Applied Geomechanics CIVL4401
Applied Geomechanics CIVL4401 Laboratory Manual 2021
The University of Western Australia
Dept. of Civil, Environmental & Mining Engineering
Applied Geomechanics CIVL4401 i Laboratory Manual
LABORATORY EXPERIMENTS
As some students are completing this unit remotely, the two laboratories this year involve analysis and reporting of videos of experiments conducted by a UWA technician and research student. Students on campus are encouraged to view the laboratory test equipment, which is in Room G81 of the Civil & Mechanical Engineering building.
Each student is required to view the posted online video of the actual experiments, view the recorded lectures relating to the theory of the laboratories and then complete and submit the laboratory reports on lms. Students are required to use the following posted “Test data” when completing the laboratory report. These files are available on lms in Week 2. The deadline for submission of the Lab1 and Lab2 reports are 9th April 2021 and 7th May 2021 respectively.
Student Number ending in
1
2
3
4
5
6
7
8
9
Data to be used for Lab1 Lab1data1 Lab1data2 Lab1data3 Lab1data4 Lab1data5 Lab1data6 Lab1data7 Lab1data8 Lab1data9
Data to be used for Lab2 Lab2data1 Lab2data2 Lab2data3 Lab2data4 Lab2data5 Lab2data6 Lab2data7 Lab2data8 Lab2data9
PREPARATION OF LABORATORY REPORTS

Laboratory reports are intended to give you practice in writing technical documents that are readily digested and understood by others. This is a core expertise required for a professional engineer.

Make the report as short as possible while still giving all the information and interpretation required.

Correct grammar and sentence construction are required.

Neat diagrams and tables should be used where necessary to present experimental data
and the results of calculations.

Tables, graphs and other diagrams can be grouped together at the end of the text, or incorporated into the text. Choose one approach or the other (don't mix figures in the text and at the end).
Prof. Barry Lehane Dept. of Civil, Environmental and Mining Engineering The University of Western Australia
Applied Geomechanics CIVL4401 ii Laboratory Manual

An appropriate number of significant digits should be used when numerical answers are quoted e.g. friction angles should be quoted to the nearest one degree.

All calculations should be clear, with input parameters listed and their sources shown, assumptions clearly stated, and the relevant formulae given. Where calculations are repetitive, place the results of the calculations in a table, clearly showing the variation in input parameters.

The report must have a cover page with your full name as it would appear in the student record system, title of experiment, group number, date experiment performed, name of demonstrator, etc. clearly shown.

The report should be laid out clearly. For guidance, you could use headings such as:

Introduction  a brief description of the purpose of the experiment and its
relevance to engineering practice (about one paragraph).

Experimental procedure  a 10 line summary or overview of the laboratory experiment and procedure.

Results – presentation of plotted results of the experiment.

Calculations/Theory – presentation of any required calculations and a brief
description of expected behaviour.

Discussion  a discussion of the results, comparison with theory or expected behaviour, address specific questions raised in the instruction sheets.

Summary and/or Conclusions  a concise summary of the report, either in one paragraph or in point form.


The following general marking scheme will be used:

Presentation 10%

Spelling and grammar 10%

Layout 20%

Technical Content 60%


Penalties applied to late submissions will follow standard faculty guidelines.
Prof. Barry Lehane Dept. of Civil, Environmental and Mining Engineering The University of Western Australia
Applied Geomechanics CIVL4401 1 Laboratory Assignment 1 LABORATORY EXPERIMENT 1: EMBEDDED RETAINING WALL
1. INTRODUCTION
Almost all new urban developments incorporate basements, which are usually single or double level (i.e. 3 to 6m deep) but can have ten or more levels. Basement construction is normally performed after installation of peripheral retaining walls and subsequent excavation of the soils within the confines of the walls. Popular walls employed for basement construction include sheetpile, secant pile, contiguous pile and diaphragm walls; these walls are referred to as embedded walls as they transfer out of balance forces through bending stiffness and differ from gravity retaining walls (also covered in CIVL4401) which are not used to construct basements in urban environments.
The removal/excavation of soil leads to outofbalance lateral soil stresses (e.g as the stress on the excavation side drops to zero). A new equilibrium set of lateral soil stresses acting on the inside and outside of the wall is made possible due to the bending capacity of the retaining wall. These stresses cannot be smaller than the active stress (which is not generally zero) and cannot be larger than the passive stress. This laboratory experiment seeks to demonstrate the existence and reasons for these Active and Passive earth pressure limits.
The experiment examines the simple case of a vertical embedded wall supporting a coarse grained soil. Two types of failure are simulated: (i) passive failure, by driving the wall horizontally against the soil mass and (ii) active failure, by withdrawing the wall by an equal amount at top and bottom. The experiment is carried out on a sample of uniform dense sand, the properties of which are given on the laboratory test sheet provided.
Prof. Barry Lehane Dept. of Civil, Environmental and Mining Engineering The University of Western Australia
Applied Geomechanics CIVL4401 2 Laboratory Assignment 1 Retaining wall
Displacement transducer
H
Sand
Rubber seal
F1
Load cells
F2
Crank handle
Figure 1. Schematic Diagram of Apparatus
Prof. Barry Lehane Dept. of Civil, Environmental and Mining Engineering The University of Western Australia
Applied Geomechanics CIVL4401 3 Laboratory Assignment 1
2. PROCEDURE (PLEASE SEE RECORDING ON LMS)

2.1 ?Measurement of the friction angle between the wall and the soil, δ.

Place the stainless steel block provided on the hinged steel plate with the sand coated surface downwards.

Raise the plate until the block slides down the surface under its own weight.

Take a note of the inclination of the steel plate, this is equal to δ.


2.2 ?Initial atrest lateral stresses
1. With the wall in the initial vertical position, zero the reading on the displacement transducer and load cell outputs.

Proceed to fill the container, which has width, b=0.15m, by placing layers of sand (t<50mm). Use the plastic plate and the pneumatic hammer to compact each layer vigorously. Layers should have uniform thickness and compaction should be applied over the entire surface to obtain a homogeneous sample. Mark each layer after compaction with an edge line of iron oxide. Finally, measure the height that the sand surface has reached (between 2530 cm).

Measure the load cell readouts when the backfill has been placed and the retaining wall is at the initial rest condition and calculate the initial total force acting on the wall.

DigiDAQ software is used to capture all necessary data for this and future stages.


2.3 ?Active failure (Observe load cell measurements but load cell recording is not required)

Withdraw the wall by turning the cranks slowly in a clockwise direction.

Observe the load cell readings; these are very close to zero (typically expect a minimum force= active force of about 0.15 kN). However the load cell resolution is not accurate enough for confidence to be placed in the measurement obtained.

Measure and sketch the failure surface that develops in the sand. Measure the horizontal distance between the wall and the projection of the failure surface with the ground surface in order to determine the experimental wedge failure angle.


2.4 ?Passive failure

Set up a new sample repeating all steps described in Section 2.2 above.

Drive the wall horizontally (and slowly) against the soil mass by rotating the crank in an anticlockwise direction.

Calculate the total force and plot a complete graph of total force vs. horizontal displacement.

Prof. Barry Lehane Dept. of Civil, Environmental and Mining Engineering The University of Western Australia
Applied Geomechanics CIVL4401 4 Laboratory Assignment 1
4. Measure and sketch the failure surface that develops in the sand. Measure the horizontal distance between the wall and the projection of the failure surface with the ground surface in order to determine the experimental wedge failure angle.
Finally, remove the sand from the container and check the load cell zeros after the cell is emptied.
2.5 Converting measured forces (F1 & F2) to pressure coefficients
Ensure that the reading you take before placing sand is taken as the zero reading.

TotalforceF=F1+F2

F(initial)=1?2γK0 H2 b

F (minimum)= active force = 1?2 γ KA H2 b (not necessary to record due to
poor load cell resolution – but note that the active force is greater than zero)

F (maximum)= passive force = 1?2 γ Kp H2 b
3. ANALYSIS AND PRESENTATION OF RESULTS

Present the results of the active and passive test carried out on a graph of force versus horizontal displacement.

Compare the lateral forces for the active and passive cases calculated using Rankine's method (δ=0) with the experimental result.

Compare the measured active and passive earth pressure coefficients with those predicted using the Caquot & Kerisel charts (provided in course notes).

Compare the lateral force for the passive case calculated using Coulomb’s method (δ >0) with the experimental result.

Compare the shapes of the failure surfaces observed in the experiment with those assumed in the theoretical analysis.

Comment on the comparisons made in Items 2,3,4 &5.
Prof. Barry Lehane Dept. of Civil, Environmental and Mining Engineering The University of Western Australia
Applied Geomechanics CIVL4401 5 Laboratory Assignment 1
4. SUMMARY OF RELEVANT THEORY

4.1 ?Coefficient of earth pressure at rest (K0)
K0 = σ?h0/σ?v0 (ratio of horizontal to vertical effective stress) K0 = 1 sin φ' for normally consolidated soil (Jaky’s formula) Note that compaction can increase the lateral stress coefficient, K0

4.2 ?Rankine's method for cohesionless soils (δ=0)
H
Lateral Earth Force, F = ∫(Kγz)dz per metre width of wall0
Where for δ=0: Active Ka = (1  sinφ?)/(1 + sinφ?)Passive Kp = (1 + sinφ?)/(1  sinφ?)

4.3 ?Coulomb's Method for cohesionless soils
(i) Active force of a wedge of soil (unit weight= γ) using a graphical method x
A
W
φ? R
C
Failure plane
Force polygon FA
RW
FA 
δ
z
θ
B
Figure 2. Coulomb's Graphical Method for Determining FA
Prof. Barry Lehane Dept. of Civil, Environmental and Mining Engineering The University of Western Australia
Applied Geomechanics CIVL4401 6 Laboratory Assignment 1 Forces:
W = 0.5 x z γ (selfweight of failing wedge) acts vertically downward
R = unknown (friction on failure plane through soil) acts at angle φ? below normal to the
assumed failure surface (BC)
FA = unknown (force on retaining wall) acts at angle δ below normal to back of wall FA cos δ = horizontal component of active force
Assume a failure plane angle. Knowing the angles φ and δ, and the force W, it is possible to draw the force polygon. The magnitude of FA is scaled off the force polygon. To find the maximum value of FA, superimpose the force diagrams corresponding to different locations of the failure surface, BC, and construct the envelope for FA. Alternatively, this can be done algebraically by considering force equilibrium and solving the equations and plotting FA versus the failure wedge angle (or the dimension x).
1234
(ii)
Spreadsheet method to determine active force of a wedge of soil
FA
Trial failure surfaces
Figure 3. Graphical determination of maximum value of FA
To solve this problem using excel, use the force polygon to determine the relationship between FA and φ?, γ, δ and θ, where θ is the angle that the presumed failure plane makes to the vertical (see Figure 2). Vary the θ angle (<45o) to find the maximum value of FA. This is the active force per metre width of wall.
Prof. Barry Lehane Dept. of Civil, Environmental and Mining Engineering The University of Western Australia
FA
3 2
1
R
4 Maximum FA
W
Applied Geomechanics CIVL4401 7 Laboratory Assignment 1
(ii) Spreadsheet method to determine passive force of a wedge of soil
This is determined as for the active case except that the minimum value of Fp is determined rather than the maximum value. As indicated in Figure 4, for the passive condition, the wedge moves upwards and therefore the reaction R lies at an angle of φ? above the normal on the slip surface and the passive force acts an angle of δ above the normal to the wall.
Determine the relationship between Fp and φ?, γ, δ and θ, where θ is the angle that the presumed failure plane makes to the vertical (see Figure 4). Vary the θ angle (> 45o) to find the minimum value of Fp. This is the passive force per metre width of wall.
x
A
C
Failure plane
φ?
z
FP 
δ
W
R
Force polygon
FP R
W
Θ
B
Figure 4. Forces on passive wedge
Prof. Barry Lehane
Dept. of Civil, Environmental and Mining Engineering The University of Western Australia
Applied Geomechanics CIVL4401 8 Laboratory Assignment 1
RETAINING WALL EXPERIMENT DATA SHEET FOOTING EXPERIMENT DATA SHEET – SEE EXCEL SHEETS POSTED ON LMS
Soil description: Uniform medium grained dense sand
Effective friction angle = 45° (assumed, dense and low stress level) Bulk unit weight (γ) = 17 kN/m3 (assumed)
Friction angle between soil and wall, δ = ________(as measured)
Test no. 1 Test no. 2
Disp Top Base Total Disp Top Base Total Load Load Load Load Load Load
mm kN kN kN mm kN kN kN
Prof. Barry Lehane Dept. of Civil, Environmental and Mining Engineering The University of Western Australia
Applied Geomechanics 4401 1 Laboratory Assignment 2
LABORATORY EXPERIMENT 2: FOOTINGS ON SAND 1. INTRODUCTION
Footings are used to transfer the load from a structure to the underlying soil. In designing such footings, the engineer has to take into account the restrictions imposed by the geometry of the site and the bearing capacity and settlement parameters of the underlying soil.
In this experiment, the bearing capacity of shallow footings on sand is to be investigated. The influence of footing geometry is examined using three footings having the same area but different shapes: one square, two rectangular (side ratio 2.5:1) and one circular. In addition, the effect of loading a circular footing eccentrically is to be examined and the result compared with the concentrically loaded case. Finally, the effect of shape is investigated with a fourth rectangular footing (side ratio 7:1). The experiment is carried out on uniform medium grained sand in both loose and dense states.
Before starting the experiment, estimate the failure loads for the three footings on dense and loose sand using Terzaghi's bearing capacity factors and the estimated friction angles. Use φ = 40° for dense sand and 32° for loose sand; note that these angles are higher than normally adopted for full scale foundations due to the high levels of dilation occurring at the low effective stresses present in the laboratory. Assume the unit weight of the (dry) sand to be 16.9 kN/m3 for dense sand and 15.5 kN/m3 for loose sand.
2. PROCEDURE (PLEASE SEE RECORDING ON LMS)

Prepare the dense sand sample by placing layers of sand (t < 50 mm) and compacting them with a pneumatic hammer. Layers should have uniform thickness and compaction should be applied over the entire surface to obtain a homogeneous sample.

Carefully place the footing on the projection of the LVDT with the centreline of the tank. Carefully install the LVDT and the load frame over the first footing.

Attach the frame holding the displacement transducer to the walls of the tank and adjust it so that the transducer is directly over the loading frame.

An electronic readout has been calibrated to read the vertical displacement in mm. Once the setup is completed, take the initial readout as a reference value.

Carefully place a load on the load frame and record the vertical displacement when the settlement has stopped. Plot the loaddeformation response. In choosing load increments, take increment loads lower than 10% of the estimated failure load or use the minimum weight available. This loading scheme should be follow strictly to avoid early failure of the footing which may require repetition of the experiment (including repreparation of the sample !). Some footings on very loose sand could fail due to the weight of the load frame (3.84 kgf)

Repeat this procedure until failure occurs, reducing the load increments towards failure. Then, remove the load frame and the footing.
Prof. Barry Lehane Dept. of Civil, Environmental and Mining Engineering The University of Western Australia
Applied Geomechanics 4401 2 Laboratory Assignment 2

Ideally a new sand sample should be made for each footing test. However, to save time, if the next test involves a compacted (dense) sand, compact the surface of the sand with the pneumatic hammer prior to that test. If the next test involves a loose sand, fully loosen the sand to a depth of at least 100 mm and replace; care is required to achieve a level surface.

Repeat steps 2 to 7 for each of the remaining footings for dense and loose sand states. Repeat any tests if necessary. The long rectangular footing should be orientated so that its long side is parallel to the long direction of the containment box.
3. ANALYSIS & PRESENTATION OF RESULTS
For each soil density, plot the results of all four tests on a graph of average vertical stress (y axis) versus vertical displacement (xaxis), distinguishing clearly between each test. Estimate the ultimate bearing capacity, qf, corresponding to each test on the graph.
What was the effect of footing shape on qf? Which shape gave the highest value of bearing
capacity and why? Comparisons between prelab predictions and experimental results should focus on mechanisms, discussion on theoretical assumptions and engineering aspects in general. Comment on observed patterns of deformation.
Use the bearing capacity equation to back analyse the friction angle for the sand in the dense and loose states, taking the test results on the centrally loaded circular and strip footings. Deduce an approximate (average) friction angle for dense and loose sand. Comment on the results obtained.
Prof. Barry Lehane Dept. of Civil, Environmental and Mining Engineering The University of Western Australia
Applied Geomechanics 4401 3
4. BEARING CAPACITY EQUATION
B = width of footing
σ?v = effective stress adjacent to footing N = bearing capacity factor
γ = bulk density
Bearing capacity factors
Nc, Nq and Nγ derived from chart overleaf
Shape correction factors
Laboratory Assignment 2
qf = sc dc ic Nc c + sq dq iq Nq σ?v + sγ dγ iγ Nγ (γB/2)
sc ≈ 1 + 0.2(B/L) sq ≈ 1+(B/L)tanφ?
For circular footings, sc=1.2, sq=1+tan φ? and sγ=0.6
Depth correction factors
dc ≈ 1+ 0.4 D/B for D/B≤1
dq ≈ 1 +2tanφ’(1sinφ’)2 D/B for D/B ≤1
dγ =1 for all D/B
(Simplified Meyerhof) inclination factors
ic =iq =(1–α/90o)2 iγ =(1–α/φ’)2
Unit weight (γ) and σ’v
Use γ’ if water table is at a level higher than a distance B below formation level Foundation width
Use effective foundation width, B', when there is an applied moment (M) on a rectangular foundation. Eccentricity (e) of applied load = M/V and B’ = [B 2e]. (For circular footing use chart displayed on the next page to determine the area reduction coefficient due to eccentric loads).
Other corrections factors
Tomlinson (2001) provides formulae for correction factors for footing inclination (bc, bq and bγ) and ground surface inclination (gc, gq and gγ).
Prof. Barry Lehane Dept. of Civil, Environmental and Mining Engineering The University of Western Australia
sγ ≈ 1 0.2(B/L)
dc ≈ 1+ 0.4 tan1[D/B] for D/B ≥1
dq ≈1+2tanφ’(1sinφ’)2 tan1[D/B] for D/B >1.0
whereα=tan1 (H/V)
Applied Geomechanics 4401 4 Laboratory Assignment 2
Prof. Barry Lehane Dept. of Civil, Environmental and Mining Engineering The University of Western Australia
Applied Geomechanics 4401 5 Laboratory Assignment 2
Prof. Barry Lehane Dept. of Civil, Environmental and Mining Engineering The University of Western Australia
Applied Geomechanics 4401 6 Laboratory Assignment 2
FOOTING EXPERIMENT DATA SHEET – SEE EXCEL SHEETS POSTED ON LMS
Soil description: Uniform medium grained sand Footing area = ________ mm2
Weight of hanger = ________ kg Footing:
Footing:
Sand density [D/L]: Weight of footing:
Load Stress (kg) (kPa)
Footing:
Sand density [D/L]: Weight of footing:
Load Stress (kg) (kPa)
Sand density [D/L]: Weight of footing:
Vertical Disp..
(mm)
Vertical Disp..
(mm)
Load Stress
(kg) (kPa) (mm)
Vertical Disp.
Test details: 1.____________________ 4.____________________ Prof. Barry Lehane
2. ____________________ 3. _____________________ 5. ____________________ 6._____________________
Dept. of Civil, Environmental and Mining Engineering The University of Western Australia