GUIDELINES FOR REPLACEMENT OF BAKAU PILES FOR COMMON DID STRUCTURES

Bakau pile is widely used in Singapore primarily to support small box culvert. There are however cases whereby Bakau pile is used to reinforce the ground below a raft or footing for a small building a few storey high. The Bakau pile is essentially the trunk of a tree, with branches trim off. It is not treated. The bark of the trunk are still attached and it is not perfectly straight i.e. in its natural form. It is cheap and easy to install. However, it is only used in soft soil such as marine clay, peatly clay etc. I think it will have problem going thru' soil with SPT N value of 10 or thereabout depending on actual site condition. The Bakau pile commonly used are 75 mm in diameter and 5m in length. For installation beyond 5m, the piles are joined using a metal sheath and nails. Bakau pile are rarely (if ever) installed more than 10 m in length. The rule of the thumb we use here is a capacity of 1 metric ton per Bakau pile. There are some indicative values for the dry stresses and moduli of elasticity for Bakau wood. The values are as follows: Bending and tension parallel to grain - 7.24 Compression parallel to grain - 5.52 Compression perpendicular to grain -0.55 Shear parallel to grain - 0.62 Mean Modulus of elasticity - 9300 Minimum Modulus of elasticity - 5500 (all units in N/mm2) Note: 1)the values are indicative and varies from pile to pile 2)the valuse are for a moisture content not exceeding 19%

GUIDELINES FOR REPLACEMENT OF BAKAU PILES FOR COMMON DID STRUCTURES

Introduction Bakau piles have been extensively used in DID works especially in the soft marine clay areas. However, indiscriminate felling of the bakau has threatened the mangrove environment to an extent that a review on the use of bakau is necessary. DID, in response to the call to preserve the mangrove environment considered the possibility of using other alternatives in areas where traditionally bakau piles are used. Current findings however, showed that the replacement of bakau bakau piles with timber or concrete piles would generally result in higher cost of a structure. This is due to the relatively high price of the latter in spite of the smaller numbers required. The law of supply and demand will also affect the price of timber and concrete piles. A complete ban of bakau piles may cause a sudden surge in demand on the timber and concrete piles and therefore a drastic increase in the prices. This may even cause the problem of supply to meet the demand. Therefore, a complete ban is not advisable. The use of bakau piles would still be allowed in certain types of works such as river training and bank protection works for rivers, drains, canals and coastal bunds. Due to budgetary or technical constraints, the use of bakau piles for other structures can still be allowed subject to the approval of the Head of the relevant functional division. Similarly, the replacement of bakau piles may have to be in stages depending on the depending on the budget allowed. The following provides guidelines on the replacement of bakau piles with other commercially available piles, for common DID structures. Replacement of Bakau Piles by Timber or Concrete Piles New DID structures are designed either from first principles or using adapted standard/available plans. For both cases, the bearing capacity of the foundation need to be checked against the expected loading from the structures. Generally, if the bearing capacity is smaller than twice the expected loading for granular soils and about three times for cohesive soils, any of the following decisions, among others, may be considered: 1. 2. 3. 4.

Remove the weak soil and replace with better soil Carry out soil stabilisation works Relocate the structure Use piled foundation

Settlement of the structure is also an important consideration. Sometimes piles are used to reduce the uneven settlement to an acceptable value. For piled foundation, considerations for replacement of bakau piles are divided into two categories: 1. 2.

New designs Adaptation of standard/available plans

New Designs For new designs where piles need to be used for the foundation, engineers are to use other alternatives to bakau piles, such as timber and concrete piles except where it is justifiable to use bakau. The piles can be frictional piles, end bearing piles or a combination of both, depending on the soil types, tolerable settlement etc. The number of piles to be use can be determined using the normal method of pile foundation design with an appropriate factor of safety. Adaptation of Standard/Available Plans Some Standard Plans include the bakau pile foundation. These bakau piles are to be replaced with other alternatives such as timber or concrete piles unless it is justifiable to retain the bakau pile foundation. In adapting these plans for the foundation, one basic condition that has to be satisfied is that the soil conditions are approximately similar. A description of the soil types or soil parameters used for design is necessary. If it is available, then the engineer has to check against the soil type on which the structure will sit on. If the soils are approximately similar, a simple method (though not an economical one) for replacement of bakau piles can be used. This is based on the assumption that the adhesion factor is constant for all piles materials for cohesive soils. The equivalent number of alternative piles is then simply equal to the number of bakau piles multiplied by the surface area of the bakau pile divided by the surface area of the alternative pile in contact with the soil. The above method using timber and concrete piles as friction piles are not using the inherent capacities of the piles to the fullest, therefore resulting in uneconomical pile foundation. If time permits, the piled foundation should be redesigned. The equivalent number of timber or concrete piles of some dimensions can be easily obtained by using the reduction factor (R) prepared as shown in table 1. The reduction factor is actually the ratio of the surface area of a bakau pile (As bakau) to the surface area of the alternative pile (As). The table serves as a comparison for the number of alternative piles to the number of 100 mm average diameter by 5m length bakau piles. Both the loads and soil conditions are identical. Thus, if 100 mm by

100 mm by 5m length piles are to replace 100 numbers of bakau piles with the above dimension and 0.5m spacing, needs a minimum of 78 numbers. These 78 numbers will need to be rearranged to suit the particular structural geometry and loading and a new spacing is obtained. The arrangement of the piles should follow as close as possible the original layout. Additional piles may be used to suit the geometry of the foundation. An example is given in appendix A. It is important to note here that the reduction factors given in Table 1 are based on the capacity of replacement piles of certain dimensions against bakau piles of average diameter 100 mm by 5m long. The bakau piles and the alternative piles are both friction piles. For piles of different dimensions, the reduction factors have to be recalculated. It is also important to highlight that the calculations are only preliminary and for single piles, the group effect should be checked whenever necessary. If the soil types are not similar, the foundation should be redesigned where the design process is similar to that of a new design. This is also applies to plans that do not have information on the foundation soils and those that do not have foundation details.

Replacement pile dimension (m)

length ( m)

Reduction Factor in the number of piles needed

0.100 x 0.100

5

0.78

0.100 x 0.100

6

0.65

0.125 x 0.125

5

0.63

0.125 x 0.125

6

0.52

0.150 x 0.150

5

0.52

0.150 x 0.150

6

0.44

Table 1 : Reduction Factors for 100 mm average diameter by 5m long bakau piles Note : To reduction factor R, is derived using, the formula: Q = α  Where Q

α



= Ultimate skin friction of the pile

= Adhesion factor cu = Cohesion As = Surface area in contact with soil

With the following assumptions: (a) The adhesion factor between the different pile material does not vary significantly for cohesive soils. (b) Cohesion is constant for a particular soil, Q α As Since As for the replacement piles are usually greater than that of the bakau piles normally used, the capacity of each replacement pile is greater. For the same loads to be supported, it follows therefore, that the number of piles to be used can be decreased and the spacing can be increased accordingly. The reduction factor is given by: R = Q bakau -----Q or R = Asbakau ------As

Appendix A Example of calculation for replacement piles Suppose 125 mm by 5m length timber piles 157 mm numbers of bakau piles of an outlet structure. The average diameter of the bakau piles are 85 mm and the average length is 5m. The general spacing of the bakau piles is 0.5m c/c. Q = α cu As As for bakau piles, : π x 0.085 x 5 = 1.335 sq.m As for timber piles : 4 x 0.125 x 5 = 2.5 sq.m Thus, Qbakau = 1.335 α cu kN Qtimber = 2.5 α cu kN Reduction Factor : Qbakau / Qtimber = 1.335 α cu / 2.5 α cu = 0.53

Total number of bakau piles : 157 Therefore, the number of timber piles needed : 0.53 x 157 = 84 # This is the minimum number needed to be able to satisfactorily replace the bakau piles. This can then be rearranged to suit the same geometry as before to find the required spacing (Refer to Fig.1) As shown in the figure, 86 numbers with an average spacing of 0.7m c/c satisfactorily replace the bakau piles to achieve the same factor of safety as the original design.

Figure 1

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