Pressuremeter Testing

Pressuremeter Testing

Paul J. Cosentino, Ph.D., P.E.

10/23/2018

I.Overview of In-situ testing:

Advantages

larger samples tested
less disturbance
much faster than lab tests

Disadvantages

can not control initial state of stress during testing (i.e. o’)
many times stresses induced during testing are horizontal while building loads are vertical
many times results are empirical

Types (most common)

pressuremeter (PMT)
Monocell and Tricell Models
cone penetrometer (CPT)
Mechanical, Electrical and Piezocone Models
dilatometer (DMT)
vane shear (VST)
standard penetration test (SPT)
Hammers vary from donut to safety to automatic

II.Pressuremeter (PMT) testing

Introduction

developed in 1954 by Ménard at University of Illinois
insert long cylindrical balloon type device into soil and during inflation with water measure response of soil

Theory and Data Reduction

during inflation long cylinder expands radially producing plain strain conditions
injected water volumes can be converted to volumetric strains or radial strains to produce a stress-strain plot that can be analyzed
typically elastic moduli (E), lift-off (or at-rest) (poh) and limit pressures (pL) are determined from plots

Uses

lateral loads on foundations especially piles and drilled shafts (or piers)
empirical bearing capacity (i.e. ultimate soil capacity) predictions
empirical settlement predictions (have been shown to be more reliable than Terzaghi’s One-dimensional consolidation predictions)
elastic moduli for finite element programs, pavement designs, immediate settlement predictions

Advantages

fast testing: field testing can be completed in 10-20 minutes
fast analysis: computerized data reduction (APMT) can be completed in a few minutes
large sample tested (10 to 18-inches length depending upon model used)
test simulates lateral loads on piles and piers
simple procedures available to determine settlement, bearing capacity, etc.,
relatively simple testing procedure, especially with automation
equipment relatively inexpensive ($12,000 to $20,000); therefore costs can be recouped quickly
new FDOT procedures for pushing saves a SIGNIFICANT amount of time
new instrumentation software also save SIGNIFICANT time

Disadvantages

test hole MUST be carefully prepared, if pre-bored
membrane failure causes ½ day delay
requires specialist to conduct test

Overview of test procedure

Prepare borehole and lower probe to desired test depth (30 to 120 minutes) or

Hydraulically push cone or Pencel Pressuremeter to desired depth (5 to 10 minutes)
Inject equal volume increments of water; wait for system to stabilize and record corresponding pressures (5 or 60 cm3 depending upon model)
Test is complete once either 90 cm3 or 1200 cm3 is injected (depending upon model)
Apply three calibrations to raw data;

one for inherent membrane resistance (i.e., resistance of balloon);
a second for system or volumetric expansion (i.e. the tubing expands and membrane contracts)
and a third for the test depth (i.e., hydrostatic pressure at test depth must be added to pressure read from control unit gage)

III. Pressuremeter Models

There are several PMT models currently available. They vary based on the length to diameter ratio and whether they are tri-cell or mono-cell probes.

Ménard first developed a tri-cell probe as shown below. There are two outer cells, called guard cells, which are first expanded to ensure plain strain conditions during testing with a center cell. The center cell is expanded at predetermined pressure increments to complete the test. The disadvantages of the Ménard probe are that 1) a stress controlled test is conducted resulting in few data 2) the testing procedure is complex and 3) that a gas supply is required to conduct the test.

To simplify the testing procedure Briaud developed a mono-cell probe. This simplification yields a strain-controlled test producing more data points as about 20 equal volume increments of water are injected into the probe and the corresponding pressures are recorded.

There are two mono-cell models currently available, the standard size PMT known as the TEXAM and the cone penetrometer size version, known as the PENCEL PMT. A schematic of a typical mono-cell PMT is shown below.As the actuator is turned a known volume of water is forced into the probe through nylon tubing and pressures are recorded from the pressure gage. For the TEXAM; 60 cm3 volume increments up to 1200 cm3 are injected while for the PENCEL; 5 cm3 increments are injected up to 90 cm3.

The smaller PENCEL PMT is depicted below (Figure 3). The diameter of the probe is 1.35-inches which is nearly the same the diameter of the Cone Penetrometer (CPT). It allows this probe to be attached to cone rods and pushed into the soil. This feature allows a significant number of tests to be conducted quickly. A photograph of the internal components of the control unit is shown on the following page (Figure 4). It details the plumbing used to run the water from the cylinder to the probe. It also includes the latest digital instrumentation that
enables operators to digitally acquire the reduced stress-strain data. The volume counter runs from 0 to 135 cm3 and the pressure gage typically included reads pressures up to 2500 kPa (about 310 psi).

Figure 4 Internal components of PENCEL Pressuremeter Control Unit

Figure 5 Typical reduced pressuremeter data with definitions of key portions

A typical set of results is shown in Figure 5. This plot shows pressure versus volume of water injected into the probe. The initial slope and rebound slopes (Si and Sr) are used to determine elastic moduli. Often the initial modulus is related to the soil response for drilled shafts and the rebound slope is related to the soil response for driven piles. The unloading data can also be used to determine moduli over a variety of strains. This data could in turn be used to evaluate the soil responses in complex loadings that require finite element analyses.

The data in Figure 5 indicates that after the soil reaches the existing at rest pressure of about 110 kPa, it displays a relatively linear response up to about 400 kPa and a nonlinear response typical of granular materials up to a limit pressure of about 650 kPa. To correctly estimate a limit pressure, Baguelin et al (1978) suggest that the data be extrapolated to twice the initial volume of the hole. This process requires two steps; a) the initial probe volume must be found based on the probe dimensions and b) the additional volume until the probe expands to the sides of the borehole (i.e. the At-Rest Soil Pressure point) should be added to the volume from part a. This number s then doubled and the plot extrapolated to the estimated limit pressure.

IV Applicable Pressuremeter Theories

There are two key parameters that define any material, the stiffness and the strength. The stiffness is based on the elastic response of a material and for the pressuremeter test the soil response which is nonlinear must be evaluated.

The basis for the pressuremeter theories is the assumption that the pressuremeter probe causes the soil to expand according to plane strain conditions. Plane strain typically occurs when one dimension is significantly very long compared to other dimensions (Holtz and Kovacs 1981). The pressuremeter probe is thus considered to be an infinitely long cylinder, expanding uniformly in the radial direction. This assumption allows the soil moduli to be determined based on linear elastic theory according to the equation:

(1a)

where,E = Young’s modulus

= change in stress

= change in volume related to

= average volume over

= Poisson's Ratio (typically assumed to be 0.3 for unsaturated conditions and 0.5 for saturated)

The relative radius increase in probe radius can be substituted into Equation1a, yielding the following equation used in analysis to determine moduli (Tucker and Briaud 1986):

(1b)

where,= radius increase at point 1

= radius increase at point 2

V. Bearing Capacity with the Pressuremeter

Menard (1963a and 1963b) developed the basic bearing capacity equation using the net limit pressure (p*L) from PMT data. This pressure is defined as the difference between the limit pressure (pL) and the lift-off pressure(poh). The basic equation is:

qL = k p*Le + qo

with p*Le is the equivalent net limit pressure within the zone of influence of the footing which can be found as the nth root of the product of the individual net limit pressures in the zone to ± 1.5B above and below the footing depth or:

qo is the overburden pressure in terms of total stress and

k obtained from Figure 66 after Briaud (1986b) modification of the original charts shown in Figure 65. He in these charts is the embedment depth which can be found as:

with Dbeing theembedment depth of the footing and p*Libeing the net limit pressure in a zi thicklayer.

Figure 66 is for square or round footings. For strip footings k from Figure 66 should be divided by 1.2. See Briaud (1992) for additional information on inclined, eccentric and footings on slopes.

VI. Settlement with the Pressuremeter

Menard and Rousseau (1962) developed the basic settlement equation from PMT data. It is composed of two parts a deviatoric component and a spherical component. Several empirical factors are required to perform the calculations but the basic equation is:

(2)

where: s = total footing settlement

Ed = pressuremeter modulus within the deviatoric zone of influence

Ec = pressuremeter modulus within the spherical zone of influence

q= net bearing pressure of the footing

d = shape factor for deviatoric term from the figure below

d = shape factor for spherical term from the figure below

 =rheological factor from the Table 1 below

Figure 6 Variation in Menards shape factors versus footing dimensions

To perform the calculations, divide the soil layers beneath the footing into layers B/2 thick. Us the PMT modulus within the first layer for Ec and an average modulus over a depth of 16 layers each B/2 thick for Ed. Briaud (1992) recommends a harmonic mean calculation for this deviatoric modulus.

Table 1Menard Rheological Factor 

VII. Applications for Laterally Loaded Piles

The Robertson et al, Pushed in PMT Method (1986)

Robertson et al. (1986) suggested a method that uses the results of pushed-in PMT to evaluate p-y curves of a driven pile. According to the authors, the results provide an excellent comparison with lateral loaded pile test measurements. The pressure component of the PMT curve is multiplied by anα-factor to obtain the corrected p-y curve. Using finite element analysis Byrne and Atukorala (1983) confirmed this factor, which was initially suggested by Hughes et al. (1979), Robertson et al. (1986) corrected the factor α near the surface assuming that the PMT response is affected by the lower vertical stress. The factor increases linearly up to a critical depth, which is assumed to be four pile diameter (Dc = 4) as shown in Figure 6.

Figure 7 Correction Factor “α” versus Relative Depth (From Robertson et al,. 1986)

To obtain the p-y curve, the PMT curve is re-zeroed to the lift-off pressure that is assumed to be equivalent to the initial lateral stress around the pile. The stress is multiplied by the pile width and the strain component is multiplied by the pile half width. For a small strain condition is assumed equal to where R and V are radius and volume of the PMT respectively.

Since the installation of the pushed in PMT produces an initial pressure on the probe, an unload/reload sequence is often used. The portion of the corrected PMT curve from the beginning of reload through the maximum volume is recommended for determining p-y curves of driven piles, while the initial slope from the PMT tests is recommended for constructing p-y curves for augured piles. The following equations outline the process for driven piles:

a) Determine the initial radius of the probe:

(2)

b) Calculate the initial volume of the probe (Vo):

(3)

c) Determine P in units of force / length:

First a correction factor, , is applied to P according to Figure 6, where the relative depth is the depth from the ground surface to the center of the membrane. Note that for for sands andfor clays and if then  can be found as follows:

(4)

(5)

Then

(6)

where: Bpile = pile diameter or width.

d) Determine y in units of length according to the following equation:

(7)

The Briaud et al, Method (1992)

Briaud et al (1992) recommended that the p-y curves be constructed from the addition of the front and side resistance components along the pile. The total soil resistance P as a function of lateral movement y of the pile, is given by the equation:

P = F + Q(8)

where

F= friction resistance

Q= front resistance

Briaud suggested for the full displacement driven piles, that the reload portion of the PMT curves be used. Graphically, the p-y curve is shown as the addition of the F-y curve and the Q-y curve in Figure 7.

Figure 8 Front and side resistance components for P-y curve construction

Smith (1983) showed excellent correlations between the pressures obtained from the PMT response and those acting on the pile. The front pressure contribution, Q, is found from the net limit pressurepL* determined as:

(9)

where; pL is the limit pressure and p0 is the horizontal stress at rest pressure obtained from the PMT curve. The frontal resistance, Q is obtained by choosing pressure points from the reduced PMT plot and using the equation:

(10)

The side friction, F(side), of the pile is taken as a constant with depth and is given by the equation:

(11)

To obtain the associated lateral pile deflections, choose PMT deflections and apply the following equation. The deflections must be less than those obtained from the PMT test and would equate to the change in radii obtained during expansion.

(12)

Where: Q(front)= soil resistance due to front reaction with unit of force /unit length of pile

F(side)= soil resistance due to friction resistance with unit of force /unit length of pile

p(pmt)= pL* = net pressuremeter pressure

B(pile)= pile width or diameter

τ(soil)= maximum soil shear stress-strain at the soil-pile interface

S(Q)= shape factor ( = 0.8 or π/4 for circular piles, 1.0 for square piles)

S(F)= shape factor ( = 1.0 for circular piles, 2.0 for square piles)

y(pile) = lateral deflection of the pile

y(pmt) = increase in radius of the soil cavity in the PMT test or radial displacement.

R(pile) = pile half-width or radius

R0(pmt) = R0= initial radius of the soil cavity in PMT test

This method does rely on an accurate estimate of the shear strength, which could be found from other field-testing performed during the site investigation.

The displacement of soil around the laterally loaded pile is also influenced by the ground surface. A reduction in the corrected PMT pressures is recommended for values near the ground surface. A critical depth (Dc), to which pressures and displacements are influenced, depends on the pile load, diameter and stiffness. Briaud suggested using a relative rigidity factor, RR, given by:

(13)

EI = pile flexural stiffness (E= pile modulus, I = pile moment of inertia)

= pile diameter or width

pL* = net PMT limit pressure

Briaud et al. (1992)relationship results in relative rigidities slightly greater than 10 for most laterally loaded piles in soft clays and the resulting critical depth will be near 4, therefore Robertson’s value of 4 is recommended. The critical depths for the PMT as recommended by (Baguelin et al., 1978) are 15 PMT diameters for cohesive soils, and 30 PMT diameters for cohesionless soils.

The Briaud et al. (1992) suggested reduction factor  is shown in Figure 8 as a function of relative depth (z/zc). The PMT curve is then corrected by using:

(14)

Figure 9 Briaud’s recommended PMT pressure reduction factor for values near the ground surface

VIII. References

Cosentino, Paul J., Edward Kalajian, Ryan Stansifer, ,J Brian Anderson, Kishore Kattamuri, Graduate Research Assistant, Sunil Sundaram, Graduate Research Assistant, Farid Messaoud, Thaddeus J. Misilo, Marcus A Cottingham (2006) Final Report, Standardizing the Pressuremeter Test for Determining p-y Curves for Laterally Loaded Piles, Florida Institute of Technology, Civil Engineering Department, Florida Department of Transportation, Contract Number BC-819.
Briaud, J.L., 1997. “Simple Approach for Lateral Loads on Piles”. Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol. 123, No. 10 pp. 958-964.
Robertson, P. K., Campanella, R. G., Brown, P. T., Grof, I., and Hughes, J. M., (1985). “Design of Axially and Laterally Loaded Piles Using In Situ Tests : A Case History”, Canadian Geotechnical Journal, Vol. 22, No. 4, pp.518-527.
Robertson, P.K., Davis, M.P., Campanella, R.G.. (1989). “Design of Laterally Loaded Driven Piles Using the Flat Dilatometer”, ASTM Geotechnical Testing Journal, Vol.12, No. 1, pp30-38.
Robertson, P.K., Hughes, J.M.O., Campanella, R.G., and Sy, A., 1983. “Design of Laterally Loaded Displacement Piles Using a Driven Pressuremeter”. ASTM STP 835, Design and Performance of Laterally Loaded Piles and Piles Groups, Kansas City, Mo.
Robertson, P.K., Hughes, J.M.O., Campanella, R.G., Brown, P., and McKeown, S., 1986. “Design of Laterally Loaded Displacement Piles Using the Pressuremeter”. ASTM STP 950, pp. 443-457.
Robertson, P.K., and Hughes, J.M.O., 1985. “Determination of Properties of Sand from Self-Boring Pressuremeter Tests”. The Pressuremeter and Its Marine Applications, Second International Symposium.
Briaud, J-L., 1992, The Pressuremeter, ISBN 90 6191 125 7
Menard, L., 1963a, Calcul de la Force Portante des Foundations sur la Base des Resultats des EssaisPressuometriques, Sols-Soils, Vol 2, Nos 5 and 6 June.
Menard, L., 1963b, Calcul de la Force Portante des Foundations sur la Base des Resultats des EssaisPressuometriques, SecondePartie, Expementaux et Conclusions Sols-Soils No. 6 September.

Homework Questions:

Part I Pressuremeter Testing

What is the required Length to Diameter ratio needed to produce plain strain responses in soils?
How many cubic centimeters of water are injected into the TEXAMPMT probe during each loading increment? The PENCEL Pressuremeter probe during each loading increment?
Using the data shown in Figure 5 determine the following:
Lift-Off Pressure
Initial Elastic Modulus
Elastic Reload Modulus
Limit Pressure