), Rock Quality Description (RQD) and method of
forming the socket. Coring would give a high friction. The allowance friction resistance should
be limited to 2.5% of the unconfined compressible strength. The lower of the unconfined
compressive strength of rock or concrete should be used unless confirmed by instrumented pile
tests. The value is reasonable when compared with some commonly used allowable adhesion of
about 0.05 qu for other sound rocks.
The socket friction is significantly higher when coring is used instead of chiselling technique.
Neoh (1997) has reported an ultimate socket friction of more than 3.8 MPa for micropiles using
air flushed down-the-hole percussion hammer in limestone having RQD of more than 90% and
the ultimate socket friction of 1.5MPa in limestone having RQD of 0-4%.
The presence of cavities and their influence on the performance of the foundation
particularly, in relation to potential collapse of cavities, must be analysed. An example is shown
in the Fig. 3.
CONSTRUCTION
Understanding of the design is important to ensure successful installation of piles for their
intended purposes. Adequate site supervision from consultant and input from the design team
throughout the construction are essential particularly for the limestone formation. A supervision
plan should be drawn up by the design team to guide the supervision team for the pile installation
and construction control. The purpose of the plan is to highlight the critical components of the
pile installation and construction control together with its checklist.
A qualified and experienced team should be assembled for competent supervision. Assess
the construction methods with the actual ground conditions and possible variation within the site.
It is also necessary to verify that the design assumptions are compatible with those used in the
designs.
The piling platform has advantage to be levelled and overlaid with a layer of lean concrete
especially on the cohesive materials to have a good working platform for the movement of plant
and machinery over a site especially during monsoon seasons. The lean concrete platform has
the benefit of ensuring minimum down time after each rain and enhances pile installation to
achieve the normal pile verticality especially during monsoon seasons.
The damage of concrete pile during installation is very common especially when the
overburden consists of loose or soft soils as illustrated in Fig 1a. Continuing piling of a tilted
pile would break the pile. Damaged concrete piles as high as 25% to 40% have been reported by
Omar (1985), Neoh (1997) and Ting & Ladchumanan (1974). Sehested and Wong (1985)
reported the use of rock shoe (Oslo Point) reduces the damage of H-piles significantly.
A recent project undertaken by the author has indicated the percentage of damaged piles
has been controlled to 1.5% from the initial 15%. The case involves some 5000 piles points of
reinforced concrete piles of 250m x 250m and 300m x 300m with a length variation from 17m to
76m and the average of 30m.
Damage of piles during installation could easily pass undetected and resulted in failures
of superstructures due to excessive settlement of the columns supported by piles that had
achieved set or refusal and subsequently deflected. Detection becomes difficult as it is a
common perception to accept piles of great variation in length, some even varies by tens of
metres between two adjacent piles. Hence full-time competent site supervision is a must
to detect the common signs of pile damage such as deviation, tilting, rotation and set of refusal
follows by further penetration.
The damage is usually more significant when diesel hammer is used. This is because the
energy of subsequent impact is greater upon the preceding blow near or at refusal. Diesel
hammer should be avoided unless the overburden consists of a reasonable thick layer of medium
dense sand or stiff soils above the bedrock to buffer the high impact of the hammer.
The use of large strain dynamic pile test to calibrate the permissible drop height to
prevent damage is most useful in ensuring the success of pile installation for driven piles and
also to serve as a useful tool for quality control during the pile installation and detection of
damage piles. In certain cases, the drop height has to be reduced to 100mm and tapping in for
proper seating to prevent damage of concrete pile with an Oslo point. The short term maximum
compressive and tensile stresses should be limited to 0.8 fcu and 0.1 fcu respectively. The above
case with only 1.5% of damaged piles was achievable with competent site supervision input from
the design team. The percentage of large strain dynamic pile test was reduced from the initial
10% of the piles to 5% subsequently.
Although the design of bored piles, barrets and micropiles generally ignores the base
resistance, adequate full-time and proper site supervision is still much needed to ensure the
design socket length is achieved. Otherwise the socket length may be significantly reduced by
accumulation of the soils at the base of the pile from the in-filled cavity or collapsible soil above
the toe even though temporary casing as shown in Fig. 2 is used.
The figure also shows that casing is unable to totally protect the collapsible soils from
entering the bored hole due to the uneven bedrock unless the material at and around the casing is
jet grouted. This is of course seldom done, simply because the casing would have to be left in
place. The loss of stabilizing fluid during drilling and grouting/concreting is common in
limestone with fissures, channels or cavities. In this situation, pre-grouting in stages has shown
to be effective.
Overbreak for micropiles is difficult to estimate especially in fractured limestone or
limestone with fissures because the significant loss of grout through fissures. The overbreak for
bored piles usually varies 10% to 40%.
CONCLUSIONS
Foundations in limestone need extra knowledge and special skill in the investigation,
analysis, design and installation. Geotechnical investigation is usually carried out in three
stages, namely preliminary, detailed and during construction.
In addition to the conventional boreholes, geophysical method for profiling and
detecting problematic areas such as slump zones, cavities and channels are particularly useful for
a big site. Boreholes should extend beyond the stress influence zone. In most cases, the
boreholes should extend at least 10m beyond the cavity free zone. For highrise structures, some
boreholes should extend to a depth of 100m and significantly beyond the influence for block
failures.
The analysis of foundation particularly highrise structure should go beyond the analysis
and design of single or group piles effect. The influence of cavities and potential of collapsed
roof of cavities should be analysed. Cavities that would affect the performance of the foundation
should be treated by compaction grouting or structural bridging.
In addition to the required skill and knowledge of the construction team, plant and
equipment, full-time site supervision by the experience and competent staff and input from
design team is imperative for the construction to reduce wastage and damage. Hence, delay
during construction could be eliminated, particularly driven piles in limestone formation to
identify damage due to ‘kicking’ of piles when they strike the inclined bedrock with loose or soft
overburden.
Special care should also be taken for bored piles, barrettes and micropiles to prevent
excessive accumulation of collapsible soil forming soft toe at the base of pile and reduces its
socket length.
In addition to the normal confirmation tests for piles, bored piles and barrettes should
have provision and pre-installed tubes to confirm the required socket length by coring later.
Acknowledgement
The author acknowledges and thank the staff of Gue & Partners Sdn. Bhd., for their
contribution and assistance particularly Ir. Liew Shaw Shong, Ir. Tan Yean Chin and Encik Zukri
bin Mat Noor (SSP Geotechnics Sdn Bhd) for the examples used in this paper, and Ms. Barbara
Ng for the many series of word processing.
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