Practical Soil Dynamics

Engineers prefer to use codes for design of structures subjected to cyclic and dynamic loads. However, design codes are very brief concerning the seismic response of underground structures (foundations, tunnels, pipelines) and when they provide recommendations on the best practice these recommendations are limited to usual types of structures (buildings) and ground conditions. The users of British Standards are aware that compliance with them does not necessarily confer immunity from relevant statutory and legal requirements. Often engineers need to seek advice and help from specialists in soil dynamics. Because the issues in soil dynamics are rather complex, the specialist use simple considerations and methods not least for checking of the results of more complex analyses. Hence, engineers can use simple considerations and methods for assessment of severity of a problem before engaging specialists for solution of the problem.

Earthquakes and their effects represent the greatest and most frequent influential factor for ground and structural damage. Less damaging and frequent factor of structural damage and effect on people and processes are the industrial activities.

Ground motions are recorded (a) for quantitative assessments of their effects on structures, processes and people, (b) for checking of predicted values, (c) when required by legislations and in other cases.

Soil is a complex system of grains of different shapes, sizes and minerals as well as voids filled with water and/or air. Soil is heterogeneous, anisotropic and inelastic medium with its properties dependent on previous loading history, state of stress and its path in static condition. In cyclic condition, soil properties can also depend on the amplitude, number of cycles and even frequency of loading if fine grained.

Fast slope failures are important because they frequently leave no time available for their remediation or even escape of people. Several types of fast slope failures occur, such as:

• Slides on single or multiple surfaces

• Rock falls and rolls

• Soil and rock avalanches, debris run-outs and fast spreads

• Soil flows

Flows caused by liquefaction of sandy slopes and the effects on adjacent structures are among the most significant hazards caused by earthquakes. EN 1998-5 (2004) (and other publications world wide) specifies procedure for determination of liquefaction potential of sandy soil with horizontal or gently inclined surface using an empirical method but not for ground slopes during earthquakes.

Massive retaining walls use their own weight to transfer nearly horizontal lateral soil forces on the wall into nearly vertical forces on ground under the wall. Sketches of cross sections of massive retaining walls are shown in Fig. 7.1. The massive walls are also called gravity walls.

Most massive retaining walls performed well during earthquakes with exception of a few cases some of which are described in the case histories in Section 7.5.

Slender retaining walls have emerged with developments in reinforced concrete and steel production as they use their flexural stiffness to transfer nearly horizontal active lateral soil forces on the wall into nearly horizontal passive forces onto the ground in front of wall. Sketches of cross sections of slender retaining walls are shown in Fig. 8.1. The walls are also called embedded.

Shallow foundations transfer loads from structures into soil via the foundation undersides mainly and could be of pad, strip and raft shape. They are used when ground is firm or when loads are relatively small. The factors of safety for shallow foundations in static condition are usually high so that their performance in cyclic condition is usually satisfactory except in liquefiable and soft/loose soil in which case the build up of excess pore water pressure and reduction of soil strength to a small residual value could cause foundation failure and loss of serviceability as described in Section 9.5. Modern shallow foundations are made of (reinforced) concrete although older types were made of brick work or stone masonry, Fig. 9.1.

EN 1998-5 (2004) specifies that the effects of dynamic soil-structure interaction shall be taken into account when P-δ effects are important (i.e. the bending moment caused by axial force times column deflexion), for structures with massive or deep-seated foundations, such as bridge piers, caissons and silos, for slender tall structures, such as towers and chimneys, and for structures supported on very soft soil, with average shear wave velocity less than 100 m/s. The code also states that piles and piers shall be designed to resist the following two types of action effects:

(a) Inertia forces from the superstructure.

(b) Kinematic forces arising from the deformation of the surrounding soil due to passage of seismic waves.

Traditional belief is that tunnels are not affected much by earthquakes except if an active tectonic fault moves across a tunnel. Dowding and Rozen (1978) studied the response of 71 tunnels in rock to earthquake motions. The damage ranged from cracking to closure in total 42 cases. Sharma and Judd (1991) compiled a database on the response of 192 tunnels during 85 earthquakes throughout the world; 94 of the tunnels suffered from small to heavy damage. More than half the damage reported was caused by events that exceeded magnitude 7 of the Richter scale, and nearly 75% of the damage reported occurred within 50 km of the earthquake epicentre. There was no damage in tunnels where the horizontal peak ground acceleration was up to 0.2g. In most cases where damage was reported, the peak ground accelerations were larger than 0.4g. The data show that shallow tunnels are at greater risk during earthquakes than deeper tunnels; roughly 60% of the total cases had overburden depths less than 50 m and suffered some damage. Ground type is also important; 79% of the openings excavated in soil were reported to have suffered some damage. Dean et al. (2006) reviewed data of 1108 tunnels worldwide with diameters larger than 3 m, of which only twelve were subjected to earthquakes with magnitudes greater than 6 that caused the peak horizontal ground accelerations in excess of 0.16g since 1980. Only four tunnels were reported damaged none of which had precast concrete tunnel linings.

Ground vibration caused by industry can result in cracking of structures, disruption of processes and annoyance to people. Vibration consideration involves its source, propagation path and its recipient. A staring point of vibration consideration should be definition of acceptable vibration effect on a recipient. For example, both ANSI S3.29 (1983) and BS 6472 (1992) recommend the same basic root mean square (r.m.s.) accelerations in the vertical direction for critical working areas such as hospital operating theatres and precision laboratories shown in Fig. 12.1. The r.m.s. acceleration is the square root of the average of sum of squares of componential accelerations. Both codes recommend the multiplication factor of 4 of the basic r.m.s. acceleration for offices, and 8 for workshops for continuous (and intermittent vibrations and repeated impulsive shock according to ANSI S3.29, 1983) and 128 for both offices and workshops for impulsive vibration excitation (with duration less than 2 s) with up to 3 occurrences a day. These two codes differ only concerning the multiplication factors of the basic r.m.s. accelerations for residential buildings as shown in Table 12.1. In addition, BS 6472 (1992) recommends the use of the same multiplication factors for the peak velocity.