A pole embedded in the ground: how the embedment is checked

Which scheme this is about

A post set in the ground works as a bar in an elastic medium. In a foundation on a slab the resistance comes from the mass of the concrete and the pressure under the base, but here the horizontal force and the wind moment are taken by the lateral resistance of the soil along the embedment depth. So the scheme depends on the diameter or width of the post in the ground and on the soil properties, not on the mass of a foundation.

The scheme occurs in two forms. In the first, a low pole is lowered into a pit and concrete is poured around its lower part, as described in Flanged or direct-set lighting poles. In the second, a tall pole on a flange stands on a single post-foundation, which is a steel tube in a bored hole filled with concrete. The underground part of such a post is checked in the same way as a pole without a flange, and the flange connection at the top is calculated separately.

Foundation types for masts, towers and line poles, choice by soil and embedded parts are covered in Foundations for masts, towers and line poles, and what exactly is passed at the interface with the foundation is in Foundation under a pole per DBN V.2.1-10. Here we cover only the embedment check.

What data the check needs

The check needs the loads from the pole and the soil data. The loads are taken at ground level separately for each group of limit states, because the reliability factors in them differ.

First group: stability against overturning

Under a horizontal force the post rotates about a point at some depth: above it the soil resists on one side, below it on the opposite side. The check is: the design horizontal force must not exceed the limit force Qu multiplied by the working condition factor of the embedment and divided by the reliability factor for the purpose.

The limit force depends on the depth and width of the embedment, the unit weight of the soil, the friction angle and cohesion, the friction of soil against the post walls and the crossbars, if there are any. For the first group the design soil values are used, which are somewhat lower than the characteristic ones: in the example the internal friction angle is reduced from 29° to 26°. The position of the rotation point also comes out of the calculation: in the example it lay at a depth of about 0.69 of the embedment, that is roughly 3.4 m out of 5 m.

The check itself is simple in meaning: the acting force is compared with the limit force and one looks at what percentage of it is used. Finding the limit force is harder, and its method and factors are what the designer of the base checks against the current edition of the standards.

Second group: rotation angle of the post

Even when the soil does not fail, the post rotates under the wind and the whole pole leans with it. So deformations are checked: the rotation angle of the post under the characteristic load must not exceed the limit. The limit depends on what stands on the pole: it is set by the standards, the equipment maker or the design brief (more on the permissible tilt in Foundation under a pole per DBN V.2.1-10).

For a post without crossbars the rotation angle in the example was calculated by this formula: the angle β equals 0.75 · Q · (6α + 3) · υ divided by the product of the soil deformation modulus E and the square of the embedment depth d. Here Q is the shear for the second group, α is the ratio of the reduced height H to the depth d, and υ is a dimensionless factor read from a chart by the ratio of the design width to the depth. The design width is larger than the diameter of the post, because the soil next to it is drawn into the work as well.

Every thousandth of a radian of rotation means 1 mm of displacement per metre of height. So even a small angle on a tall pole gives a noticeable displacement of the top, to which the deflection of the pole itself is added. In the example, to reduce residual soil deformation near the surface, the upper part of the hole is closed with a reinforced concrete “plug”.

Conditional numerical example

The example is conditional: a post of 1.5 m diameter in a bored hole, embedment 5 m, medium sand, no crossbars. The values are converted from tonnes-force to kilonewtons and rounded.

A concrete-filled tube: strength check

If the post is a steel tube filled with concrete, the strength of its section is checked separately. The moment in the underground part is not equal to the moment at the ground surface: going down from the surface it first grows, because the lever arm of the force keeps increasing, and then falls once the soil starts to take it. So the section is examined at the depth where the moment is largest, not near the surface.

The section is treated as a steel-concrete one. It is assumed that sections remain plane, that concrete in the tension zone does not work and that the tube carries the tension. In the compression zone both concrete and steel work. Tests have shown that the gain from the concrete filling in bending is small, so it is allowed for by factors only slightly above one. The moments in the section are compared with the bending capacity under the relevant section of the standards on steel-concrete composite structures.

In the zone of the ground surface, where the tube is exposed to moisture and air, corrosion protection is needed; more on galvanising in Hot-dip galvanising.

Who does what

The designer of bases and foundations chooses the embedment scheme, the depth and diameter, determines the design soil characteristics from the survey and carries out the checks described above. That is his part of the project, not ours.

We make poles, masts, steel shell piles and embedded parts and prepare KM and KMD drawings: KM and KMD design, steel fabrication. We design a foundation under a separate order and for a separate fee, or only pass the loads to the foundation: the moment, shear and axial force at ground level for both groups of limit states. Erection is done by our partner construction company.

What to send for the embedment check

The check needs:

• the height and type of the pole, a list of equipment with mass and wind area;
• the wind region, the ice region and the terrain type;
• the loads at ground level for both groups of limit states, if they have already been calculated;
• the report of the engineering-geological survey: section, soil properties, water level, frost depth;
• the permissible rotation angle of the pole top from the equipment maker, if it is stricter than the standard one.

Standards and validity

The designer of the base checks the embedment against the current editions of these documents (as amended): DBN V.2.1-10:2018 «Bases and foundations of buildings and structures. Basic provisions», DBN V.1.2-2:2006 «Loads and actions. Design standards», DBN V.2.6-198:2014 «Steel structures. Design standards» and DBN V.2.6-160:2010 «Steel-concrete composite structures. Basic provisions». The method and factors used in the example have to be checked against the current text of the standards: this article does not replace a design.