Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities

Table 8-1: Seismic zones (Hannigan et al. 2016)

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• 8.3.2 Liquefaction

Table 8-1: Seismic zones (Hannigan et al. 2016). Acceleration Coefficient, 𝑺𝑺 𝑫𝑫𝑫𝑫 Seismic Zone 1 2 3 4 𝑆𝑆 𝐷𝐷1 ≤ 0.15 0.15 < 𝑆𝑆 𝐷𝐷1 ≤ 0.30 0.30 < 𝑆𝑆 𝐷𝐷1 ≤ 0.50 0.50 < 𝑆𝑆 𝐷𝐷1 8.3.2 Liquefaction Ground failure due to liquefaction and/or seismic settlement beneath and near structures supported by deep foundations can cause significant damage to these structures. Liquefaction may occur for relatively large magnitude earthquakes in loose, saturated sands. However, other materials (e.g., some gravels and silts) can also liquefy. Several factors affect the liquefaction potential, including (i) geologic age and origin of deposit (ii) fines content and plasticity index (iii) degree of saturation of deposit (iv) depth below ground surface and (v) soil penetration resistance. The consequences to a deep foundation due to liquefaction include loss of lateral and vertical capacity and lateral spreading or lateral flow. Liquefaction potential should be addressed for structures where the peak earthquake acceleration will be greater than g (Hannigan et al. 2016). Foundations should be designed to accommodate loss of resistance, increased loads, displacements, and drag force resulting from liquefaction. An alternate option is to mitigate liquefaction potential through ground improvement techniques. Where liquefaction potential exists, foundations should be designed to extend below the zone of liquefaction with adequate resistance in the underlying deposits. Residual strength properties can be assigned to liquefiable layers to evaluate compression and uplift resistances during an earthquake. Both axial and lateral loads should be analyzed using the residual strength of liquefiable soils. Residual strength can be estimated by applying an equivalent clean sand blow count, determined using Equation 8-5, to Figure 8-1 (Hannigan et al. Equation 8-5) Where Ne = Equivalent clean sand blow count. N SPT N value corrected for energy and overburden stress. N corr = Correction for percent fines (see Figure 8-1). (𝑁𝑁 1 ) 60−𝑒𝑒 = (𝑁𝑁 1 ) 60 − 𝑁𝑁 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 120 Download 6.03 Mb. Share with your friends: