Lab 6: Properties of sediments.

Names________________________________________________________________ Date_____________________

 

Introduction: By the term sediments we are referring more generally to any collection of solid particles at the earth's surface. Sand at the beach, mud in a lake, soil, boulders at the foot of a cliff, volcanic ash ­ these are all examples of naturally occurring aggregates. Sedimentology is the study of sediments . The interactions between sediments and fluids produces a great variety of landforms such as deltas, dunes, alluvial fans, valleys and more. While the veneer of sediments on the earth is quite thin, sediments are very extensive and an important part of the surface environment. Sediments are a common reservoir for ground water, a common substrate for plants, microbes and are a crucial component of many ecosystems. They can show some very counterintuitive behavior. Though composed of solids they can flow. Think of sand in an hour glass. In traditional introductory physical geology and geography labs rocks and minerals are investigated, but not sediments. Usually it is only in advanced sedimentology labs that students are introduced into sediments. Yet, this does not mirror their importance, especially from a system science perspective. What you learn in this lab will be used in subsequent labs.


Description of individual grains
.

The properties of individual grains contribute to the properties of aggregates as a whole. Obviously the size of a grain is important. It takes more enery to transport a larger grain. There are many other important properties: including grain roundness, grain sphericity, grain composition, grain density, grain hardness.

In your own words what is the difference in shape between the particle in sample A and sample B?

Use the chart and by visual comparison, attach the appropriate term to the particle shape.
Term for sample A grain. ______________________ Term for sample B grain. _________________

Taken from Mazullo et al. (1988) http://www.ga.gov.au/odp/publications/197_IR/chap_02/c2_f8.htm

Use the calipers to measure the longest axis and the shortest axis of three grains in sample A and B in millimeters.

 

 short axis grain 1

 long axis grain 1

 short axis grain 2

 long axis grain 2

 short axis grain 3

 long axis grain 3

 average grain 'diameter'
 range in grain diameter
 Sample A                
 Sample B                

Note that the difference in the longest axis and shortest axis is also some measure of how much the grain departs from a perfect sphere, i.e. it is a measure of the departure from sphericity.

Sediment is transported from some source area by water, wind, ice or some combination of the above. Sediment that is closer to its source is called proximal, while that farther away is distal. As a thought exercise, what changes do you expect to find in the above grain characteristics with distance from the sediment source along a river channel.

 

 

Which of the above samples do you think is more proximal and why?

 


Statistical descriptions: Imagine a handful of sand. How many grains? If we were to measure the grain size, sphericity, and other traits of each grain we would have a detailed description of the grains, and a lot of numbers. If that handful of sand is like other handfuls of sand taken from the source then this collection of numbers could be considered representative. We might think of it as a sample population. However, the long collection of numbers wouldn't be very useful. Statistics is a way to boil down all those numbers into a handful of useful numbers. We need to think about what are the important attributes of that list of numbers. Lets focus just on grain size. To explore this conceptually think of the following questions. Are most of the grains close to one size, or are some grains closer to one size, and others closer to another distinct and different size. In the first case your sample is likely unimodal, in the second bimodal. For a unimodal distribution of grains you can ask two other questions. What value are most of the grains closest to? We usually take the average as the best answer to this question, but it can get more complicated. The average is known as measure of central tendency. There is more than one such measure. The next question you can ask is how close are the values to the measure of central tendency, or what is the variation around this measure. The numbers that capture this are know as measures of dispersion. The range, the difference in value between the lowest and highest value is one measure of such dispersion (although the measure of standard deviation is much more commonly used).

 

Average grain size: A scale has been developed to classify sediments on the basis of average grain size. You are already familiar with it in a sense. Gravel is coarser than sand, which is coarser than silt, which is coarser than mud. Use the comparative visual chart attached (or provided) to estimate the average grain size of samples C and D.

 

Describe what you think is the relationship between grain size and the velocity of a water current needed to move material of that average grain size.

 

Sorting focuses on the concept of dispersion. The more variation in grain size in a sediment the more poorly sorted it is while the less variation the better sorted it is. Which sample is better sorted and which is more poorly sorted.

 

 


Water in sediments

Water can be seen both seeping into the ground and issuing forth as springs or seeps. Considering this common experience and the importance of water, it is not surprising that some concept of groundwater, and of the hydrologic cycle, has been part of human lore for a long time. Leonardo de Vinci, the Italian genius and painter, gave some thought to this, some of which is captured in the following quote from his notebooks;

" It is the property of water that it constitutes the vital human of this arid earth; .... But that which crowns our wonder in contemplating it is, that it rises from the utmost depths of the sea to the highest tops of the mountains, and flowing from the opened veins returns to the low seas; the once more, and with extreme swiftness, it mounts again and returns by the same descent, thus rising from the inside to the outside, and going round from the lowest to highest, from whence it rushes down in a natural course. Thus by these two movements combined in a constant circulation, it travels through the veins of the earth." ( Richter, translator, 1970)

The study of water within the ground is known as geohydrology. For this lab we will focus on ttwo basic geohydrologic properties of porosity and hydraulic conductivity.

Porosity simply refers to the amount (%) of void space in a rock body. Such space can naturally be occupied by gases or fluids. Two basic types of porosity are intergrain porosity, the void space between grains in the rock, and fracture porosity, the void space of fractures in the rock. Sedimentary rocks (sandstones, siltstones, etc.) are often characterized by being dominated by the former and crystalline rocks (granites, gneisses, schists, basalts, etc.) are characterized by the later, but both can coexist and be significant. Porosity of geologic material can vary from close to 0% up to unusual cases of 60%. We will focus on intergrain porosity of loose sediments.
While porosity is an important aspect, it is only one of several factors that determines how water flows through a porous medium. Permeability refers to the ability of a material to transmit water for given "boundary conditions" (pressure driving flow, length of flow path, etc.). It is a material property of a rock body, as is density, or the speed with which certain vibrations (e.g. seismic waves) pass through the rock, and thermal conductivity. A rock with high permeability will transmit a greater amount of water for a given set of conditions than will a rock with low permeability. As will be seen in this lab, two materials of similar porosity, but differing elsewise, may have notably different permeabilities due to the other factors involved. For the purpose of this lab we will look at hydraulic conductivity, a property closely related to permeability.

What sediment characteristics control porosity and permeability? Specifically, how does grain size effect porosity? How does sorting effect porosity and permeability? Can you think of toher factors that will effect porosity and permeability?

We will experiment with three sediment samples, one of Platte River channel alluvium, and two artificially constructed ones, determining both their porosity and their permeability. One sample is well sorted, but coarser grained. one sample is well sorted but finer grained, and one sample is more poorly sorted, with coarse and fine grains. Which sample is which?

well sorted, but coarser grained = _________________________ visual estimate of porosity ________________%

well sorted, but finer grained = ___________________________ visual estimate of porosity ________________ %

more poorly sorted = ______________________________ visual estimate of porosity __________________%


Fill the graduated beaker with the sample sediment up to certain level and record that level in terms of ml. Then starting with a known quantity of water fill the beaker with water until the sediment is totally saturated, but with no excess water above the sediment. Record, in ml, how much water it took to saturate the specimen. From the two numbers you have recorded for each sample calculate the sample porosity.


 

volume (in ml) of sediment
 volume of water (in ml) needed to saturate  % porosity
 sample E      
 sample F      
 sample G      

If you had an aquifer that was 100 feet thick that underlay a certain area with the porosity of sample E, and you were to magically pump all that groundwater up to the surface instantaneously, how many feet high would the water be? _________ The conclusion is that very sizable amounts of water are stored in the ground in some places.

Note as you are filling the cylinders the varying time required to allow the water to percolate downward. This gives insight into their relative permeabilities. Which sample is the quickest and which the slowest?

quickest --> ___________________ slowest --> _______________________

Measuring hydraulic conductivity:

We are going to experiment (hopefully) with a simple device to measure a variant of permeability using Darcy's law. Darcy devised this law when trying to understand how big a sand filter was needed for the city of Paris (to prevent the rampant disease). The law can be stated as follows - the amount of flow in terms of unit volume per unit time is a function of a constant for a given geologic medium, known as the hydraulic conductivity, multiplied by the cross-sectional area of flow times the quantity obtained by dividing the head difference (the force driving the flow) by the length of the flow path. This is Darcy's law in words. Below is Darcy's law in its more familiar, mathematical form (choose the one you wish to learn and use):

Q = K · A · (h/l)

where Q is flow (volume per unit time), K=hydraulic conductivity, A=cross-sectional area of flow, h=head difference (driving force), l=length of flow path. Hydraulic conductivity is closely and directly related to permeability.

Initiate flow of water through the Darcy tube. The hole in the upright tube keeps the head (water column driving flow) at a constant value. Wait until the flow through the system steadies, then measure Q using a watch showing seconds and a graduated beaker to measure outflow volume(or the like). You might want to make several Q vs. time measurements to determine consistency and error. Measure the other parameters of the apparatus so that you can compute the hydraulic conductivity. Use the chart below to work through to the answers. Make sure all your units are correct. You can also use Excel to do so.

   sample E  sample F  sample G
 volume output      
 time      
 discharge (Q)      
 h      
 l      
 diameter      
 cross sectional area      
 hydraulic conductivity      

Which sample is the highest and why? Which sample is the lowest and why?

 

 


Slope stability of sediments.

Slopes of sediments can move down hill under the incluence of gravity in a process known as mass wasting. A slope that exhibits such behavior is deemed unstable, while a slope that doesn't is called stable. The mass wasting can take a variety of forms (from rock falls, to slumbs, to slides, to debris flows), and is a major environmental concern in some areas.

The angle that separates a stable from an unstable sediment surface slope is known as the angle of repose. It is one measure of sediment 'strength'. What do you predict the angle should be? _______________ How much should it vary from one sediment type to another? ____________________ What might be factors that cause a variation in the angle of repose? ______________________________________

In this experiment you should do the following:

What is the average angle of repose for the 2 sediment types? a) _____________ b) ______________

What is the range in measurements of angle of repose for your 5 repeat trials? a) _____________ b) ______________ .

Take the spritzer and wet the top of the sediment and repeat your 5 measurements? How do your results differ from the 'dry' run and why?

 

 

Measure the angle of the sediment surface on the front (steep side) of the dune in the photo below. Also measure the other side (left side) of this dune. Steep angle of dune face ________. Shallow angle of dune face__________ .

Compare these angles to your results.