Geology Laboratory
Rock Mechanics and Seismology Laboratory
Members of laboratory group (no more than three):
Rock Mechanics
1. During a site investigation, a boring was advanced to a depth of 30 ft(9.1 m) at which point rock was encountered. The ground water table (GWT) was encountered at 30 ft(9.1m), the depth of rock, during the investigation. The engineering geologist decided to advance the boring into the rock by drilling to a depth of 16.4 ft (5 m).The recovered rock cores were positioned in a rock core box from top down as shown in Figure 1. The beginning of the rock core was located in the upper left hand corner of the box and each following piece of the core was placed from left to right on each row with increasing depth. Each row of the rock core box represents a depth of the 3.2 ft (1 m) of drilling depth, therefore the total drill depth that can be contained in the box is 16.4 ft (5 m)
Figure 1 - Recovered Rock Cores
Using Figure 1 determine the amount of recovered rock in pieces of 10 cm or longer in length. Determine the Rock Quality Designation (RQD) for the recovered rock cores using Equation 1. Using Table 7.7provide a description of rock quality.
RQD=(∑of the rock core lengths greater the 10 cm)/(∑of the total drill depth) x 100%... Eq 1
Total Amount of Recovered Rock = ___________ (ft) ________ (m)
RQD = ___________
Description of Rock Quality = ___________
2. The profile of the soil stratum was developed from the boring during the site investigation is shown in Figure 2. Calculate the existing vertical stress (σv) on the rock bed (at a depth of 30 ft) using Equation 2:
"σv=γaha+γbhb +γchc ... Eq 2"
Figure 2 - Soil Profile
Existing Vertical Stress (σv) = ___________ (psf) ________ (kg/cm2)
From the recovered rock cores,two unconfined compression tests were conducted on test specimens, which were2 in. in diameter and 4 in. long. The load verses displacement data for the two tests are presented in Tables 1 (sandstone) and 2 (gneiss).
Table 1 - Unconfined Compression Data for Sandstone
|
Axial Deformation (in)
|
Compressive Load (lb)
|
|
0
|
0
|
|
0.0012
|
3140
|
|
0.0024
|
6280
|
|
0.0044
|
12560
|
|
0.0064
|
18840
|
|
0.0084
|
25120
|
|
0.0112
|
31400
|
|
0.0144
|
32970
|
|
0.016
|
31400
|
Table 2 - Unconfined Compression Data for Gneiss
|
Axial Deformation (in)
|
Compressive Load (lb)
|
|
0
|
0
|
|
0.001332
|
7534.964
|
|
0.002664
|
15069.93
|
|
0.004884
|
30139.86
|
|
0.007104
|
45209.78
|
|
0.009324
|
60279.71
|
|
0.012432
|
75349.64
|
|
0.015984
|
79117.12
|
|
0.01776
|
75349.64
|
Develop the compressive stress (σa in psi) verses axial strain (ε in in. /in.) plots for both tests using equations 3, 4 and 5. Present these plots in the same graph for comparison. Determine the ultimate unconfined compressive strength (σa) for each rock type. Calculate the Modulus of Elasticity (Et50) at 50% of σausing thecorresponding strainat 50% of σaand Modulus Ratio (Mr) for each rock type using equations 6 and 7. Using Tables 7.3, 7.4, and Figure 7.28 to determine the Strength Classification of the Intact Rock, the Modulus of Elasticity Classification of the Intact Rock and plot the Mr for each rock type.
Area=πr^2 ... Eq 3
σa=Force/Area ... Eq 4
∈=(change in length)/(initial length)... Eq 5
Et50=σa 50% /? ... Eq 6
Mr=Et50/σa ... Eq 7
Unconfined Compressive strength (σa) of sandstone = ________ (psi) ________ (kg/cm2)
Unconfined Compressive strength (σa) of gneiss = ________ (psi) ________ (kg/cm2)
Modulus of Elasticity (Et50) of sandstone = ________ (psi) ________ (kg/cm2)
Modulus of Elasticity (Et50) of gneiss = ________ (psi) ________ (kg/cm2)
Modulus Ratio (Mr) of sandstone = ___________
Modulus Ratio (Mr) of gneiss = ___________
Do the Modulus Ratios correlate well with respective figures in your textbook for the rock types?
Strength Classification of the Intact Rock of sandstone = _____________________
Strength Classification of the Intact Rock of gneiss = _____________________
Modulus of Elasticity Classification of the Intact Rock of sandstone = __________________
Modulus of Elasticity Classification of the Intact Rock of gneiss = __________________
Additional triaxial testing was conducted on the gneiss. Three triaxial tests were performed on intact specimens of the gneiss at confining pressures of 1000, 2000, and 3000 psi (70.3, 140.4, and 210.9 kg/cm2). The test data for the three triaxial tests are presented in Figure 3.
Figure 3 - Triaxial Compression Tests on Intact Specimens of Gneiss
Develop the Mohr Circles for each of the three tests using equations 8, 9 and 10. Determine the cohesion (c) and friction angle (Φ) that best fits the test data for the gneiss as defined by equation 11.
σ3=minor principal stress ... Eq 8
σ1=major principal stress= σ3+ΔP ...Eq 9
ΔP=deviator stress ... Eq 10
τ=c+ σ tan??Φ ...Eq 11?
Cohesion (C) of gneiss = ______________ (psi) ________ (kg/cm2)
Friction Angle (Φ) of gneiss = ______________ (°)
Using equation 12 calculate the unconfined compressive strength (σa) for the gneiss.
σa = 2 C tan (45 +" Φ/2 ") ....Eq 12
Unconfined compressive strength (σa) of gneiss = ______________ (psi) ________ (kg/cm2)
How does this σa compare to the actual unconfined compression test on the gneiss?
Knowing that the tensile strength of rock is typically 5 to 10% of σa, estimate the tensile strength (T0) of the gneiss.
Tensile strength (T0) of gneiss = ______________ (psi) ________ (kg/cm2)
5) Using the an RQD value calculated from Part 1 and the Unconfined Compressive Strength determined from Part 3 for the gneiss determine the Rock Mass Rating (RMR) for atunnelwhich is to beexcavated 50 ft (15.2 m) below the ground surface in the bedrock using the following information. The spacing of the discontinuities were found to be 380 mm apart and consisted of slightly weathered, slightly rough surfaces separated by less 1mm. The rock mass inflow per 10 m of tunnel length was observed to be approximately 70 liter/minute, withconsiderable outwash of joint fillings.The strike parallel to the tunnel axis had a dip of 35°. Provide the Rock Mass Rating, Class Number, Description, and Meaning of Rock Mass Class using the Tables 7.8 and 7.9 provided in the Appendix.
Rock Mass Rating = _____________________
Class Number = __________________
Description = __________________
Provide the meaning of the Rock Mass Class with regard to stand-up time, cohesion and friction angle of the rock mass.
Seismology
We will be using a "virtual" seismogram laboratory developed at UCLA for this lab. Go to this website:
http://www.sciencecourseware.com/virtualearthquake/VQuakeExecute.html?x=131&y=80 to get started.
This page has additional explanations and tutorials if you run into a snag:
http://www.sciencecourseware.org/eec/earthquake/
The USGS has an interactive website that discusses the 1906 San Francisco EQ. You can import a few files into Google Earth and look at historic pictures of the earthquakes. It is really neat!
http://earthquake.usgs.gov/earthquakes/
Assignment/Questions
1. What is a seismic wave?
What are the two types of seismic waves that seismologists use? Describe each type of wave and the differences between them.
3. What earthquake did you select?
4. Do a screen capture or cut and paste the following results:
a) Comparison of your location of the Epicenter to the actual location of the Epicenter with the summary table of the results.
b) The Richter's nomogram with your estimated magnitude.
c) Virtual Seismologist Certificate of Completion showing your finalized tabulated results. Please do not e-mail a certificate to the instructor (5 points OFF if you do!!!).
5. Answer problems 13, 14, 15, 16 and 17 from Chapter 8 from the Kehew textbook.
APPENDIX
Attachment:- Rock Mechanics and Seismology Laboratory.pdf