Lab 3 Triaxial Loading Test

Summary

In this lab, students perform triaxial measurements to determine the elastic properties of rocks. They apply axial stress and confining stress using fluid pressure, measuring the rock's response. The process involves preparing the sample, setting up the loading frame, fracturing the rock, and analyzing data to find stress-strain relationships, modulus of elasticity, and rock strength. The experiment aims to understand how confining pressure affects rock behavior.

Takeaways

• 🪨 The lab focuses on triaxial measurements to determine the elastic properties of rock under confined conditions.
• 📏 Similar to laboratory one, axial stress is applied, but the main difference is the use of a confining vessel filled with fluid.
• 💧 A fluid pump is used to apply confining pressure, with differential stress being the difference between axial and confining stresses.
• 🔧 The workflow involves measuring the sample, preparing the assembly, fracturing the rock, and analyzing the data.
• ⚙️ A membrane is used to isolate the sample from the confining fluid, and components like end caps, plastic o-rings, and vacuum grease are crucial for sealing.
• 🚰 Once the sample is prepared and placed in the vessel, air is removed from the system to ensure only water remains in the confining chamber.
• 🔍 The key measurements in the process include load and displacement, which help determine elastic properties and the point of rock fracture.
• 📉 The analysis focuses on converting force and displacement into stress and strain to calculate Young's modulus during loading and unloading.
• 📐 By performing several measurements, more circles can be combined to estimate the rock's cohesion and internal friction angle.
• 💥 The peak stress during fracture represents the maximum axial stress, and the angle at which the rock fractures can be predicted based on confining pressure.

Q & A

What is the main objective of Laboratory Number Three?

The main objective of Laboratory Number Three is to perform triaxial measurements to determine the elastic properties of the rock.

How is the principle used in Laboratory Number Three similar to Laboratory One?

The principle used in Laboratory Number Three is similar to Laboratory One in that both involve applying axial stress to a rock sample to induce deformation and fracturing.

What is the role of the confining stress in the triaxial measurements?

In triaxial measurements, confining stress is applied through a fluid to the rock sample, influencing the actual stress and vertical strain experienced by the rock.

What is the differential stress and how does it relate to the axial stress and confining stress?

Differential stress is the difference between the axial stress and the confining stress, which is a crucial parameter in analyzing the rock's behavior under stress.

What is the purpose of the fluid pump in the laboratory setup?

The fluid pump is used to increase the confining pressure in the system by allowing the confining fluid to be pressurized.

What are the steps involved in measuring the rock sample in Laboratory Number Three?

The steps include ensuring the sample is straight, measuring the diameter at different points, checking that the interfaces are parallel, and weighing the sample.

How is the rock sample prepared for the triaxial test?

The sample is placed in a membrane within a vessel, with components such as end caps, o-rings, vacuum grease, and water added to isolate it from the confining fluid.

What is the significance of the bleed valve and pump valve during the setup?

The bleed valve and pump valve are used to control the air pressure within the vessel, ensuring that air is displaced and the system is filled with water.

How is the confining pressure applied and monitored during the test?

The confining pressure is applied by connecting the vessel to the pump and using it to increase pressure. It is monitored by observing the pressure readings on the console.

What is the purpose of the strain gauge in the laboratory setup?

The strain gauge is used to measure the displacement of the rock sample during the test, which is crucial for determining the rock's elastic properties.

How is the data from the triaxial test analyzed to determine the rock's properties?

The data from the test is analyzed by converting the measurements of force and displacement into stress and strain. This allows for the calculation of the rock's elastic modulus and peak strength at different confining pressures.

Outlines

00:00

🔬 Introduction to Laboratory 3: Rock Mechanics Testing

The script introduces Laboratory 3, which focuses on determining the elastic properties of rock through triaxial measurements. The lab employs a similar principle to Laboratory 1, where axial stress is applied to a rock sample to understand its deformation and fracturing. The key difference is the application of confining stress through fluid in a vessel, creating a scenario where both axial stress and confining stress are considered. The lab includes the use of a Floyd pump to increase confining pressure. The workflow involves measuring the sample, preparing the assembly, fracturing the rock, and analyzing data. Special attention is given to ensuring the sample's straightness and parallelism, and the sample's weight is also measured.

05:00

🛠 Setting Up the Triaxial Test Vessel

This section details the preparation of the triaxial test vessel for the rock sample. Components such as the vessel, membrane, end caps, o-rings, vacuum grease, and water as the confining fluid are assembled. The process involves applying vacuum grease to the vessel's inner walls, placing end caps, and using o-rings to isolate the sample from the confining fluid. The vessel is filled with water after ensuring that the pump and plate valves are closed. The setup is completed by sealing the vessel with metal and golden o-rings, and adjusting it with screws. The vessel is then placed in the loading frame, connected to a pump, and air is removed from the system.

10:02

📈 Operating the Loading Frame and Measuring Confining Pressure

The script describes the operation of the loading frame, focusing on the measurements of load and displacement. The setup involves ensuring the bleed valve is closed and the pump valve is open to connect the pump and vessel. The confining pressure is increased step by step, monitoring the pressure and force to ensure they correspond. The process includes setting a target pressure, such as 50 psi, and observing the system's response. Once the desired confining pressure is reached, a strain gauge is placed and secured. The system is leveled, and the sample, vessel, and strain gauge are checked for proper alignment before commencing measurements.

15:04

📊 Analyzing Rock Strength and Elastic Modulus

The final section outlines the analysis of the rock's strength and elastic modulus. The process involves increasing force until the rock fractures, indicated by a drop in force. Safety measures are emphasized, including reducing the confining pressure to zero and ensuring the axial load is near zero before disassembling the setup. The rock sample is then extracted, photographed, and its data file is retrieved for analysis. The analysis focuses on converting force and displacement measurements into stress and strain, calculating the rock's modulus during loading and unloading, and determining the rock's strength at specific confining pressures. Multiple measurements can be used to calculate an envelope that provides information on cohesion and the internal friction angle, which in turn helps determine the rock's fracturing angle.

Keywords

💡Confining Pressure

Confining pressure refers to the pressure applied to the sample from all sides while it is submerged in fluid within a vessel. This pressure is crucial for simulating subsurface conditions in rock testing, influencing how the rock reacts under stress. In the video, confining pressure is increased using a fluid pump, allowing for the differential stress to be applied and analyzed.

💡Axial Stress

Axial stress is the force applied along the axis of the rock sample, causing deformation and potentially leading to fracture. In the video, axial stress is applied to measure the rock's elastic properties. The difference between axial and confining stress forms the differential stress, which is key in determining the rock's behavior under pressure.

💡Elastic Modulus

The elastic modulus is a measure of a material's ability to resist deformation under stress. In this context, it helps determine how the rock behaves elastically (before fracturing). The video outlines how the triaxial measurements allow for the determination of the rock's elastic modulus by applying stress and measuring strain.

💡Triaxial Measurement

Triaxial measurement refers to the process where a sample is subjected to three different principal stresses, with the axial stress being applied along with confining pressure from all sides. In the video, this method is used to analyze the elastic properties and strength of the rock under realistic subsurface conditions.

💡Differential Stress

Differential stress is the difference between the axial stress and the confining stress applied to the rock. This value helps in understanding how close the rock is to failing under pressure. In the video, differential stress is critical for determining the rock's fracture point, as the rock is subjected to increasing stress until it fractures.

💡Fracture

Fracture refers to the point at which the rock breaks under stress. In the video, the rock is loaded with stress until it reaches its fracture point, which provides data on the rock's peak strength and helps in understanding its mechanical properties. The moment of fracture is key to determining the rock’s failure under applied forces.

💡Strain Gauge

A strain gauge is a device used to measure the amount of strain (deformation) in the rock as it is subjected to stress. In the video, it is placed on the sample to monitor changes in length, providing data on how much the rock deforms before it fractures. This information helps calculate the rock’s elastic and mechanical properties.

💡Loading Frame

A loading frame is the apparatus used to apply axial stress to the rock sample. In the video, the loading frame holds the vessel containing the sample and applies stress at controlled rates to simulate subsurface conditions. The stress and displacement measured through the loading frame are crucial for analyzing the rock's behavior under pressure.

💡Cohesion

Cohesion refers to the component of shear strength that holds the particles of a material together. In the video, after multiple tests, the cohesion of the rock is calculated from the data, along with the internal friction angle. These parameters help understand the rock's mechanical stability under various stress conditions.

💡Internal Friction Angle

The internal friction angle is a measure of the shear strength of a rock due to the friction between particles. In the video, this angle is calculated based on the results of multiple triaxial measurements, providing insight into the angle at which the rock is likely to fracture under applied stress.

Highlights

Introduction to laboratory three focusing on confined compression and elastic modulus determination using triaxial measurements.

Comparison with laboratory one, where unconfined compressive strength (UCS) was measured by applying axial stress.

The main difference in this lab is the use of a fluid vessel to apply confining stress on the rock sample.

The importance of differential stress, calculated as the difference between axial stress and confining stress, is highlighted.

Preparation of the rock sample for testing, including the use of a membrane and end caps to isolate the sample from the confining fluid.

Workflow of the experiment involves four steps: measuring the sample, preparing the assembly, fracturing the rock, and analyzing the data.

Key setup process includes ensuring the sample is straight, measuring the diameter at different points, and confirming the interfaces are parallel.

Vacuum grease and plastic o-rings are used to isolate the sample from the fluid and ensure proper sealing in the vessel.

The process of filling the vessel with water to remove air before applying pressure is explained in detail.

The lab uses a pump to apply confining pressure, and the importance of safety checks during this step is emphasized.

The operation of the loading frame involves moving the vessel up to apply stress, with adjustable speed settings for different stages of the test.

The confining pressure is gradually increased step by step until the desired value is reached.

Final measurements include load and displacement, and the procedure continues until the rock fractures.

After the rock fractures, the system must be safely depressurized before disassembling the vessel.

The analysis phase includes calculating Young's modulus, peak strength, and constructing Mohr circles to estimate cohesion and internal friction angle.

Transcripts

basic properties of the rock hey

everybody

welcome to laboratory number three

laboratory number three is confined

compression

and elastic modulus determination

in this laboratory we are going to

perform

triaxial measurements to undermine

elastic properties of the rock

the principle that we use in this

laboratory

is similar to the approach here that we

follow

for laboratory one when we are trying to

calculate ucs

so it means

that again we are going to apply

an axial stress to a rock

we are going to apply

stress deforming the rock

and fracturing the rock but the main

difference

now is that we are going to place our

sample

in a vessel with fluid and we are going

to

apply through a fluid a confining

stress over the sample

consequently we are going to have again

an actual stress and a vertical strain

and now our junk monologues and

our big stress are going to be function

not only on the properties but

also on the confining that we are

applying in the rock

another parameter that is going to be

useful in our analysis

is the differential stress which is the

difference

between the axial stress and the

confining stress

compared to laboratory one we are adding

one more component in our analysis which

is the floyd pump

here the we are going to have again

the sample where we are applying the

differential stress

and as i mentioned before we are going

to have uh

the sample submerged into a confining

vessel

in in this this confining vessel is

going to be connected

to a fluid pump and this fluid pump

is going to allow us to increase

the confining pressure in the system

and once we set up the confining

pressure

until till the value that we want we are

going to apply our differential stress

and perform our measurements

the workflow that we follow in the

laboratory

is similar as the workflow that we

follow for laboratories one

and two it's going to have again

step one where we measure the sample

step two when we prepare the assembly

and the sample

and we will where we set up our loading

frame

step three where we fracture the rock

and finally uh

step four where we analyze our data

when we measure the sample the step that

we follow

is exactly the same as the one that we

observe for laboratory one

we are going to control again that the

sample is

straight and we are going to control

that the sample

has nearly the same values at different

uh points of our measurement

the diameter is going to be the average

of these points

then through measuring the length at

different angles

we are going to control that the

interfaces are

parallel in the rock again we are going

to measure at different angles

and we are going to average our

measurements

finally we are going to weight the

sample that we have now we are going to

initiate the setup of our loading frame

so the first step in this setup

is just controlling through a level that

the

bottom and the top of the

loading frame are parallel one

to each other now it's time to prepare

the vessel

with our sample so

now we have to add several components to

prepare the vessel that we are going to

put in the loading frame

so the components that we are using are

the vessel the membrane that we are

going to put the sample to us

isolate it from the fluid and

the end caps the plastic o-rings

the vacuum grease the water which is

going to

work as our confining fluid

the meron metal and golden o-rings

just to isolate the

confining fluid from the outside and the

screws that we are going to use just to

adjust the

vessels so

our first step is first putting the

sample in the membrane

and we are going to put the sample in

the middle so we have

nearly one inch at each side

of the of the membrane

now what we do is we are going to put

every component

until the sample looks like this

and to achieve this first we are going

to put vacuum grease

in the inner walls this areas

now after we put the vacuum grease we

are going to place the end caps

the short end cap is going to be at the

bottom and the long

end cap is going to be at the top

finally we are going to increase the

ceiling by putting plastic o-rings

at each side of the sample

this way we are going to

isolate our sample from the confining

fluid

now we are going to take the vessel if

we take

a look inside the vessel what we are

observing is that we have

a small spot in here which is going to

be the end gap slot

we have to place our sample

in here and we are going to fill it with

water before doing that we have to check

that our pump

and plate valves that are to the side of

the vessel

are closed once we check that they are

closed

we can fill the vessel with water

along the inner thinner section that

we have in here so the way that

our vessel with our sample

membrane it looks it's going to be like

this

once we done this we can

close the vessel to do that

first we are going to take our metal

o-ring

in this mineral or ring we have to put

a vacuum grease

in the inner side on top

and on the bottom toward the

bottom of the metal ring once we do that

we have to place them

metal or ring after that

we have to place the golden o-ring we

can use the screws that women

that we see in here at the back

just to fix this golden old ring

once we do that what we are going to

obtain is the closed vessel

with our two

valves closed now we can put this vessel

in the loading frame once we put the

vessel in the loading frame we are going

to connect it

with the pump and we are going to take

out the air

that is still remaining in our vessel

to do this first we are going to

open both the bleed and the pump

bounce and we are going to start using

our pump

so first we are going to hit

our constant flow and our lining here is

going to change into cfa

we are going to place a and we are going

to

input that a flow rate of 10 milliliters

per minute and we are going to

click enter and

first before

before hitting run we are going to check

that we have a low pressure in here

and we start hitting run

and what we are doing is we are

filling with water from the pump to

displace the air

that we have in here so what we are

going to control is that the plane

valve is plenty with air

and once what that water star

coming out we can hit stop

if we see that this pressure in here

start increasing when we are doing this

we have to

to hit stop immediately just because of

safety once we

take out all the water and all of the

air

and see that the blade bulb is

it is just reducing water

and it means that our system is

closed and filled with water and now we

can

close the bleed vault

now we can start operating our

loading frame so just a preview from our

rotary 1

what we can see is that again

our two measurements that we are going

to be interested about

are going to be load load 1 and

displacement 2

and that by pressing up or down

we can move our frame

either up or down and that to change

the speed we are going to click speed

and with f2

f3 and f4 we are going to change the

speed as we are using

during the seating we are going to use

again 0.22 inch per minute

and during the displacement speed we are

going to use again 0.02

inch per minute

now

based on that we are going to move up

our frame until it touch

the upper part of the frame

we will realize that because

force in our board is going to

slightly increase again

once we do that we have to

set up the pressure that we are going to

use for the measurement

to do that the first step is just check

that our bleed valve

is closed and that our pump

valve is open so the

pump and the vessel are connected

now we are going to use a console

pressure increase which means that we

are going

to press constant pressure a and we are

going to set up

as objective pressure 50 psi

we are going to hit enter and run and

these values should increase

what we are going to observe are two

main things

first we have to observe that the

pressure in here

is going to increase up to 50 psi

and it shall remain at 50 psi

also what we have to check is that

the actual force that we are reading

in the in our loading frame is going to

increase as well this is because we are

applying sound confining on the sample

and this force is going to

be transmitted to the force that we are

applying

to a sample in here

so if if we see that

these two indicators then it means that

we are

good to go and we can continue

increasing the confining pressure to

that we are going to repeat

steps two and four reaching

step by step the pressure and not in a

single run

until we achieve our desired confining

pressure

once we are at the our desired confining

pressure

we have one last step to which is place

and hold the string gauge

this uh we will see that this is done

when we place our strangle gauge

and displacement should slightly

increase

a picture of how it should look like

is like this where we have our vessel

touching the upper part of our loading

frame

and the strain gauge also

place and fix at the top

now we have to do

just a double check that we already did

the leveling

we see the sample we prefer the vessel

and the strain gauge is placed

which means that we are ready to measure

the first step that we are going to do

is just

increasing our force so we are moving

up our vessel until our force

increases up to two thousand pounds

then we are going to decrease this value

to

[Music]

thousand fifteen thousand and

fifteen hundred uh pounds and then we

are going to increase

the stress until rock fracture

where we are going to observe a drop in

the force

once we observe this stroke we have to

stop

our measurement now again because of

safety

is very important to before

disassembling

we have to reduce the vessel confining

pressure to zero

and to release the pressure that we have

there

in the system also we have to double

check that

our axial load is nearly zero or

in the same range of values once we

do that then we can take out the vessel

and we can extract it disassemble and

clean it

and now we can recover and take the

picture of the sample

and extract the data file for analysis

the sample once we extract it it will

look

like this

now let's move to that analysis again

as we observe for lower router v1 we are

going to have two main measurements

force and displacement

from these measurements we are going to

measure the

jungle models and the peak strength

that we have for that specific confining

pressure

so again we are turning these parameters

of force and vertical displacement into

stress and strain

and based on the trajectory that with it

we can estimate uh junk money's

modulus loading and jump modulus

during unloading and based on the big

string

that we read we can

see how the the the strength

of the rock for that specific combining

question

if we perform the measurement for

uh say for several measurements

then we can obtain different

values and different

more circles combining all of them

we can calculate an envelope

which is going to tell us the cohesion

and the internal friction angle

and from that internal friction angle

actually we can calculate

which is this angle at the at which

the rock is being fractured

here what we can see is that actually

the peak stress

is going to be our sigma 1 and the

confining pressure is our

is going to be our sigma 3.

finally when we compare this data with

other data what we observe is that as we

mentioned before

this angle is in this point in here

is going to tell us the fractal angle

that we shall

expect based on the confining that we

are applying to a rock