Innovative Substructures on Soft Ground

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Johnathan Lindsey
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Ir. Dr. Gue See Sew & Ir. Tan Yean-Chin Gue & Partners Sdn Bhd (www.gnpgroup.com.

1 Ir. Dr. Gue See Sew & Ir. Tan Yean-Chin Gue & Partners Sdn Bhd ( Introduction In Malaysia, the construction of industrial structures, commercial and residential buildings on soft ground and hill-site has increased tremendously for the last 15 years due to depleting good land at cities like Kuala Lumpur, Penang and Johor Bahru. In addition, highrise development in the city also often entails the need for a deep basement to maximise use of space despite the implementation of a mass transit system to reduce cars into and out of the cities. Often these developments will require innovative design and construction to make them costeffective and shorter construction time but without sacrificing safety. This paper presents some of the innovative substructures designed by the Authors firm and also from some of the projects involved by the Authors.. Housing Developments on Soft Ground Area The development of national road networks, residential and commercial properties have encroached into areas underlain with very soft soils (e.g. alluvial soils, marine clays, etc.). In this formation, usually the competent layer (stiff or dense soils) and bedrock are very deep (sometimes more than 60m deep) and resulting in higher cost of foundation. Geotechnical works in deep deposit of highly compressible soft clay is often associated with problems such as excessive differential settlement, negative skin friction and bearing capacity failure. Conventionally, piles are introduced to address the issue of bearing capacity and excessive differential settlement. Piles are often installed into competent stratum or set in order to achieve the pile bearing capacity and to limit the differential settlement by reducing the overall settlement of a structure. However, this solution only addresses short-term problem associated with soft clay as pile bearing capacity will be significantly reduced with time due to negative skin friction (down drag force acting the piles). Few centimetres of ground settlement along the pile is sufficient to fully mobilise the negative skin friction acting on the pile. This option often reduces the cost-effectiveness of such conventional solution as correctly the piles have to be downgraded (using lower allowable capacity thus more numbers of piles or larger pile size) to cater for the down drag force acting on these piles due to settling ground with time. In addition to that, problems such as continuing settlement of the ground will cause large gap to form between the building supported on rigid piles and the settling ground. Figure 1 shows a typical problem encountered using the piled-to-set foundation on settling ground. The large differential settlement (between the ground and the building will also cause breakage of services (e.g. water supply, sewerage lines, etc.). Exposed Pile Magnitude of Settlement Closed-Up View of the Gap Figure 1. Typical example of continue settlement of the ground causing formation of large gap between the building with piled-to-set foundation and the settling ground. In a housing development project of 1200 acres at Bukit Tinggi, Klang which is on very soft ground termed as Klang Clay (Tan, et al. 2004), the Authors have designed an innovative foundation solutions to solve this problems. The site is underlain by alluvial deposits which generally consist of very soft to firm silty CLAY up to a depth of 25 to 30m with presence of intermediate sandy layers. The silty CLAY stratum is generally underlain by silty SAND. Klang Clay can be divided into two distinct layers at a depth of 15m. The competent hard layer with SPT N greater than 50 blows is only encountered at depth of 35m to 40m. Page 1 of 7

This innovative foundation uses short settlement reduction piles coupled with strip-raft foundation to support 2-storey to 5-storey buildings on soft ground.

This system can also be termed as floating piled raft foundation.

The floating piled raft foundation is designed to limit differential settlement and it consists of short piles strategically located at areas of concentrated load and interconnected with a rigid

2 This innovative foundation uses short settlement reduction piles coupled with strip-raft foundation to support 2-storey to 5-storey buildings on soft ground. When designing the foundation system, short piles (length of pile is a quarter to a half of the depth to hard layer with SPT>50, depending on the load of the structures). This system can also be termed as floating piled raft foundation. The conceptual comparisons of the conventional foundation system and floating piled raft foundation for low rise buildings are shown in Figure 2. The floating piled raft foundation is designed to limit differential settlement and it consists of short piles strategically located at areas of concentrated load and interconnected with a rigid system of strip-raft to control differential settlement (Tan, et al & Tan, et. al. 2005). This system is the hybrid of piled raft design combining creep piling and differential settlement control piling defined by Randolph (1994). The objective of the design is to provide an optimum piled raft foundation system that takes into consideration the bearing capacity contribution of the raft and the piles are introduced mainly to limit differential settlement. The general approach is to increase the stiffnesses of areas where the settlement is expected to be the largest by introducing settlement reducing piles. Cost comparison has also been carried out to compare this innovative system with conventional foundation system that has been properly designed with negative skin friction consideration. This innovative system is more costeffective, easier to construct and no long term serviceability problems. Actual Completed Houses 25m to 30m thick soft compressible layer (Klang Clay) Foundation System = 9m length of 150mm x 150mm reinforced concrete (RC) square piles interconnected with 350mm x 600mm strips and 150mm thick raft. Dense sand layer Figure 3. 2-storey link houses on floating piles. Page 2 of 7 Figure 2. Conceptual Comparison of Conventional Piled-to-Set Foundation Floating piled-raft foundation. This foundation system has shown to be very effective as demonstrated by the monitoring results on the completed structures. Figure 3 shows the completed 2-storey link houses with schematic of the foundation depth relative to the thickness of the soft compressible subsoil. Figures 4 and 5 show the typical layout of the foundation system for 2-storey link houses and cross section of the strip raft

foundation system respectively. The floating piles foundation system is also extended to support 5-storey low cost apartments at the same site as shown in Figure 6.

This is because the shear wall frame of the building will increase the rigidity and stiffness of the whole foundation system thus shallower strips and short piles are sufficient to satisfy the design

3 foundation system respectively. The floating piles foundation system is also extended to support 5-storey low cost apartments at the same site as shown in Figure 6. The floating piled raft foundation has been further optimized and more cost effective if they are coupled with shear wall system used for the buildings. This is because the shear wall frame of the building will increase the rigidity and stiffness of the whole foundation system thus shallower strips and short piles are sufficient to satisfy the design requirements. Completed 5-storeys Apartments Soft compressible layer ( 25 to 30 m) Piles with varying length (18m, 21m and 24m) Stiff layer Figure 6. Schematic of piled raft system with varying pile lengths superimposed on completed low cost apartments. MALAYSIA Site Figure 4. Typical layout of foundation system for terrace houses. (Tan, et al. 2004) SUMATRA Steel Reinforcements are not shown Figure 7. Location of palm oil mill in Sumatra, Indonesia. Figure 5. Cross-section of strip raft foundation system. (Tan, et al ) Tank Structures on Soft Ground Area A 120 ton per hour palm oil mill has been constructed over the sand filled platform with an area of about 83,000m2 on soft swampy ground. Figure 7 shows the location of the proposed site, which is about 50km away from Sg. Guntung of the Province of Riau, Sumatra, Indonesia At the proposed site, there are seven numbers of heavy steel tank structures for the storage of processing water and processed palm oil. The site is underlain by recent alluvium and coral reefs of Quaternary age. The original and the surrounding ground conditions of the site are generally flat with reduced level of RL+8.3m. The water level is almost at the original ground surface. Page 3 of 7

Adopted Figure 9. Comparison of Conventional Piled-to-Set Foundation Floating piled-raft foundation Figures 8 shows the subsoil description from the boreholes.

Underneath the organic materials, the subsoil mainly consists of very soft normally consolidated clayey deposit of 34m thick is followed by 12m thick medium stiff clay overlying the white medium

5m sand bed coated with bitumen strips in order to have uniform seating between the coned-down tank base and the reinforced concrete raft of 500mm thick.

4 Adopted Figure 9. Comparison of Conventional Piled-to-Set Foundation Floating piled-raft foundation Figures 8 shows the subsoil description from the boreholes. Generally, the top one metre of the subsoil is organic materials of peat and decayed tree roots at the surface. No obvious dessicated weathered crust has been observed. Underneath the organic materials, the subsoil mainly consists of very soft normally consolidated clayey deposit of 34m thick is followed by 12m thick medium stiff clay overlying the white medium dense fine sand and dense clayey sand. The steel tanks are seated on 0.5m sand bed coated with bitumen strips in order to have uniform seating between the coned-down tank base and the reinforced concrete raft of 500mm thick. Piled rafts with different pile lengths have also been used as more cost effective foundation replacing the conventional piles to set foundaion system as the support for 2500Ton oil storage tanks on very soft and compressible alluvial clayey soil of about 34m thick as shown in Figure 9. The storage tanks sit on a 20m diameter and 500mm thick reinforced concrete (RC) circular raft. The pile points have been strategically located beneath the RC raft. Varying pile penetration lengths have been designed to minimize the angular distortion of the thin RC raft and the out-of-plane deflection at the tank edge. A total number of 137 of 350mm diameter hollow circular prestressed concrete (PC) spun piles with concrete strength of 60MPa have been designed and installed to support the tank through the RC raft. (Liew, et al. 2002). In order to monitor and validate the actual performance of the piled raft during water-loading test, strain gauges and horizontal inclinometer and settlement markers have been installed in the piles and also the raft. Seven working piles were instrumented with strain gauge at the pile top. Figure 10 shows the details of the piled raft for the instrumented tank structures. These tanks have performed better than our prediction in terms of total and differential measured settlement. Figure 8. Subsoil Conditions at Oil Palm Mill Figure 10. Layout of Instruments for Piled Raft with varying Pile Lengths (from Liew, et al., 2002) Page 4 of 7

Fig. 12. Mushrooms and undulating surface on highway. Figure 11.

maintenance and repaving. The protruding parts of the embankment with pilecaps as if punching through the embankment look like mushrooms and therefore, the term is used to describe the problem.

Meanwhile, Figure 8 shows the differential settlement ( mushroom ) observed between the area with and without pilecaps after excavation at a depth of about 300mm.

The area where the mushroom problems are prominent is predominantly in areas underlain by Quaternary age deposits and comprises of marine deposits such as clay, silt and sand with sea shells.

5 Fig. 12. Mushrooms and undulating surface on highway. Figure 11. Completed Tank Structures Innovative Solution to Address Mushroom Problems on Soft Ground Area A piled embankment with individual pilecaps was constructed in the 1980s as part of the highway in Malaysia. The original design principle of this solution was intended to rely solely on the arching of the embankment materials to transfer the load to the pilecaps as the soft compressible subsoil between the pilecaps settled under consolidation. However, shortly after the expressway was opened to traffic, the embankment continued to experience large differential settlement in the form of localized depressions that required regular maintenance and repaving. The protruding parts of the embankment with pilecaps as if punching through the embankment look like mushrooms and therefore, the term is used to describe the problem. Figure 12 shows features of the mushroom problem. Meanwhile, Figure 8 shows the differential settlement ( mushroom ) observed between the area with and without pilecaps after excavation at a depth of about 300mm. The Authors were involved in the investigation of the causes of problems and design of long term remedial measures. The area where the mushroom problems are prominent is predominantly in areas underlain by Quaternary age deposits and comprises of marine deposits such as clay, silt and sand with sea shells. The alluvium deposits mainly consist of very soft to soft silty Clay and clayey/sandy Silt with the presence of intermittent sand layers with some sea shells and wood remnant. The modelling of mushroom problems was carried out using two-dimensional (2D) and three-dimensional (3D) finite element method (FEM) programmes for geotechnical analysis to determine the possible causes of the problems. A typical FEM model adopted for the investigation is shown in Figure 14. Results from the FEM analyses have shown that the differential settlement of the embankment ranges from 64mm to 156mm with angular distortion as high as high 4% (1/25). This is in excess of the recommended values of 1% (1/100) by BS8006 (1995). Typical results from the FEM analyses showing the mushroom problem are shown in Figure 15. Note : The thicker bitumen layer means larger settlement and many topping up had taken place. Fig. 13. Differential settlement observed after excavation for construction of remedial works. Less settlement directly above pilecap Larger settlement between pilecaps Fig. 14. Typical 3D FEM Model of Embankment. Piles and Pilecaps Modelled. Fig. 15. Results of 3D FEM analyses showing mushroom problems top view bottom view. Page 5 of 7

6 (c) (d) (e) (f) Fig 16. Milling works up to 300mm deep in progress After milling (c) Laying of steel reinforcement (d) Concreting (e) Completed RC Raft (f) Pavement completed, traffic re-opened. In summary, results of FEM analyses have shown that the mushroom problems arise due to the ineffective arching mechanism influenced by the following factors: a) Unsuitable fill materials b) Large pile spacing to the height of fill The occurrence of the mushroom problems has necessitated regular repaving works to ensure the riding comfort and safety of the highway. However, repaving works are only a short-term solution as the embankment continues to settle due to additional loads from the pavement. Therefore, an effective remedial design for the mushroom problems must satisfy the following criteria: a) Minimum disturbance to operation of the expressway b) Simple and fast to construct c) Cost effective d) Minimum long-term maintenance. After reviewing many feasible options such as a high strength geogrid with granular infill (on top of pilecaps or at shallow depths) and a reinforced concrete raft (either on top of pilecaps or at shallow depths), it was found that a reinforced concrete (RC) raft at shallow depth offers the best solution to the mushroom problems satisfying the above criteria. The design was checked using FEM analyses to ensure the required long-term angular distortions of less than 1% (1/100) as recommended by BS8006 (1995). Results of FEM analyses have indicated that the angular distortion of the embankment is below 1% upon construction of the RC raft. The RC raft solution at shallow depth essentially involves the following simple construction sequence: a) Excavating of the pavement for a minimal depth for the concrete raft and wearing course. This is typically less than 500mm as the thickness of the concrete raft is approximately 300mm and thickness of the wearing course is 50mm (total 350mm) and can be easily and speedily carried out using a milling machine. b) Laying of steel reinforcement and casting of concrete. c) Laying of the wearing course. The simple construction sequence is very important for this site due to its location along a busy expressway. The simple construction sequence minimizes lane closure for the construction works. In addition, the remedial solution is easy to construct and does not require a specialist contractor. The typical construction sequence of the works is shown in Figures 16. Page 6 of 7

7 Conclusion Innovative designs are the outcomes of exploring various options or alternatives to conventional solutions. The various options need to be explored in terms of technical suitability, cost, time and ease of construction or installation as well as minimal long term maintenance. These should not be compromised on the quality, safety and good engineering practice. This tall order is possible if we invest in giving your best resources where the value engineering works are needed most. Commitment to in house research and development is also a contributing factor. References 1. Gue, S. S., Tan, Y. C. Liew, S. S. (2002), Cost Effective Solutions for Roads and Factories Over Soft Marine Deposits, CAFEO2002, Cambodia, 2-5 September, Liew, S. S., Gue, S.S. & Tan, Y.C. (2002), Design and Instrumentation Results of A Reinforcement Concrete Piled Raft Supporting 2500 Ton Oil Storage Tank On Very Soft Alluvium Deposits, Ninth International Conference on Piling and Deep Foundations, Nice, 3rd 5th June, Randolph, M.F. (1994) Design Methods for Piled Rafts,: State-of-the Art Report, Proc. 13th Int. Conf. Soil Mech. Found. Engng, New Delhi Vol. 4, pp Tan, Y.C. Gue, S.S., Ng, H.B. & Lee, P.T. (2004a), Some Geotechnical Properties of Klang Clay, Proc. of Malaysian Geotechnical Conference 2004, Selangor, pp Tan, Y. C., Chow, C.M. & Gue, S.S.(2004b), A Design Approach for Piled Raft with Short Friction Piles for Low Rise Buildings on Very Soft Clay, 15th SEAGC, Bangkok, Thailand. 6. Tan, Y.C., Chow, C.M. & Gue, S.S. (2005), Piled raft with Different Pile Length for Medium-Rise Buildings on Very Soft Clay, (to be published) Proc. 16 th Int. Conf. on Soil Mechanics & Geotechnical Engineering, Osaka, Japan. Page 7 of 7

DESIGN AND INSTRUMENTATION RESULTS OF A REINFORCEMENT CONCRETE PILED RAFT SUPPORTING 2500 TON OIL STORAGE TANK ON VERY SOFT ALLUVIUM DEPOSITS

DESIGN AND INSTRUMENTATION RESULTS OF A REINFORCEMENT CONCRETE PILED RAFT SUPPORTING 25 TON OIL STORAGE TANK ON VERY SOFT ALLUVIUM DEPOSITS Shaw-Shong Liew, See-Sew Gue and Yean-Chin Tan, Gue & Partners