Big Dam Bridge

Opened in 2006, the pedestrian and bicycle bridge built over Murray Lock and Dam between Murray Park in Little Rock and Cook’s Landing Park in North Little Rock is a major addition to the Arkansas River Trail, connecting several miles of hiking and biking trails on both sides of the river.   The 15.7 mile Big Dam Loop of the 88-mile Arkansas River Trail System, crosses the river at the Big Dam Bridge on the west with an eastern crossing over a renovated Rock Island Railroad bridge downtown at the Clinton Library.   An extension route to Pinnacle Mountain State Park will connect to the 225 mile Ouachita Wilderness Trail.

The official dedication for the bridge was September 30, 2006.

The project’s official name  is Pulaski County Pedestrian & Bicycle Bridge – Murray Lock and Dam—however, it is known as the Big Dam Bridge and is the world’s longest bridge specifically constructed as a pedestrian/bicycle bridge. At 4,226 feet (1288 m.) in length, the bridge rises to 65 feet over the surface of the Arkansas River and 30 feet over the dam.  The span over the river is 3463 feet (1055 m.), with the ramps on either side of the river accounting for the rest of the length.

It is lit at night with 169  “Illumivision Lightwave LX” fixtures installed at the base of the 13 piers over the dam.  Each light is a “wall washing LED that generates color-changing effects.”

Other dam bridge facts:

• The bridge has 679 feet of walled embankments.
• There are eight observation areas with benches.
• It’s designed to support two 36″ utility pipelines.
• The structure contains over 3 million pounds of steel.
• The bridge and associated trails assists in the connection of over 7,000 acres of city, county, state, and federal park land.
• Oversight of design and construction was provided by the U.S. Army Corps of Engineers to ensure the project would not interfere with navigation, though no corps funds were used in the construction.
• Funding was through a combination of federal and state transportation funds as well as local funding from Pulaski County and the cities of Little Rock and North Little Rock.
• It was constructed with “weathering steel girders” to minimize requirements for future maintenance.
• The bridge is gently sloped to comply with Uniform Federal Accessibility Standards.
• Using Murray Lock and Dam as foundation saved an estimated $10 million in construction costs

Murray Lock and Dam and the McClellan-Kerr Arkansas River Navigation System (MKARNS) facts:

• Essentially a series of navigation pools connected by locks, the waterway enables vessels to overcome a 420-foot difference in elevation from the Mississippi River to the head of navigation at Catoosa, Oklahoma.
• There are 18 locks; 5 in Oklahoma and 13 in Arkansas. All lock chambers are 110-feet wide by 600 feet long.
• Size of towage accommodated: more than 8 jumbo (35 ft. X 195 ft.) barges with double lockage using tow haulage (Tow haulage equipment on a lock can pull the first cut through by itself, so that the towboat can stay in its original pushing position and lock through with the second cut.)
• A typical 8-barge 12,000-ton is equivalent to 400 semi-trucks or 120 railroad freight cars.
• The maximum lift from one navigation pool to another ranges from only 14 feet at lock No. 4 near Pine Bluff to as much as 54 feet at the Dardanelle Lock.
• The locks are operated 24 hours per day and handle both commercial barges and recreational vessels.
• Bridges over the channel have a minimum vertical clearance of 52 feet 98% of the time. Actual vertical clearance above the normal level of the navigation pool is normally more than 52 feet.
• Murray Lock and Dam is #7 going upstream, 125.4 miles from the Mississippi.  Under normal pool conditions, the lower pool level is 231 ft above sea level and the upper pool is 249 ft., for a nominal change in elevation of 18 feet.

www.thuyloivn.com st

Soil nail wall - Soil nailing - soil nailing walls

Soil nail walls consists of installing passive reinforcement (i.e., no post-tensioning) in existing ground by installing closely spaced steel bars or sections (i.e., nails) and placing a front face support. Soil nails are later grouted if they are installed in drilled holes. Ungrouted soil nails are also possible if nails (or steel sections) are driven into the ground. Figure 1 shows typical details of a soil nail wall:

Figure 1: Typical soil nail wall arrangement

Soil nail wall construction proceeds from the top to bottom, and head plates are installed on each nail.  Shotcrete or concrete is typically applied on the excavation face to provide continuity when a soil nail wall is constructed.  For a soil nail wall the general construction procedure involves:

a) Excavate for the first nail (soil must be sufficiently self standing)

b) Install the 1st nail.

c) Construct 1st phase shotcrete on soil face (optional if shotcrete is constructed) with wire mesh or other reinforcement if required.

d) Install soil nail head plate (with or without other attachments, Figure 2)

e) Construct 2nd phase shotcrete (depending on staging specifications).

f) Excavate to next soil nail level, and install next soil nail, shotcrete etc.

g) Repeat steps c) through f) until the final excavation level is reached.

h) Construct additional permanent facing if required.

* Drainage filters and drainage pipes are also commonly installed during construction.

 

Figure 2: Typical soil nail head plate and shotcrete details (US Practice)

Soil nailing is typically used to stabilize existing slopes or excavations where top-to-bottom construction is advantageous compared to other retaining wall systems. For certain conditions, soil nailing offers a viable alternative from the viewpoint of technical feasibility, construction costs, and construction duration when compared to ground anchor walls, which is another popular top-to bottom retaining system.

Soil nail walls are particularly well suited to excavation applications for ground conditions that require vertical or near-vertical cuts and have been shown to be particularly well suited in the following temporary or permanent applications:

• Roadway cut excavations.
• Road widening under an existing bridge end.
• Repair and reconstruction of existing retaining structures.
• Temporary or permanent excavations in an urban environment.

Soil nailing has proven economically attractive and technically feasible when:

• The soil in which the excavation is constructed is able to stand unsupported in a 1- to 2-m (3- to 6-ft) high vertical or nearly vertical cut for one to two days.
• All soil nails within a cross section are located above the groundwater table
• If soil nails are below the groundwater table, and the groundwater does not adversely affect the face of the excavation, the bond strength of the interface between the grout and the surrounding ground, or the long-term integrity of the soil nails (e.g., the chemical characteristics of the ground do not promote corrosion).

Soil nail advantages

Soil nail walls exhibit numerous advantages when compared to ground anchors and alternative topdown construction techniques. Some of these advantages are described below:

• Requires smaller right of wat than ground anchors as soil nails are typically shorter;
• Less disruptive to traffic and causes less environmental impact compared to other construction methods.
• Provide a less congested work place, particularly when compared to bracedexcavations.
• There is no need to embed any structural element below the bottom of excavation as with soldier beams used in ground anchor walls.
• Soil nail installation is relatively rapid and uses typically less construction materials than ground anchor walls.
• Nail location, inclination, and lengths can be adjusted easily when obstructions (cobbles or boulders, piles or underground utilities) are encountered. On the other hand, the horizontal position of ground anchors is more difficult to modify almost making field adjustments costly.
• Since considerably more soil nails are used than ground anchors, adjustments to the design layout of the soil nails are more easily accomplished in the field without compromising the level of safety.
• Overhead construction requirements are smaller than those for ground anchor walls because soil nail walls do not require the installation of soldier beams (especially when construction occurs under a bridge).
• Soil nailing is advantageous at sites with remote access because smaller equipment is generally needed.
• Soil nail walls are relatively flexible and can accommodate relatively large total and differential settlements.
• Measured total deflections of soil nail walls are usually within tolerable limits.
• Soil nail walls have performed well during seismic events owing to overall system flexibility.
• Soil nail walls are more economical than conventional concrete gravity walls when conventional soil nailing construction procedures are used.
• Soil nail walls are typically equivalent in cost or more cost-effective than ground anchor walls when conventional soil nailing construction procedures are used.
• Shotcrete facing is typically less costly than the structural facing required for other wall systems.

Soil nail disadvantages

Some of the potential disadvantages of soil nail walls are:

• Soil nail walls may not be appropriate for applications where very strict deformation control is required for structures and utilities located behind the proposed wall, as the system requires some soil deformation to mobilize resistance. Deflections can be reduced by post tensioning but at an increased cost.
• Existing utilities may place restrictions on the location, inclination, and length of soil nails.
• Soil nail walls are not well-suited where large amounts of groundwater seeps into the excavation because of the requirement to maintain a temporary unsupported excavation face.
• Permanent soil nail walls require permanent, underground easements.
• Construction of soil nail walls requires specialized and experienced contractors.

Soil nail wall construction sequence

(Resource: http://www.deepexcavation.com/en/soil-nail-wall)

Hoover Dam

Hoover Dam spans the Colorado River in Black Canyon between Arizona and Nevada, some 30 miles southeast of Las Vegas Nevada. Constructed in the 1930s, the concrete arch-gravity structure was intended to prevent flooding as well as provide much-needed irrigation and hydroelectric power to arid regions of states like California and Arizona. It was originally known as Boulder Dam, but was renamed in 1947 in honor of Herbert Hoover, who as U.S. secretary of commerce and the 31st U.S. president proved instrumental in getting the dam built. At 726 feet high and 1,244 feet long, Hoover Dam was one of the largest man-made structures in the world at the time of its construction, and one of the world’s largest producers of hydroelectric power.

REGION & BACKGROUND

From its source in the Rocky Mountains of north-central Colorado, the mighty Colorado River travels southwest more than 1,400 miles to the Gulf of California, joining with other water sources (including the Green River and the Little Colorado River) and carving out the majestic Grand Canyon along its way. The Colorado River Basin includes parts of seven western states (Arizona, California, Colorado, Nevada, New Mexico, Utah and Wyoming) and 2,000 square miles in Mexico.

Did You Know?

Building Hoover Dam required more than 5 million barrels of concrete. The finished dam contains enough concrete (4.5 million cubic yards) to build a two-lane highway from Seattle, Washington to Miami, Florida.

Beginning in the late 19th century, attempts were made to harness the natural power of the Colorado in order to provide irrigation and allow for settlement in the arid Southwest. In 1905, massive flooding caused by melting snow from high in the Rocky Mountains broke through the existing canals built just a few years earlier, completely submerging nearby farms. By the early 1920s, it had become clear that the Colorado would need to be controlled in order to prevent springtime flooding and channel the water where it was needed for irrigation, as well as provide hydroelectric power for people living in the region.

EARLY PLANS

Arthur Powell Davis, head of the U.S. Bureau of Reclamation (the federal agency given responsibility for irrigation in the West) drew up plans for an ambitious dam-building project in 1922. Black Canyon was chosen out of two prospective locations for the dam; the other was Boulder Canyon, and for some reason the planners continued to call the project Boulder Dam. Before a dam could be built, however, political work was necessary to resolve the competing claims on the river by different western states. As U.S. secretary of commerce, Herbert Hoover negotiated the Colorado River Compact, which divided the river basin into two regions, lower (Arizona, Nevada and California) and upper (Wyoming, Utah, New Mexico and Colorado) that would make the building of the dam possible.

More than 200 engineers worked to design the dam that would be constructed in Black Canyon. It would be the highest concrete arch dam in the United States, and the largest building project that the federal government had ever undertaken. In 1928, after years of lobbying to get a dam-building bill through Congress, the legislation was finally approved as the Boulder Canyon Project Act. Hoover, who that same year was elected as the 31st president of the United States, signed the bill into law in 1929.

A MASSIVE BUILDING PROJECT

By the time construction of Boulder Dam began in 1930, thousands of prospective workers had flooded the region, many of whom had lost their jobs during first years of the Great Depression. A total of 21,000 men worked on building the dam over the course of its construction (around 5,000 at any one time) and the region’s growing population turned Las Vegas from a sleepy town to a bustling city.

Blistering summer heat and a lack of adequate shelter and services combined with difficult and dangerous working conditions to create a volatile situation, and conflicts arose between the construction firm, Six Companies, and dam workers and their families. The Bureau of Reclamation would later estimate that 107 workers lost their lives while building the dam. Despite these problems, the massive project proceeded relatively quickly, and by the fall of 1935 Boulder Dam was completed.

A NATIONAL LANDMARK

Some 12,000 people attended the ceremony on September 30, 1935, when President Franklin D. Roosevelt dedicated Boulder Dam. (Twelve years later, the dam would be renamed for Hoover in honor of his efforts on behalf of the project.) At 726 feet (221 m) high–twice the height of the Statue of Liberty–and 1,244 feet (379 m) long, the dam weighs more than 6.6 million tons. At its base, where the maximum water pressure is 45,000 pounds per square foot, are huge generators that could produce up to 3 million horsepower and provide electricity for three states. The building of the dam created Lake Mead, which extends for 550 miles of shoreline and 247 miles of area, and is one of the largest man-made lakes in the world.

Hoover Dam was the tallest dam in the world when it was finished, and remained the largest producer of hydroelectric power in the world until 1948. Today, it is no longer the tallest, the largest by volume or the largest hydroelectric power producer, but remains among the biggest and best-known dams in the world. A National Historic Landmark, Hoover Dam draws some 7 million tourists a year, and another 10 million visit Lake Mead for boating, sailing, fishing and other recreation.

http://www.history.com

Tower Cranes

Most tower cranes have to be assembled and erected on site prior to use and can be equipped with a horizontal or luffing jib. The wide range of models available often make it difficult to choose a crane suitable for any particular site but most tower cranes can be classified into one of four basic groups thus:

1. Self Supporting Static Tower Cranes † high lifting capacity with the mast or tower fixed to a foundation base - they are suitable for confined and open sites.

2. Supported Static Tower Cranes † similar in concept to self supporting cranes and are used where high lifts are required, the mast or tower being tied at suitable intervals to the structure to give extra stability.

3. Travelling Tower Cranes † these are tower cranes mounted on power bogies running on a wide gauge railway track to give greater site coverage - only slight gradients can be accommodated therefore a reasonably level site or specially constructed railway support trestle is required.

4. Climbing Cranes † these are used in conjunction with tall buildings and structures. The climbing mast or tower is housed within the structure and raised as the height of the structure is increased. Upon completion the crane is dismantled into small sections and lowered down the face of the building.

All tower cranes should be left in an `out of service' condition when unattended and in high wind conditions, the latter varying with different models but generally wind speeds in excess of 60 km p.h. would require the crane to be placed in an out of service.

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