Tipologia e dimensionamento estrutural das fundações de Torres de Linhas de Alta Tensão em diferentes condições geotécnicas Typology and structural design of High Tension Electric Lines foundations for different geotechnical conditions Ana Raquel Pereira Afonso IST, Instituto Superior Técnico, Lisbon, Portugal Key Words: Foundations, High tension electric line, Footing, Nailing, Pad and chimney October 2015 Introduction With the cities development, the need of producing and transporting electric energy to the population center has become critical. This energy needs to be transported by high tension electric lines. It has a great importance in development of economics activities, which causes a constantly evolution of technics, material and equipment (Decreto Regulamentar nº1/92 de 18 de Fevereiro). The construction of the line depends on topography and geology of ground foundation. The position and distance between towers depends on topography as well as other factors. The tower foundation depends on topography too, and geologic composition of foundation ground. A large number of investigations to the ground foundation are very important to a better characterization of the soil. A complete characterization of soil samples allows the optimization of the choice of the type of foundation and respective design. The need is therefore identified to systematize the methods of design of each type of foundation for each type of soil. The main types of foundation are: isolated footing, nailing and pad and chimney foundation with circular and rectangular section. For the design of foundations the code Regulamento de Segurança de Linhas Eléctricas de Alta Tensão (Decreto Regulamentar nº1/92 de 18 de Fevereiro) [1] can be used, where safety coefficients and some parameters related to soils are defined. The present study discusses the design methods based in principles of stability using coefficients and parameters present in this code and other methods from studies of the soil-foundation interaction behavior, including Sulzberger, Clouterre and Biarez and Barraud. 1
Case of study This present study has the description of the calculation methods of foundations and the design of foundations for real samples of soil with a chosen tower. The chosen tower (A60D2) for design has the loading described in Table. This tower has a greater height and demanding loading. Table 1 460D2 Tower loading A60D2 Tower Tension T Shear force Compression C Shear force height (m) Long V l,t Trans V t,t Long V l,c Trans V t,c 30 1533,15 169,33 174,55 1720,04 174,55 194,24 Design methods There are two types of foundations: isolated foundations and fractionated foundations. The design of isolated foundation is conditioned by overturning stability. The design of fractionated foundations is conditioned by uplift resistance. Isolated footing Stability design The overturning moment is calculated with loading of base of the tower considering the worst edge (in red). Fig. 1 Loading and isolated footing M der = 2 C ( B 2 L 2 ) 2 T (B 2 + L 2 ) (2 V l,t + 2 V l,c ) H The footing stability is guaranteed by own-weight. M est = PP sapata ( B 2 ) According to the code, the safe coefficient to overturning is 1,5. M est M inst 1,5 Sulzberger method [2] The Sulzberger method differs from previous by considering the reactions from soil which benefit the footing stability. The method is based on the principle that for a slope gradient which tan α < 0,01, soil has elastic behavior. Small displacement of the pad create stress in vertical excavation walls. The surface soil resistance is null and grow up proportionally to depth. 2
The soil reactions are represented in figure bellow. T d = T K γ d T k = T ult ξ Pad and chimney ; T ult = π D d α q s L b Pad and chimney with a circular section is like a pile with an enlargement at the base, even the reinforcement is like a pile only in vertical, the base has not reinforcement. Fig. 2 Soil stresses These stresses are included in the calculation to resisting moments through soil parameters. Nailing This type of foundations needs a good soil resistance, it s indicated for rock bloc with a level of RQD higher than 75%. The nailing resistance is assured by three elements involved, the steel bar, the cement grout and the soil. There are three types of failure: Pad and chimney with a rectangular section is a single footing founded at a big depth that has a chimney to connect with the structure of the tower. The design of both kind of pad and chimney is calculated using the same methods, since the behavior of interaction with soil is the same. This kind of foundation when subjected to uplift load, the base mobilizes a truncated cone of soil which works like part of uplift resistance of the foundation. Method based on angle of Regulamento [1] Steel bar failure Grout-soil interface failure Slippage of bar-grout interface Based on the principle of truncated cone formation, the code provides values to angle of incline, β, in Quadro Nº5.1-artigo 74º. Corrosion is a risk in this type of foundation, to prevent corrosion requires a good covering of bar and a water-cement ratio between 0,4-0,5. Nailing axial strength P γ = f syd A p Grout-soil interface resistance Fig. 3 - Truncated cone with angle β [3]. Based on Recommendations Clouterre 3
Uplift resistance is calculated with the sum of own-weight of the foundation and the weight of truncated cone. (V s γ s ) + PP fund > T i Biarez and Barraud [4] The method developed by Biarez and Barraud was based in experimental studies with reduced models with river sand and unsaturated clay. The behavior of pad and chimney depends of kind of soil, the method divided in two kind of soils: 1 st Category clay soils with higher saturation and internal friction ϕ 15º 2 nd Category powdery soils or unsaturated clay with high internal friction ϕ > 15º The depth of foundation influence the failure surface. If the depth of foundation is bigger than critical depth (Dc) the shear is localized. The parameters M are determined with abacus. 1 st category soils Fig. 4 1 st category soils behavior D Dc uplift resistance: Q ft = S L [C M c + γ D (M φ + M γ ) + q M q ] + P For rectangular section R = Re = p/8 D > Dc uplift resistance is a sum of two resistances: For rectangular section R = Re = p/8. Q ft2 = S L [C M c + γ (D D c )(M φ + M γ )] + P F(D Dc) For rectangular chinmey Rf = Ref = p f /2π. Q ft = Q ft1 + Q ft2 Q ft1 = S L [C M c + γ D c (M φ + M γ ) + qm q ] + P (DC) + P S 4
2 nd category soils Fig. 5 2 nd category soils behavior D < Dc uplift resistance: Soil characterization Q ft = S L [C M c + γ D (M φ + M γ ) + q M q ] + P Sample of soil S1 For rectangular section R = Re = p/2π. D > Dc uplift resistance: Q ft = Q ftb + Q ftf + P Q ftb is the resistance caused by localized shear. The powdery soil at this depth causes a failure surface with a circular form around the two sides of the base. Rock RQD Maximum compressive stress σ adm (kpa) Specific gravity γ s (kn/m 2 ) Siltstone 75-100% (5m) 1000 kpa 19,0 Q ftb = (S b S f ) m M (γ D tan φ + C) Q ftf is the resistance of chimney that act like a pile with α = - ϕ/8. Sample of soil S2 Rock Siltstone Q ftf = S L [C M c + γ D (M φ + M γ ) + q o M q ] RQD 50-75% (3m) For rectangular section R = Re = p/2π. Maximum compressive stress σ adm (kpa) Specific gravity γ s (kn/m 3 ) 800 kpa 19,0 5
Sample of soil S3 Soil Siltstone decomposed NSPT (average) 12,25 Table 2 - Isolated footing dimensions A (m) 12 Maximum compressive stress σ adm (kpa) Specific gravity γ s (kn/m 3 ) 600 kpa 18,5 B (m) 12 H (m) 3 Internal friction ϕ (º) 30 Using the Sulzberger method with consideration of soil stresses in stability to overturning, the dimensions are reduced to: Design of foundations For each type of soil, presents the main results of the foundation design that was considered most appropriate. However, other solutions were studied. Type of soil S1 Due to high level of RQD, soil S1 is a rock with good characteristics and high strength. Nailing foundation seems be the better choice. Considering nailing 16ϕ25, the tension in each nailing is 143,73 kn. The tensile strength is 213,53 kn, conditioned by nailing axial strength. A (m) 11 B (m) 11 H (m) 3 For the project design was considered the bigger dimensions, for security reasons. Type of soil S3 Due to weak conditions of soil, because of the degradation state of the rock, pad and chimney foundations seems the better option. Pad and chimney with circular section Table 3 - Pad and chinmey dimensions d (m) 1,1 D (m) 2,5 h1 (m) 6 h2 (m) 6,5 h (m) 6,7 Type of soil S2 Once at 3m of depth, soil present conditions for foundation, it was considered the solution of isolated footing. With stability design method without consideration of soil stresses, the dimensions of the footing are: b (m) 0,5 c (m) 0,2 Δ (m) 0,5 Comparing the results of uplift resistance of the two methods in study: 6
Uplift resistance by angle β of the code [kn] 2482,56 Uplift resistance by Biarez and Barraud method 3764,74 The method using the angle of the code is conditional. Pad and chimney with rectangular section Comparing the results of uplift resistance of the two methods in study: Uplift resistance by angle β of 2496,78 the code [kn] Uplift resistance by Biarez and 3698,51 Barraud method The method using the angle of the code is conditional. Table 4 - Pad and chimney dimensions b (m) 0,8 B (m) 3,0 a (m) 0,5 h (m) 5,0 c (m) 1,0 Table 5 - Results analysis Nailing Isolated Footing Pad and Chimney circular Pad and Chimney rectangular Soil S1 Rock Little degradation Good strength Soil S2 Median degradation Some strength Soil S3 High degradation Soil Low strength Legend: Suitable Not advised Very suitable Can be considered 7
Nailing foundation is the better solution for soil like S1, because takes vantage of rock strength. On the other hand, its construction implies skilled labor and specialized equipment. Isolated footing is an option that involves large amounts of material, but the excavation depth is low and doesn t need skilled labor. Pad and chimney with circular section has the problem of reinforcement, the fact of the reinforcement is only vertically and not exist in the base, compromises the tensile strength of the base. The equipment needed is very specialized. Pad and chimney with rectangular section is a good solution for different types of soil. However, the design methods in study only considered soil and not rock, furthermore the excavation of rock is not a good option. To ensure the intended effect of soil behavior, the excavation must be done completely vertical. of investigations to the ground in order to be representative of the entire line. Bibliographic References [1] Regulamento de Segurança de Linhas Eléctricas de Alta Tensão, Decreto Regulamentar nº 1/92 de 18 de Fevereiro, 1992. [2] P. LABEGALINI, Projectos Mecânicos das Linhas Aéreas de Transmissão, Blucher. [3] Projet national Clouterre, Recommandations Clouterre, Presses de Ecole Nationale des Ponts et chaussées, 1991. [4] J. BIAREZ e Y. BARRAUD, The Use of Soil Mechanics Methods for Adapting Tower Foundations to Soil Conditions, CIGRE, Paris, 1968. Conclusions In this present study, the stability methods based on coefficients and parameters of the code [1] gives more conservative results when compared with methods of geotechnical studies which considered the interaction and reactions from soil to foundation. A complete study of foundation soil characteristics is fundamental for a minimization of construction costs. It s essential to perform a considerable number 8