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Chapter 6 Excavating, Grading and compacted fill

by: dana Notetaker

Chapter 6 Excavating, Grading and compacted fill CAE 370

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Geotechnical engineering 7th edition notes
Geotechnical Engineering
Alejandro Montenegro
Class Notes
Geotechnical Engineering principles and practices




Popular in Geotechnical Engineering

Popular in Engineering and Tech

This 7 page Class Notes was uploaded by dana Notetaker on Wednesday March 2, 2016. The Class Notes belongs to CAE 370 at University of Miami taught by Alejandro Montenegro in Spring 2016. Since its upload, it has received 36 views. For similar materials see Geotechnical Engineering in Engineering and Tech at University of Miami.


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Date Created: 03/02/16
CHAPTER 6 Excavation, Grading, and compacted Fill 1. When we remove soil or rock, we make a cut or excavation 2. When we add soil or rock, we create a fill embankment 3. Virtually highways require earthwork to create smooth grades an alignments to provide proper surface drainage. 4. EARTHWORK is important especially in hilly areas. 6.1 Earthwork construction: 1. The fundamental objective of earthwork construction is to change the ground Surface from initial configuration to final configuration 2. These changes are necessary to maintain proper surface drainage 3. Earthwork must not create slope stability problems in hilly and mountainous areas In 1960: building codes began to include stricter requirements for fills, and required builders to hire geotechnical engineers. What are the requirements for compacted fills?  They must have sufficient shear strength to support their weight and external loads, lack of strength leads to landslides, bearing capacity failures and rutting of pavements.  The fills must be stiff to avoid excessive settlement since soft fills damage the building.  Fills must satisfy the above requirements even when they become wet.  They must have a low hydraulic conductivity to restrict the flow of water.  It is desirable to have fills made of soils that are not frost- susceptible. 6.2 Construction Methods and Equipment: Historic Methods: 1. Builders have been making cuts and fills for thousand years using large number of slaves and animals in india and sri lanka.  In Kalaweva in Sri lanka was created by 19 km long, 21 m tall earth dam built in AD 459.  A larger dam would have created a huge reservoir, however it was never used because the canal was to feed the reservoir was built running uphill where builders expertise in earthwork (Schuyler,1905) 2. In 1907, large steam powered equipment was introduced to build the Panama Canal which was the largest earth moving project. This project have faced many problems but after the geological studies were resumed, locks had been added to raise the canal 25 m above sea level. The canal was completed in 1914 and is considered a great civil engineering achievement and a dramatic case study demonstrating the importance of understanding geology. 1. Culebra cut required 75,000,000 m^3 of excavation and was finished in 1914. 2. This project faced difficult geological conditions. Hydraulic Fills: 1. 20 century, earthwork projects were introduced using hydraulic fillings. 2. This method consisted of mixing soil with water & conveying the mixture through pipes & flumes. 3. No compaction equipment was used 4. It was popular in 1900 and 1940, especially for earth fill dams since the earthmoving equipment was small and unpowered for such large projects. 1. Last Hydraulic fill built in United States was at Ft. Peck Dam in Montana. A large 3,800,000 m^3 landslide occurred during construction in 1938 and was blamed on the poor quality of hydraulic fill. 2. Another problem was in 1971, when the san Francisco dam near Los Angeles failed during an earthquake measuring. The failure was due to liquefaction of hydraulic fill soils. 3. Another construction of San Pablo dam using hydraulic fill techniques in 1918. Modern Earthmoving: a key development in modern earthmoving equipment was the introduction of the tractor. th  20 century had more advances in earthmoving equipment during the period 1920-1965.  Its goal is to move large quantities of earth at a very low cost.  For example; 1914, the excavation costs for the panama canal was about $0.79/yd^3, by adjusting this figure for inflation, it becomes $17/yd^3 in 2010. This is similar to work using modern equipment is less than ¼ of that adjusted cost. *What is a tractor or a crawler?  It converts engine power into traction and moves itself and other equipment.  The first tractor was developed thr military and agriculture purposes in the 20 century.  They were mounted to tracks to allow movement over very rough terrain.  Modern track mounted equipment is considered to be powerful but operates in low speed no more than 11 km/hr.  Wheel mounted tractors are greater operating speeds 50 km/hr. They have less traction and not suited for rough terrains. *What are the accessories added to a tractor? 1. Bulldozer: movable steel blade attached in the front of a tractor and is used for cutting, moving, speeding and mixing. 2. Loader: consists of a bucket attached in the front and is used to pick up, transport, light excavations and deposit soil. 3. Hoe: a bucket used for digging pits or trenches. 4. Backhoe 5. Excavator Conventional Earthwork:  This term is used to describe excavation, transport, placement and compaction of soil or soft rock in areas where equipment moves freely. *What is the process of conventional earthwork? What are the steps of conventional earthwork? 1. Clearing and grubbing:  Remove vegetation, trash and undesirable materials  Remove stumps, roots, buried objects and contaminated soil. 2. Excavation 1. Remove rock or soil 2. Excavated materials are used to make fills 3. Excavating at offsite barrow pits. 4. Excavate for removal of underground structures 3. Transport and placement  Dump trucks are used over public highways and move faster than scrapers  Wagons  Conveyor bells 4. Moisture conditioning  The soil must be at a proper moisture content before it is compacted and will not compact well if its too wet or too dry. 5. Compaction Control in sites should not exceed 12 in , horizontally and must be moisture conditioned. 1. Why can’t we rely on the incidental compaction?  Some constructions are intentionally designed to have low contact pressure between tires and tracks  Incidental traffic usually follows common routes, so their compactive effort is not uniformly distributed across the fills. 2. What are the method used for the compaction equipment? pressure impact vibration manipulation 6. Fine grading Careful trimming and filling to produce desired configuration. 7. Small backfills 8. Utility trenches 6.3 Soil Compaction Concepts: *What are the properties of a fill that soil compaction leads to? Increased shear strength, which reduces the potential slope stability problems such as landslide Decreased compressibility, which reduces the potential for excessive settlement. Decreased hydraulic conductivity. Decreased void ratio. Increased erosion resistance. *What are the factors that the degree of compaction depends on? The type of soil being compacted. The method of compaction. The compactive effort. The moisture content of the soil being compacted. 6.4 Soil compaction standards and Specifications: Proctor compaction test. 1930’s, R.R. proctor, an engineer in Los Angeles developed a method of assessing compacted fills which modeled the compaction provided by the compaction equipment of that era. Maximum dry density Moisture content. Standard proctor test. Modified proctor test. During 1940’s and 1950’s, trucks and aircraft traffic increased in weight and frequency. Geotechinal engineers found that fills compacted using the standard proctor test were no longer providing adequate support for modern trucks and aircraft. In addition, heavier and efficient moving earthwork were being developed. 6.7 Earthquake quantity compactor: Grading plans: When planning earthwork construction, civil engineers begin with a topographic map of the existing ground surface then develop a new topographic map showing a proposed ground surface When the proposed elevations are higher than the existing ones, new fills will be required. When the proposed elevations are lower that the existing, a cut will be required. Designer engineers also uses two sets of elevations to compute the anticipated quality of a cut and fill. If extra soil needs to be hauled to the site, then the project has a net import. When neither the network nor export is required, then the earthwork is balanced. Bulking and shrinkage: The excavation will occur in an area called the bank. The key to earthwork quantity computation is that , given a certain amount of soil (no matter how it may have been changed through such earthwork operations as excavation, compaction and moisture conditioning), the amount of solids in the soil remains the same. The net change in volume from the bank compacted conditions, or the shrinkage factor ( dV/Vf = [ (yd,f/yd,c)-1 ]* 100 ) Where, dV= change in volume during grading. Vf = volume of fill Yd, f= average dry unit weight of fill. Yd, c = average dry unit of cut (bank soil) Note that: A positive shrinkage indicates a net decrease in volume A negative value indicates a net increase in volume (called swelling) Deep Fills:  What are the settlement problems in deep fills stem? 1. They are very heavy, need large compressive stresses. 2. Loose natural soils were usually removed before placement of the fills. 3. The fills became wetter due to infiltration of water from the rain, irrigation and broken utility pipelines. 4. The wetting of lower portions of the foil and the underlying of the soils often causes them to compress and the thickness often changed over short distances. 5. If the fill is made out of expensive soils, the wetting caused heaving in the top zone, which leads to offsetting the compression of the underlying fill.  What are the new methods of designing and constructing projects? a. Design must be based on assumption that some wetting will occur. b. When using the expensive soils, its better to be more carefully controlled. c. The Removal of loose natural soils underneath fills need to be more aggressive. d. The requirement for 90% compaction may not be sufficient for the lower portion of deep fills. CHAPTER 7 7.1


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