Paper on Settlement of Pile Foundation and Raft Theory

Settlement of Pile Foundation and Raft Theory


1.INTRODUCTION High rise buildings are usually founded on some form of piled foundation which is subjected to a combination of vertical, lateral and overturning forces.                         Combined pile-raft foundations can be a particularly effective form of foundation system for tall buildings because the raft is able to provide a reasonable measure of both stiffness and load resistance. This paper sets out a limit state design approach for tall building foundation systems, with attention being focused on piled raft foundation systems. Some of the advantages of piled rafts are outlined, and then the principles of the design approach are set out. A published case history involving a 9-pile group is then analysed to compare the performances of a pile group and a piled raft. An example of the application of the design approach is then described for a proposed tower on reclaimed land in Incheon, South Korea. The city with the tallest high-rise buildings in Germany is Frankfurt am Main. The construction of these buildings is a demanding and complicated task for all areas of civil engineering, especially for geotechnical engineers when considering settlement-sensitive Frankfurt clay. Due to the high loads of the new structures, the risk of a high degree of settlements and tilts of not only the new buildings themselves, but also of the adjacent structures have to be taken into account. The subsoil of Frankfurt am Main mainly consists of nonhomogeneous, stiff and overconsolidated tertiary ”Frankfurt clay” with embedded limestone bands of varying thicknesses. This layer is underlain by ”Frankfurt limestone”, which consists of limestone and dolomite layers as well as algal reefs, marly calcareous sands and silts and marly clay. The rather thin top layer consists   
       ABSTRACT In urbun areas the principal question related to design of high rise buildingOn  settlement sensitive soils is the cost optimized reduction of settlements to avoid damage and to minimize the deformations of adjacent structures and the high rise building itself.A conventional method for reduction ofsettlement foundations on settlement sensitive soils is to design and build a pile foundationbased on stiff layer like rock or sand.To handle the problems encountered in the design of foundations for high rise buildingdevelopment of new technical and economical foundations is made.                                     
                                                Piled raft foundations are increasingly being recognised as an and effective foundation system for tall buildings.  This paper sets out some principles of design for such foundations, including design for the geotechnical ultimate limit state, the structural ultimate limit state and the serviceability limit state.
  The advantages of using a piled raft will then be described with respect to two cases: a small pile group subjected to lateral loading, and then the design of the Inchon Tower in South Korea. Attention will be focused on the improvement in the foundation performance due to the raft being in contact with, and embedded within, the soil.  2.RAFT FOUNDATION In conventional rigid method raft is assumed to be infinitely rigid and bearing pressure at the bottom of the raft follows a planer distribution where
the centroid of the bearing pressure coincides with the line of action of resultant force of all loads acting on the raft.1)Column loads of all columns coming from the superstructure are calculated as per standard practice.2)Loads include live and dead loads.3)Determine the line of action of resultant of all loads.4)Mat is treated as whole in each of the two perpendicular directions.5)Total shear force acting on any section cutting across the entire raft is equal to arithmetic sum of all forces and reactions to the left or right of the section.To gain an understanding of the significance of including the raft in the foundation analysis, a relatively simple example of a field test is considered first. A lateral load test was performed by Rollins and Sparks (2002) on a group of 9 piles having a cap that was buried in fill as shown in Figure 1. The piles were 324mm outside diameter steel pipe piles with a wall thickness of 9.5mm (elastic modulus of the steel Esteel = 200GPa), and were driven to about 9.1m. Prior to testing, the piles were instrumented and then the pipe filled with concrete. The piles were spaced at a nominal spacing of 3 pile diameters. The pile cap was 2.74m square and 1.22m deep and extended 0.3m beyond the outer edge of the piles. Horizontal load was applied to the pile cap using two hydraulic jacks such that the load was applied 0.4m above the base of the pile cap.The soil consisted of various layers of clay, sand and sandy silt. The soil profile and the soil parameters used in the analysis are shown in Figure 2. The unit weight of the fill was 24kN/m3 and that of the soil was taken as 18kN/m3. The passive pressure against the face of the pile cap was treated as a load on the pile cap that increased with displacement and then reached a constant value at passive failure. 4.CONCEPT  OF   RAFT  USING  SETTLEMENT  REDUCING  PILES This method was first proposed by burland in 1977. 2)For the idealized condition of uniform loading,the settlement profile of the raft foundation is of bowl shaped. 3)Settlement is largest in the centre and smallest at the edge. 4)Settlement reducing piles are therefore introduced introduced in the centre of raft to reduce raft settlement at the centre. 5)It also reduces differential settlement.    9.TEMPORARY SURCHARGING OF EARTH PLATFORM. In certain projects temporary surcharging and preloading technique with or without vertical drains is adopted to control long term settlement of subsoil under the loads from films and buildings to be placed on the top of it.The design of temporary surcharging and preloading technique requires carefull determination of subsoil and profileand its properties for field variation.     Several important desigd criterta for this method: 1)Stability should be checked with preloading or surcharged load. 2)Preloading or surcharging should be designedto the choosen construction                                 Period. 3)The option should be economical. 4)Does not cause any damage to adjoining structure and utility. 5)Surcharging is the process of removal and fill of sand. 6)Proper proportions are varied according to its utility and use. CARE TO BE TAKEN: 1)Surrounding structures and utility should not be affected. 2)Proper planning of construction programme for cost effective use of    materials. 3)The option should be economical. 4)Settlement after construction should be within range of tolerance.  1)Surcharging is the process of removal and fill of sand.2)Proper proportions are varied according to its utility and use.CARE TO BE TAKEN:1)Surrounding structures and utility should not be affected.2)Proper planning of construction programme for cost effective use of materials.3)The option should be economical.4)Settlement after construction should be within range of tolerance.
  3.CONCEPT OF PILED RAFT  Settlement of pile foundation has caused various problems of increased stresses on piles.Loads from the structure are assumed to be entirely supported by the piles.Pile capacity has also been significantly reduced with time due to negative skin friction and other problems associated with it.Piled raft foundation system using friction piles as settlement reduceris technically superior foundation system as bearing capacity of both raft and piles are taken in to consist.The piles in the piled raft foundation system consists of relatively short friction pileslocated to enhance the bearing capacity of raft and also to enhance and control differencial settlement.Highest building in Frankfurt am Main which is built on a CPRF is the 256 m high Messeturm, which was constructed between.The initial settlements, which were calculated for a shallow foundation, were about 40-50 cm with a differential settlement of about 15.The settlement observed for the CPRF was about 13 cm until 2000 (Reul 2000). Designing CPRFs requires the qualified understanding of the different interactions as presented In comparison with a conventional foundation design of a pile group, a new design philosophy has been applied for CPRFs. Piles are now used up to a load level which is much higher than the permissible design values for the bearing capacities of comparable single piles. The performance of the entire foundation system has to therefore be evaluated, taking into account the different effects of soil-structure interaction. The distribution of the total building load between the different bearing elements of a CPRF is described by the CPRF coefficient αCPRF (Equation 2),   8.GENERAL USE OF PILE AND RAFT.
 For first high rise buildings which were built on shallow foundations settlements of about 20 and 30cm had been observed.Due to the problems of settlement and tilting considerable efforts had been made to correct the settlement behavior of these buildings during the construction of shallow foundation. It was attemted during the construction to keep the towers from drifting out of plumband it was finally accepted although not recognized.1)Pile foundation is mainly used on soft ground.
2)Piles are generally used where hard strata is not available.3)Settlement problems are not created again.4)successful life of  building is achieved. Piles are now used up to a load level which is much higher than the permissible design values for the bearing capacities of comparable single piles. The performance of the entire foundation system has to therefore be evaluated, taking into account the different effects of soil-structure interaction. The distribution of the total building load between the different bearing elements of a CPRF is described by the CPRF coefficient αCPRF   4.TYPES OF PILES USED Piles can be classified according to the type of material forming the piles, the mode of load transfer, the degree of ground displacement during pile installation and the method of installation. Pile classification in accordance with material type (e.g. steel and concrete) has drawbacks because composite piles are available. A classification system based on the mode of load transfer will be difficult to set up because the proportion of shaft resistance and endbearing resistance that occurs in practice usually cannot be reliably predicted. In the installation of piles, either displacement or replacement of the ground will predominate. A classification system based on the degree of ground displacement during pile installation, such as that recommended in BS 8004 (BSI, 1986) encompasses all types of piles and reflects the fundamental effect of pile construction on the ground which in turn will have a pronounced influence on pile performance. Such a classification system is therefore considered to be the most appropriate. In this document, piles are classified into the following four types :(a) Large-displacement piles, which include all solid piles, including precast concrete piles, and steel or concrete tubes closed at the lower end by a driving shoe or a plug,i.e. cast-in-place piles.(b) Small-displacement piles, which include rolled steel sections such as H-piles and open-ended tubular piles. However, these piles will effectively become largedisplacement  piles if a soil plug forms.(c) Replacement piles, which are formed by machine boring, grabbing or hand-digging. The excavation may need to be supported by bentonite slurry, or lined with a casing that is either left in place or extracted during concreting for re-use.   5.DESCRIPTION OF PILES End bearing piles-They are the most common types of piles which are used on soft ground.End bearing piles are used on hard materials such as hard rock dense sand gravel etc.                             
Timber piles-in ancient times where use of concrete and steel was minimum timber was the most economical material used.Although it has a very low resisting power is only used for inferior works.Now a days its use is avoided with increase use of steel and concrete. Friction piles:These are the only most important piles which are used on soft ground where hard strata is not available.These piles are also called as settlement reducing piles. Tension piles:These are used in case of chimneys transmission towers to resist the uplift forces. Precast reinforced concrete pilesPrecast  reinforced  concrete piles are not common nowadays in Hong Kong. Thesepiles are commonly in square sections ranging from about 250 mm to about 450 mm with a maximum section length of up to about 20 m. Other pile sections may include hexagonal, circular, triangular and H shapes.                      6.MONITERING OF PILED RAFT FOUNDATIONS  Installing the measuring devices and monitoring the performance of foundation structures not only serves stability and safety aspects but alsoserves to document movements that occur and gurantee serviceability through out all the construction stages including the phase of operation.           
               The following paramaters can be measured by geotechnical and geodetic measurements with in the context: 1)Settlements are usually measured by geodetic measurements where as in order to to determine the time dependent load developments a carefull analysis of actual weight of the building is necessary. 2)Load share between pile and raft foundation can be measured indirectly by measuring the effective contact pressure and pile forces as well. 3)The value and distribution of total contact pressure and the water distribution below the raft foundation.
 7. ADVANTAGES OF PILED RAFT FOUNDATIONS  Piled raft foundations utilize piled support for control of settlements with piles providing most of the stiffness at serviceability loads, and the raft element providing additional capacity at ultimate loading. Consequently, it is generally possible to reduce the required number of piles when the raft provides this additional capacity.
                            
                                         In addition, the raft can provide redundancy to the piles, for example, if there are one or more defective or weaker piles, or if some of the piles encounter karstic conditions in the subsoil. Under such circumstances, the presence of the raft allows some measure of re-distribution of the load from the affected piles to those that are not affected, and thus reduces the potential influence of pile “weakness” on the foundation performance. Another feature of piled rafts, and one that is rarely if ever allowed for, is that the pressure applied from the raft on to the soil can increase the lateral stress between the underlying piles and the soil, and thus can increase the ultimate load capacity of a pile ascompared to free-standing piles (Katzenbach et al., 1998).                                       A geotechnical assessment for design of such a foundation system therefore needs to consider not only the capacity of the pile elements and the raft elements, but their combined capacity and interaction under serviceability loading. The most effective application of piled rafts occurs when the raft can provide adequate load capacity, but the settlement and/or differential settlements of the raft alone exceed the allowable values. Poulos (2001) has examined a number of idealized soil profiles, andfound that the following situations may be favourable:• Soil profiles consisting of relatively stiff clay
  6. MONITORING OF COMBINED PILE RAFT FOUNDATIONS  Installing measuring devices and monitoring the performance of foundation structures only serves not safety and stability aspects, but also serves to document movements that occur and guarantee serviceability throughout all the construction stages, including the phase of operation. The monitoring of a CPRF might comprise the excavation, foundation and surrounding area. The type and quantity of instrumentation and measurements depend on the requirements defined by the complexity of the CPRF, its geotechnical and geometric boundary conditions and the subsoil situation. The following parameters might be measured by geotechnical and geodetic measurements within the context of the observation of a CPRF:
• The load-settlement behaviour of the foundation: Settlements areusually measured by geodetic measurements, whereas in orderto determine the time-dependent load developments, a carefulanalysis of the actual weight of the building is necessary (e.g., byanalysing the delivery notes for concrete and steel).• The load share between the raft and foundation piles: It canonly be measured indirectly by measuring the effective contactpressure and the pile forces as well.• The value and distribution of the total contact pressure and thewater pressure below the foundation raft.• The bearing behaviour of the foundation piles comprising the pileforces at the pile heads within the pile group, the distribution ofskin friction along the pile shaft and the remaining pile forces atthe pile toe (tip resistance      12. CONCLUSIONS By using CPRFs as a foundation for high-rise buildings in the settlement-sensitive Frankfurt clay, a considerable settlement reduction of more than 50% compared to raft foundations can be achieved. Due to its enhanced design philosophy, a CPRF reducesthe costs for piles by more than 60 % compared to a conventional pile foundation.During the design process of a CPRF based on finite element analysis, strong co-operation between the geotechnical and structural. This paper sets out the principles of a limit state design approach for piled raft foundations for tall buildings. Ultimate limit state, serviceability limit state and cyclic loading conditions are addressed. Information on the groundwater regime is necessary for the design and selection offoundation type and method of construction. Artesian water pressures may adversely affect shaft stability for cast-in-place piles. For developments close to the seafront, the range of tidal variations should be determined. In a sloping terrain, there may be significant groundwater flow, and hence the hydraulic gradients should be determined as far as possible since the flow can affect the construction of cast-in-place piles, and the consideration of possible damming effects may influence the pile layout in terms of the spacing of the piles.  2. RAFT FOUNDATIONS   A raft foundation is usually continuous in two directions and covers an area equal toor greater than the base area of the structure. A raft foundation is suitable when theunderlying soils have a low bearing capacity or large differential settlements are anticipated.It is also suitable for ground containing pockets of loose and soft soils. In some instances, the raft foundation is designed as a cellular structure where deep hollow boxes are formed in the concrete slab.The advantage of a cellular raft is that it can reduce the overall weight of the foundation and consequently the net applied pressure on the ground. A cellular raft should be provided with sufficient stiffness to reduce differential settlement. Raft foundations are relatively large in size. Hence, the bearing capacity is generally not the controlling factor in design. Differential and total settlements usually govern the design. A common approach for estimating the settlement of a raft foundation is to model the ground support as springs using the subgrade reaction method. This method suffers from a number of drawbacks. Firstly, the modulus of subgrade reaction is not an intrinsic soil property. It depends upon not only the stiffness of the soil, but also the dimensions of the foundation. Secondly, there is no interaction between the springs. They are assumed to be independent of each other and can only respond in the direction of the loads. BSI (2004) cautions that the subgrade reaction model is generally not appropriate for estimating the totaland differential settlement of a raft foundation. Finite element analysis or elastic continuum method is preferred for the design of raft foundations.   10. Programme and Cost
  The design engineer frequently has a choice between a number of technically feasiblepiling options for a given site. The overall cost of the respective options will be a significant consideration. The scale of the works is a pertinent factor in that high mobilisation costs of large equipment may not be cost effective for small-scale jobs. The availability of plant can also affect the cost of the works. Contractors may opt for a certain piling method, which may not be the most appropriate from a technical point of view, in order to optimise the material, equipment and plant available to them amongst the ongoing projects. The cost of piling in itself constitutes only part of the total cost of foundation works. For instance, the cost of a large cap for a group of piles may sometimes offset the higher cost of a single large-diameter pile capable of carrying the same load. It is necessary to consider the cost of the associated works in order to compare feasible piling options on an equal basis. A most serious financial risk in many piling projects is that of delay to project completion and consequential increase in financing charges combined with revenue slippage. Such costs can be much greater than the value of the piling contract. The relative vulnerability to delay due to ground conditions, therefore, ought to be a factor in the choice of pile type. 11.RAFT FOUNDATION A photo for practical guidance and site knowledge of raft foundation