material, strain is induced. Strain, ε, caused by propagation of a compression wave (P-wave) can be determined if the particle velocity, v , measured in the direction of P wave propagation, and the wave speed, c P are known. ε = vP cP Similarly, the shear strain, γ, can be calculated by dividing the particle velocity measured perpendicularly to the direction of wave propagation with the shear wave speed, c .S γ = vS cS Shear strain is an important parameter when assessing the risk of settlement in granular soils or disturbance of cohesive soils. A threshold strain level, γ , exists t below which it is unlikely that any rearrangement of soil particles will occur and, therefore, the vibrations will not generate an increase of pore water pressure in water-saturated sands. It has been shown that soil disturbance will not occur if shear strain are below a threshold value of γ ≈0.001%. When this level is exceeded, t the risk of particle rearrangement and thus settlement increases (see Figure 3). At a shear strain level of 0.01%, vibrations can start to cause settlement, and this value should not be exceeded. Significant risk of settlements exists when the shear strain level exceeds 0.1%. It is important to note that shear modulus and shear wave speed are affected by shear strain. The shear wave speed decreases with increasing shear strain and this reduction depends on the fines content (plasticity index) of the soil. The reduction of shear wave speed is more pronounced in gravel and sand than in silt and is even smaller in clay. In the following example, piles are driven in the vicinity of a building founded on medium dense sand. It is assumed that the sand has an average shear wave speed of 200 m/s (656 ft/s). From Figure 3 it is possible to determine the three damage threshold levels: no risk (0.001% shear strain) = 2 mm/s (0.08 in/s); low risk (0.01% shear strain) = 20 mm/s (0.8 in/s); and high risk (0.1% shear strain) = 75 mm/s (2.9 in/s). In practice, a planning engineer can require pile driving tests where the expected vibration levels can be determined as a function of pile penetration depth and at different distances. This information can be used to assess the risk level with respect to settlement in the sand. If the predicted vibration level exceeds the “low risk” level, a detailed monitoring program should be implemented. This simple example illustrates that it is possible to assess the risk of settlement when sandy soil is subjected to ground vibrations. Of course, a more detailed analysis can be performed which also takes into account other important factors, such as number of vibration cycles, etc. However, for many practical purposes, a simple assessment of the settlement risk in combination with field monitoring will suffice. Conclusions Driving of piles or sheet piles can cause damage to building foundations or installations in the ground, such as sewage pipes and tanks. Some of the observed damage may not directly be related to vibrations but to static ground movement. Vibrations can cause settlement in loose granular soils, a fact which is not appreciated in most building vibration standards. Differential settlements of the ground below a building are often the main reason for damage in building foundations, where damage can propagate up the building structure and be interpreted as vibration damage. Therefore, it is important that the risk of settlement in granular soils due to ground vibrations is included in a risk analysis. Another important aspect which frequently is overlooked is the type of building foundation. Of particular significance is the vulnerability of buildings with mixed foundations where one part of the building is founded on stiff ground or piles, and the other part on soft or loose soil. A basic method is proposed to estimate the risk of settlement due to ground vibration in granular soils. Even in very loose sand and silt with a shear wave speed of about 100 m/s (328 ft/s), settlements are unlikely to occur when the peak particle velocity is below 1 mm/s (0.04 in/s) However, the risk of settlement increases when the peak particle velocity exceeds about 10 mm/s (0.4 in/s). In the case of important projects, pile driving tests and vibration monitoring will provide val- uable information. For additional information, please refer to the conference proceedings for the complete paper as well as the companion paper: “Part 2 – Review of Vibration Standards.” Printed proceedings are avai lable at ht tp: / /www.df i .org/ publications.asp?goto=100#P100 for $145 for members or $195 for nonmembers. Figure 3. Assessment of settlement risk in sand DEEP FOUNDATIONS • SEPT/OCT 2014 • 81
