Chapter 6 Flashcards
What is earth work?
Changing the configuration of the ground surface by removing soil and or rocks by either cut or excavation or adding soil or rocks by back filling or embankment.
Example of projects requiring earth work.
Highways
Rail roads
Building construction
What are properly graded tract less prone to
Land slides
Excessive settlements
Flooding
What is the fundamental objective of earthwork construction?
Is to change the ground surface from its initial configuration, typically described by a topography map, to some final configuration, as described on a new topographic map known as grading plan. Also to create slope stability on hilly terrain.
Fills
Must have sufficient shear strength to support own weight and external weight
Must be sufficiently still to avoid excessive settlement- eve when wet
In areas that prone to freezing, fills must be made with non- frost susceptible soil.
Historic Methods and Equipment in construction
Slaves, animals and carts were used.
Hydraulic Fills
Process of mixing soil and water together and then transporting it to its designated location and allowing it to settle. After it settles, the remaining water is directed away. This is no longer used
Modern earth moving equipments
Tractor- moves its self plus other equipment, first designed for military and agriculture. Designed for rough terrains
Modern earth moving equipments
Wheel-mounted tractors: operates at greater speeds, not well suited for rough terrains, equipments can be attached like bulldozer. Can be used for cutting, moving and spreading.
Conventional earthwork
Is described at the excavation, transport, placement, and compaction of soft rocks and or soils in areas where equipment can move freely
Clearing and grubbing
Clearing(Above Ground)
Grubbing( Below Ground)
Step one in earthwork projects: removal or trash, debris and vegetation from site. Most of these materials would have detrimental effect on fills, and must be hauled offsite.
Clearing and Grubbing
Inorganic debris such as chunks of concrete, bricks, or asphalt pavement does not need to be hauled away. They can be reused during filling.
Excavation
Occurs in areas where the proposed ground surface is lower than the existing ground surface. The excavated material is used for site fills, or site fills material is obtained from borrow “sites”.
Transport and placement
Bulldozers and Wheel loaders can be used for long hauls but are very no economical.
Transport and Placement
Scrapers are very different at moderate- length hauls, and earth work projects falls into this category
Transport and placements
Dump Trucks: can be used instead of scrapers especially when the soil is being excavated by scrapers. Ability to travel on public highways.
Transport and placement
Wagons: used for hauling soils over long distances on highways
Transportation and Placement
Soil transportation equipment depends on the haul distance. When ever it arrives on a site that requires filling, it must be laid out into “lifts” typically 200 mm thick. Each “lift” must be moisture conditioned and compacted before next lift is placed.
Moisture conditioning
Soils must be at the proper moisture content before compacting.
They wont compact well if too dry or too wet.
If moisture is needed, soil is sprayed with water by a water truck.
After being sprayed, it is then mixed by a bulldozer for uniformity.
Moisture Conditioning
Mixing good wet soils with dry soils is very difficult, especially when mixed wit clays because it results in alternating clumps instead of uniformity
Method used for this is to spread wet soil over large areas and allow it too dry naturally.
Compaction
Use of equipment o compress soil into smaller volume, thus increasing dry unit weight
Soils and water are virtually incompressible, so during compaction the volume of air is reduced while the volume of the soil and water remains unchanged.
Compaction
In the stages of development, animals were used for compaction.
Earth dam near Santa Fe New Mexico used 115 goats.
Rollers were initially pulled by horses which was replaced by tractors in 20 th century
Compaction
Basically anything the moves across a compaction site does some form of compression, like pickup trucks
We cannot depend on accidental forms of compaction because some pick up trucks amongst other compaction equipment were designed to have low pressures between tires and soils which allowed them to move across soft ground more easily
Compaction (Pressure)
The compact pressure between the ground and the equipment is most important factor resulting in compaction of the ground underlining
Compact (Impact)
Some equipment imparts a series of blows to the soils, such as by dropping weight. This adds a dynamic component to the compactive efforts
Compaction (Vibration)
Vibratory compaction equipment utilizes eccentric weights or some other device to induce strong vibrations into the soil, which can enhance its compaction
Compaction (Manipulation)
Compaction equipment that imparts some shearing forces to the soil can also contribute to better compaction. This action is called “Kneading” or “manipulation”. Excessive manipulation such as overly wel fill, can be detrimental. When such fills are simply being moved around with no compaction, we have a condition called pumping.
Compaction Machines
Speed foot Rollers: consists of one or more rotating drums with numerous feet that concentrate its weight onto a small area. They compact soils by pressure manipulation
Tamping foot rollers
Similar to sheep foot rollers, except they have larger feet with a correspondingly smaller contact pressure
Pneumatic rollers
Are heavy units resting on several tires. The contact pressure is typically about 600 kPa. Since each tire is able to move independently, this device is good at finding small soft spots that rigid compaction equipment such as sheepfoot rollers can miss
Vibratory rollers
Similar to sheep foot rollers with vibration mechanism
Smooth steel wheel rollers
Leaves smooth compacted soil surfaces.
Fine Grading
After last lift has been placed and right elevation achieved - this completes rough grading. Fine grading which consists of careful trimming and filling to produce the desired configuration. Requires different equipment
Utility trenches
Are need for trenches when installing utility pipes, sewer pipes and electrical lines. Holes are back filled, but no fully compacted due to sensitivity. Allowed to settle naturally.
“Jetting” process of mixing sand with water, and injecting it into trenches around pipes, while allowing gravity and lubrication to settle it in trenches.
Small Backfills
Used for earth work in small confined areas
Soil Compaction Concepts
Increased shear strength: reduces potential for slope stability
Decreased compressibility: reduces potential for excessive settlement
Decreased Hydraulic conductivity: Inhibits the flow of water through soil
Decreased void ratio: reduces amount of water that can be held in soil, maintain soil strength
Increased erosion resistance: helps maintain ground surface in serviceable condition
