Why do we use Large Stones in Construction

Why do we use Large Stones in Construction

 

Stone has been used for many years to protect embankments, levees, river banks, and engineered features against erosion and in the construction of dams, breakwaters, and other large structures. Advantages over other materials and designs are often contingent on low cost of large-scale production and processing of stone and placement on the structure.

1. Slope Protection


a. Slope protection generally means the engineered feature composed of large-stone material constructed as a relatively thin overlay on a slope otherwise vulnerable to erosion. A bedding layer is usually included. The large-stone material is commonly called riprap. At the heart of some riprap design is the characteristic of physical flexibility. Riprap adjusts to minor flank erosion or undercutting and continues to function in its protective role.


b. One key consideration in slope protection is stone size, and the specifications for riprap should be detailed in regard to median size, gradation, and allowable tolerances. The cost advantage may be even greater where gradation requirements allow quarry-run material to be used. Even here, the importance of well defined specifications must be made clear since there are still limitations on oversize or undersize components that may require at least some separation processing.

At the other extreme from quarry-run material is riprap of narrow size range for manual placement in a keyed or fitted-stone arrangement. This labor-intensive and costly method of protecting slopes is practiced only rarely today but may be encountered in maintenance of features constructed years ago.

 

2. Training Structures

River training structures are relatively short, linear features constructed near the bank of a channel to control the pattern and velocity of flow. Examples are spur dikes and groins perpendicular to the bank and vane dikes more or less parallel. Rock training structures may be unzoned or earth-cored but are always designed for low cost, sometimes at the sacrifice of longevity. Groins are also used along coastlines to control sea drift currents and sand deposition. Coastal jetties train the flow of river outlets and may secondarily function as groins and breakwaters.

 

3. Retention Dikes

Retention dikes are designed to impound saturated materials such as from dredging. Elaborate, zoned designs are sometimes used where the waste is contaminated and the stability and the control of leachates are of high priority. Where wave attack is predictable, two or more zones of large stone are commonly incorporated, with the most critical usually being the outer armor.

As designs prescribe larger and larger stones, the problems of quality and cost increase dramatically. Smaller sized stone materials are usually more than adequate to remain stable and support the superimposed layers. Emphasis may then be redirected from high-quality stone to quantity and the need to provide large volumes of core stone.

4. Breakwaters and Jetties


a. Large breakwaters and jetties provide the outstanding examples of construction with large stone. The term rubble mound, though somewhat entrenched in the technical literature, seems inappropriate to describe these engineered structures which are commonly designed with several massive zones of different materials.

The special demands for protection against ocean wave attack lead to the ultimate among numerous possible requirements, that is very large, dense, and durable individual stones. At some large size, different for each project, the cost of constructing armor with stones exceeds that of construction with man-made concrete units such as tetrapods and dolosse. For projects near this cutoff, an especially thorough investigation of stone sources is needed. Quarries at great distance are often included, and the cost of transportation and handling must be estimated carefully.


b. Structures in deep water present special construction problems. Placement is partly remote and obscure so that quality control, quality assurance, and measurement for payment are comparably more difficult than in construction above water.

5. Zoned Embankments


a. Construction of zoned rockfill dams has constituted the largest use of large stone in some CE districts. The demands regarding material quality are usually different than for rubble mounds or even for retention dikes, and
weak rock can be used. Instead, the focus is on the immense volumes required.

b. Much of the stone material can come from excavations required as part of the project, particularly, excavations for spillways and outlet works. Quarries or stone borrow areas may be needed at least as supplementary sources. An accurate estimate of processing requirements is essential to assure sufficient volumes where suitable deposits are thin or otherwise in short supply or the material quality is marginal.

The attendant blasting, handling, and placement sometimes degrade some of the material and can cause shortfall in acceptable stone. 

6. Other Uses

Other uses of large-stone material are subordinate as a group to those identified above. Nevertheless, special uses can be significant.


a. Energy Dissipators. Energy dissipators may be constructed from derrick stone placed in plunge pools to reduce erosion of foundation soil or bedrock. The high-energy environment may preclude long service life without
additions of fresh stone.


b. Structure Protection. Stone placed around the upstream end of a bridge pier is an example of protection of structures. Stone placed to stabilize and protect dikes or breakwaters primarily composed of concretefilled caissons is another example.


c. Masonry. Masonry construction is included in this manual for completeness. The high cost of this labor-intensive method largely disfavors its use today, but stone masonry still exists in CE structures and facilities and occasionally can be repaired with essentially new masonry construction.
Dimensional and cut stone for masonry are produced in a relatively few, specialized quarries.


d. Landscaping Stone. Landscaping stone is not part of an engineered structure or feature as such, but it essentially completes some CE projects. Guidance in selecting stone and in estimating or inspecting stone work for landscaping is potentially useful on a broad scale.

 

Spillway Function and Classification

Spillway Function and  Classification

 

Project functions and their overall social, environmental, and economic effects may influence the hydraulic design of the spillway. Optimization of the hydraulic design and operation requires an awareness by the designer of the reliability, accuracy, sensitivity, and possible variances of the data used.

The ever-increasing importance of environmental considerations requires that the designer maintain close liaison with other disciplines to assure environmental and other objectives are satisfied in the design. 

Spillway Function

The basic purpose of the spillway is to provide a means of controlling the flow and providing conveyance from reservoir to tailwater for all flood discharges up to the spillway design flood (SDF). The spillway can be used to provide flood-control regulation for floods either in combination with flood-control sluices or outlet works, or in some cases, as
the only flood-control facility.

A powerhouse should not be considered as a reliable discharge facility when considering the safe conveyance of the spillway design flood past the dam. A terminal structure to provide energy dissipation is usually provided at the downstream end of the spillway. The degree of energy dissipation provided is dependent upon the anticipated use of the spillway and the extent of damage that will occur if the terminal structure capacity is exceeded.

The standard project flood is a minimum value used for terminal structure design discharge. The designer must keep in mind that damage to the dam structure that compromises the structural integrity of the dam is not acceptable. Acceptance of other damages should be based on an economic evaluation of the extent of damage considering the extremely infrequent flood causing the damage.

Spillway Classification

Spillways are classified into four separate categories, each of which will serve satisfactorily for specific site conditions when designed for the anticipated function and discharge.

1. Overflow Spillway

This type of spillway is normally used in conjunction with a concrete gravity dam. The overflow spillway is either gated
or ungated and is an integral part of the concrete dam structure. 

Figure 1 : Chief Joseph Dam overflow spillway

2. Chute Spillway

This type of spillway is usually used in conjunction with an earth- or rock-filled dam; however, concrete gravity dams also employ chute spillways. In these cases the dam is usually located in a narrow canyon with insufficient room for an overflow spillway. The chute spillway is generally located through the abutment adjacent to the dam; however, it could be located in a saddle away from the dam structure. 

Figure 2 : Mud Mountain Dam

Figure 3 : Wynoochee Dam

3. Side Channel Spillway

This type of spillway is used in circumstances similar to those of the chute spillway. Due to its unique shape, a
side channel spillway can be sited on a narrow dam abutment. Side channel spillways generally are ungated; however, there is no reason that gates cannot be employed. 

Figure 4 : Townshend Dam side channel spillway

4. Limited Service Spillway

The limited service spillway is designed with the knowledge that spillway operation will be extremely infrequent, and when operation occurs, damage may well result. Damage cannot be to the extent that it would cause a catastrophic release of reservoir water.

 

Types of Concrete Gravity Dams

Types of Concrete Gravity Dams

 

Introduction

Gravity dams are solid concrete structures that maintain their stability against design loads from the geometric shape and the mass and strength of the concrete. Generally, they are constructed on a straight axis,
but may be slightly curved or angled to accommodate the specific site conditions.

Gravity dams typically consist of a nonoverflow section(s) and an overflow section or spillway. The two general concrete construction methods for concrete gravity dams are conventional placed mass concrete and RCC.


1. Conventional concrete dams


Conventionally placed mass concrete dams are characterized by construction using materials and techniques employed in the proportioning, mixing, placing, curing, and temperature control of mass concrete (American Concrete Institute (ACI) 207.1 R-87). Typical overflow and nonoverflow sections are shown on Figures 1 and 2.

Figure 1 : Typical dam overflow section

 

Figure 2 :  Nonoverflow section

Construction incorporates methods that have been developed and perfected over many years of designing and building mass concrete dams. The cement hydration process of conventional concrete limits the size and rate of concrete placement and necessitates building in monoliths to meet crack control requirements.

Generally using large-size coarse aggregates, mix proportions are selected to produce a low-slump concrete that gives economy, maintains good workability during placement, develops minimum temperature rise during hydration, and produces important properties such as strength, impermeability, and durability. Dam construction with conventional concrete readily facilitates installation of conduits, penstocks, galleries, etc., within the structure.


Construction procedures include batching and mixing, and transportation, placement, vibration, cooling, curing, and preparation of horizontal construction joints between lifts.

The large volume of concrete in a gravity dam normally justifies an onsite batch plant, and requires an aggregate source of adequate quality and quantity, located at or within an economical distance of the project.


Transportation from the batch plant to the dam is generally performed in buckets ranging in size from 4 to
12 cubic yards carried by truck, rail, cranes, cableways, or a combination of these methods. The maximum bucket size is usually restricted by the capability of effectively spreading and vibrating the concrete pile after it is dumped from the bucket. The concrete is placed in lifts of 5- to 10-foot depths. Each lift consists of successive layers not exceeding 18 to 20 inches. Vibration is generally performed by large one-man, air-driven, spud-type vibrators.

Methods of cleaning horizontal construction joints to remove the weak laitance film on the surface during curing include green cutting, wet sand-blasting, and high-pressure air-water jet. Additional details of conventional concrete placements are covered in EM 1110-2-2000.


The heat generated as cement hydrates requires careful temperature control during placement of mass concrete and for several days after placement. Uncontrolled heat generation could result in excessive tensile stresses due to extreme gradients within the mass concrete or due to temperature reductions as the concrete approaches its annual temperature cycle.

Control measures involve precooling and postcooling techniques to limit the peak temperatures and control the  temperature drop. Reduction in the cement content and cement replacement with pozzolans have reduced the temperature-rise potential. Crack control is achieved by constructing the conventional concrete gravity dam in a series of individually stable monoliths separated by transverse contraction joints.

 

2. Roller-compacted concrete (RCC) gravity dams


The design of RCC gravity dams is similar to conventional concrete structures. The differences lie in the construction methods, concrete mix design, and details of the appurtenant structures. Construction of an RCC dam is a relatively new and economical concept.

Economic advantages are achieved with rapid placement using construction techniques that are similar to those employed for embankment dams. RCC is a relatively dry, lean, zero slump concrete material containing coarse and fine aggregate that is consolidated by external vibration using vibratory rollers, dozer, and other heavy equipment.

In the hardened condition, RCC has similar properties to conventional concrete. For effective consolidation, RCC must be dry enough to support the weight of the construction equipment, but have a consistency wet enough to permit
adequate distribution of the past binder throughout the mass during the mixing and vibration process and, thus,
achieve the necessary compaction of the RCC and prevention of undesirable segregation and voids.


What is Compass Surveying

What is Compass Surveying

 

 

INTRODUCTION

Compass surveying is the branch of surveying in which directions of survey lines are determined with compass and the lengths of the lines are measured with a tape or a chain. In compass surveying the direction of survey line called the bearing of line is defined as the angle made by the line with the magnetic meridian. In practice compass is generally employed to run a traverse. Traverse consists of series of straight lines connected together to form an open or closed traverse.

COMPASS

The commonly used instrument for compass surveying is Compass. A compass is small instrument which consists essentially of magnetic needle, a graduated circle and a line of sight. When the line of sight is directed towards a line, the magnetic needle points towards magnetic meridian and the angle which the line makes with the magnetic meridian is read at the graduated circle.

Compass consists of cylindrical metal box of about 8-12 cm diameter in the center of which is a pivot carrying a magnetic needle which is already attached to the graduated aluminum ring. The ring is graduated and is read by reflecting prism. Diametrically opposite to the prism is the object vane hinged to the box side carrying a horse hair with which the object in the field is bisected.

 TYPES OF COMPASS

There are two forms of compass as under

  1. Prismatic compass
  2. Surveyors compass

The two compass are almost same except few differences so far as their construction is concerned. Prismatic compass uses WCB (0⁰­­-360⁰) circular ring while as the surveyors compass uses Quadrantal Bearing (0⁰­­-90⁰) circular ring system. Besides bearing system, the former has graduated ring attached to magnetic needle as the result of which when compass box and sight vane is rotated the needle remains stationary on the other hand in surveyors compass graduated ring being attached to compass box moves with it as the box is rotated

 

ADJUSTMENT OF COMPASS (WORKING)

Working of compass involves three steps:

  1. CENTERING
  2. LEVELLING
  3.  OBSERVING THE BEARING

Centering involves to align the compass is such a way the Centre is placed vertically over the station point. It is done with the help of tripod stand and is checked by dropping a small pebble below the Centre of compass.

Levelling is done so that the graduated ring swings quite freely. It is done with the help of ball and socket arrangement and can be checked by rolling a round-pencil on the compass box

Observing the bearing once centering and levelling has been done, raise or lower the prism until the graduations on the ring are clearly visible when looked through the prism. Afterwards turn compass-box until the ranging rod at the station is bisected by horse-hair of objective vane. At this position note down the reading.

COMPASS TRAVERSING

Whenever in traversing compass is used for making angular measurements, it is known as compass traversing or compass surveying. In compass traversing, the compass is used to determine the direction of survey lines of the framework of the traverse for measuring the angles which these lines make with the magnetic meridian.

The process of chaining and offsetting is the same as in chain surveying and running the check lines is not necessary. Compass traverse may be closed or open. Close traverse starts from one traverse station and closes either on same or on another traverse station whose location is already known. On the other hand an open traverse starts from one station and closes at other station whose location is neither known nor established.

 

EQUIPMENT USED IN COMPASS SURVEYING

  • PRISMATIC COMPASS
  • MEASURING TAPE
  • RANGING RODS
  • PLUMB BOB
  • CHAIN
  • CROSS STAFF

 

MEASUREMENT OF INCLUDED ANGLE

In a compass (prismatic) which uses Whole Circle Bearing (0⁰ to 360⁰) system the included angle is given by

Included angle   =  F.B of next line  —  B.B of previous line at same station

 

PRECAUTIONS

Compass surveying is used in case of rough surveys where speed and not accuracy is main consideration. One of the biggest disadvantages of compass is that magnetic needle which gives bearing of lines is disturbed from its normal position in presence of materials such as iron-pipes , current carrying wires , proximity of steel structures , transmission lines etc. called sources of local attraction.

Top 10 Questions You May Have about LiDAR

Top 10 Questions You May Have about LiDAR

 

1. Why Is LiDAR Such a Valuable Tool?

The value of LiDAR lies in the fact that it virtually places you on your target site without having to leave the office. Combining imagery with LiDAR point cloud data is the next best thing to being there. Plus, it’s fast, accurate, and affordable.

2. Can I Collect Other Information While I’m Gathering LiDAR Data?

You name it — cameras, video imaging systems, multi-spectral and hyper-spectral imaging systems can all be mounted on and operated with most current LiDAR systems. Because you can operate these multiple systems using the same components, you’ll save time and money, a big benefit to corridor mapping projects such as transportation, pipeline, and transmission mapping.

Mobile mapping systems typically include two or more cameras within the system. One drawback: These passive systems require a light source, which means your collection times are lim-ited. Unlike LiDAR, they can’t see in the dark.

3. Do I Need Breaklines If I’m Using LiDAR?

Not necessarily. It all depends on the requirements of the product you’re generating. Typically, LiDAR is very good at defining the surface, as long as the sample spacing is ade-quate and there isn’t too much vegetation. Most features and terrain are very well defined in LiDAR data.

The rule of thumb relative to breakline usage comes down to edge recognition needs in the surface. If you have key elements, such as a back of curb line or a lip of gutter line, you will want to collect breaklines. If you’re only producing large-scale contour maps, you may not need the accuracy that breaklines provide.

4. Where Can I Find an End-to-End Solution for LiDAR Data?

Try Autodesk. Specifically, you’ll find an end-to-end solution in AutoCAD 2011, AutoCAD Labs, AutoCAD Civil 3D, Map 3D, and Navisworks products. Another robust data extraction solution for feeding all these applications with classified LAS and featurized GIS is the Virtual Geomatics solution.

5. Classical Photogrammetric Data Collection Works for Me — How Does LiDAR Compare?

LiDAR is about 40 percent less expensive than classical pho-togrammetric collection. And it takes less time to collect, process, and extract the needed information from LiDAR com-pared with traditional methods.

6. Is LiDAR Data Accurate Enough to Use on Road Overlay Projects?

You bet, as long as you use the appropriate collection method with the sufficient survey control. Just be careful when you pick your method and plan the control.

7. What Is Corn-Rowing?

You can’t eat it. The term corn-rowing refers to an artifact of LiDAR sampling that typically occurs at the edges of scans and overlapping data areas. It’s caused when LiDAR points are sampled close together and the difference in the sampled points is greater than the relative accuracy. With proper col-lection and filter processes that remove the points causing the trouble, you can minimize corn-rowing.

8. What Type of LiDAR Data Do I Really Need?

The best way to determine what LiDAR products you need is to really understand the application for the data. Call a qualified LiDAR collection agency for a recommendation on the appropriate collection methods based on your specific requirements. Here’s the info you’ll need to pass along:

✓ Accuracy requirements for data.

✓ Extraction requirements — do you just need points, or linear features, too?

✓ End products you’ll require. Do you need a triangulated surface, a classified LAS, and/or GIS features?

9. What Is an Intensity Image?

An intensity image is a monochromatic (shades of gray) image of the illumination (energy) returns from the LiDAR system. These images can be used for generating planemetric and breaklines by using LiDARgrammetry. The intensity image is typically a Geotiff, and the accuracy of the image is a function of the horizontal accuracy of the LiDAR, along with the inter-polation of the point to a raster image.

10. You Just Said LiDARgrammetry—What’s That?

LiDARgrammetry is the process of using intensity images to generate synthetic stereo pairs, much like the stereo pairs used in photogrammetry. The data generated from LiDARgrammetry tends to be only as accurate as the LiDAR from which it’s generated. There are varying opinions regard-ing the usefulness of this information and how accurate it is. Still, it’s a good byproduct of LiDAR, and whether it’s useful to you just depends on the scope of your project.

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