Deep Foundations 2. Load Capacity of a Single Pile

Transcription

1 Deep Foundations 2 Load Capacity of a Single Pile All calculations of pile capacity are approximate because it is almost impossible to account for the variability of soil types and the differences in the quality of construction practice. The ultimate pile capacity Q ult consists of two components: skin/shaft friction or side shear Q f and end bearing at the pile tip or base Q b. Q ult = Q f + Q b The allowable pile capacity is expressed as: Q all = Q ult /factor of safety A minimum factor of safety of 2.5 is typically maintained. 1

2 Load Capacity of a Single Pile Load Capacity of a Single Pile Some pile movement is needed to mobilize skin friction and end bearing. Pile load tests on driven piles have shown that a vertical pile movement of 2.5 mm to 10 mm is needed to fully mobilize skin friction. For driven piles, end bearing resistance is fully mobilized when the vertical pile displacement is about 8-10% of the pile tip diameter (for diameters cm in diameter, movements between 32 mm to 120 mm). 2

3 Load Capacity of a Single Pile Similar response is characteristic of bored piles. Generally, full mobilization of skin friction and end bearing does not occur at the same displacement. Skin friction is mobilized at about 10% of the displacement required to mobilize end bearing. Concept of Mobilized Resistances 3

4 Methods for evaluating capacity of deep foundations 1. Full-scale load tests on prototype foundations. 2. Analytical methods based on soil properties obtained from laboratory and/or insitu tests. 3. Dynamic methods based on pile driving data or wave propagation. Analytical Methods 4

5 Analytical Methods For a cylindrical pile of uniform diameter (D), penetrating a homogenous soil, Q f is given by: Q f =.s u..d.l Where L is the pile length embedded in soil. There is a wide range of variability in the values of reported in literature. Analytical Methods In this course the following values of will be adopted: Bored piles: = 0.3 to 0.4 Driven piles: = 1 for very soft clay (s u = kpa) = 1 for soft clay (s u = kpa) = 0.8 for medium stiff clay (s u = kpa) = 0.6 for stiff clay (s u = kpa) = 0.4 for very stiff clay (s u = kpa) 5

6 Analytical Methods The end bearing capacity is found by analogy with shallow foundations as expressed by: Q b = q b.a b = N c (s u ) b A b Where q b is the pile base (tip) resistance; N c is a bearing capacity factor typically taken equal to 9; (s u ) b is the soil undrained shear strength at the pile base; A b is the pile cross sectional area at the pile base. The undrained shear strength should be obtained within 2 pile diameters below the pile tip. Analytical Methods The method is based on an effective stress analysis and is used to determine the short term and long term pile load capacities of coarse grained soils and the long term response of fine grained soils. Friction along the pile shaft is computed using Coulomb s friction law, where the frictional stress is given by f s =. x\ = tan( i \. x \ Where is the coefficient of friction between the pile and soil; x\ is the lateral effective stress; i \ is the interface effective angle of friction. 6

7 Analytical Methods Analytical Methods The end bearing capacity is calculated by analogy with the bearing capacity of shallow footings as expressed below: Q b = q b.a b = N q ( z ) b A b Where N q ( z ) b is the base resistance; N q is the bearing capacity coefficient as a function of \ ; ( z ) b is the effective vertical stress at the pile base; A b is the pile cross sectional area at the pile base. 7

8 Pile Groups The most practical situations, piles are used in groups. They are arranged in geometric patterns at a spacing s. The piles are connected at their heads by a concrete pile cap. The spacing between piles in a pile group should be kept as large as possible to avoid driving problems and overlap of stresses which may cause a reduction in pile capacity or excessive settlement. large spacings are impractical since they will require massive and heavy pile caps. The optimum spacing between piles should be between 2.5 and 3 times the diameter of pile. 8

9 Pile Groups To prevent any side deformation of piles under load, tie beams in two directions should by used to connect a single pile-cap to adjacent pile caps. Theoretical and experimental investigations have shown that friction pile groups may fail as a unit before the load per pile reaches the maximum capacity of a single pile. This means that the load-carrying capacity of a group of friction piles will be less than the sum of the individual pile capacities. Pile Groups Each individual pile is supported by the surrounding and underlying soil. The pile imposes a region of stress influence on the soil, which is greatest immediately adjacent to the pile, and decreases with increasing distance from the pile. If multiple piles are used in a group, the regions of stress influence overlap, and the capacity of the soil to support the piles may be reduced. In addition to the effect of stress overlap, the capacity of a pile may be influenced by the installation of neighbouring piles. 9

10 Efficiency of Pile Groups It is common to not allow for any increase in capacity due to densification effects. However, pile group capacity losses are an effect which engineers must be careful to account for. Pile group capacity loss is by convention calculated using a pile group efficiency factor,. = Q ult.group /(n.q ult ) Where n is the number of piles in the group, Q ult.group is the total load capacity of the group of piles and Q ult is the load capacity of a single pile. Efficiency of Pile Groups A check of pile-group efficiency (group action) can be made by considering the group to act as one large pile whose length equals to the length of individual piles and its cross-sectional area equals the area enclosed within the outer perimeter of the group. The capacity of this large pile is computed in the same way as the friction pile capacity is computed. If this capacity (P') of the large pile is equal to or greater than the sum of individual pile capacities (np) the group is 100% efficient. If not the group efficiency is less than 100% and this condition can be treated by increasing the pile length and/or increasing the pile spacing. 10

11 Considerations for Pile Groups When piles are resting on a strong layer underlain by a weaker one, it is important to check that the bearing capacity of the underlying layer is not exceeded so that the group does not punch through the bearing layer as illustrated below. Shallow footing analogy may be applied. Negative Skin Friction Negative skin friction is a force developed between the soil and the pile in the downward direction due to soil compressibility. This force may be large enough so that, in conjunction with the applied load from the superstructure, the piles will settle excessively and foundation failure may occur. Negative skin friction occurs in response to relative downward deformation of the surrounding soil to that of the shaft, and will not develop if downward movement of the drilled shaft in response to axial compression forces exceeds the vertical deformation of the soil. 11

12 Negative Skin Friction The potential for negative skin friction is greatest when the soils in the upper zones of the subsurface profile can settle and where the lower portion of the shaft is founded in a relatively rigid material such as hard/dense soil or rock. Examples of soils that will undergo settlement after pile construction include loose sand, soft to medium stiff clay, recently-placed fill, and soils subjected to earthquake induced liquefaction. Negative Skin Friction 12

13 Negative Skin Friction Negative Skin Friction 13

14 Negative Skin Friction The negative skin friction can be estimated emperically from data given in figure where the weight of soil enclosed in the shaded area is to be added to the structural loading. Negative Skin Friction 14

15 Displacement of Pile Groups Friction Piles After checking the group action of the friction pile group, the load support.ed by the pile foundation is assumed to be transferred at the lower third point of the pile length on an area equal to the area enclosed by the piles. The settlement will be considered as that due to the consolidation of the thickness (H) and not of the whole thickness of the clay layer,. Displacement of Friction Piles 15

16 Displacement of Pile Groups End Bearing Piles The settlement of a pile group is larrer than the settlement of the single pile and depends on the size of the pile group, The larger the pile group; the deeper the stress bulb penetrates the bearing stratum, and consequently the settlement of the group will be much larger than that of a test pile although each pile of the group is carrying the same allowable load determined from test pile. Displacements of Bearing Piles 16

17 Pile Load Test In spite of the most thorough efforts to correlate drilled shaft performance to geomaterial properties, the behavior of drilled shafts is highly dependent upon the local geology and details of construction procedures. This makes it difficult to accurately predict strength and serviceability limits from standardized design methods such as those given in this manual. Site-specific field loading tests performed under realistic conditions help in improving the accuracy of the predictions of performance and reliability of the constructed foundations. Conventional Pile Load Test Setup FHWA-NHI

18 Pile Load Test-ECP According to the Egyptian Code of practice: It is advisable to drive test piles and to carry out a loading test for each piles. There are two methods for performing the pile load test: الحمل على مراحل Maintained Load test :معدل الھبوط الثابت Constant Rate of Penetration 0.4 mm/minute for piles resting in clay 2 mm/minute for piles resting in sands. Maintained Load Test-ECP Applied load (% of design load) Duration 25 1 hour 50 1 hour 75 1 hour hours hours hours minutes minutes minutes minutes minutes 0 4 hours 18

19 Interpretation of Pile Load Conceptually, bearing capacity failure is defined when a constant stress is reached. However, foundation load tests do not always reach a well-defined peak stress because of practical limitations on field equipment and test setups, or because a progressive failure allows repositioning of soil particles beneath the foundation, thereby the highest stress is not fully achieved. This creates ambiguity in defining the true bearing capacity, Interpretation of Pile Load Modified Chin s Method - Hyperbolic Asymptote One of the simplest forms to represent non-linear curves is the hyperbola as only two constants are required. Fitting a simple hyperbola to load-displacement test data has been used for evaluating the bearing capacity of piles (Chin, 1971). The simple hyperbolic relationship between stress q and pseudo-strain e s (s/b) is: 19

20 Interpretation of Pile Load Modified Chin s Method - Hyperbolic Asymptote where ki = initial stiffness at zero displacement and qult = ultimate load (asymptote of the hyperbola). The parameters ki and qult are determined objectively by plotting the transformed axes: es/q versus es, which is represented by a straight line given by: where 1/ki = y-intercept for zero displacement, and 1/qult is the slope of the straight line. Thus, the hyperbola requires two constants (ki and qult) that are determined and have physical significance: the initial stiffness ki = q/s at s = 0, and the asymptote qult at infinite displacements ) ( s). Interpretation of Pile Load Modified Chin s Method - Hyperbolic Asymptote 20

21 Interpretation of Pile Load Modified Chin s Method - Hyperbolic Asymptote According to ECP, the ultimate axial pile capacity is: Q ult = 1/1.2 s Design of Pile Design Load - ECP The ultimate pile is not less than 2 times the design load considering dead and live load, 1.75 times the design load considering also wind loads and 1.5 times the design load taking earthquake loading into consideration. Additionally, a pile load test is considered satisfactory if the measured pile settlement after 12 hours under 1.5 times the design load does not the following equation: s = 0.02 d + 0.5QL/AE d=pile diameter, Q =1.5 design pile load, L = pile length, A = pile cross sectional area, E = pile material modulus of elasticity. 21

22 Eccentric Loading The load per pile (i) P i can be determined from the following formula: The above equation is valid provided that the pile cap is rigid and the pile settlement into the soil is proportional to the load it carries. 22