Introduction: (Initial Observation)
Dams are among the structures that are subject to extreme forces. Many small and large dams have broken in the past and almost every time such accidents are contributed to structural defects in original design or structural defects caused later as a result of poor maintenance.
When a dam breaks, millions of dollars of investment are lost and it creates a sudden flood. The flood destroys farms and houses and properties and takes many lives. Without a dam. many cities will lose electricity and water for an extensive period of time.
The structure of a dam or the way that it’s parts are designed and interconnected is the main factor affecting the strength of a dam.
This project studies on different structural designs for dams. You will make models and measure and compare the strength of each design.
Find out about dams and design considerations in structuring a new dam. Read books, magazines or ask professionals who might know in order to learn about the pressure of water in different depths and the way that such pressure will enter forces into different sections of a dam. Keep track of where you got your information from.
What you will learn in this project?
You will learn that the main purpose of structural design and engineering is to ensure that structure in whole has enough strength to resist all forces to the structure. That includes permanent forces such as weight, as well as temporary forces such as changing load, wind and earthquake. A secondary purpose is minimizing the amount of material and labor used for construction without jeopardizing the strength and reliability of the structure.
You will also learn that the forces in different parts of the structure vary depending on the loads and the design. For example in dams, forces to the lower parts of the dam are much more than the forces to the upper sections of the dam.
Finally you will learn that every construction material such as steel beams, concrete and wood have different strengths in presence of different forces. Among them Compression strength and tensile strength are the two major factors used in design and calculating the strength of a structure. In most structural designs we try to divert forces that may bend the material to forces that pull or push the material in a way compatible to the tensile strength or compression strength of material.
As you see the knowledge about the structure is not just about dams. The same skills is used in designing bridges, tunnels and buildings. Since your focus in this project is the structure of dams, it is good if you also make yourself familiar with some dam designs.
Arch dams are made from concrete. They are curved in the shape of an arch, with the top of the arch pointing back into the water. An arch is a strong shape for resisting the pushing force of the water behind the dam. Arch dams are usually constructed in narrow, steep sided valleys. They need good rock for their foundations, and for the sides of the valleys, to resist the forces on the dam.
Buttress dams are made from concrete or masonry. They have a watertight upstream side supported by triangular shaped walls, called buttresses. The buttresses are spaced at intervals on the downstream side. They resist the force of the reservoir water trying to push the dam over.
The buttress dam was developed from the idea of the gravity dam, except that it uses a lot less material due to the clear spaces between the buttresses. Like gravity dams, they are suited to both narrow and wide valleys, and they must be constructed on sound rock.
Embankment dams are made mainly from natural materials. The two main types are earthfill dams and rockfill dams. Earthfill dams are made up mostly from compacted earth, while rockfill dams are made up mainly from dumped and compacted rockfill. The materials are usually excavated or quarried from nearby sites, preferably within the reservoir basin.
A cross-section (or slice) through an embankment dam shows that it is shaped like a bank, or hill. Most embankment dams have a central section, called the core, made from an impermeable material to stop water passing through the dam. Clayey soils, concrete or asphaltic concrete can be used for the core.
Earthfill – Earthfill materials for a dam include clays, sands, gravels and silts, and soils made up from a mixture of these.
Rockfill – Rock is blasted using explosives to break it down into small enough pieces to use in embankment dams. The size of the pieces of rock vary from very small (about 2 mm) up to about 600 mm. Sometimes, the blasted rock needs to be crushed as well to get the right range of sizes.
Rockfill dams are permeable. They can have a core or an impermeable cover on the upstream face. Materials used for the cover include reinforced concrete and asphaltic concrete.
Embankment dams are usually chosen for sites with wide valleys. They can be built on hard rock or softer soils, as they do not exert too much pressure on their foundations.
A gravity dam is made from concrete or masonry, or sometimes both. It is called a gravity dam because gravity holds it down to the ground stopping the water in the reservoir pushing it over.
A cross-section (or slice) through a gravity dam will usually look roughly triangular.
Gravity dams are suited to sites with either wide or narrow valleys, but they do need to be built on sound rock.
In order to calculate the amount of forces to different sections of a dam, you need to calculate the water pressure in different sections.
To learn about pressure and conversion of different units of pressure see the following link:
What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.
The purpose of this project is to see how does the structural design affect the strength of a dam.
When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.
Independent variable is the design or model of the dam. Dependent variable is the strength of the dam. Controlled variables are the size, water capacity and the structural material of the dam.
Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis. This is a sample hypothesis:
My hypothesis is that the arc dam provides the highest level of strength while using the least amount of construction material. However construction of a arc dam may not be feasible in many locations because of absence of hard rock for the foundation and the sides.
Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”
The main experiment for this project is to build different models of dam and compare their strength. When testing the strength, you may use water or any other force as long as you can control or measure the amount of force.
To construct the dam model, you may use balsa wood or Styrofoam sheets. In any case you must use low thickness and low strength material, so your design will provide the strength not the material. Both Styrofoam and balsa wood can be cut using a utility knife and can be glued using wood glue.
This is what you do in the experiment #1. However we have proposed a few additional experiments that you may choose to perform after completing your main experiment. Note that you may choose to change the sizes or dimensions as you wish, but keep it challenging.
Make three dams in a valley 2 feet wide and one foot high. Your designs include a straight wall, an arc dam and a buttress dam. DO NOT try an embankment dam and a gravity dam because these two rely on the mass of material and your purpose is minimizing the use of material.
You can always test the strength of a dam the same way that you test the strength of a bridge. In other words you place your model dam like a bridge between two tables that are two feet apart. The upstream part is faced up so you can place a weight on that side. The downstream side is faced down. However you may want to test the dam using water.
In this case you will need a wooden box that one side of that is missing. The dimensions of this missing side is 1′ x 2′.
You mount the dam that you make in this open side. To make your wooden box, you may purchase cut to size pieces from a hardware store and connect the pieces together using nail and wood glue. In our design we used an extra stick on the top to hold the sides in place.
The open side can optionally have some 1/2″ moldings on the walls to hold the dam in place. In this way you can remove one dam and replace it with the other very easily. Otherwise you need to secure each dam with glue and nail. Testing the dam with water is very hard because you will need to spend a lot of time to make your dam leak proof. The other problem is that water has a limited amount of pressure. You will not be able to modify the force and compare the strength. In my experiment a Styrofoam wall broke by the pressure of water but two other dams did not, so I could not find which model dam is stronger. Later I tested the dams with external forces to compare their strength.
How to build a buttress dam?
Depending on the material that you choose (Styrofoam or balsa wood), you can come up with your own measurements. Following is a sample:
Get a 24″ x 12″ piece of Styrofoam board as the dam main wall. The thickness of the foam that I used was 0.75″ but I think it is known as 1″ foam. To make buttresses, cut rectangles of 6″ x 12″ and then cut each rectangle diagonally to make two buttresses. You will need seven to 10 buttresses. Use another piece of 7″ x 24″ foam board as the foundation.
In my sample, I did not have a 7″ x 24″ foam board to use as foundation. Instead I had smaller pieces of foam that I used to connect the bases of buttresses. This served as a foundation, but not as well.
Glue one long edge of the main wall and mount it on the foundation board (align it with one side and hold it vertically). Glue two short sides of a buttress and connect it to the wall and foundation in the center of the dam. This will help the wall stay vertically. Connect the next two buttresses in the sides, making sure you leave one inch space from each side. This space is required for further tests and to connect the dam to the walls of the valley.
The same way add 4 or 6 more buttresses between the existing buttresses. Keep the distances equal. So you will use a total of seven or 9 buttresses.
Let everything dry in a warm room for about 24 hours.
Next day check the seams and refill them with glue. Wait another day so the glues will dry again.
This time the dam will be ready for test. Place the dam between two tables (I used boxes) and place an empty plastic jar on the upstream side of that.
Open the jar and start adding water to the jar. Continue that until the dam breaks. Each liter water is 1 kilogram, so using the volume you can know the maximum weight that the dam could hold. Use that as the strength of the dam.
Do the same test with a plain 12″ x 24″ Styrofoam board and compare the results.
How to build an arc dam?
If you are using Styrofoam, you can not just bend the foam board to an arc. Stress in the board may weaken the board. You need to cut pieces of the arc from foam board and glue them to each other.
Draw an arc similar to the above arc on the foam board and cut that. This will be your template to draw and cut more arcs. Make sure the thickness of your arc is the same as the thickness of the foam board so you can compare the results. In our example the thickness of the arc had to be 0.75″.
You may use a string and a pencil as a compass to draw the arc.
In my experiment I had to cut 16 arcs and glue them on each other to make a 12″ high dam. If you are using balsa wood, you can keep the wood in water, hot water or steam to make it soft. Then you bend the soften wood and let it dry while bent.
When gluing the arcs to each other, cover one side of one arc with a thin layer of glue and the place it on the other. You may temporarily use pins to hold the pieces together while the glue is being dried.
Picture in the right shows two arcs glued to each other. Entire dam took 16 arcs.
After your arc dam is ready you will test it by placing it between two tables and placing weight on the upstream side of the dam. I noticed that weight is trying to widen the dam and this is not supposed to happen because hard rocks on the sides of the valley are not expandable. So I made a wooden frame to prevent the change of width while testing. You can make your wooden frame any way you like, the following drawing shows how a wooden frame will hold the dam.
Such a frame can be used to test other dams as well.
Your results table may look like this:
Strength of a 2 feet wide, 1 foot high dam
|Dam Style||Dam material weight||Dam strength||Strength/material ratio|
You may also use your results table to draw a bar graph. Each bar represents the strength of a certain type dam including straight wall.
In this experiment you will simply perform a calculation to find out the amount of force entered to a dam with certain size. For example a dam similar to the buttress dam that you made in the previous experiment has an area of 288 square inches and every square inch of dam is under a pressure which is equal to the water pressure at that depth.
In metric system it is easier to know water pressure at different depths. The specific gravity of water is 1 g/ml. So the water at the depth of 11 cm has a pressure of 11g/cm2.
Pressure changes by depth. So you need to calculate the average pressure. Simply measure the pressure at the bottom. multiply it by the height and divide it by two.
For example in a dam 30 cm deep, the maximum pressure is 30 g/cm2. So the average pressure is 15 g/cm2. To calculate the total force of water to the dam, multiply the area of the dam by this pressure. For example in a 30cm x 60 cm dam, the area is 1800 cm2. Total force to the dam at sea level is 1800 x 15 = 27000 g.
So the total force to the dam is 27kg.
In all dam designs we use the compressive strength of material to make our design. Even in constructing reservoirs in flat areas the same properties are the key to the design (such as embankment dams). In this experiment we try to use the tensile strength of material to construct a water barrier or tank.
Bags made of natural or synthetic fibers rely on the tensile strength of the strings. Bags can not hold water because they are pores. Plastic bags however do not have strength. If you attempt to fill up a large plastic bag with water, it will break. By combining these properties we can make a model of a reservoir. To do that place a plastic garbage bag inside a fabric bag. In this way the tensile strength of fabric will hold the plastic bag and prevent it from breaking a part.
The plastic bag that you use can be very cheap and thin plastic, however it’s diameter must be more than the diameter of your fabric bag. Plastic pools used in homes are also designed based on the tensile strength of fabric and leak proof ability of plastics.
Materials and Equipment:
List of material can be extracted from the experiment section. This is a sample list of materials. Your final list of materials may be different from this list:
- Styrofoam 1″ thick boards. (come in 12″x36″ boards. Available in Hardware stores)
- Utility knife
- White glue or wood glue
- Weights (Available at MiniScience.com, but you may improvise your own weights in the form of plastic containers field with water or sand)
- Scale (like kitchen scale) used to test or prepare weights.
Results of Experiment (Observation):
Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.
Which design dam had the highest strength?
How do you decide which design to use based on the area that you may want to construct a dam?
Compare the strength of straight wall dam with the strength of different designs that you made.
How about Hoover dam? is it an arc dam or gravity dam? Or maybe a combination of both?
In experiment #2 you will calculate the total force to a dam. In other experiments you may perform some simple calculations to decide what should be the dimensions of the pieces. Write your calculations in your report.
Summary of Results:
Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.
It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.
Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.
Related Questions & Answers:
What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.
If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.
If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.
List of References