result in movement of the adjacent soils within the zone of influence of the excavation. While the soil type and strength determine the zone of influence, it is generally defined within a lateral distance of up to three times the depth of the excavation away from the face of the excavation (Figure 1). Similar charts are available for lateral movements of the SOE system. Where portions of buildings or below-grade structures are supported on soils within the zone of influence, such as slabs-on-grade and shal - low foundations, these structures are susceptible to differential vertical and horizontal movements. The relative movement of structures can be quantified using the following two terms: • Angular Distortion: Sometimes referred to as relative rotation, this is a measure of the shearing distortion and is often approximated as the rotation, due to differential settlement, of the straight line joining two reference points on the structure, such as the foundation elements. • Horizontal Strain: Sometimes referred to as lateral distortion, this is a measure of the relative horizontal movement of two reference points on the structure. Angular distortion and horizontal strain can cause damage to struc- tures, and different types of building construction are inherently more susceptible to distress. Less flexible systems like masonry bearing wall buildings are such an example. Damage can range from cosmetric dis- tress to structural distress to loss of building functions. Examples of damage include cracks in walls, slabs, or finishes, racking of doors or windows, and loss of weathertightness.
Boscardin and Cording (1989) provide a visible damage classification system that relates severity, damage type, and approximate crack widths. The visible damage classification system is characterized into the follow - ing categories based on the ease of repair of the damage: negligible, very slight, slight, moderate, severe, and very severe. The general required repairs for each classification and crack width magnitudes are based on analysis of empirical measurements and observations. • Negligible – “Hairline cracks.”Approximate crack widths are less than 0.1 mm. • Very Slight – “Fine cracks easily treated during normal redecoration. Per- haps isolated light fracture in building. Cracks in exterior brickwork visible upon close inspection.” Approximate crack widths are less than 1 mm. • Slight – “Cracks easily filled. Re-decoration probably required. Several slight fractures inside building. Exterior cracks visible, some repointing may be required for weathertightness. Doors and windows may stick slightly.” The approximate crack width is less than 5 mm. • Moderate – “Cracks may require cutting out and patching. Recurrent cracks can be masked by suitable linings. Tuck-pointing and possible replacement of a small amount of exterior brickwork may be required. Doors and windows sticking. Utility service may be interrupted.Weathertightness often impaired.” The approximate crack widths are 5 mm to 15 mm or several cracks greater than 3 mm. • Severe – “Extensive repair involving removal replacement of sections of walls, especially over doors and windows required. Windows and door frames distorted, floor slopes noticeably. Walls lean or bulge noticeably, some loss of bearing in beams. Utility service disrupted.” The approximate crack widths are 15 mm to 25 mm, but the classification also depends on the number of cracks. • Very Severe – “Major repair required involving partial or complete re-con- struction. Beams lose bearing, walls lean badly and require shoring. Windows broken by distortion. Danger of instability.” The approximate crack widths are usually greater than 25 mm, but the classification also depends on the number of cracks. Further, Clough, W. and T. O’Rourke (1990) include charts that relate angular distortion and horizontal strain for each of the different visible damage categories and excavation soil type (Figure 2). Charts for both cohesive and cohesionless soils are available. These charts are based on analytical models and data collected over years for a variety of SOE systems, including cross- lot struts supporting sheet pile, soldier pile and lagging, and concrete diaphragm walls the authors consider to be of “average workmanship.” These charts enable an engineer to cor- relate a category of visible building damage, as described above, to quantifiable measures of building movement.
Photo 2: Crack Resulting from Movement of SOE System
Two notable research papers compare the relationships between soil type, distance from the face of the SOE system, and depth of excava- tion to damage to adjacent structures. These papers summarize predic- tions for vertical and lateral movements for various soil types based on the depth of excavation and distance from the earth support system. They also predict the building damage severity based on soil type, the amount of angular distortion, and amount of lateral strain of a building or below-grade structure during SOE movement.
Figure 2: Relationship of Damage to Angular Distortion and Horizontal Strain for Excavations in Soft to Medium Clay
Made with FlippingBook Annual report