SOIL ENGINEERING: TESTING, DESIGN, AND REMEDIATION phần 5

9.1.1 FILL ON SOFT GROUND When the natural soils have a very low bearing capacity and it is necessary to place a relatively heavy structure on it, it is possible to place a structural fill to distribute the imposed load. A thorough investigation is required to justify such an undertaking. Some of the factors to consider are outlined below: 1. To know the extent and thickness of the soft soil strata 2. The compressibility of the soft soil strata must be determined 3. Under certain loads, it is necessary to estimate the time required to complete the consolidation 4. The location of the water table is sometimes necessary to control the feasibility of such a project 5. The feasibility of the installation of a dewatering system 6. The availability of suitable fill material 7. The tolerable amount of settlement 8. The type of compacting equipment available The most difficult problem confronting a geotechnical engineer is the erection of structures on very soft organic clay or silt. Such problems often rise during the construction of highways or railroads. Natural deposits of this type are common in regions formerly occupied by shallow lakes or lagoons. The deposits usually consist of peat moss or other types of marsh vegetation. Such soils may not be able to sustain the weight of a fill more than few feet in height. Fill on such foundations may continue to settle excessively for many years or decades. During the construction of the Tibet Highway, a vast area of marshy ground was encountered (Figure 9.1). The deposit extended many square miles and was located at elevations above 18,000 ft. It was believed that the area constitutes part of the sources of the Yellow River. The deposit was so soft that it did not sustain even horseback riders. The subsoil contains about 50% silt and clay with a liquid limit of more than 100. Since no granular soils were available, ditches were dug along both sides of the proposed roadway to a depth of about 10 ft to lower the water table. The excavated material was allowed to dry, then used as fill. The completed road was able to support truck traffic. 9.1.2 REMOVAL AND REPLACEMENT OF EXISTING FILL If the soft clay layer is thin and near the ground surface, it is more economical to remove the clay layer and replace it with compacted structural fill. Such an operation should be limited to the following conditions: That the soft clay layer exists within about 10 ft below ground surface That an excessive amount of settlement can take place if such a layer is not removed ©2000 CRC Press LLC FIGURE 9.1 Marshy ground near Tibet. That under the soft layer, high bearing capacity soils exist, such as bedrock or stiff clays That imported fill material is within economical reach More critical than the soft clay layer, the existing fill may consist of by-products other than soils. These are trash materials such as building debris, concrete, bricks, ashes, cinders, and organic matter. Some of such materials can be used as fill, but the separation is difficult and the cost of such an operation can seldom be justified. Coarse mine tailings are generally considered a good source of fill as long as they do not contain radioactive matter. The use of such materials as fly ash, chimney dust, etc., should be determined after laboratory testing. When any of the above trashy material is suspected to be present, field engineers should be aware of the following: 1. The location of such material is erratic and cannot be determined. Drilling of test holes can sometimes miss the fill. In such cases, the geotechnical engineer faces an angry client. Laypersons may not understand bearing pressure, but they certainly can recognize trash material. 2. The trash fills can be very old and extend to a great depth. In one case, a historical building more than a century old in Denver, Colorado suddenly showed cracking on walls. Upon careful drilling, decomposed coffins were found at a depth more than 20 ft below the ground surface. 3. The extent of trash removal must not be limited to the proposed building line. It is sometimes necessary to remove as much as 15 ft outside the ©2000 CRC Press LLC FIGURE 9.2 Typical pattern of cracks due to deep-seated settlement. building line in all directions. This is to prevent the trash material from affecting the stability of the foundation within the pressure bulb influence. 4. The presence of deep-seated trash fills can sometimes be revealed from the examination of the pattern of cracking of existing buildings. Patterns of cracking from differential settlement or heaving generally are in a diagonal pattern, while deep-seated fill settlements usually are in a hori-zontal pattern, as shown in Figure 9.2. 9.1.3 RECOMPACTION OF NATURAL SOFT SOILS Such an operation is limited when the low-bearing soils are located within about 10 ft below the ground surface, and also when the bedrock is deep and pile or pier construction is costly. The use of such a system can best be illustrated by the following project: Project Column load Subsoil Water table Ten-story hotel building. Unknown at time of investigation. Five to ten ft of loose silty or clayey sand. Average penetration resistance N = 5, with a few N = 2 underlain by 20 to 30 ft of medium dense clayey sands, with average penetration resis-tance N = 15. Claystone bedrock is at a depth of about 45 ft. Stabilized at a depth of 58 ft. The upper 10 ft of silty sands has a low-bearing capacity with a possibility of shear failure and cannot be used to support a high column load. The bedrock is deep and a pier foundation system is costly. ©2000 CRC Press LLC FIGURE 9.3 Stouffer’s hotel, built with footings on compacted fill. The most economical foundation system is to remove the upper 10 ft of low-bearing capacity sands and replace re-compacted, with the following requirements: Excavation Compaction Control Bearing capacity Remove at least 10 ft of the existing silty sands. Removal should extended at least 10 ft beyond the building line in every direction. The removed soil should be re-compacted to at least 100% standard Proctor density at optimum moisture content. Full-time inspection control is required by an experienced field engineer with frequent density tests performed. Footings placed on the controlled structural fill should be designed for a bearing pressure of 5000 pounds per square foot. The completed structure is shown in Figure 9.3. The building is 20 years old with negligible settlement. 9.2 COMPACTION The compaction of fill increases the bearing capacity of foundations constructed over them. Compaction also decreases the amount of undesirable settlement of structures and increases the stability of the slopes of the embankments. 9.2.1 COMPACTION TESTS The standard Proctor test as described in Chapter 5 is most commonly used. Highway and airport engineers choose to use the modified compaction test that offers high compaction effort. The Corps of Engineers adopted another set of standards for controlling its fill. It is possible to control the fill with one set of standards by varying ©2000 CRC Press LLC the degree of compaction from, say, 90% to 100%. A great deal of confusion can be avoided by adopting a standard compaction test for all fill. With the development of heavy rollers and their use in field compaction, the standard Proctor test was modified to better represent field conditions. The soil is compacted in five layers with a hammer that weighs 10 lbs. The drop of the hammer is 18 in. The number of hammer blows for each layer is kept at 25 as in the case of the standard Proctor test. The modified Proctor test is used most of the time by the Bureau of Reclamation as well as the U.S. Army Corps of Engineers. Still, the procedures are not without loopholes. In one case, the soil compaction in the field was obviously inadequate, yet all field density tests indicated higher density than the Proctor maximum density requirement. After many months of litigation and investigation, it was found that the technician performing the Proctor density test placed the compaction cylinder on the open tailgate of his pickup truck instead of on a solid surface. Consequently, the maximum density was as much as 5 lbs lower than the actual value. This case indicates that no testing can replace experience and common sense. A hand-dug hole is made in the fill and the removed soil collected for weight measurement. There are several methods for determining the volume of the hole. For many years, the consultants in the Rocky Mountain area chose the “sand replace-ment method,” where the hole is filled with pre-calibrated Ottawa sand through a special sand cone device (ASTM D-1556). With this method of testing, the field engineer is able to gain a good feel of the condition of the fill while digging the test hole. However, the result of the test cannot be known until the laboratory completes the testing. The earth-moving contractor will not be able to know whether the compaction meets the required criteria for at least 24 h. In recent years, the nuclear method for compaction control was widely used. Both the bulk density and the moisture content of the fill can be measured using controlled gamma radiation techniques. The apparatus generally consists of a small, shielded radiation source and a detector (Figure 9.4). The intensity of transmitted or back-scattered radiation varies with density and moisture content. Calibration charts are used which relate detected radiation intensity to values recorded from soil with known intensity. In 1980, the device was adopted by ASTM. Consultants prefer to use such a device, as it reduces laborious hole digging. Contractors welcome such a device, as it provides immediate results. Some unrea-sonable test results have been attributed to a wrongly calibrated instrument. Field engineers should review the report carefully before submission. Field manual density determination methods are still used by consultants. Proctor tests can be performed only with soils containing at least 6% of fines. For clean granular soil (SP-GP), relative density should be used. Since relative density and Proctor density originated from totally different concepts, no correlation should be established between them. When preparing specifications for a certain project, some tend to copy the old specification without giving consideration to the difference between standard Proctor, modified Proctor, or relative density, which results in specifying an unnecessary degree of compaction and boosting the cost of the project. In some unfortunate cases, the problem has to be decided in court. ©2000 CRC Press LLC ... - tailieumienphi.vn