roger a. failmezger, p.e., f. asce, d. ge geo-chicago
TRANSCRIPT
USING THE DILATOMETER TEST TO MAKE ACCURATE SETTLEMENT PREDICTIONS
Roger A. Failmezger, P.E., F. ASCE, D. GEGeo-Chicago Lecture Series
May 10, 2019
DMT Push Truck—First in North America
1971 Dodge Carrier—Ran 55mph Downhill
2006 Kenworth Carrier with Automatic Transmission
Replaced carrier after hydraulic brakes failed when leaving a site
Geotechnical engineers are essentially predictors—the better that we can predict the outcome—the better our designs are
Good Predictions => Good Engineering
Geotechnical Engineers?
Outline
◊ Performance and Financial Failures• Why we need accurate measurements and
design models◊ Compare Different Tests and Settlement
Prediction Methods• Standard Penetration Test• Cone Penetrometer Test• Laboratory Consolidation Test• Pressuremeter Test• Dilatometer Test
◊ Case Studies
Types Of Failures◊ Performance Failure Outcome has an undesired consequence
◊ Financial Failure Poor design model and parameter evaluation creates overly
costly design because of high uncertainty/lack of knowledge Settlement Evaluation Need Static Deformation Test => Dilatometer, Pressuremeter,
Lab Consolidation Test◊ Never Want the Test to be Your Source of Uncertainty◊ Accurate Design Balances Performance and Financial
Failures– Select the Appropriate Probability of Success
Fundamentals of Probability◊ Mathematicians have presented probability theory as being
much more complicated than it really is—and thus we shy away from using it
◊ The event always occurs—therefore its probability is 100% Flip a coin—it will land as either a head or tail Engineering design—either a successful outcome or undesired
outcome◊ Engineering probability distribution curves tend to be bell-
shaped with a more likely chance of outcome occurring near the center
◊ Total area beneath the probability curve must always equal 1.00 or 100% Figure out how much is in the success zone and how much in the
failure zone Use trapezoidal method to compute the areas Probability of Success + Probability of Failure = 1.00
Probability of Success◊ Because of uncertainty, design solution is not deterministic,
but rather probabilistic solution based on a bell-shaped curve
◊ Bell-shaped curve is defined knowing:1. Average Value2. Standard Deviation3. End Limits
◊ End limits for normal probability distribution curve are +/-∞ and for log-normal curve are zero to +∞ -- not realistic
◊ Beta probability distribution curve gives engineer best solution—Engineer chooses the end limits!
◊ Probability of success is the area under curve in the success zone
Same Soil—Different Tests and Modeling3.0
2.5
2.0
1.5
1.0
0.5
0.0
Beta
Pro
babi
lity
Dis
tribu
tion
Valu
e
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
Factor of Safety
1.20 0.1215 0.051.20 0.303 0.2671.20 0.607 0.409
AverageFactor of
SafetyStandardDeviation
FailureArea
HIGH QUALITY MEASUREMENTS
POOR QUALITY MEASUREMENTS
FailureZone
FAIR QUALITY MEASUREMENTS
SuccessZone
Lack of KnowledgeMore Expensive Design Probability curves shift to the right for same Success
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Beta
Pro
babi
lity
Dis
tribu
tion
Valu
e
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
Factor of Safety
1.20 0.1215 0.051.50 0.303 0.052.00 0.607 0.05
AverageFactor of
SafetyStandardDeviation
FailureArea
PROBABILITY OF SUCCESS = 0.95MIN/MAX LIMITS = AVERAGE + 3 S.D.
EFFICIENT DESIGN
VERY EXPENSIVE DESIGNFailureZone
COSTLY DESIGN
SuccessZone
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Beta
Pro
babi
lity
Dis
tribu
tion
Valu
e
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
Factor of Safety
LOW UNCERTAINTYAverage F.S. = 1.2
S.D. = 0.122
HIGH UNCERTAINTYAverage F.S. = 2.0
S.D. = 0.607Failure Zone
F.S. < 1.0Area = 0.05
Financial Failure = 0.786 or 78.6%
One certainty with Geotechnical Engineering: If you don’t make accurate measurements and use good models, you will certainly have financial failure!
If owner discovers that you are designing for a financial failure of 79%, then you will lose client!
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6Standard Deviation
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
Prob. of Success = 90% FS = 1.2823 * SD + 1.012Prob. of Success = 95% FS = 1.5926 * SD + 1.013Prob. of Success = 99% FS = 2.1481 * SD + 1.000Prob. of Success = 99.9% FS = 2.5395 * SD + 1.000Prob. of Success = 99.99% FS = 2.7473 * SD + 1.000Prob. of Success = 99.999% FS = 2.8594 * SD + 1.000
Heterogeneous
Homogeneous
Plot Average Factor of Safety and its Standard Deviation to Determine Probability of Success
Factor of Safety Design Chart
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
Beta
Pro
babi
lity
Dis
tribu
tion
Val
ue
0 5 10 15 20 25 30 35 40Settlement (mm)
22 1.820 0.083 0.0520 3.033 0.152 0.0516 5.446 0.340 0.0512 7.148 0.596 0.0510 7.748 0.775 0.058 8.177 1.022 0.05
AverageSettlement
StandardDeviation
FailureArea
Probability of Success = 0.95 For Threshold Settlement = 25 mmMin/Max Limits = Average + 3 S.D.
HOMOGENEOUS
HETEROGENEOUS
ThresholdSettlement= 25 mm
Beta Probability Distribution Curves for Settlement Analyses
UnsuccessfulZone
Coefficientof Variation
0 1 2 3 4 5 6 7 8 9 10 11 12Standard Deviation
24
22
20
18
16
14
12
10
8
6
4
2
0
Ave
rage
Set
tlem
ent (
mm
)
Probability of Success = 90%Probability of Success = 95%Probability of Success = 99%Probability of Success = 99.9%
Beta Distribution Becomes "U" Shaped for Settlements < 7 mm.
(i.e. meaningless)
Heterogeneous
Homogeneous
Dashed Line:Skewed Right to
Reverse "J" ShapedDistribution
Solid Line:Bell ShapedDistribution
25
Settlement Design Chart
Shallow Foundation Design Provided that the footing is sufficiently
wide, settlement, rather than bearing capacity, controls the design
Often, engineers design all footings with the same bearing pressure as the “worst case scenario” footing
Even if settlement predictions are accurate, other footings settle less than the “worst case” one, resulting in differential settlement
Often Engineers incorrectly recommend one allowable bearing pressure for entire site
Shallow Foundation Design (cont.)
Goal is to minimize the differential settlement by designing each footing to settle the same amount
Dilatometer tests performed at close intervals can be used for accurate settlement predictions
Dilatometer test is a calibrated static deformation test
Contouring Because soundings are only performed at a
finite number of locations, engineer must interpret what will likely occur between locations
Contouring is a numerical interpretation that can use mathematical algorithms, such as kriging and others
Contour maps may show holes, peaks, ridges or valleys—these anomalies may be areas for additional testing for better design
D-1[0.47"/11.9 mm]
{6.0'/1.83 m}
D-4[0.52"/13.2 mm]
{5'/1.52 m}
D-6[0.51"/12.9 mm]
{4 ft/1.22 m}
D-2[0.53"/13.5 mm]{6.0 ft/1.83 m}
D-3[0.46"/11.7 mm]{5.5 ft/1.68 m}
D-5[0.50"/12.7 mm]
{5 ft/1.52 m}
D-7[0.48"/12.2 mm]
{4 ft/1.22 m}
D-8[0.55"/14.0 mm]{4.5 ft/1.37 m}
D-9[0.50"/12.7 mm]{10 ft/3.05 m}
D-10[0.47"/11.9 mm]{10 ft/3.05 m}
D-11[0.53"/13.5 mm]{9.0 ft/2.74 m}
D-12[0.47"/11.9 mm]{9.0 ft/2.74 m}
D-13[0.52"/13.2 mm]{9.5 ft/2.90 m}
D-14[0.48"/12.2 mm]{8.5 ft/2.59 m}
D-15[0.50"/12.7 mm]{8.0 ft/2.44 m}
D-16[0.52"/13.2 mm]{8.5 ft/2.59 m}
D-17[0.46"/11.7 mm]{8.5 ft/2.59 m}
DMT Sounding[ ] Settlement{ } Footing Width
LEGEND
Numeric Example
Contour Map Showing Settlement Across the Site
0 20 40 60 80 100 120 140 160 180 2000
20
40
60
80
100
120
140
0 20 40 60 80 100 120 140 160 180 2000
20
40
60
80
100
120
140
11.6
11.8
12.0
12.2
12.4
12.6
12.8
13.0
13.2
13.4
13.6
13.8
14.0
SETT
LEM
ENT
(mm
)
Comparing different tests for settlement analyses
(After Marchetti, 2001)
Importance of stress history
Standard Penetration Test Dynamically drive a 2-inch diameter
split barrel sampler with 140 pound hammer falling 30 inch, counting the number of blows
While the inspector meticulously counts the number of blows, the energy that the hammer generates is often not calibrated and varies significantly
ADSC-ASCE-FHWA Load Tests at GT
05
1015202530354045505560
0 5 10 15 20 25 30 35 40
Depth (fe
et)
SPT Penetration Resistance (bpf)RAW N‐VALUES
Rig X
Rig Y
Rig Z
Assumed ER = 85% ER = 62% ER = 42%(After Dr. Paul Mayne, 2019)
ADSC-ASCE-FHWA Load Tests at GT
0
5
10
15
20
25
30
35
40
45
50
55
60
0 5 10 15 20 25 30 35 40
Depth (fe
et)
SPT Penetration Resistance, N60 (bpf)CORRECTED USING ASSUMED ENERGY RATIOS
Rig X
Rig Y
Rig Z
(After Dr. Paul Mayne, 2019)
Standard Penetration Test Predicting modulus from
N60-value has wide range of correlation factors
Primary Disadvantages1. the energy not being measured2. dynamically penetrating the soil3. the soil being strained to failure4. remolding of the soil
Best case scenario for SPT:Using N60 values in sand
Coeff. Of Var. =0.67 – quite highFor 95% certainty sett less than 25 mm,then ave sett =7.5 mm
(After Burland and Burbridge, 1985)
Financial Failure Will Certainly Occur!
Cone Penetration Test Quasi-statically hydraulically push a 10
cm2 projected tip probe into the soil at a constant rate of 2 cm/sec.
A computer measures and records data from the calibrated electronic sensors at depth intervals of 1 to 5 cm and plots their values on the screen.
Constrained deformation modulus is computed by multiplying the tip resistance by an factor.
Cone Penetration Test Primary Disadvantages
1. strains the soil to failure2. remolds the soil 3. stress history unknown
Ground Improvement changes factor
Scatter for factor to obtain deformation modulus from tip resistance
Need site specific correlation values to get modulus
M = ()(qT)
(After Marchetti, 1996)
(After Marchetti, 1996)
= factorfactor not constant: increases with ground improvement due to increases in lateral stresses
Lab Consolidation Test Advantages:1. With high
quality sample can measure the deformation properties
Disadvantages:1. Too costly and
time consuming to perform numerous tests
2. Difficult to test cohesionless soil
Pressuremeter Test Advantages:1. Static deformation
test2. Empirical design
method based on large database of case studies
3. Can test dense/hard soils when loads are high Slotted casing for
gravel formations
Disadvantages:1. Test takes 1+
hours to perform2. Vertical spacing
1+ meter Can miss thin soft
layers
Dilatometer Test Advantages:1. Economical to perform
numerous tests at close intervals
2. Strains soil to intermediate levels similar to the structure
3. Measures the effect of soil stress history
Disadvantages:1. Gravel causing point loads
against membrane or damaging membrane
1 10 100 1000
DEFORMATION MODULUS -- OEDOMETER DATA, M (bars)
1
10
100
1000
DE
FOR
MA
TIO
N M
OD
ULU
S --
DM
T D
ATA
, M (b
ars)
ResidualAlluvial
SITES WERE LOCATED MAINLY IN VIRGINIA, USA
Dilatometer penetration causes has much smaller shear and volumetric strain to soil
Shape of dilatometer blade disturbs the soil less than conical shape
Dilatometer Testing Pointers
Pressurization rate should be relatively slow near “A” and “B” readings so that pressure in blade is same as shown on gauge
Thrust measurements can alert operator of potential shifts in “A” and “B” readings when comparing with previous readings
Low thrust readings are indicative of soft soils
Schmertmann,1986 and Hayes, 1986
Coeff. Of Var. = 0.18
Quick clayey siltNiche silt?
DMT Settlement Analysis Factors Because constrained deformation modulus
is in the denominator of settlement equation, low values have a much greater impact than high values
By performing tests at close interval spacing, the engineer can detect thin soft layers, which often control design
In very soft layers, DMT at 10-cm intervals provide better layer definition and more accurate settlement predictions
Settlement Accuracy Ratio of predicted/measured
settlement = 1.15; coefficient of variation = 0.29
If eliminate data from quick silts and driving DMT blade, predicted/measured ratio = 1.06; coefficient of variation = 0.18
Case Studies
White Flint WMATA Site
6 dilatometer soundings were performed to top of decomposed rock
Schmertmann’s (1986) method was used to predict settlement at each sounding
DMT were performed at 20-cm depth intervals and each test was considered a layer for the analyses
Column Load Footing Width Applied Bearing Pressure Predicted Settlement
(kips / kN) (ft / m) (ksf / kPa) Sounding (inch / mm)
850 / 3780 10.5 / 3.2 7.71 / 369 D-5 0.24 / 6.1
850 / 3780 10.5 / 3.2 7.71 / 369 D-6 0.37 / 9.4
1000 / 4448 13 / 4 5.92 / 283 D-1 0.70 / 17.8
1000 / 4448 11 / 3.4 8.26 / 396 D-2 0.33 / 8.4
1400 / 6227 15 / 4.6 6.22 / 298 D-1 0.84 / 21.3
1400 / 6227 13 / 4 8.28 / 397 D-2 0.38 / 9.7
1400 / 6227 13 / 4 8.28 / 397 D-3 0.57 / 14.5
1400 / 6227 13 / 4 8.28 / 397 D-4 0.82 / 20.8
1400 / 6227 13 / 4 8.28 / 397 D-5 0.40 / 10.2
1400 / 6227 13 / 4 8.28 / 397 D-6 0.51 / 13.0
1500 / 6672 16 / 4.9 5.86 / 281 D-1 0.84 / 21.3
1500 / 6672 13.5 / 4.1 8.23 / 394 D-2 0.38 / 9.7
1500 / 6672 13.5 / 4.1 8.23 / 394 D-5 0.42 / 10.7
1500 / 6672 13.5 / 4.1 8.23 / 394 D-6 0.53 / 13.5
2000 / 8896 18 / 5.5 6.17 / 296 D-1 0.98 / 24.9
2000 / 8896 16 / 4.9 7.81 / 374 D-2 0.41 / 10.4
2000 / 8896 16 / 4.9 7.81 / 374 D-3 0.72 / 18.3
2000 / 8896 16 / 4.9 7.81 / 374 D-4 0.89 / 22.6
2000 / 8896 16 / 4.9 7.81 / 374 D-5 0.50 / 12.7
2000 / 8896 16 / 4.9 7.81 / 374 D-6 0.57 / 14.5
Summary of Settlement Analyses
0 1 2 3 4 5 6 7 8 9 10 11 12Standard Deviation
24
22
20
18
16
14
12
10
8
6
4
2
0
Ave
rage
Set
tlem
ent (
mm
)
Probability of Success = 90%Probability of Success = 95%Probability of Success = 99%Probability of Success = 99.9%White Flint WMATA Analyses
Beta Distribution Becomes "U" Shaped for Settlements < 7 mm.
(i.e. meaningless)
Heterogeneous
Homogeneous
Summary for Beta Probability Distribution Analyses for SettlementWith a Threshold Settlement = 25 mm
and Min/Max Limits = Average + 3 S.D.
Dashed Line:Skewed Right to
Reverse "J" ShapedDistribution
Solid Line:Bell ShapedDistribution
25
Sensitive and conservative owner satisfied with design
Performed 20 dilatometer test soundings adjacent to existing footings inside Lynchburg Hospital to compute additional settlement for proposed increase from 3 to 5 stories. Found only 4 out of 20 columns will need underpinning!
Lynchburg Hospital
Performed dilatometer test soundings inside existing Museum of American Educator to determine which existing footings needed underpinning for new loads
James Madison University—Phillips Hall
Tested every column location for new building
Ocean City Warehouse Buildings Three adjacent sites were investigated Two outside ones with DMT by one consultant Middle one with SPT by different consultant Similar Loads for the three structures
Two Sites with DMT predicted ~0.5 inches of settlement for shallow spread footings
Embankment load test showed 0.5 inches Consultant for Middle Site recommended stone
column ground improvement Wasted ~$800,000
Hoskins Creek –Route 360 Bridge
Virginia DOT Replacing Existing 50-year old Bridge
Originally Planned to Construct New 15-foot High Embankment Adjacent to Existing Bridge
DMT Data Predicted 5 feet of Settlement VDOT Cored Asphalt Pavement Next to
Existing Abutment Retrieved 5 feet of asphalt core 50 years of Overlays
Non-believers became believers in DMT accuracy for settlement prediction
Highly compressible clays
Hoskins Creek Bridge
Project Name Cost Savings Using DMT
MD Live! $2,000,000Towson Circle $200,000
Retirement Community, Glen Mills, PA
Significant but unknown
Xfinity Live! $500,000Obery Court $200,000
Residences at Rivermarsh Significant but unknown
Residences at River Place $80,000
Project Name Cost Savings Using DMT
Westminister Village $100,000Ocean Landing Shopping
Center$750,000
Old Towne Crescent $150,000Fox Run Village $100,000
Monarch Landing $150,0001336 H Street, Washington $80,000
Residences at River Place
Building 1: changed design toShallow spread footings instead of piles
Building 2: left design as timber piles
Pushing DMT with Drs. Schmertmannand Crapps at Kennedy Space Center
Used for assessment of crawlerway soils for new space shuttle load of 25,000 kips—highest load
The Importance of Soil Ageing
Backfill Natural Soil
Total Unit Weight (pcf)
118.6 111.9
Void Ratio 0.75 0.84
Natural Soil and Backfill had same Gradation
CONCLUSIONS Use dilatometer tests for predicting settlement of
shallow foundations Case studies show DMT has lowest coefficient of variation
Engineers should make the measurements because they understand the design needs
Technicians only collect numbers—do not have educational background to understand their meaning or make any field changes
Encourage you to vote for Dr. Jean-Louis Briaudfor ASCE President this month!