SOIL ENGINEERING: TESTING, DESIGN, AND REMEDIATION phần 9

FIGURE 14.10 Typical landslide along a highway cut area. drainage patterns, deforestation, flood, excessive rainfall, or earthquake-induced flooding can cause massive or localized landslides. Some of the smaller-magnitude landslides can be stabilized by improving drainage. Whenever possible, engineers should try to avoid the potential slide risk area. A typical major landslide along a highway in California is shown in Figure 14.10. During the location of the Burma Road, due to insufficient time for detailed survey, a section of road was located in a potential landslide area. After the road ©2000 CRC Press LLC was open to traffic, a massive slope failure took place. At one time, as many as six slides in the cut slope occurred on the same day. Clearing the slides became a major task. At last, the engineers gave up and – at great expense – relocated 10 miles of the existing road to a stable area. A granular soil that is looser than the critical density may pass into a state of complete liquefaction if failure starts. Some of these failures may be referred to as mud flow. Such a flow occurs rapidly and the mass that moves may continue to flow to the lower ground a considerable distance away. Flow slides can take place in slopes as flat as 5 to 10°, and may result in slides of great magnitude. Almost every stiff clay is weakened by a network of hairline cracks or “slick-ensides.” If the spacing of the joints is wide, the slope may remain stable even on steep sloping ground during the dry season. However, if water is allowed to seep into the cracks, the shearing resistance of the weakened clay may become too small to counter the force of gravity and the slide occurs. 14.5 MAN-MADE SLOPES Man-made slopes are necessary in the construction of highways, railroads, canals, and other projects. In high ground, open cuts with adequate slopes will be necessary and on low ground the stability of fill must be considered. Geotechnical engineers seldom pay attention to the design of man-made open cuts and fill. They cover the design with the standard construction specifications. For small projects with no seepage problems, such procedures may be adequate. However, for larger projects, such as locating a section of new highway, the designs of man-made slopes become critical. The cost of an over-designed slope can exceed the cost of a long-span bridge. At the same time, steep slopes can give maintenance crews years of headaches. Experience has shown that slopes at 1 1/2 (horizontal) to 1 (vertical) are usually stable. The sides of most railroad and highway cuts less than 20 ft use such slopes. The standard slopes for water-carrying structures such as canals range between 2:1 and 3:1. 14.5.1 SLOPES IN SAND Instead of failing on a circular surface, sand slopes fail by sliding parallel to the slope. Sand located permanently above the water table is considered stable and cuts can be made at standard angles. Slides occur only in loose saturated sands that liquefy. When the slope angle exceeds the angle of internal friction of sand, the sand grains slide down the slope. The steepest slope that a sand can attain, therefore, is equal to the angle of internal friction of the sand. The angle of repose of sand as it forms a pile beneath a funnel from which it is poured is about the same as the angle of internal friction of the sand in a loose condition. Geotechnical engineers seldom have the opportunity to study the stability of a slope of sand, unless there is a sudden rise of the water table or after an earthquake. ©2000 CRC Press LLC 14.5.2 SLOPES IN CLAY The stability of a slope of clay can be expressed by Coulomb’s shearing strength and shearing resistance relationship. s = c + s tan f where s = shear strength, psf c = cohesion, psf s = effective normal pressure, psf f = angle of internal friction A chart of the stability number for different values of the slope angle b is shown in Figure 14.11. For homogenous soft clay with f = 0, the stability number depends only on the angle of the slope b and on the depth of the stratum. If the value of f is greater than about 3°, the failure surface is always a toe failure. The above figure can be used to determine the stability number for different FIGURE 14.11 Chart for finding the stability of the slopes in homogenous unsaturated clays and similar soils (after Liu). ©2000 CRC Press LLC value, of f, by entering along the abscissa at the value of b and moving upward to the line that indicates the f angle and then to the left, where the stability number is read from the ordinate. 14.5.3 SLOPES OF EARTH DAM The design of a dam shell consists of the selection of the fill material on the basis of its strength and availability for construction. Generally, the material is obtained from borrow pits. Rock waste from a dam core excavation can also be used. For the upstream side of the dam, consideration should be given to the seepage problem. Thus, the degree of compaction of both soil and rock should be under control. The slopes of most dams are established on the basis of experience and consid-eration of the foundation conditions, availability and properties of material, height of dam, and possibly other factors. Typical upstream slopes range from 2.5(H) to 1(V) for gravel and sandy gravel to 3.5(H) to 1(V) for sandy silts. Typical downstream slopes for the same soils are 2(H) to 1(V) to 3(H) to 1(V). A seepage analysis is made on the trial design to determine the flow net and the neutral stresses within the embankment and the foundation. Safety against seepage erosion and the amount of leakage through the dam are computed. Stability analyses are made of both faces of the dam, using the method of slices. The upstream face is usually analyzed for the full reservoir, sudden drawdown, and the empty reservoir before filling. The downstream face is analyzed for the full reservoir and minimum tailwater and also for sudden drawdown of tailwater from maximum to minimum if that condition can develop. The construction of the Greyrock Dam embankment at Greyrock, Wyoming is shown in Figure 14.12. 14.6 FACTOR OF SAFETY The designs of cut and fill have been studied by many leading academicians. With the use of the computer, all aspects of dam design can be accomplished accurately and quickly. All studies involve some basic assumptions, as follows: The soil is homogenous both in extent and in depth. A single angle of internal friction and cohesion values can represent the entire soil mass under investigation. The assumed seepage condition will remain unaltered. There will not be any external disturbance affecting the stability. The type of affiliated structure has not been determined. There will not be any unexpected surcharge load. In order to cover the above uncertainties, geotechnical engineers assign various values of “factor of safety” in an effort to cover the possibility of failures. Sower listed in Table 14.1 the factor of safety applied to the most critical combination of forces, loss of strength, and neutral stresses to which the structure will be subjected. ©2000 CRC Press LLC FIGURE 14.12 Embankment compaction, Greyrock Dam, Greyrock, Wyoming. TABLE 14.1 Suggested Factor of Safety Safety Factor Significance Less than 1.0 Unsafe 1.0–1.2 1.3–1.4 1.5 or more Questionable safety Satisfactory for cuts, fills; questionable for dams Safe for dams Sower further stated that under ordinary conditions of loading, an earth dam should have a minimum safety factor of 1.5. However, under extraordinary loading conditions, such as designing a super flood followed by a sudden drawdown, a minimum factor of safety of 1.2 to 1.25 is often considered adequate. In addition to the uncertainty involved in the factor of safety, engineers must consider “cost”; with an exception of dam design, if cost is not a factor, the risk of a slope failure can be greatly reduced. The factor of safety values suggested by Sower should be considered as design guides. The factor of safety against sliding is determined by dividing the sum of forces tending to resist sliding by the force tending to cause sliding. The slide-resisting ©2000 CRC Press LLC ... - tailieumienphi.vn