SOIL ENGINEERING: TESTING, DESIGN, AND REMEDIATION phần 4

Upon completion of excavation, the stress condition in the soil mass will undergo changes. There will be elastic rebound. Stress releases increase the void-ratio and alter the density. Such physical changes will not take place instantaneously. If construction proceeds without delay, the structural load will compensate for the stress release. Thus, this will not be a significant amount. 6.4.4 PERMEABILITY The permeability of the soil determines the rate of ingress of water into the soil, either by gravitational flow or by diffusion, and these in turn determine the rate of heave. The higher the rate of heave, the more quickly the soil will respond to any changes in the environmental conditions, and thus the effect of any local influence is emphasized. At the same time, the higher the permeability, the greater the depth to which any localized moisture will penetrate, thus engendering greater movement and greater differential movement. Therefore, the higher the permeability, the greater the probability of differential movement. 6.4.5 EXTRANEOUS INFLUENCE The above-mentioned basic factors, although difficult to predict, can be evaluated theoretically. At the same time, extraneous influences are totally unpredictable. The supply of additional moisture will accelerate heave, for instance, if there is an interruption of the subdrain system to allow the sudden rise of a perched water table. The development of the area, especially residential construction, can contribute to a drastic rise of the perched water table. Various methods have been proposed to predict the amount of total heave under a given structural load. These include the double oedometer method, the Department of Navy method, the South Africa method, and the Del Fredlund method. Recently, with the advance of suction study, Johnson and Snethen claimed that the suction method is simple, economical, expedient, and capable of simulating field conditions. Some fundamental differences between the behavior of settling and heaving soil are as follows: 1. Settlement of clay under load can take place without the aid of wetting, while expansion of clay cannot be realized without moisture increase. 2. The total amount of heave depends on the environmental conditions, such as the extent of wetting, the duration of wetting, and the pattern of moisture migration. Such variables cannot be ascertained, and conse-quently, any total heave prediction can only be speculation. 3. Differential settlement is usually described as a percentage of the ultimate settlement. In the case of swelling soils, one corner of the structure may be subject to maximum heave due to excessive wetting, while another corner may have no movement. No correlation between differential and total heave can be established. ©2000 CRC Press LLC 6.5 BUILDING ADDITIONS Take great care when designing a new addition adjacent to or abutting an existing building. This is especially important when the existing structure is owned by another person. The new footings can exert an additional load on the existing footings and cause settlement and cracking. Whenever possible, it is wise to consult with the original engineer or the owner and study the initial design. If common walls are used, eccentric loading will be expected. When the new and the old structures are not on the same level, the lateral load from the existing structure should be consid-ered. The bearing capacity as calculated for isolated footings should be drastically reduced. Similar precautions should be taken even when the new construction is isolated from the existing structure. The owner of the neighboring structure can claim that the weight of the new construction has caused the settlement of the neighboring structure. It is therefore important to have a conference with the neighboring building owners before starting the excavation. A prudent engineer takes pictures of the neighboring structure to avoid possible future litigation. Documented photographs can prove that the distress or cracking of the neighboring building existed before the new construction. Another important consideration in the design of footings is the property line. The building owner wants to make use of every foot of his property. Without the knowledge of the adjacent property owner, the footing construction may extend beyond the property line. The error may not be detected until years later when the excavation of the neighboring property is started. The court can order the demolition of the building or order the payment of a substantial compensation. It is very rare for a geotechnical consultant to be sued for overdesign, but neglecting to pay attention to the site condition can haunt the engineer. Details such as neighboring structures, property lines, drainage patterns, slope stability, or the rise of water table may be more important than the accuracy of the bearing capacity numbers. REFERENCES F.H. Chen, Foundations on Expansive Soils, Elsevier Science, New York, 1988. B.M. Das, Principles of Geotechnical Engineering, PWS Publishing, Boston, 1994. P. Rainger, Movement Control in Fabric of Buildings, Batsford Academic and Educational, London, 1983. D. R. Sneathen and L. D. Johnsion, Evaluation of Soil Suction from Filter Paper, U.S. Army Engineers, Waterway Experimental Station, Vicksburg, Mississippi, 1980. W.C. Teng, Foundation Design, Prentice-Hall, Englewood Cliffs, NJ, 1962. K. Terzaghi, R. Peck, and G. Mesri, Soil Mechanics in Engineering Practice, John Wiley-Interscience Publication, John Wiley & Sons, New York, 1996. U.S. Department of the Interior, Bureau of Reclamation, Soil Manual,Washington, D.C., 1970. R. Weingardt, All Building Moves — Design for it, Consulting Engineers, New York, 1984. ©2000 CRC Press LLC 7 Footings on Clay CONTENTS 7.1 Allowable Bearing Capacity 7.1.1 Shape of Footings 7.2 Stability of Foundation 7.2.1 Loaded Depth 7.2.2 Consolidation Characteristics 7.3 Footing on Soft or Expansive Clays 7.3.1 Raft Foundation 7.3.2 Footings on Expansive Soils 7.3.3 Continuous Footings 7.3.4 Pad Foundation 7.3.5 Mat Foundation References The design of footings on clay has been the concern of engineers since the beginning of soil engineering. The classical theory of ultimate bearing capacity developed by Terzaghi more than 60 years ago is still the basic theory used by engineers. In referring to footings on clay, the correct description should be footings on fine-grained soils. These include lean clay, fat clay, and plastic silt; the analysis can sometimes be extended to clayey sands (SC) and sandy silt (ML). The basic require-ments of designing footings on clay are that the design should be safe against shear failure and the amount of settlement should be tolerable. The shear consideration is theoretically important; it seldom takes place in actual construction. When such failure does occur, it receives attention from the public. The silo tilting in Canada certainly is a good example. Consultants are generally conservative and the cost of a slightly bigger footing seldom affects the total construction cost. As discussed in the previous chapter, what constitutes a “tolerable settlement” is hard to define. Judgment and experience of the consultant are probably more important than figures and equations. 7.1 ALLOWABLE BEARING CAPACITY The ultimate bearing capacity is defined as the intensity of bearing pressure at which the supporting ground is expected to fail in shear. The allowable bearing capacity is defined as the bearing pressure that causes either drained or undrained settlement or creep equal to a specified tolerable design limit. In plain consulting engineer’s language, allowable bearing capacity refers to the ability of a soil to support or to hold up a foundation and structure. 0-8493-????-?/97/$0.00+$.50 © 1997 by CRC Press LLC ©2000 CRC Press LLC In 1942, Terzaghi expressed the ultimate bearing capacity of footing on clay with the following general equation: qult = cNc + g DNq + 0.5 g BNg where q = ultimate bearing capacity, psf g = unit weight of soil, pcf c = cohesion, psf D = depth of foundation below ground, ft. B = width of footing, ft. Nc, Nq, Ng = bearing capacity factors. The bearing capacity factors are shown in Figure 8.2. The third term of the equation refers to the friction of the soil. For clay, where f = 0, the term is eliminated. The second term of the equation is referred to as the depth factor. It depends on the construction requirement. In probably 90% of the cases, footings are placed at a shallow depth. Therefore, for footings on clay, the net-bearing capacity can generally be defined as the pressure that can be supported at the base of the footings in excess of that at the same level due to the surrounding surcharge. qd = cNc where qd is the net ultimate bearing capacity. Prandtl determined the value of Nc, for a long continuous footing on the surface of the clay deposit where the friction angle is assumed to be zero, as 5.14. A great deal of research has been conducted in recent years on the bearing capacity factors. The ratio between footing width and footing depth appears to be an important controlling factor. In general geotechnical practice for low rise structures, the footing width is on the order of 24 to 30 in. For frost protection, the building code generally specifies a 30-in. soil cover. Consequently, the D/B ratio is generally less than one, and the Nc value should be on the order of 5.5 to 6.5, as shown in Figure 7.1. Using a factor of safety of three, the allowable soil bearing pressure qa for footings on clay would be qa = c Nc For f = 0 or very small, the unconfined compressive strength is twice the cohesion value of clay. Thus, aa = Nc qu = qu ©2000 CRC Press LLC FIGURE 7.1 Bearing capacity factors for foundation on clay (after Skempton). where qu is the unconfined compressive strength. For most structures the consultants are dealing with, it will be sufficient to assume that the allowable soil-bearing pressure for footing on clay is equal to the unconfined compressive strength. In using the unconfined compressive strength values for footing designs, the following should be considered: Average value — It is a mistake to determine the value by averaging all the data obtained from the laboratory. Experience should guide the consultant in selecting the most reliable and applicable ones. Water table — The vicinity of the water table or the likelihood of the development of a perched water condition should be of prime importance in selecting the design value. Most foundation failures take place, not due to underdesign, but due to the failure to recognize the possibility of the saturation of the footing soils. Drainage — It is common practice to provide drains along the footings with the intention of keeping the foundation dry. Such drains may not have an adequate outlet, or sometimes the outlet has been blocked. As a result, the soils beneath the footing can be completely saturated for years without detection. Soft layer — The presence of a soft layer sandwiched between relatively firm clays should not be ignored. During exploratory drilling, such a layer can be overlooked by the field engineer. If such condition is suspected, the bearing capacity should be reduced. ©2000 CRC Press LLC ... - tailieumienphi.vn