Determining the pile load capacity by the 4D-8D method

Science erwinman December 9, 2016 0 17
The foundation of a building can be made by means of concrete piles. To that end, are formed on the construction or piles driven into the ground. Depending on the composition of the soil may vary by installation method, pile length and cross-sectional shape on the basis of a probe made in the work ring, determine the load-bearing capacity. What is the 4D-8D method and how is this calculated the pile carrying capacity?

Determine pole Capacity: 4D-8D method

  • Need probes
  • Asking the qc, avg values ​​over route
  • Pile tip resistance
  • Maximum shaft friction
  • Pole Capacity

Need probes

To determine what the soil bearing capacity has to be taken soundings. Usually, which should be up to 15 m or more can be performed depending on locally known condition of the substrate. A probe ring is a graphical representation of the structure of the soil, in which the resistance is measured. To this end, a cone is pressed to the ground, with which a build-up resistance can be determined, expressed in MPa. If graphics are known or can be done on the basis of an assessment of the soil structure, which allows the pile bearing capacity can be determined. See the following theoretical probe ring with soft clay layers on the surface, after which different layers of sand alternate. In this chart, the following classification in terms of soil structure using Table 1 NEN 6740 is determined as follows.

Asking the qc, avg values ​​over route

For each probe ring holds that peak values ​​of more than 15 MPa always have to be truncated. If, in addition, peak values ​​of 12 MPa apply than than those should also be truncated if it is less than 1 m thick. If in previous sounding the pile tip level is placed at 18.6 m is then calculated how the pile bearing capacity. To this end, held three border areas, for which the following applies:
  • qc, I avg = the mean cone resistance about 0.7 to 4 * D * D under pile tip level. The average value should be so determined that the value is at least about the range indicated;
  • qc; II gem = bottom of the range I to pile tip level at which the value qc is equal to or less than lower value. It may mean that the start to the minimum level over the range I, but may also be equivalent to the range I;
  • qc; III avg = the average cone resistance of pile tip level to 8D above, wherein the value qc is never higher than lower value. For avegaarpalen applies here is that the average qc; value is 2 MPa or less.

Pile tip resistance

On the basis of previous sections a pole point resistance may be calculated by the following formula:
  • p, r, max, point = 1/2 * α, β p *, s * s * with it;
  • α, P = pile class factor;
  • β; s = shape factor pole standard 1.0. If the point has been increased, the influence on the surrounding floor part, so that pile bearing capacity is decreasing;
  • s = factor for odd shaped spring which is different from width. This factor is normally 1.0.
Based on previous sounding and foundation level at 18.6 m below ground level, the following can be determined:
  • p; r; max; point = 1/2 * 0.8 * 1 * 1 * = 5.2 MPa / m2.

Maximum shaft friction

The maximum length over which shaft with friction into account may be is placed over part of the pile tip level to a maximum 8D, in which the pole is in sandy soil. It passes a clay or loam layer then there is a reduced α; s, however, the value qc must be relatively high. On soft clay no shaft friction is granted and the overlying layers as well. The average q; c, z; a value over the range in the probe ring is 7.8 MPa.
  • p, r, max; shaft = α; s * qc; z; a gem;
  • α; s = pile class factor.
In the previous example so it finds a shaft friction:
  • p; r; max; shank = 0.006 * 7.8 = 0.0468 MPa.

Pole Capacity

In order to determine the pile load-bearing capacity, the following is calculated:
  • Fr; fr = max, max, + Fr point; max; shaft;
  • Fr, max; point = Apunt * pr; max = 1/4 * π * 0.3 ^ 2 * 5.2 = 0.367 MN = 367 kN;
  • Fr, max; shaft = Os, avg * ΔL * pr; max; shaft = π * 0.3 * 2.4 * 0.0468 = 0.106 MN = 106 kN;
  • Fr; max = 367 + 106 = 473 kN;
  • Fr, max; rep = * fr, max = 0.72 * 473 = 341 kN;
  • Fr, max d = pile load capacity = fr; max; rep / 1.2 = 341 / 1.2 = 284 kN.

A single pole may therefore 284 kN record if it is built on 18.6 m below ground level. If larger loads are to be recorded than is necessary to apply a larger diameter. Mind you normally will be more probing per property, making the ξ factor is higher and the capacity is more favorable.