Lighting Pole Foundations: Types and Construction

Cast-in-place foundation

A cast-in-place reinforced concrete foundation is made on site as a concrete block. The work goes in this order:

• excavating the foundation pit;
• setting up the formwork;
• placing the reinforcement;
• installing the anchor cage with a template, through which the pole is fixed to the foundation;
• fitting a condensate drain tube;
• running the wiring for the later connection to the power cable;
• pouring the concrete;
• backfilling the pit.

Pile shell foundation

A foundation of single pile shells is made by lowering a steel pile into a pre-drilled pit and filling it with concrete. The pile shell is made at the factory, usually from a tubular section. The pole is fixed to the foundation by a flange connection. The sequence is:

• drilling a pit 100 mm wider than the diameter of the pile shell;
• placing the pile shell in the pit;
• fitting a condensate drain tube;
• running the wiring for connection to the power cable;
• filling with concrete the inside of the pile and the space between the walls of the pit and the pile.

The drain tube and the wiring are fitted through technological holes in the pile shell.

Precast block foundation

A foundation in the form of a precast concrete block is made by lowering the block into a prepared pit and backfilling it. The block is made at the factory: a reinforced unit with an anchor cage already fitted for fixing the pole and a technological opening (window) for the condensate drain tube and the wiring. Steps:

• excavating the pit;
• placing the concrete block in the pit;
• fitting a condensate drain tube;
• running the wiring for connection to the power cable;
• filling the technological opening with cement-sand mortar;
• backfilling the pit.

How foundations are designed

Lighting pole foundations are designed from the customer’s technical specification to DBN V.2.1-10:2018. Under it, foundations are calculated for two groups of limit states: the first for bearing capacity, the second for deformation. The design takes into account:

• the results of geotechnical surveys;
• the seismicity of the construction area;
• the purpose, structural and process features of the poles and foundations and their operating conditions;
• the loads acting on the poles and foundations;
• the existing buildings and the effect of new construction on them;
• environmental requirements;
• the size of the plots;
• a technical and economic comparison of the possible options to choose the most advantageous one.

Which foundation to choose

The choice depends on the soil, pole height, luminaire and wind loads, and the site conditions (schedule, equipment, utilities).

Anchor unit and template

The pole is fixed to the foundation with a flange on anchor bolts. To make the flange holes match the anchors, the bolts are set in a template that fixes their position before the concrete is poured. The template also defines the height of the anchors above the surface and the verticality of the pole after installation.

The power cable enters through a service hole in the foundation (a condensate drain tube); the tube is installed before pouring.

The foundation as an earth electrode: what the PUE says

Under the Rules for the Arrangement of Electrical Installations (PUE), 2026 edition, cl. 1.7.115, reinforced-concrete structures of buildings and facilities that are in contact with the ground may be used as natural earth electrodes, including foundations in non-aggressive, weakly aggressive and moderately aggressive environments. For overhead-line poles the clause separately names the underground parts of reinforced-concrete and metal poles together with their foundations, provided the concrete is not waterproofed with polymer materials. Overhead outdoor-lighting lines are built to the requirements of Chapter 2.4, “Overhead power transmission lines up to 1 kV” (cl. 6.3.26), and that chapter states that a metal pole on a reinforced-concrete foundation must have a metallic bond between its metalwork and the foundation reinforcement (cl. 2.4.33).

The PUE requires four points to be settled by calculation (cl. 1.7.116): whether the current density in natural earth electrodes is acceptable, whether the reinforcing bars of the foundation must be welded, whether the anchor bolts must be welded to them, and whether the foundation may be used at all in a strongly aggressive environment.

An artificial earth electrode of hot-dip galvanised steel buried in the ground must meet the minimum sizes of Table 1.7.5 (cl. 1.7.117). Vertical round electrode: diameter 16 mm, coating 70 µm. Horizontal electrodes: round, diameter 10 mm with a 50 µm coating, and strip, cross-section 90 mm² and thickness 3 mm with a 70 µm coating. Aluminium earthing conductors must not be laid in the ground (cl. 1.7.120). The designer takes the final values for a specific site from the current text of the PUE.

Typical foundation mistakes

• inaccurate anchor position without a template: the pole cannot be seated on the flange without rework;
• insufficient compaction of concrete and backfill, damaged reinforcement cover;
• no cable tube or a tube in the wrong place;
• groundwater, heaving soils and frost depth not taken into account;
• installing the pole before the concrete has gained enough strength;
• earthing not done or damaged anti-corrosion coating on the anchors.

What is needed to calculate a foundation

Send the pole data (height, type, number and weight of luminaires), the construction region and, if available, the results of the geotechnical survey. From this we calculate and propose a foundation. A foundation design can be ordered together with the poles.