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1. INTRODUCTION
The design of overhead line tower or substation gantry structure foundations must be such as to safely sustain and transmit to the ground the combined dead load, imposed load and wind load in such a manner as not to cause any settlement or movement which would impair the stability of the structure or cause damage. The settlement is a result of the transfer of load from the structure to the soil layers. Essentially settlement must be minimized to an acceptable level for the design life of the structure and adequate factors of safety applied to ensure this. Foundation design requires information on the properties of the soil and in particular its compressibility, moisture content, plasticity characteristics, friction between soil particles and for fine soils its undrained shear strength. This section describes typical soil investigations and foundation design. Such design is the responsibility of the civil engineer.
The details described in this section are intended to give the transmission and distribution electrical engineer an appreciation of the factors involved.
--2 SOIL INVESTIGATIONS
Ground investigations are carried out by geotechnical experts using bore holes, trial pits and penetrometer tests.
Investigations take the form of in situ and laboratory tests. In situ tests include standard penetration tests to provide data on the relative density to sand for the more coarse-grained soils.
Laboratory investigations on soil samples taken from boreholes or trial pits will measure grain size, density, shear strength, compressibility, chemical composition and moisture content such that the soil can be categorized.
Figures 1a and 1b give a useful guide for estimating soil types based on cone penetrometer end resistance and friction ratio. Examples of Middle East and UK substation soil penetrometer site investigations are given in FIGs. 2a and 2c. Soil chemical test results, grain-size distribution, consolidation and plasticity are given in FIGs. 3a to 3d, respectively.
Guidance on geotechnical design is provided in Eurocode 7 (EN 1997).
FIG. 1a Guide for estimating soil type from penetrometer testing.
FIG. 1b Guide for estimating soil type from penetrometer testing.
FIG. 2a Soil investigation borehole profile ( not shown)
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FIG. 2b Dutch cone penetrometer and accessories.
Cone rod
Friction jacket Union sleeve Mantle Lock washer Cone angle 60° All dimensions are in millimeters Mantle cone Friction jacket cone Sounding tube with friction reducing ring Sounding tube Pressure rod; Retaining sleeve
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--3 FOUNDATION TYPES
The results from the soil investigation allow the civil engineer to decide which type of foundation will most economically support the structure. The actual practical solution will take into account the access requirements for a piling rig, availability and transportation of materials.
Large concrete raft type platforms as shown diagrammatically in Fig. 4a are used where the upper layers of soil have relatively low bearing capacities.
This type of foundation will evenly distribute the load over a wide area thus avoiding potential bearing capacity failure and ensuring that any settlement will be acceptable and even. FIG. 4b shows a close-up view of 'pad and chimney' foundation steelwork during concrete pouring operations.
Pile foundations, FIGs. 5a and 5b, are necessary for very poor soils and are used where the weight of the structure is likely to cause bearing capacity failure or excessive settlement of the upper layers. The pile foundation transmits the load to the lower and more stable areas of ground. Several piles may be required for each tower foundation depending upon the load capacity. Bored, cast-in-situ piles typically utilize temporary steel casings which are bored into the ground and removed during concreting.
Alternatively, the piles can be steel or precast concrete sections which are driven into the ground. Less common are galvanized steel screw anchor piles which are screwed into the ground until the required resistance is achieved.
They are then left in the ground and the tower attached directly to the screw anchor stubs. They are more expensive than traditional piling methods because of the material costs but they provide an extremely fast method of construction. The pile installation machine consists of a hydraulic unit attached to a JCB, Poclain or similar type of digger. FIG. 5b shows an auger for forming bored cast-in-situ concrete piles at a 400 kV substation site.
Denser soils with high bearing capacity only require small rectangular foundations or strip footings to transfer adequately the structure weight to the soil. Undercutting arrangements ensure stability in uplift conditions (see FIGs. 6a and 6b).
Hard ground or rock conditions require anchor foundations where reinforcement is grouted into predrilled holes in the rock.
Simple distribution pole foundations may not require concrete. The pole is lowered into a predrilled hole with compacted soil backfill.
FIG. 2c Dutch cone penetration test.
FIG. 3a Soil chemical test results (coming soon)
FIG. 3b Soil grain size distribution.
--4 FOUNDATION DESIGN
The wind loading on the towers will result in uplift or compression forces being transmitted through the tower legs to the foundations. Terminal or heavy angle towers will have tower leg foundations remaining in uplift or compression although a check is necessary to ensure broken wire conditions do not reverse the effect. Straight line or light angle towers can have the loading reversed depending upon the wind direction and therefore the foundations for each tower leg must be capable of restraint in both modes (see FIG. 7). EN 50341 requires 'special attention ... to be paid to the interaction of loads resulting from active soil pressures and the permanent weight of foundation and soil'. It also warns of the buoyancy effect of ground water on soil and foundations.
EN50341-3-9 also has specific requirements on precautions relating to uplift.
FIG. 3c Soil consolidation test results.[coming soon]
FIG. 3d Soil plasticity test results.
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FIG. 4a Overhead line power and pole foundations.
Undercut pad and chimney foundation Rock anchor foundation Pad and chimney foundation Enlarged pad and chimney foundation for poor soils and large leg loads Pyramid block foundation Guyed tower foundations
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FIG. 4b Pad and chimney foundation steelwork _ concrete pour in progress.
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--5 SITE WORKS
--5.1 Setting Out
Accurate setting out is essential in substation and overhead line work in order to match the prefabricated assemblies and structures to the foundation holding-down arrangements.
--5.2 Excavation
The specifications issued with the tender documentation associated with the works must include details for shoring up trenches and excavations in order to safeguard workers from collapse. In addition, it should be noted that cable trench depth, cable surround and backfill material have a direct effect upon cable ratings. In a similar way foundation depths may affect the final height of overhead line supporting structures and associated clearances. Therefore if cable trench or overhead line structure excavation depths differ from those specified then the civil engineers must be made aware that the electrical characteristics may be compromised.
--5.3 Piling
Sample load checks on a random selection of piles must be included in the civil works specifications in order to prove that the predicted load bearing capability has been obtained in practice. This is particularly important for bored piles. For driven piles or screw anchor foundations the torque converter or resistance limit is set on the pile driving machine and this must be regularly checked for correct calibration.
The load tests measure settlement under pile loading and recovery on removal of the test load. For a satisfactory proof load test on a trial pile, loads at 23 working load should not result in excessive settlement. Routine checks during construction works on actual foundation piles at 1 1/2 x working load are typical.
--5.4 Earthworks
An economic design will attempt to match cut and fill earthwork quantities.
This might be possible by using the 'cut' to prepare the substation level switchyard. As long as cable or line entries to the substation are still possible the resulting 'fill' may then be used to form embankments around the switchyard thereby reducing the environmental impact.
Soils must be deposited in shallow layers. Voids are reduced and the ground consolidated by compacting each layer in turn. The final soil density of the 'fill' should be within 90% of the optimum density obtained by a standard Proctor compaction test.
FIG. 5a Auger for forming bored cast-in-situ concrete piles
--5.5 Concrete
Concrete specification is the responsibility of the civil engineer. The electrical engineer needs to appreciate the fundamentals and the terminology.
Concrete design is based on the required characteristic strength measured in N/mm^2. Concrete gains strength over time as the concrete 'cures'. Concrete cube samples are taken from each batch and measured for strength in a materials laboratory often after 7 and 28 days. Results from the tests taken after 7 days will give a good guide to the 28-day characteristic strength. The curing period will of course impose constraints upon the timing for the installation of switchgear and steelwork structures on the foundations. Concrete may be quoted in terms of its aggregate size and 28-day characteristic cube strength as, say, 30/20, meaning 30 N/mm^2 strength and 20 mm nominal maximum aggregate size. FIG. 8 shows concrete site investigations taking place during construction of the Yanbu-Medina 380 kV transmission line in Saudi Arabia. The 'slump cone' gives details of the nature and consistency of the concrete. The cube moulds are used to form the concrete cubes for determination of concrete strength.
FIG. 5b Auger for forming cast-in-situ concrete piles _ 400 kV substation.
FIG. 6 (a) Standard pad foundation; (b) undercut pad foundation.
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Wind direction or Wind direction
Compression, Uplift
FIG. 7 Uplift and compression on tower foundations.
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FIG. 8 Concrete cube and slump cone samples being prepared _ Yanbu-Medina
380 kV transmission line, Saudi Arabia ( Balfour Beatty Projects and
Engineering).
The overhead lines standard EN50341 refers to Eurocode 2 (EN 1992) for concrete foundation design.
Concrete durability depends upon the degree of exposure, the concrete grade (or strength) and the cement content. A high density, alkali-resistant concrete will better resist the effects of moisture penetration. Since concrete is a porous material reinforcement bars within the concrete will be subject to possible corrosion if safeguards are not taken. Adequate concrete 'cover' to the reinforcement should be included in the specifications in order to reduce moisture or salts penetrating to the rebar. Should the rebar corrode it will expand and the considerable forces involved cause the concrete to crack.
The amount of water present in a mix has a large influence on the strength and durability of concrete. Too much water will not only decrease strength and durability but could also be the cause of shrinkage cracks as the concrete dries out. The optimum water content of the mix should be determined by trial mixes thereby allowing the slump to be defined. This desired slump can then be checked on site by means of a slump test.
Curing is the stage of concrete construction where chemical reactions under controlled conditions ensure that the concrete correctly gains its design strength. Concrete will gain strength rapidly at first and an initial 'set' takes place within a few hours and a good strength after 3 days. About 60% of final strength is gained after 7 days and full strength is assumed to have been achieved after 28 days although the process goes on for many months.
The concrete should be kept damp during the curing process by using modern chemical sprays, or the more traditional wet sacking. Concrete surfaces must be protected from dry windy conditions or intense sunlight by polythene sheeting or sacking screens in order to avoid rapid surface evaporation.
Uncontrolled cracking of a concrete member can seriously affect its structural integrity. Cracks in reinforced concrete members should not generally exceed 0.3 mm in width in order to avoid possible deterioration of the rein forcing steel. Cracking must, therefore, be controlled. This is initially done by the designer in his prudent spacing of reinforcement and also on site by adequate curing as described in SECTION 5.5.4. On large-volume concrete pours insulated shutters and adequate cooling may also be necessary in order to prevent thermal cracking.
The electrical engineer may be called upon to assist with site supervision and should look out for the following points regarding concrete foundation works:
1. Materials, including rebar, should be checked for cleanliness and grading.
2. Sand and aggregate storage conditions must be such as to keep materials clean. Materials must be clearly identified in order to avoid accidental mixing of wrong components. A batching plant (even if not fully auto mated) should have a concrete hardstanding and bunkers for the easy delivery of materials by lorry and segregation of different grades of aggregates. Cement must be stored in a damp-proof building and used in the order in which it is delivered _ first in, first out.
3. Volume or weight measurement of the different concrete components may be used to obtain the required mix. Weight measurement is to be preferred since the volume of a fixed amount of fine aggregate can vary considerably depending upon its moisture content. Full bags rather than part bags of cement should be used in any one mix. The mixing of structural concrete must always be done by machine.
4. Trials on the specified mix should be done in advance before full construction work begins on site in order to prove that the local materials match up to the expectations of the specifications. Such tests must form part of the overall concrete cube strength program that should continue throughout the construction period. It is essential that additional water is not added during the work since excess water will produce a porous concrete with low strength.
5. Skilled carpenters should be used to prepare firm shuttering for concrete placement. Concrete compaction must be achieved with the aid of mechanical vibration equipment. This also allows even mixing between the different pours of concrete into the foundation. Such a process should be as continuous as possible in order to avoid weak joints between the pours.
6. The curing process must be controlled by ensuring that the concrete is kept moist.
--5.6 Steelwork Fixings
Overhead line towers connect to stubs which form an integral part of the foundation. Stubs or fixing bolts are locked into the foundation reinforcement as part of the design.
Large items of substation plant such as power transformers may merely sit on the foundation and no special fixing arrangements are required. In environmentally sensitive areas vibration pads may be required between the transformer and the base, and then it is essential that no rigid fixing exists between the plant and the concrete mass. Stability is achieved by the weight of the transformer itself. For smaller transformers with wheel fixings arrangements are made to lock the wheels in position. Larger transformers tend to be skid mounted and steel runner plates may form part of the foundation design such that the transformer may be slid into position without dam age to the concrete surface (but again, special precautions may be needed to avoid damaging vibration pads).
Substation steelwork structures or switchgear may be connected to the foundations either by setting fixing bolts into the concrete foundation or by leaving pockets in the foundation for future grouting-in of fixing bolts.
Accurate setting out is essential when the fixing bolts form an integral part of the foundation. A wooden template should be used to hold the bolts in the correct position during concrete pours. Pockets left in the foundation allow more flexibility. Bolts may be adjusted in position in the pockets and then grouted-in in their final position after matching with the switchgear or steel work. It is essential that the correct bolts are used and that all connections are correctly tightened.