OVERVIEW OF THE SEISMIC CONCEPTUAL DESIGN FOR BRIDGE FOUNDATION IN PULAU PINANG

AHMAD M.F.,
SULONG T.J.T.,
SALLEH S.,
IBRAHIM A.
Universiti Teknologi MARA Pulau Pinang

INTRODUCTION 

Frequent earthquake in the region has created jitters, resulting in the government to require seismic loading to be considered in the planning of the critical infrastructure in Malaysia. Apart from vibrations, which were frequently felt in Kuala Lumpur and Pulau Pinang, areas such as North Perak and Kedah were notably impacted by tsunami event of 2004 due to Sumateran earthquake. Pulau Pinang is a key industrial and trading center for Malaysia. ICT manufacturing industries are well developed on the island. Pulau Pinang also has ports that become the import and export center for the northern region of Peninsular Malaysia and southern of Thailand. The two main bridges located in Pulau Pinang that are part of this study are Penang Bridge and Butterworth Outer Ring Road (BORR). Penang Bridge connects Prai area in the peninsular and Penang island, and BORR connects Sg Dua Interchange of the North South Highway with Penang Bridge (Figure 1).

Peninsula Malaysia is located on the stable Sunda Shelf with low to moderate seismic activity level. Even though the country is actually free from earthquake activities, but the northern area in Malaysia had been frequently affected by tsunami of 2004 due to the Sumateran earthquake. A number of major faults have been recognized in the Peninsula. They had shown important movement along the strike of the fault plane in post Triassic and some have offset the lower Cretaceous formations. The geological opinion stated that they are now inactive. The nearest of these major faults is the Bok Bak Fault, which is about 70 km from Pulau Pinang, where the Penang Bridge and BORR were constructed (Figure 1). This fault shows lateral motion presumably arising from the general east-west compression of the area. There is no evidence from the geological map of faulting of significant consequence in the vicinity of the site (Chin, 1998).

Table 1 shows sample of borehole results for Penang Bridge and Butterworth Outer Ring Road (BORR).

TABLE 1: Sample borehole results for the bridges 

BH 31 and DB/2 are located within mangrove area of the Prai coastal area. The soil profile for the Penang Bridge is shown in Figure 2.

Figure 2: Simplified Soil Profile from Glugor Interchange to

Prai  Interchange for Penang Bridge

CONCEPTUAL DESIGN

This study will focus on the following elements of conceptual design: 

i)                    Peak Ground Acceleration

ii)                   Code of Practice

iii)                 Pile Material

iv)                 Foundation Protection

Peak Ground Acceleration

The seismic design of any major structures considering seismic loading is intended to provide safety against failure and consequent injury or loss of life during any earthquake that considerably affect the structure. There are two different earthquake intensities specified for design purposes:

1.      Design Earthquake, or the largest earthquake expected to happen during the life of the structure. No damage is to occur during Design Earthquake.

2.      Maximum Credible Earthquake, the largest earthquake that could affect the structure. Normally taken as twice the design earthquake. Yielding in ductile manner is acceptable during Maximum Credible Earthquake.

Peak ground acceleration adopted for Penang Bridge and BORR was 0.075g. The Design Earthquake is an earthquake of magnitude 7 at a distance 300 km. The Maximum Credible Earthquake is twice the intensity with magnitude 8 at a distance 300 km. The magnitude 7 will give damping of 5 % for both Penang Bridge and BORR and might induce significant motion at the bridge site (Figure 3). 

Table 2 shows the code of practices that was used by the consultant in the design of the two bridges.

TABLE 2: Code of Practice for design

 Penang bridge was completed in the 1980’s whereas BORR has just been completed and yet to be handed over.

Pile Material 

The pile foundation for Penang bridges were pre-stressed pre-tensioned reinforced concrete pile (spun pile). However the foundation for BORR used both spun pile and hollow steel pile. Table 3 provides the comparison on the type of pile used in two bridges.

TABLE 3: Comparison on the type of pile used between both bridges 

Type of Protection

For Penang Bridge, four artificial islands (Figure 4) were designed and built in about 49 feet (15 m) depth of water to serve protection to the main piers of the cable-stayed concrete at the navigation channel. Since the islands will be easily eroded by wave caused by wind and wash from navigation, long gentle slopes of riprap are provided. A sea wall with steep slopes using sand, as fill was also adopted. Figure 4 shows the details of slope protection for artificial islands built for Penang Bridge. 

Figure 4: Details of Slope Protection for artificial islands built for Penang Bridge

The protection provided for the foundation of both bridges is summarized in Table 4.

TABLE 4: Comparison between the protections provided for the foundations of the two bridges in Pulau Pinang

CONCLUSION AND RECOMMENDATION FOR FURTHER STUDY

The conclusions and recommendations on the conceptual designs are as follows: 

i)        Peak Ground Acceleration

Peak ground acceleration adopted was 0.075g for the bridges. This is compared to the value of 3.89 gal presented by Adnan et al (2002). Though BORR is a new project, there is still a disagreement on what should be the proper PGA value for the local soil. Thus there is a potential to study on the attenuation of local soil due to the seismic loading. 

ii)       Code of Practice

There is a difference in the choice of code of practice for the studied bridges in Penang. The effect in the choice of standard has not been studied. Note however that the main engineering work for BORR was done in the United Kingdom. The cost and engineering impact of using various international standards on seismic loading is a potential area of study. 

i)        Pile Material

The choice of pile material also differed between Penang Bridge and BORR. It was clear that larger diameter of 1m and 1016 mm was used in the design at critical locations to provide sufficient shear modulus against lateral load. The piles were all friction piles. Study could be undertaken to optimize pile material in local soil condition. This could be in conjunction with ground treatment work and pile coatings. 

ii)       Foundation Protection

Both bridges provided a lot of protection to the pile-cap to prevent from vibrating. With these measures, it is possible to assess the pile under seismic loading to be due to lateral load only. Work could be initiated to determine soil shear modulus of local Penang soil and thus finally the deflection of pile due to lateral load only.  

Acknowledgement 

The authors would like to acknowledge the assistance given by Jabatan Kerja Raya and Malaysian Highway Authority to complete this paper.