Foundations for Masts, Towers and Power Line Poles: Types, Soil, Anchor Cages

Which foundations are used for masts, towers and power line poles

Besides weight, a structure transfers to the ground a horizontal force and a moment from wind, ice and conductor tension. The moment mostly sets the foundation size: a tall, light structure may need a stronger base than a short, heavy one. The types differ in how they resist it:

See also: lighting poles, catenary masts, power line poles, telecom towers.

How to choose a foundation by soil and load

The choice starts with loads (height, area of equipment or conductors, wind and ice regions under DBN V.1.2-2:2006, pole type) and ground (layers, water level, freezing depth, frost-heave susceptibility, chemical aggressiveness). The foundation is then checked under DBN V.2.1-10 against the two groups of limit states: bearing capacity and deformation; for towers the allowable tilt, set by the equipment manufacturer, matters too. Wind pressure W0 for regional centres is in the article Wind Regions of Ukraine, and the choice of height is in Height of a Mast or Tower.

On anchor, angle and dead-end poles of a line the difference in conductor tension adds to the wind load, so their foundations are usually larger.

Anchor cages and anchor bolts: what the joint consists of

An anchor cage is a group of anchor bolts that templates hold in position while the concrete gains strength. The pole flange goes over the bolts and is tightened with nuts, so its holes must match the bolts without adjustment. Sizes are in the article on foundation anchor cages; cages of the FPO, FPM and FKM series can be ordered separately from poles. For a sense of size: the FPMV-810-1200 cage for a PMV-20 mast (see Floodlight Masts) has 16 M36 anchors, 1200 mm long, on an 810 mm diameter bolt circle; telecom towers 25–40 m use 12–20 bolts, M36–M42 (see Telecom Towers).

What matters in the joint:

• the upper and lower templates hold the anchor spacing and verticality and keep the cage from shifting during concreting;
• the flange rests on levelling nuts or a levelled surface, with the gap grouted with non-shrink mortar if needed;
• the cable duct and earthing lead go in before concreting; the thread is shielded from concrete splashes until erection, and its corrosion protection above the concrete is set by the design.

Tolerances at installation of anchor cages

Anchor position tolerances come from the foundation design and the pole drawing. For a flange it matters more that the bolts keep their positions relative to each other than that the group sits exactly on the axis: the flange hole is only slightly larger than the bolt. That is why the cage is assembled and cast with templates. Every bolt is checked and recorded in the as-built diagram, and deviations are corrected before erection, not by adjusting the flange on site.

A useful reference is the acceptance-control table for concrete structures in the national guidance document DSTU-N B V.2.6-203:2015, the row on the location of anchor bolts:

These are general values for concrete structures, not a rule for a bolt group under a flange: the relative position of the bolts is limited by the clearance in the flange hole and is set by the drawing. The guidance is a recommendation and does not replace the design documents, and a stricter tolerance on the drawings governs.

Geotechnical survey: what is needed before design

Without a survey the soil properties have to be assumed, which risks recalculation or an inadequate base. The survey must give a geological section showing weak interbeds, soil properties from field and laboratory tests, the groundwater level (present and highest possible), aggressiveness towards concrete and metal, freezing depth, frost heave and seismicity.

Besides the report, the designer needs the loads at the base: axial force, moment and shear. For our towers, axial force and moment are on the page Telecom Towers.

Selection example: a 30 m telecom tower on weak ground

A hypothetical example. For a 30 m tower our catalogue has two versions: with 200 kg of equipment and a base moment of 346 kN·m, and with 320 kg and 1113 kN·m. The axial force differs by less than a factor of two (35 and 65 kN), the moment by more than a factor of three: the moment decides which foundation is needed. On site the upper metres are made ground and plastic soil, dense sand lies below, and groundwater is close to the surface. The reasoning:

• a slab or pier at shallow depth is ruled out: the base would sit in the weak layer and below the water table, and uneven settlement would give a tilt unacceptable for antennas;
• that leaves piles to the dense sand: one large-diameter pile under the flange, or a cluster with a cap into which the cage is cast (pile axes, as a rule, at least three pile diameters apart); for the smaller moment a single pile is considered, for the larger a deeper pile or a cluster;
• because groundwater is shallow, the cap is waterproofed and the concrete is of higher watertightness.

Checking concrete and anchor cages before the pole is erected

The foundation is checked before the pole goes on it, again against the guidance DSTU-N B V.2.6-203:2015:

• before concreting, the base, reinforcement and embedded items are inspected and accepted as work that will be covered up, and the cage is braced so that the pressure of the mix cannot move it;
• the mix must match the strength class and the frost-resistance and watertightness grades in the design; adding water at the point of placing is prohibited;
• concrete is placed in horizontal layers of equal thickness, and the vibrator is rested neither on the reinforcement nor on the embedded items;
• in heat (above 25 °C) curing starts straight after placing: the concrete is kept under a moist cover or under a continuous water spray, not watered periodically, as a rule until it reaches about 70 % of the design strength;
• in frost (mean daily temperature below 5 °C, minimum below 0 °C) winter measures are needed; at a temperature below minus 10 °C, for heavily reinforced structures, bars over 24 mm in diameter, rigid rolled sections and metal embedded parts, the metal is warmed to a positive temperature and the mix beside it is vibrated for longer (by at least 25 % compared with summer concreting); strength in frost is, as a rule, checked on specimens made near the place of placing.

Erection starts when specimen tests confirm the strength stated in the design. There is no single waiting time: the design age of concrete is usually 28 days, and in cold weather the concrete gains strength more slowly.