Format for the laboratory report.

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22 févr. 2014 (il y a 3 années et 1 mois)

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GEOS 334
Sedimentology & Stratigraphy
-

Laboratory Exercise
2


This laboratory exercise has

three

parts. In Part I, you will practice a basic technique for

determining grain size
(>63um)
using the RoTap
(sieving)
method. In Part II you will create a grain
s
ize

chart. Part

III is a set of questions that
everyone answers a
s part of the laboratory report
.


The goals of this series of laboratory exercises are as follows:



Learn particle size analytical techniques for sands;



Graph data on arithmetic and logarith
mi
c scales, both by hand and by
computer;



Perform graphical statistical analyses using a spreadsheet;



Interpret results;



Develop a well
-

written laboratory report following the guidelines.


You will
work

in groups of no more than

3 people
-

Make sure you take

the time to observe what
the other groups are doing.
Since

you will be writing up a

short lab report and interpretation based
on other’s data so it is crucial that you understand what errors are

possible, and
when they are
most likely to be
introduced.


T
he procedures for Parts I and II
will

be
best if
finished
during

lab time. Each group will complete a

spreadsheet
using Excel
. Have a group representative e
-
mail your group’s spreadsheet to
me. I

will check th
e data then e
-
mail it
back
to you
a couple of d
ays later.


E
very

student
will

write an
individual laboratory report based on all

datasets.
You are responsible for

obtaining

all

the data

sets
. Finish graphing by hand at home. Turn

these i
n with your lab report
by a week from the start
of the lab
. As par
t of your laboratory report, determine the statistics for each

of the

datasets
graphically, USING A SPREADSHEET.
Statistical calculations can be found on p. 41
-
52 of Folk
and in handout.
Be sure to
check your formulas. The labor
atory report is due
no late
r than Sept.
20
th
.


Format for the laboratory report.

Your report should be
a Max of
2 pages

TYPED
, plus amendments

(graphs)
.

Purpose
. A short paragraph explaining why you performed the experiments

(saying “because I
had to” does not count)
.

Methods
: Mater
ials and Equipment. A brief description of the methods and equipment used

(Do
not copy this lab step by step)
. Describe

the materials used.

Experimental Procedure
. Detail any deviations from the procedure outlined in the instructions.

(Specially true if yo
u have more greater than 2mm fragments than others).

Results
. Results must include: 1) All the

datasets, 2) a table of graphic statistics for all
the
datasets; 3)

histogram and cumulative

frequency plot for each of the
datasets. Describe the results
in a s
hort

paragraph basically reiterating what is already shown by the graphics. It is ESSENTIAL
that you refer to

the graphs of the data
-

these are your primary means of presenting the data. Don’t
worry about

embedding graphics

in your report
-

it’s a waste of
your time. However, you must
attach the labeled (with captions)

graphs with your report and refer to them by number in your text
(use your textbook as a guide if you are

not sure

how to properly reference material
).

Discussion
. Discuss
the
analysis;

then i
n
terpret. Use ALL the questions of Part III

provided as a
basis for

your discussion. For Analysis, discu
ss any
experimental error, strengths or weaknesses
of the design, and

relationship to your objective. For Interpretation, discuss your data in terms of
potential depositional

environment. Think in terms of sorting, as determined by skewness, standard
deviation, etc. Justify your

interpretation.

Conclusion
. A short summary of what you now know.



Part I. Coarse Grain Size Analysis (Sieving)


RoTap. Sieving

is appropriate to particle sizes down to 1/16 mm

(63

m)
, when electrostatic
attraction interferes.


Equipment and Materials

Mechanical sample splitter

Balance

Wire
-

mesh sieves

Sieve Shaker

Brush for cleaning sieves

Binocular microscope


Determination
of particle
-

size distribution.

1. Obtain ~
100 to
200 g sample. Split it mechanically to reduce it to an appropriate size (~40
-
70

g).

2. Weigh it to the nearest 0.01 g and record the weight on the form.

3. Assemble the CLEANED sieves plus a pan and lid.

4.

Pour the sample into the top, coarse sieve. Cap it and RoTap for 10 minutes.

5. Remove

nested sieves and carefully put

contents of each sieve onto a sheet of
white

paper

6. Clean sieve screen by quickly inverting it and slamming it sharply and evenly on t
he paper
(hitting

it at an angle will damage the mesh). NEVER force a grain through (some are supposed to
be

lodged on the screen). NEVER touch the screen with anything other than the brush.

7. Weigh each fraction and record the weight. Save the fine fract
ions.

8. Determine the total weight retained. Compare it to the starting weight to determine sieve loss.

9. Determine weight percent (weight retained per class/ total weight) * 100.

10. Plot as a histogram by hand.

11. Determine cumulative weight percent d
ata. Plot by hand.

12. Transfer data to a spreadsheet. Complete and email to
me

13. Obtain data from the other groups. Using the instructions attached, determine graphical
statistics

for all

datasets using a spreadsheet.

14. Complete laboratory report by
answering questions first.



Part II. Grain Size Chart

Estimating grain size is difficult until you get a feel for it. For this reason you are going to

construct
a grain size chart with RoTap samples.

1. Obtain a cardboard template.

2. Label the holes as c
oarse silt, v. fine sand (4 phi), fine sand (3 phi) medium sand (2 phi),

coarse
sand (1phi) and very coarse sand (0 phi).

3. Glue the bottom of the circles; add a thin layer of sand. Let it dry.


Fine grained particles

Because of their small size, silt and

clay particles (finer than 4

or 62

m diameter) cannot be
measured by sieving. Most methods developed to measure small particle diameters involve
“sedimentation” in which particle size is estimated from the rate at which particles sink in fluid
medium. This rate can be determined by

measuring the weight or volume of accumulated
sediment, the decrease in fluid density, or the decrease in turbidity of the suspension.


There are three methods of analysis of silts and clays: pipette, hydrometer, and decantation
methods. The pipette metho
d is more common and more accurate than the other two. All are
based on the settling velocity of the particles, computed on the basis of Stoke’s Law.


1. the pipette method: a small volume of suspension is obtained, evaporated and the residue is
weighed. T
he residue represents the range of grain sizes suspended at the given time;

2. the hydrometer method: the density of the suspension is measured, which depends on the
amount of sediments in suspension;

3. decantation method: all the grains still suspended a
fter a given time are poured off, dried and
weighed.



For pipette analysis, all coarse material (> 4

or 62
.5


m diameter) must be removed from the
sample either by wet or dry sieving. If wet sieving, be sure not to exceed a total volume of 1000 ml
of suspension. The optimum amount of sediment to work with is 15 g (although it is possible to run
an analy
sis on a sample of 5


10 g). Too large a sample leads to grain interference and possible

flocculation, and too small a sample leads to very small residues and greater experimental error
during weighing. Also, any organic matter should be removed with an o
xidizing agent (hydrogen
peroxide).


The pipette method is based on the idea that fine sediment is uniformly distributed throughout the
1000 ml column, and we draw off exactly 20 ml at the stated times, then the amount of mud in
each withdrawal is equal to

1/50 of the total amount of mud remaining in the column at that given
time and at that given depth (i.e., the amount of mud finer than the given diameter; all particles
coarser than the given diameter will have settled past the point of withdrawal). The f
irst withdrawal
is made so quickly after stirring and at such a depth that particles of all sizes are present in
suspension; therefore if the initial withdrawal weight (minus the dispersant weight) is multiplied by
50, then you can obtain the weight of the

entire amount of mud in the cylinder. Then, if you draw
off a sample at a settling time corresponding to a diameter of 6

, and multiply it by 50, then we
know that the product represents the number of grams of mud still in suspension at this new time,
the
refore the grams of mud finer than 6

.


Table for Settling Times computed according to Stokes Law for temperatures near 20°C.


Diameter

Velocity

Depth


Times of Settling

(

)

(mm)


(cm/s)

(cm)

(hours)

(minutes)

(seconds)

4

1/16


0.349


20


0


00


20

5

1/32


0.0872

10


0


01


55

6

1/64


0.0217

10


0


07


41

7

1/128


0.00545

10


0


31

8

1/256


0.00136

10


2


3

9

1/512


0.00034

7


5


43

10

1/1024

0.000085

7


22


53

11

1/2048

0.000021

5


65


25


As you can see t
his procedure is time and labor consuming, so we will no perform it, but we will
need to compute calculations based on Stoke’s Law.


PART III

Questions:

1. What is the difference between accuracy and precision?

2. What factors in these methods might affect accuracy?

3. What factors might affect precision?

4. What do the data indicate about the energy of the system?

5. What is a likely environment of deposition for your sample? Justify it.

6. Does the composition
/ mineralogy of the s
ediment change by size fraction?

7. What assumptions are made in the RoTap method of mechanically sieving the sample?


Given Stoke’s Law:

Settling velocity assumes a spherical shape and is valid for particles 3

and smaller:

Vs =
[d
2
(

1
-

2
)g]/18



1
= density of sphere (Quartz = 2.65 g/cm3)


2
= density of liquid water (at 20°C, = 0.998 g/cm3)

g = gravitational constant (980.17 cm/sec2)

d

= grain
diameter


= viscosity of water at 20°C = 1.00 centipoises

1 centipoi
se = 0.01 gm/cm
-
sec


8. What would be the effect on the settling velocity for analyses conducted on the moon where the

gravitational constant is only 1/6 of that of earth? CALCULATE AND SHOW WORK.

9.
In very cold water (~ 1°C), the viscosity is 1.75 centip
oises. How would this effect settling
velocity?

CALCULATE AND SHOW WORK.

10. Compare the settling velocity of quartz vs. magnetite (density = 5.2 g/cm3) sand grains in 20°C
water.

Compare grains of 63

m (4

) CALCULATE AND SHOW WORK.