But sometimes the soil surrounding the pile can stick to the surface of the pile and cause. Negative skin friction is caused by groundwater drainage and soil consolidation.
Aide to classification of piles
Advantages and disadvantages of different pile material
Can be inspected before casting and can be easily cut or extended to desired length. The lateral displacement of the soil during piling is low (steel profile H or I profile piles) can be split or screwed relatively easily.
Classification of piles - Review
Task
LOAD ON PILES
Introduction
Calculation of load distribution in pile group consists of vertical piles subjected to eccentric vertical load. Calculate the load distribution on vertical and tiered columns subjected to vertical horizontal and eccentric loads.
Pile arrangement
However, if a group of columns is subjected to lateral load or vertical eccentric load or combination of vertical and lateral load, which may cause moment force in the group, which must be taken into account when calculating the load distribution. In the first part of this section, considering the set of columns with a limited number of columns subjected to vertical and lateral forces, forces acting centrally or eccentrically, we learn how these forces are distributed to individual columns.
LOAD DISTRIBUTION
Pile foundations: vertical piles only
The only vertical load acting on the pile group is the weight of the pile cup. Measure the distance from N.A to the cutting point R on the underside of the stack.
Pile foundations: vertical and raking piles
Determine the location of the N.A for the vertical and rip piles in plane position. As shown in Figure 3.6, the lateral force, H, is kept in equilibrium by the vertical and shear piles. NB: The horizontal force, H, applies a torsional force to the vertical posts. ri measured perpendicular to the N.A of both the vertical and raking piles.
First, we determine the location of the neutral axis, N.A, of both the vertical posts and the rake posts. From Figure 3.7 we see that the number of vertical poles = 8 and the number of rake poles. We can assume that the ¢ for the rake piles b1 and b4 serve as a reference line and calculate the location of the neutral axis for the rake piles as follows:
Calculate the forces acting on each pile
- Symmetrically arranged vertical and raking piles
As we can see, the maximum load of 279kN will be carried by piles c1 and the minimum load of 233kN will be carried by piles in row a1. Just as we did in the previous cases, we first decide the location of the neutral axis for both the vertical and rafter piles. In the case of symmetrically placed piles, the vertical pile I is subjected to compressive stress by the vertical component Pv, and the rip pile Pr is subjected to tension (see figure 3.11 - 12).
The symmetrical arrangement of pile piles keeps the lateral force, H, in balance and its effect on the vertical piles is ignored. The following example compares the theoretical and actual load distribution as a result of post-pillar deflection. The displacement of the pillars in the X-X direction is measured, the left edge of the pillar cap as a reference point (see figure 3.17).
LOAD ON SINGLE PILE
Introduction
Also in this section are examples of work that demonstrate the use of formulas used to predict the bearing capacity of piles made of different types of materials.
The behaviour of piles under load
Geotechnical design methods
The concept of the separate evaluation of shaft friction and base resistance forms the basis for the "static or soil mechanical" calculation of pile bearing. In terms of soil mechanics theory, the ultimate skin friction on the pile shaft is related to the horizontal effective stress acting on the shaft and the effective transformed friction angle between the pile and the clay. The ultimate shaft resistance Rs can be evaluated by integrating the shear strength τ a of the pile soil. Where: Nc, Nq, Nγ ,= bearing capacity factors and are functions of the internal friction angle φ of the soil, the relative compressibility of the soil and the pile geometry.
If the simplified assumption is made that the drained pile-soil adhesion C¢ a is zero and that the term in eqn (4-1)...which involves Nc, Nγ which ignores the drained ultimate bearing capacity of the pile, can be expressed as. All such formulas relate ultimate bearing capacity to pile set (the vertical movement per stroke of the driving hammer) and assume that the driving resistance is equal to the bearing capacity of the pile under static loading, they are based on an idealized representation of the action of the hammer on the stack in the final stage of its embedding. This can lead to dangerous misinterpretation of the results of dynamic formula calculation as they represent conditions at the time of driving.
SINGLE PILE DESIGN
- End bearing piles
- Friction piles
- Cohesion piles
- Steel piles
- Concrete piles
- Pre-cast concrete piles
- Simplified method of predicting the bearing capacity of timber piles Consider the previous case and use the following formula
Pile length 22 m, steel pile, friction pile, external diameter 100 mm, GC2, Determine the ultimate bearing capacity of the pile. Bearing capacity of the pile can be calculated using the following formula for pile installed in clay. The first formula gives us a lower value, therefore the design bearing capacity of the pile is 0.3 MN.
If a concrete pile is supported by soil with an undrained shear strength greater than 7 MPa throughout its length, the following formula can be used to determine the bearing capacity of the pile. Calculate the shear strength of the soil at the midpoint of each 3.5 m section Cmi. Due to the cohesion force, the pile can be loaded up to 55 MPa, the bearing capacity of the pile is therefore 55 MPa.
DESIGN OF PILE GROUP
In some cases, the shaft capacity of the pile driven in sand can increase by a factor of 2 or more. In the case where the pile spacing in one direction is much greater than that in the perpendicular direction, the capacity of the group that fails, as shown in Figure 6-2 b), must be assessed. For pile groups in cohesive soil, the group bearing capacity can be calculated as a block by men of eq.
For pile groups in non-cohesive soil, the group bearing capacity as a block can be calculated using e.g. For most pile groups placed in sand, the estimated capacity of the block will easily exceed the sum of the individual pile capacities. As a conservative design approach, the axial capacity of a pile group in sand is generally taken as the sum of the individual pile capacities, calculated using the formulas in 4-8.
Pile spacing and pile arrangement
A retaining wall with a weight of 120 kN/m including the own weight of the pile cap is built on a clay pile foundation.
PILE INSTALATION METHODS
Introduction
Pile driving methods (displacement piles) Methods of pile driving can be categorised as follows
A hammer of approximately the weight of the pile is raised a suitable height in a guide and released to strike the pile head. At the top of the stroke, or at a lesser height which can be controlled by the operator, the steam is cut off and the cylinder falls freely onto the pile cap. A pile frame is not required with this type of hammer, which can be attached to the top of the pile using leg guides, where the pile is guided by a wooden frame.
When used with a post frame, back guides are bolted to the hammer to engage the leaders, and only short leg guides are used to prevent the hammer from moving relative to the top of the post. Vibratory hammers are typically electrically or hydraulically driven and consist of counter-rotating eccentric masses in a housing attached to the pile head. The amplitude of the vibration is sufficient to reduce the skin friction on the sides of the pole.
Boring methods ( non-displacement piles) .1 Continuous Flight Auger (CFA)
A feature of auger piles that is sometimes used to exploit the bearing capacity of suitable strata by providing increased footing. In the closed position, the bottom drilling tool is positioned within the flat section of the pile shaft and then expanded at the bottom of the pile to form the . the lower part shown in fig. 8-3. Typically, after installation and before concrete is poured, the man-carrying cage is lowered and the shaft and bottom of the pile are inspected. A hollow cylindrical steel shaft, closed at the lower end with a one-way valve and equipped with triangular steel ribs, is pressed into the ground by a hydraulic piston.
At the same time, concrete is pumped through the center of the shat and through the one-way valve. A typical 6 meter long pile with an 800mm diameter base and a 350mm shaft, founded on moderately dense gravel under soft, overlapping ground, can reach an ultimate capacity of more than 200 tonnes. The post is suitable for embankments, hard uprights and floor slabs, where you have a soft silt layer over a gravel layer.
LOAD TEST ON PILES
Introduction
In the CRP (constant penetration rate) method, the test pile is driven into the ground, with the load adjusted to ensure the downward movement of the pile at a constant speed. Pile failure is defined in two ways: as the load at which the pile continues to move downward without further increase in load, or according to the BS, the load at which the penetration reaches a value equal to one-tenth of the diameter . from the pile to the base. A platform is built on the head of the pile on which a mass of heavy material called a "kentledge" is placed.
The ram of a hydraulic jack, placed on the pile head, rests on a crosshead under the bridge girders, so that a total reaction equal to the weight of the bridge and its load can be obtained. Figure 9-1, the constant increasing load test. Kentledge or adjacent tension piles or ground anchors are used to provide a response to the test load applied by jack(s) placed over the pile under test. The load is increased in certain steps and is maintained at each load level until all settlements have stopped within a certain specified time period or do not exceed a certain amount.
Limit State Design
- Goetechnical category 1, GC 1
- Geotechnical Category, GC 2
- Geotechnical Category, GC 3
- The partial factors γ m, γ n, γ Rd
It is a code requirement that the project be supervised at all stages by personnel with geotechnical knowledge. That it can be permanent as in the case of the weights of structures and installations, or variable according to the imposed loads, or wind and snow loads. It can cause strengthening such as the increase in strength of a clay with long-term loading, or weakening as in the case of excavation slopes in clay over a medium or long term period.
Partial factor γ m: this factor is used as a safety factor, so that the characteristic values of the material are divided by this factor. At the ultimate limit state, depending on the given conditions, for geotechnical category 2, the values of γ m can be determined using Tables 10-1 and 10-2. Partial coefficient γ n: to ensure stability and adequate strength in the structure and in the ground, cases A, B and C are introduced in the code.
