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Strength Verification of Stabilised Soil-Cement Columns: A
Laboratory Investigation of the Push-In Resistance Test (PIRT)
Journal: Canadian Geotechnical Journal
Manuscript ID cgj-2016-0230.R1
Manuscript Type: Article
Date Submitted by the Author: 15-Dec-2016
Complete List of Authors: Timoney, Martin; Arup Consulting Engineers, formerly of National
University of Ireland, Galway McCabe, Bryan; National University of Ireland, Galway, Civil Engineering
Keyword: PIRT, KPS, stabilisation, clay/silt, strength
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Strength Verification of Stabilised Soil-Cement Columns:
A Laboratory Investigation of the Push-In Resistance Test (PIRT)
Martin J. Timoney, PhD, MIEI ([email protected])
Project Engineer, Arup, 50 Ringsend Road, Dublin 4, Ireland, formerly of the College of Engineering & Informatics, National University of Ireland, Galway, Ireland.
Bryan A. McCabe, PhD, CEng, MIEI, Eur Ing ([email protected])
Lecturer, College of Engineering & Informatics, National University of Ireland, Galway, Ireland.
Corresponding author: +353 91 492021 [email protected]
Word Count: 6797 (Introduction to Acknowledgements)
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Abstract: The Push-In Resistance Test (PIRT) is an in situ means of verifying the
strength of stabilised soil columns. During the test, a winged penetrometer penetrates the
stabilised column at a constant rate and the column strength is estimated from the
recorded probing resistance using a bearing capacity factor, N. While N values between 8
and 20 (although typically between 8 and 15) are quoted in the literature (based almost
exclusively on empirical and Scandinavian experience), there have been few field tests
and no laboratory investigations aimed at investigating the value of N and the factors
upon which it depends, thereby limiting international confidence in the method.
This paper presents the findings of a unique laboratory-scale PIRT series conducted at
NUI Galway. The development of appropriate column construction and testing methods is
discussed. The results of 11 no. PIRT tests on pre-drilled stabilised columns with
unconfined compression strengths (UCS) from 150 kPa to over 800 kPa are reported, as
well as those of a complementary cone-only series of tests to assess additional frictional
forces acting on the penetrometer. Appropriate corrections to the data for temperature and
time-consistency between the probing forces and UCS values are discussed. Test results
indicate that the strength of the column has a mild influence on the N value. Further
investigation of this finding is recommended at both laboratory and field scales.
Keywords: PIRT, KPS, cement, stabilisation, clay, silt, column, laboratory testing
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Introduction
Soil stabilisation (or soil mixing) is a method of ground improvement in which
cementitious and/or pozzolanic binder materials (such as lime, Ordinary Portland Cement
(OPC), Ground Granulated Blast Furnace Slag (GGBS), Pulverised Fuel Ashes (PFA) or
combinations of these) are mixed with a soil in situ to enhance its strength and stiffness
characteristics (EuroSoilStab 2002). Dry soil mixing (DSM) is one such method in which
the binder is injected as a dry powder and is appropriate for soils with natural moisture
contents in excess of approximately 40% (Topolnicki 2012) as this moisture content is
sufficient to instigate the binder hydration reactions.
Developed in Scandinavia in the 1970s (Bredenberg 1999), Deep Dry Soil Mixing
(DDSM) provides a method with which stabilised columns can be created in soft
clays/silts, peats and other weak soils. A mixing tool is inserted into the ground, rotating
at a specific rate to break up the soil. Once the design depth is reached, the mixing tool is
retracted as the binder is injected and mixed with the parent soil. DDSM columns can be
constructed as either single units, rows or interlocking panels, to depths of up to 30 m
(Topolnicki 2012), although depths are typically less than 10 m. Column diameters
between 0.5 m and 1.0 m are typical in European practice (Dahlström 2012), with
diameters up to 1.6 m possible using Japanese equipment (Kitazume and Terashi 2013).
Applications of DDSM include foundations for roads, railways and light commercial and
residential structures, slope and embankment stability solutions, confinement and
remediation of contaminated soils, vibration reduction solutions for rail applications and
temporary works (Dahlström 2012). In recent years DSM processes have been deployed
beyond the traditional markets of Europe (principally Scandinavia) and Japan, with
projects carried out in the USA (Burke et al. 2007; Filz 2009; Hussin and Garbin 2012),
Australia (Liyanapathirana and Kelly 2011) and Asia (Horpibulsuk et al. 2012), and
therefore it is important that there is international confidence in the methods used to
establish in situ stabilised field strengths.
The factors which influence the effectiveness of the stabilisation process have been
generalised into four categories by a number of authors (e.g., Babaski and Suzuki 1996;
Kitazume 2005) as follows:
(i) Soil characteristics: moisture content (Babaski et al. 1996; Porbaha et al.
2000; Jacobson 2002; Åhnberg et al. 2003; Marzano et al. 2009), organic
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content (Tremblay et al. 2002; Timoney et al. 2012a), pH (Kitazume 2005)
and the particle size distribution. Many soft soils are inherently variable in
nature and this variability can be reflected in the characteristics of the
stabilised soil.
(ii) Binder characteristics: the type of binder added to the soil (Janz and Johansson
2002; Axelsson et al. 2002; EuroSoilStab 2002; Timoney et al. 2012a).
(iii) Mixing conditions: the quantity of binder added to the soil (Babaski et al.
1996; Porbaha 2000; Axelsson et al. 2002; Kitazume 2005; Horpibulsuk et al.
2011) and the degree and duration of mixing (Larsson 2003).
(iv) Curing conditions: the time (Nagaraj et al. 1996; Jacobson 2002; Horpibulsuk
et al. 2003; Åhnberg 2006; Filz 2009), temperature (Marzano et al. 2009;
Åhnberg and Holm 2009) and confining pressure (Åhnberg 2007) under which
the stabilised soil cures.
Site-specific binder trials can help estimate achievable column strengths provided the
laboratory conditions are representative of the field conditions under categories (i)-(iv).
This can be very difficult to achieve in practice, as evidenced by variable field-to-
laboratory strength ratios reported by Larsson (2003); therefore in situ strength
verification is of paramount importance. Although rotary coring, in situ column shear
vane (Larsson 2006) and column extraction (Holm et al. 1999) methods are used to assess
in situ column strengths directly, the most commonly-used method remains the Push-In
Resistance Test (PIRT), also known in Scandinavia as Kalk-Pelar-Sondering (KPS) or the
Conventional Column Penetration Test (CCP).
The probing resistance recorded during the PIRT is related to the column strength using a
bearing capacity factor, N. Some guidance exists in the literature on appropriate values of
N (Åhnberg and Holm 1986; Carlsten and Ekström 1996; EuroSoilStab 2002; Larsson
2006; Trafikverket 2011), mainly based on Scandinavian experience, but with limited
supporting data.
In this paper, a reduced-scale PIRT facility developed at NUI Galway is presented. These
experiments, believed to be the first of their kind, were conducted under controlled
laboratory conditions, an advantage over previous PIRT studies. The research is intended
to provide some new insights into the N value applicable to stabilised clays/silts spanning
a wide range of stabilised strengths.
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Push-In Resistance Test (PIRT)
PIRT Details
During a PIRT, a winged penetrometer (Figure 1) is advanced into the column at a
constant rate and the force-depth profile recorded. Details and guidance on in situ
penetrometer dimensions are provided by Carlsten and Ekström (1996), Larsson (2006)
and TK Geo 11 (Trafikverket 2011), see Figure 1a. The width of the penetrometer (B) is
typically 75% of the diameter of the column to be tested or 100 mm less than the column
diameter. The leading edge of the penetrometer has a circular bulb shape to reduce
friction along the vertical plate section of the wing; this is typically either 15 mm or 20
mm in thickness (d), while the wing thickness is 5 mm less than that of the leading edge.
The cone tip of the penetrometer is typically 50 mm in diameter and sounding rod
diameters are between 36 mm and 50 mm. The larger diameter bars are used in high
strength columns where probing resistances are high and there is a risk of the bars
bending during the PIRT (Carlsten and Ekström 1996).
PIRT offers a number of conceptual advantages over the popular Cone Penetration Test
(CPT). CPT cones only probe at a point location within the column cross-section and tend
to follow the weakest path, i.e., that previously followed by the kelly bar of the mixing
tool, whereas in the case of PIRT, a chord of the column is probed. In addition, CPT
cones are more prone to deviation from vertical than the winged penetrometer,
particularly in strong columns.
Equipment used for carrying out a PIRT can be either truck-mounted (as used for CPT),
excavator/track-mounted or wheel-loader-mounted (Hussin and Garbin 2012). The
penetrometer is pushed into the column at a constant rate of 20±4 mm/sec (Carlsten and
Ekström 1996; EuroSoilStab 2002; Larsson 2006; Trafikverket 2011), similar to that used
in the CPT (Lunne et al. 1997), until it has reached a hard stratum or, in the case of
floating columns, has passed 2 m beneath the column base. Column lengths of 6 m to 8 m
can normally be tested, but in long and/or strong columns, the penetrometer can deviate
off vertical and exit the column (Halkola 1999). Current Swedish guidelines suggest that
columns longer than 6 m to 8 m (Larsson 2006; Trafikverket 2011) or columns with
undrained shear strengths greater than 300 kPa should be pre-drilled. This has been
shown to prevent the penetrometer deviating from the centre of the column as the hole,
typically 50 mm to 65 mm in diameter, provides a guide for the penetrometer tip to
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follow (Axelsson and Larsson 2003) and testing of columns up to 15 m long has been
achieved in this way (Bergman and Larsson 2014).
Interpretation of PIRT
From the results of a PIRT, the undrained shear strength (cu) of a stabilised column is
estimated using Equation 1, where N is an overall dimensionless bearing capacity factor
for the device, P is the probing force (kN) (after correction for sounding bar friction) and
A is the projected plan area of the wings and cone of the penetrometer (m2). For pre-
drilled columns, the area of the drill hole is deducted from the projected area of the
penetrometer. The relationship in Equation 1, proposed by Boman (1979), is a
simplification of the semi-empirical equation for the Iskymeter penetrometer, a device
used for sounding in soft ground (Kallstenius 1961).
A
P
Ncu ×=
1 1
Early guidance (Åhnberg and Holm 1986) recommended the use of an N value of 8 and
10 for 400 mm wide penetrometers with thicknesses of 15 mm and 20 mm, respectively,
while more recent guidance documents (Carlsten and Ekström 1996; EuroSoilStab 2002;
Larsson 2006; Trafikverket 2011) recommend the use of an N value of 10. N values
reported for PIRT in the literature from field studies are compiled in Table 1, the majority
of which originate from Scandinavian experience.
While an N value of 10 appears to have a broad (although not complete) acceptance, it is
difficult to draw a firm conclusion from the data in Table 1 due to:
(i) Variation in the stabilised materials tested
(ii) Variation in penetrometer dimensions
(iii) Differences in the test methods by which the column strength was measured
or inferred, and
(iv) Limits of strength for which the quoted N value has been derived.
Furthermore, the variation in N observed in the literature has prompted some authors
(Axelsson and Rehnman 1999; Bergman et al. 2013) to suggest that N may be a function
of the strength and stiffness properties. Axelsson and Rehnman (1999) found good
agreement between strengths (estimated using N = 10) from PIRT data and UCS tests on
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column samples, and suggest that a somewhat higher value may be required for regions
with shear strengths less than 100 kPa. Finite element modelling reported by
Liyanapathirana and Kelly (2011) indicated a dependence of N on both the rigidity index
and the probe-soil interface roughness, concluding that an N value of 12 to 14 is
reasonable.
Other relevant penetrating devices
As mentioned, the convention for interpreting PIRT data is to use a single N value, and
there is no reference in the literature, including the numerical work of Liyanapathirana
and Kelly (2011), to separate bearing factors for the wings and cone. However, T-bar
penetrometers and Deep Penetrating Anchors (DPA) provide some insight into the
relative contributions of the PIRT penetrometer components. This is a relevant frame of
reference given that the columns were pre-drilled for the tests reported here, as there was
no cone-bearing contribution to the total force.
T-bar Penetrometer:
Stewart and Randolph (1994) have shown that for a 50 mm dia. and 200 mm long T-bar
penetrometer, the relationship between undrained shear strength and T-bar resistance is
dependent on the surface roughness of the T-bar with bearing factors (Nb) of between 9
and 12 suggested for smooth and rough probes, respectively. Using a bearing factor of
10.5, recorded resistances from field T-bar tests in a soft clay deposit were used to
estimate the undrained shear strength and the results were found to be in agreement with
shear vane and triaxial test results. The authors also noted the similarities in plan area and
bearing factor with that of the Iskymeter probe.
Deep Penetrating Anchors (DPA):
Used in mooring offshore structures, DPAs consist of a cylindrical shaft with a number of
flukes or wings attached to the upper end and a rounded conical tip (O’Loughlin et al.
2004). While obviously not used for in situ testing, the similarities between DPAs and the
PIRT penetrometer are noteworthy. O’Loughlin et al. (2004) describes an equation to
calculate the pull-out capacity of a DPA to include (i) the weight of the device
(significant for DPAs), (ii) frictional resistance along the wings and shaft, and (iii)
bearing resistance along the wings and end of the shaft. Published bearing N values are
between 9 and 12 for the conical tip of the shaft and 7.5 for the flukes.
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Laboratory Experimental Work
Soil and Stabilisation Properties
An organic clayey silt from Kinnegar, Co. Down, Northern Ireland, known locally in the
Belfast area as sleech, was used as the parent soil for construction of the stabilised
columns. Extensive classification and testing of the sleech has been reported by Crooks
and Graham (1976), Bell (1977), McCabe (2002) and McCabe and Lehane (2006) and
details are provided in Table 2.
Approximately 5 m3 of sleech was obtained from between 3.0 m to 4.5 m below ground
level and transported to the laboratory in 1 m3 bulk containers. Swedish fall cone tests on
undisturbed samples, taken during sampling, indicated an undrained shear strength of
approximately 15 kPa and a remoulded strength of 4.5 kPa. Moisture contents (wi) at the
time of stabilisation ranged from 42% to 56% and in some cases are below the in situ
ranges reported by the aforementioned studies (see Table 2) due to drying out that
occurred due to reworking of the sleech during the construction process and during
storage between tests. The sleech was found to have a plastic limit of approximately 28%
and a liquid limit of approximately 75%. Results from loss on ignition tests on the sleech
used for the columns, carried out at 440 °C, ranged from 2.2% to 4.6%.
The Unconfined Compression Strength (UCS, q) is the typical measure of strength used
in DSM applications, and is equivalent to twice the undrained shear strength, i.e., q = 2cu.
It was envisaged that columns with various UCS values would be constructed, covering at
least the range of strengths (approximately 100 kPa to 600 kPa) represented in Table 1.
To assess achievable strengths, an initial series of stabilisation trials were performed in
which OPC, GGBS and lime binders were used to stabilise the sleech. From the results of
these trials, OPC at binder contents of 75, 100 and 150 kg/m3 were deemed most
appropriate for the testing programme (Timoney 2015).
Curing Basin and the Reduced-scale PIRT Penetrometer
Columns with a diameter of 200 mm were constructed, cured and tested in a 750 mm
diameter, 1.0 m high High-Density Polyethylene (HDPE) pipe sealed at the base with a 6
mm HDPE base plate. The column diameter was chosen following consideration of
possible methods to form the column and transpired to be one-quarter that of the
commonly-used diameter of 800 mm. The basin diameter was chosen based on a
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simplified axisymmetric finite element model, ensuring that the horizontal stress increase
at the basin boundary due to the expansion caused by the insertion of the penetrometer
was no greater than 5% of the increase experienced at the edge of the column (Timoney
2015).
Using guidance provided by Carlsten and Ekström (1996), two one-quarter scale PIRT
penetrometers were designed and manufactured. The stainless steel penetrometers (Figure
2) had a width (B) of 150 mm, leading edge thickness (d) of 6 mm and a cone diameter of
14 mm; the penetrometer shaft and the sounding bars were 10 mm in diameter. A bespoke
cone-only penetrometer (Figure 2a) was also developed, originally intended to assess
cone bearing resistance, but ultimately used in pre-drilled columns for a series of
sounding bar friction tests to isolate the force on the penetrometer wings only.
PIRT Column Construction
A total of 14 no. stabilised columns were constructed and subjected to PIRT testing. The
first three (PI-1 to PI-3) were preliminary trials to devise a robust test methodology;
details of the remaining 11 column tests (PI-4 to PI-14) are provided in Table 3 and
details of an additional 4 PIRTs carried out in the unstabilised sleech surrounding the
columns are provided in Table 4.
The use of a reduced-scale mixing tool was considered for column production. However,
based on verticality, binder delivery and mixing challenges encountered by Larsson
(1999), Al-Tabbaa and Evans (1999) and Kosche (2004), it was deemed that higher
quality columns could be produced using casting techniques. Due to significant deviations
of the PIRT penetrometer during initial trials, the aforementioned pre-drilling stage was
replicated to enable the penetrometer remain within the column. This was achieved by
pre-forming a hole in the column using a 13 mm diameter metal bar during the
construction process, and extracting it immediately prior to the test.
Using two semi-circular pieces of 200 mm diameter pipe, bound together with adhesive
tape, columns were constructed in four lifts within the curing basin. The stages below
correspond to those depicted in Figure 3:
(i) The form pipe was placed on a 200 mm thick layer of sleech, which represents
unstabilised soil beneath a floating column; for construction purposes the sleech
directly beneath the column was held in a 225 mm diameter, 200 mm high pipe.
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(ii) EuroSoilStab (2002) laboratory procedures for column applications (applicable
to soft cohesive soils) were followed to create each stabilised mixture, using
either 75, 100 or 150 kg/m3 of OPC. The first stabilised mix was compacted in
40mm layers within the form pipe.
(iii) A 13 mm diameter hole-form bar was inserted into the centre of the column to
create the guide hole. The form pipe was removed (Figure 4) and unstabilised
sleech was carefully moulded by hand around the column.
(iv) The form pipe was replaced around the top of the column and bound closed with
adhesive tape.
(v) A second stabilised mix was created and compacted into the form pipe as
described in step (ii).
(vi) Steps (iii) to (v) were repeated until the desired column height was reached, with
an overall construction time of 6 to 7 hours. The hole form bar was left in place
during curing and removed just prior to the PIRT.
To simulate columns at greater depths and to increase confinement around the top of the
column, PI-9S and PI-14S were cured and tested under a surcharge. This surcharge was
applied over the entire top surface of the 750 mm dia. basin, loading both the column and
the surrounding sleech. The surcharge magnitude of 13.7 kPa, limited by testing setup
constraints, was nevertheless of the same order as surcharge loading applied in practice.
PIRT Column Testing
Columns were allowed to cure for between 0.9 and 11.9 days (Table 3), with the
maximum strengths achieved at the latter time adjudged to be approaching the limit of the
testing equipment. The reduced-scale PIRT penetrometer was then pushed into the
column using the NUI Galway CPT rig, mounted on an independent reaction frame over
the test basin (Figure 5a). Column probing was carried out in two pushes, with rates
designated δ1 (first push) and δ2 (second push), with a short pause in between required to
connect additional sounding bars. In keeping with the full-scale PIRT, a target push-in
rate of 20±4 mm/sec was sought. While some of the rates shown in Table 3 are slightly
above this range, consideration of research on rate effects, including that by Brown and
Hyde (2008) and Robinson and Brown (2013), indicates that such relatively small rate
differences will have a negligible effect on strength. The push-in force and penetrometer
displacement were recorded by a 5 kN load cell and a draw wire gauge, respectively
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(Figure 5b). During testing, minor buckling of the sounding bars was observed due to
their slenderness, resulting in contact with the column, the extent of which cannot be
quantified.
A total of four reference PIRTs were also carried out in the unstabilised sleech that
surrounded a number of columns (Table 4). These were carried out in the same manner as
those in the stabilised column with the second PIRT penetrometer. In two of these, a 13
mm diameter hole was formed in the sleech immediately prior to the test, while in the
other two, pre-drilling was not replicated.
Once tested, each column was carefully exhumed from the basin and the location of any
cracks was noted. Cylindrical samples, 50 mm in diameter, were obtained from the
column where possible (i.e., from intact sections of column) and their UCS determined in
accordance with EuroSoilStab (2002) procedures. For quality assurance purposes, the
uniformity of the surrounding unstabilised sleech was assessed during the exhumation
process through its moisture content and bulk density (determined using 50 mm diameter,
50 mm long core samples) and its shear strength (determined using a 49.4 mm by 33 mm
shear vane).
Cone and Sounding Bar Tests
The cone-only penetrometer (shown in Figure 2a) was originally envisaged as a means of
ascertaining the bearing resistance of the cone element of the PIRT penetrometer in
isolation. However, as occurred in the trial PIRT tests, the cone-only penetrometer was
found to deviate excessively from verticality. As the PIRT columns were pre-drilled, the
cone-only tests enabled quantification of the frictional contribution and therefore allowed
the force on the penetrometer wings to be isolated. An additional series of 7 such tests
was carried out on 104 mm diameter pre-drilled stabilised columns. Each column was
constructed, cured and tested in a 104 mm diameter pipe without any surrounding sleech,
from a single stabilised mix, with construction taking approximately 1 hour. Pre-drilling
was replicated using the same 13 mm diameter bar as used in the PIRT series and the hole
was offset from the centre to allow for recovery of 50 mm diameter samples for UCS
testing (Figure 6). A summary of the test details is provided in Table 5. For comparison,
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a cone-only penetrometer test (see Table 4) was also carried out in the unstabilised sleech
surrounding column PI-14S.
Experimental Results and Interpretation
PIRT Probing Force Data
The variation in the uncorrected probing force profile (Pu) with height from the basin base
(h, relative to the leading edge of the penetrometer wings) is shown in Figure 7 for tests
PI-4 to PI-14S. An expanded version of the notation PI-X (Yd-Zc) is used in this and
subsequent figures, to aid in their interpretation, where X is the test number, Y is the
curing time (rounded to the nearest day) and Z is the cement content (kg/m3). The data in
Figure 7 are grouped according to curing times: (a) 1-4 days, (b) 6 days and (c) 12 days.
The following general observations can be made about the probing force profiles at the
top, middle and bottom of each column:
(i) Top of the column: At the beginning of each test, the force increased instantly as
the penetrometer began to press into the column. The delayed increase in the case
of PI-4, PI-9S and PI-14S is due to a thin layer of sleech at the top of those
columns. The force peaked and subsequently dropped as the column cracked
ahead of the penetrometer. This drop is less significant in the case of the
surcharged columns; for example, the force drops from a peak of 2.8 kN to 1.1 kN
for PI-10, but from 2.8 kN to 1.9 kN for PI-14S, even though UCS values in the
top 200 mm, corrected to the time of PIRT are similar at 688 kPa (PI-10) and 634
kPa (PI-14S). After this drop the force rises again before becoming constant or
gradually increasing with depth, albeit with fluctuations.
(ii) Middle of column: A temporary reduction in force identifies where the test was
paused to insert additional sounding bars. Interestingly, the recommencement of
probing after the addition of sounding bars does not show similar peaks and drops
in force to those occurring at the top of the column. It appears that the increased
confining stresses around the column at this level limit the amount by which the
column can crack open. In PI-12*, the varied binder content column, a reduction
in force was recorded between h = 615 mm and h = 420 mm, i.e., the vertical
extents of the 100 kg/m3 Mix 2, and showed a notable drop at h ≈ 450 mm.
(iii) Bottom of column: Within the last 50 mm of the column, the force dropped,
believed to be due to a crack forming ahead of the penetrometer and extending to
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the base of the column. In all cases, the magnitude of the probing force further
reduced upon entering the unstabilised sleech underneath the column.
Upon exhumation, the softer columns showed little evidence of crack development due to
PIRT probing, although a weak plane was noted along boundaries between individual
mixes (Figure 8a). However, the stronger columns behaved in a more brittle fashion; with
cracks typically running from the centre of the column diagonally downwards and
outwards to the face of the column at approximately 45° (Figure 8b). In places, especially
in the mid-sections of columns whose q values lay approximately between 300 kPa and
450 kPa, cracking was significant and limited the amounted of samples that could be
retrieved in these regions. From a comparison of the Pu profiles in Figure 7, it is likely
that the jaggedness in the profiles is related to the increased material brittleness and the
localised effects of cracking. The soft columns, in which few cracks were observed, show
a smoother profile.
Two of the PIRTs in unstabilised sleech were performed alongside columns also
subjected to PIRTs, i.e., PI-13-U and PI-14S-U, where U denotes that the material was
unstabilised. PI-13-U and PI-14S-U were pre-drilled. The other two tests (PI-T-U1 and
PI-T-U2) were performed in the sleech alongside an instrumented column that was
constructed solely to assess the impact of curing temperature, i.e. a PIRT was not
performed on the column. PI-T-U1 and PI-T-U2 were not pre-drilled. The probing force
profile with depth for the PIRTs carried out in the unstabilised sleech surrounding the
columns typically increased from 0.1 kN at the top to 0.25 kN at the bottom. This
increase is believed to be due the mild increase in strength (cu = 7-12 kPa) with depth and
friction due to the adhesive nature of the sleech.
PIRT Column Strength Data
In Figure 9, q values at the time of the UCS test (qmeas) are plotted for each individual
column sample against the depth of the midpoint of each sample, from which it can be
seen that a wide range of qmeas values (between 148 kPa and 816 kPa) has been obtained,
as intended, by varying the binder content and curing time.
The number of samples obtained from each column ranged from 7 to 22 with column
cracking (caused by the PIRT itself) affecting the number of samples retrievable and their
heights (with samples measuring between 75 mm and 108 mm in height).
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Column secant stiffnesses at 50% of the failure strength (E50) were found to range from 5
MPa to 110 MPa (Figure 10) and increased with both time and binder content. E50/qmeas
were found to lie between 45 and 130, comparing well with the range 50-150 found in
literature. The value of E50/qmeas reduced with increasing strain to failure; further details
are available in Timoney (2015).
Column Strength Correction
Given the inevitable delay between the PIRT on the column and UCS testing of the
column’s samples (between 4 and 10 hours), it is inappropriate to relate qmeas and Pu
values in the context of the subsequent calculation of an N value. A framework was
developed to this end to allow the q value at the time of PIRT to be estimated; this value
is referred to as the corrected UCS (qcor). The framework unites all PIRT data by
accounting for the principal variables of curing time, curing temperature, moisture
content and binder content (represented in Table 3).
Curing Temperature: Column temperatures during curing were measured as part of this
research through a series of thermistors embedded within a column constructed
specifically for this purpose (Timoney 2015). Column temperatures were compared to the
average ambient room temperature (T) recorded by the Building Management System
(see Table 3) and it was found that the column temperature equalised to ambient
temperature within 12 hours of stabilisation. Therefore in this study, the average ambient
temperature of the laboratory during curing (see Table 3) provides a reasonable measure
of the curing temperature of the column. Temperature variations were accounted for by
adjusting the curing time using Equation 2, a formula used for adjusting concrete curing
times for temperature (BSI 2008):
[ ]( )tet
n
i
T
adj ×=∑=
−+−
1
65.13273/4000 2
where Temperature-Adjusted Time (tadj) is the adjusted curing time of the column
samples and T is the average ambient temperature (°C) over a curing period of t (days).
Moisture Content and Binder Content: It has been shown that the effect of variations in
moisture content and binder content on stabilised strengths can be assessed using a single
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parameter such as water-to-binder ratio (WTBR, η) or a similar parameter (Åhnberg et al.
1995; Jacobson et al. 2005; Filz 2009; Timoney et al. 2012a, 2012b). η is defined as the
mass per unit volume of water (mw) divided by the mass per unit volume of (active)
binder (mb), and can be calculated using Equation 3 (Timoney et al. 2012a):
+
==
i
b
soil
b
w
wm
m
m
11
ρη
3
where ρsoil is the bulk density of the soil (kg/m3), mb is the binder content (kg/m3) and wi
is the initial moisture content of the unstabilised soil. Binder trials reported by Timoney
(2015) have shown that q varies approximately linearly with 1/η for cement-only binders,
rendering 1/η a suitable parameter by which to normalise q (i.e. the normalised parameter
is qη).
Values of (qmeasη)avg are plotted against their corresponding tadj values for all PIRTs in
Figure 11. This correlation is stronger than that found by plotting q against time. In
keeping with the mathematical form of the normalised strength versus time relationship
proposed by other authors for cement-stabilised soils (Nagaraj et al. (1996), Horpibulsuk
et al. (2003; 2011; 2012), Åhnberg (2006) and Filz (2009)), a best-fit natural log
relationship was arrived at and is shown in Equation 4 (R2 = 0.787). The standard
deviation in (qmeasη)avg is also shown.
(qmeasη)avg = 531.78 ln(tadj) +1010 (1 ≤ tadj ≤ 12) 4
The aforementioned qcor is ascertained for each individual sample from Equation 5, using
(i) the qmeas value, (ii) the η of the actual sample and (iii) the term ∆(qmeasη)avg, determined
as the difference between (qmeasη)avg at tadj relevant to the UCS test and (qmeasη)avg back
calculated using tadj relevant to the time at which the PIRT was carried out; both of these
values are determined from Equation 4. Corrections to the strength ranged from 2.5 kPa
to 25 kPa and, as expected, were most significant for early-age columns where the rate of
strength gain is at its highest.
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( )η
ηavgmeas
meascor
qqq
∆−=
5
PIRT Cone-Only Results
Probing force profiles for the cone-only tests (Pc) in columns are shown in Figure 12,
using the notation C-X (Yd-Zc), where X is the test number, Y is the curing time (to the
nearest 0.1 day) and Z is the cement content (kg/m3). Pc values were near-constant with
depth (with values up to 0.25 kN) indicating that minimal friction occurred in the cone-
only tests between the sounding bars and the column; bending of the sounding bars did
not occur due to the lower probing forces. Values of qcol ranged from 195 kPa to 839 kPa
and showed similar strengths with depth in any one column. E50 values of 8 MPa to 108
MPa were observed and E50/qcol was found to range from 60 to 125.
The same correction strategy described previously was applied based on data in Table 5
but with an equivalent of Equation 4 specific to the cone-only column results. The qcor
values are plotted against the average push-in force over the respective sample length
(Pc,avg) in Figure 13. A linear trend is apparent and Equation 6 (R2 = 0.926) can be used to
estimate the PIRT cone-only friction correction (kN):
Pc,avg = 0.0003qcor 6
Probing forces for the cone-only test in unstabilised sleech (C-14S-U), also shown in
Figure 12, showed a relatively linear increase with depth to 0.07 kN at 800 mm. In
keeping with the column tests, the cone was guided by a hole had been formed with the
13 mm diameter bar prior to the test.
Discussion and N value calculation
Statistical Analysis
The variability of the stabilised soil used for the columns is now discussed, which has
implications for the correlations derived from the test results (such as Equation 4) and in
turn, the N values. Coefficients of variation (CoV) were used to assess variability within
each given column, while bivariate correlation analysis and linear mixed model
regression analysis were carried out, using the software package SPSS, to statistically
determine the impact of a number of variables on the measured column strengths.
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Coefficient of Variation:
CoVs were calculated for the initial sleech moisture content (wi), density of the stabilised
column samples (ρcol), qmeas and E50 within each column and compared with values quoted
in published literature. All CoV values for wi were found to be less than 6.2%, which is
below the 8% lower limit quoted by Phoon and Kulhawy (1999) for in situ clay and silt
samples. CoV values for ρcol were less than 3.5%, within the limit of 10% quoted by the
same authors. CoV values for qmeas were between 5.6% and 17.5%, generally below the
15% to 60% values quoted in the literature (Larsson 2005) for field mixing. E50 values
fell between 19.7% and 40.5% (no CoV ranges were found in the literature with which to
compare). Similar CoV results were observed for the cone-only columns.
Bivariate Correlation:
A bivariate correlation analysis using Pearson Correlation was performed to indicate
covariates (t, mb, wi, T, ρcol, depth d, loss on ignition LOI) that may have an effect on the
dependant variable (qmeas), and the significances of each. Highly significant correlations
were observed between qmeas and curing time, binder content and temperature only; an
interaction between wi and ρcol was also noted. The effects of other variables were not
significant. An interaction between wi and ρcol was also noted, as might be expected.
Linear Mixed Model Regression:
A number of linear mixed model regression analysis were carried out, increasing in
complexity with the addition of variables. qmeas was the dependent variable and the
covariates and fixed effects considered were tcol, mb, wi, T, ρcol, sample depth (d) and LOI,
depending on the analysis being carried out. A summary of the results is provided in
Table 6.
In all models for the PIRT columns, curing time and binder content were found to have a
highly significant effect on the qmeas value; values less than 0.05 are considered
significant and values less than 0.01 are considered highly significant. The natural log of
the qmeas and the curing time was taken to remove flaring noted in the residual versus
predicted graphs. Normal distribution of the residuals was observed. Addition of wi and T
to the models showed both properties not to be significant. In the PIRT models, following
inclusion of the natural log data, all models show similar R2 values of approximately
92%.
Summary:
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The statistical analyses performed have clearly illustrated: (i) the quality/repeatability of
the column construction process and (ii) that all of the variables upon which the strength
of the column depends have been captured in the single normalised framework in Figure
11.
N Value Calculation
The value of N can be deduced by rearranging Equation 1. In order to determine the force
P due to the penetrometer wings only, the cone-only force contribution (Pc,avg) must be
deducted from the PIRT force (Pu, avg), where Pu, avg is the average probing force over the
depth range originally occupied by the sample. An exception to this is for samples at the
bottom of the column where a reduction in probing force occurred; for these samples the
reducing Pu is not considered in arriving at the Pu,avg value. Therefore Equation 1 can be
revised, in terms of UCS, as:
( )Aq
PPN
cor
avgcavgu ,,2 −= 7
The PIRT penetrometer plan area (A) of 0.00082 m2 was determined in this case as the
penetrometer width minus the diameter of the cone-only penetrometer multiplied by the
wing thickness, as any contribution of the cone has been removed by subtracting Pc,avg. In
a similar fashion, N values for the tests in unstabilised sleech (Nunstab) were back-
calculated using the cu values measured using the hand shear vane. It should be noted that
Nunstab values are very sensitive to the value of cu used in the calculation, given the low
undrained strengths of the unstabilised sleech and the small probing forces. As such, the
Nunstab values should be considered only as an approximate frame of reference for the
column N values.
N Value Discussion
In an initial plot of N value with depth, low N values (less than 7) were observed in the
top 200 mm of some unsurcharged columns. These values are due to low confining
stresses around the top of the column. This effect is depicted in Figure 14 where N values
for two unsurcharged columns (PI-6 and PI-10) are compared with those of two
equivalent surcharged columns (PI-9S and PI-14S); the higher N values shown in the
latter case are more consistent with those over the full column length.
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Average values of N for each column are plotted against corresponding average corrected
undrained strengths cu,cor (= qcor/2) in Figure 15; error bars illustrate the standard
deviation in both N and cu,cor for each test. Based on the foregoing argument, only the
surcharged values are included at shallow depths (for cu,cor > 150kPa) and a small number
of points were omitted which were adversely affected by cracking during the PIRT or
show discrepancies in their UCS test data. The average Nunstab values, also shown in
Figure 15, were found to lie between 15 and 22 (cu = 7-15kPa); however it is strictly only
the Nunstab for those two tests that also incorporated pre-drilling (i.e. PI-13-U and PI-14S-
U) that provide a reference for the N values for the stabilised sleech. The comment in the
N value calculation section regarding probing forces in low strength materials should also
be noted.
The data indicate a mild dependence of N on undrained strength (for cu,cor values up to
400 kPa), in keeping with that postulated by a some authors (Axelsson and Rehnman
1999; Bergman et al. 2013). The higher N values, recorded at lower strengths, imply an
additional contribution to the probing force occurred. In Figure 16, stabilised material can
be seen adhered to the penetrometer wings in test PI-5 and is believed to be evidence of
additional friction occurring between the penetrometer and the stabilised sleech,
particularly for a low strength column. Interestingly, the modelling of Liyanapathirana
and Kelly (2011) also identify increased adhesion between column and tool as a source of
higher N values.
It is recommended that the potential dependence of the value of N on column strength is
investigated further at laboratory and field scale, under realistic stress conditions, as this
may provide a possible explanation for the spread of values reported in Table 1.
Conclusions
The research presented in this paper is the first attempt (known to the authors) to carry out
reduced-scale PIRT, in pre-drilled columns, under laboratory conditions and the results
have helped to identify the factors which influence the relationship between stabilised
column strength and probing resistance. The following points summarise the research:
(i) A range of stabilised column UCS values (up to approximately 800 kPa, typical of
the range encountered in the field) were achieved by varying the binder content
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(75 kg/m3, 100 kg/m3 or 150 kg/m3 of OPC) and curing time (1 to 12 days).
Strengths were also influenced by moisture content and curing temperature.
(ii) The quality/repeatability of the columns produced was found to be consistently
high, with CoV values typically below ranges seen in the literature.
(iii) A new framework was successfully developed to unify all of the data in terms of
time, temperature and moisture content, thereby allowing the measured UCS
strengths to be corrected to correspond to those at the time of the PIRT. Statistical
analyses have confirmed that there are no other significant variables affecting the
column strengths besides those captured in this framework.
(iv) Low column confining stresses can significantly influence the N value as splitting
of the column resulted in lower than representative probing forces relative to the
strength of the column.
(v) The data obtained from this study indicate a mild inverse dependence of the N
value on the undrained strength of the stabilised columns; N was found to vary
from approximately 13 at low undrained strengths to approximately 8 at the
highest strengths considered in this study. This may form an explanation for the
range of N values reported from various previous studies.
While it is acknowledged that there is scope for further work to investigate other factors
that may influence N, such as scale, soil type and confinement, an appropriate set of
laboratory procedures and an interpretation framework has now been developed to do so.
While the influence of low confinement is not expected to be an issue in the field, the
potential dependence of the value of N on column strength is worthy of further
consideration at both laboratory-scale and field-scale, under realistic stress conditions, as
it may provide a possible explanation for the spread of values reported in Table 1.
Acknowledgements
The first author acknowledges the financial support provided by the Irish Research
Council for Science, Engineering and Technology under the EMBARK Initiative. The
funding provided by Keller Group to carry out the PIRT series is greatly appreciated, as is
guidance on the statistics provided by Dr. Jerome Sheahan and assistance from the civil
engineering technical staff at NUI Galway and visiting student Mr. Antoine Boutin.
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Finally, the authors are extremely grateful to Dr. Alan L. Bell for his technical insights
and support throughout this study.
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A Laboratory Investigation of PORT and PIRT. PhD Thesis. NUI Galway.
Timoney, M.J., McCabe, B.A. and Bell, A. 2012a. Experiences of Dry Soil Mixing in
Highly Organic Soils. Ground Improvement 165(1), pp. 3–14.
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Timoney, M.J., Quigley, P. and McCabe, B.A. 2012b. Some laboratory soil mixing trials
of Irish peats. In: Denies, N. and Huybrechts, N. eds. ISSMGE - TC 211
International Symposium & Short Courses; Recent Research, Advances & Execution
Aspects of Ground Improvement Works. Brussels, pp. 511–520.
Topolnicki, M. 2012. In-situ soil mixing. In: Bell, A. and Kirsch, K. eds. Ground
Improvement, Third Edition. 3rd ed. Taylor & Francis, pp. 329–434.
Trafikverket 2011. TK Geo 11; Trafikverkets tekniska krav för geokonstruktioner
(Trafikverket technical requirements for geotechnical structures; available in
Swedish and English).
Tremblay, H., Duchesne, J., Locat, J. and Leroueil, S. 2002. Influence of the nature of
organic compounds on fine soil stabilization with cement. Canadian Geotechnical
Journal 39, pp. 535–546.
Wiggers, A.G. and Perzon, J. 2005. The Lekkerkerk trial: Mixed-in-place dike
improvement in Netherlands. In: Rydell, B., Westberg, G., and Marrarsch, K. R. eds.
Deep Mixing - Best Practice and Recent Advances, Deep Mixing ’05. Swedish Deep
Stabilization Research Centre, Stockholm, Sweden, pp. 179–183.
1. LIST OF SYMBOLS
A Penetrometer plan area Avg Average cu,cor Stabilised column undrained shear strength corrected to the time of the column
test (= qcor/2) cu Undrained shear strength (= q/2) CCP Conventional column penetration test, alternative name for PIRT C-X Cone-only penetrometer experiment on cone-only column no. X d Depth from top of column dcol Depth from the top of column to centre of the sample DDSM Deep dry soil mixing dia. Diameter DSM Dry soil mixing E50 Secant stiffness at 50% of the failure stress h Height from basin base KPS Kalk-Pelar-Sondering (Lime-Column Probing), alternative name for PIRT LOI Loss on ignition
mb Binder content (kg/m3) N Bearing capacity factor relating undrained shear strength to penetrometer probing
resistance relationship NUI Galway National University of Ireland, Galway Pu PIRT Penetrometer push-in probing force (uncorrected) Pu,avg Average PIRT penetrometer push-in probing force over the depths occupied by a
column sample Pc Cone-only penetrometer push-in force Pc,avg Average cone-only penetrometer push-in probing force over the depths occupied
by a column sample PIRT Push-in resistance test PI-X PIRT experiment on PIRT column no. X
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PI-X-Y-Z PIRT experiment on PIRT column no. X, where Y is the curing time (days) and Z is the binder content (kg/m3)
q Unconfined compression strength qcor Stabilised column UCS corrected to the time of the column test qmeas Stabilised column UCS from UCS testing qmeasη Product of the stabilised column UCS (from UCS testing) and its WTBR Sleech Soft dark grey organic clayey silt soil local to Belfast, Northern Ireland St. Dev Standard deviation T Average ambient laboratory temperature during the curing period tadj Temperature-Adjusted Time tcol Time from mixing to UCS testing for column samples t Time from mixing to the PIRT UCS Unconfined compression strength wi Initial soil moisture content WTBR Water-to-binder ratio δ Push-in rate η Water to binder ratio ρcol Stabilised column sample density ρsoil Soil bulk density ∆(qmeasη)avg Change in qmeasη of a sample occurring in the time between PIRT of the column
and the UCS testing of the column’s samples References from Table 1. These are required in the document for the reference manager to compile the bibliography but are not needed for the final paper. (Halkola 1983) (Axelsson and Rehnman 1999) (Halkola 1999) (Rogbeck et al. 2000) (Axelsson 2001)
(Axelsson 2001) (Axelsson and Larsson 2003) (2001) (Edstam et al. 2004) (Wiggers and Perzon 2005)
(Burke et al. 2007) List of Figure captions
Figure 1: Typical PIRT penetrometer: a) guideline dimensions (Trafikverket 2011) & b) a 400 mm PIRT penetrometer
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Figure 1: a) 150 mm scaled PIRT penetrometer and cone-only penetrometer, b) wing profile
Figure 2: PIRT column construction process (all dimensions in mm)
Figure 3: PIRT column during construction with 13 mm dia. hole-form bar in place
Figure 4: PIRT Experimental Setup; a) test frame with CPT rig & b) load cell and draw wire gauge instrumentation
Figure 5: 104 mm dia. cone-only test column showing the pre-formed 13 mm dia. hole
Figure 6: Recorded PIRT probing force with height from basin base: a) 1-4 day columns, b) 6 day columns and c) 12 day columns
Figure 7: PIRT columns following testing; a) Extracted PI-13 column with few cracks & b) PI-10 during extraction with highlighted diagonal cracks in the column mid-section
Figure 8: Uncorrected PIRT column UCS with depth
Figure 10: Column UCS with column stiffness
Figure 11: Average column qmeasη (with St. Dev error bars) against temperature-adjusted time
Figure 12: Cone-only probing force with depth for pre-drilled stabilised columns and unstabilised sleech
Figure 13: Corrected column UCS with cone-only probing force
Figure 14: PIRT column N value at shallow depth: unsurcharged and surcharged
Figure 15: PIRT column N value with corrected shear strength
Figure 16: PIRT penetrometer at final location in PI-5 with evidence of stabilised sleech adhered to the penetrometer wings
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Figure 1: Typical PIRT penetrometer: a) guideline dimensions (Trafikverket 2011) & b) a 400
mm PIRT penetrometer
(b) (a)
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Figure 2: a) 150 mm scaled PIRT penetrometer and cone-only penetrometer, b) wing profile
(a) (b)
6 mm
150 mm PIRT
Penetrometer
14 mm Cone-only
Penetrometer
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Figure 3: PIRT column construction process (all dimensions in mm)
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Figure 4: PIRT column during construction with 13 mm dia. hole-form bar in place
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Figure 5: PIRT Experimental Setup; a) test frame with CPT rig & b) load cell and draw wire gauge
instrumentation
(a) (b)
Load cell
Draw-wire
gauge
CPT rig
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Figure 6: 104 mm dia. cone-only test column showing the pre-formed 13 mm dia. hole
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Figure 7: Recorded PIRT probing force with height from basin base: a) 1-4 day columns, b) 6 day columns and c) 12 day columns
0
100
200
300
400
500
600
700
800
900
1,000
1,100
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
He
igh
t fr
om
ba
sin
ba
se, h(m
m)
Uncorrected Probing Force, Pu (kN)
PI-4 (2d-150c) PI-5 (1d-100c)
PI-8 (4d-150c) PI-13 (1d-75c)
Column Base0
100
200
300
400
500
600
700
800
900
1,000
1,100
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Uncorrected Probing Force, Pu (kN)
PI-6 (6d-150c) PI-7 (6d-100c)
PI-9S (6d-150c) Column Base
0
100
200
300
400
500
600
700
800
900
1,000
1,100
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Uncorrected Probing Force, Pu (kN)
PI-10 (12-150c) PI-11 (12d-100c)
PI-12 (12-150*c) PI-14S (12d-150c)
Column Base
(a) (b) (c)
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Figure 8: PIRT columns following testing; a) Extracted PI-13 column with few cracks & b) PI-10
during extraction with highlighted diagonal cracks in the column mid-section
(b) (a)
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Figure 9: Uncorrected PIRT column UCS with depth
0
100
200
300
400
500
600
700
800
900
0 100 200 300 400 500 600 700 800 900
Co
lum
n D
ep
th, d
(mm
)
PIRT Column UCS, qmeas (kPa)
PI-4 (2d-150c)
PI-5 (1d-100c)
PI-6 (6d-150c)
PI-7 (6d-100c)
PI-8 (4d-150c)
PI-9S (6d-150c)
PI-10 (12d-150c)
PI-11 (12-100c)
PI-12 (12d-150*c)
PI-13 (1d-75c)
PI-14S (12d-150c)
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Figure 10: Column UCS with column stiffness
0
20
40
60
80
100
120
140
0 100 200 300 400 500 600 700 800 900
Co
lum
n S
tiff
ne
ss, E50
(MPa
)
Column UCS, qmeas (kPa)
PI-4 (2d-150c) PI-5 (1d-100c)
PI-6 (6d-150c) PI-7 (6d-100c)
PI-8 (4d-150c) PI-9S (6d-150c)
PI-10 (12d-150c) PI-11 (12-100c)
PI-12 (12d-150*c) PI-13 (1d-75c)
PI-14S (12d-150c)
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Figure 11: Average column qmeasη (with St. Dev error bars) against temperature-adjusted time
PI-4PI-5
PI-6
PI-7
PI-8PI-9S
PI-10PI-11
PI-12*
PI-13
PI-14S
y = 531.78ln(x) + 1010
R² = 0.7872
0
500
1,000
1,500
2,000
2,500
3,000
0 2 4 6 8 10 12 14
Av
era
ge
qmeasη
(kP
a)
Temperature Corrected Time, tadj (days)
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Figure 12: Cone-only probing force with depth for pre-drilled stabilised columns and unstabilised
sleech
0
100
200
300
400
500
600
700
800
900
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35
Co
lum
n D
ep
th, d
(mm
)
Cone-Only Probing Force, Pc
(kN)
C-1 (1d-100c) C-2 (2d-150c)
C-3 (6d-100c) C-4 (6d-150c)
C-5 (12d-150c) C-6 (1.6d-150c)
C-7 (12.7d-100c) C-14S-U
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Figure 13: Corrected column UCS with cone-only probing force
y = 0.0003x
R² = 0.92560.00
0.05
0.10
0.15
0.20
0.25
0.30
0 100 200 300 400 500 600 700 800 900
Av
era
ge
Co
ne
-On
ly P
ush
-In
Fo
rce
. Pc,avg
(kN
)
Corrected Column UCS, qcor (kPa)
C-1 (1d-100c)
C-2 (2d-150c)
C-3 *(6d-100c)
C-4 (6d-150c)
C-5 (12d-150c)
C-6 (1.6d-150c)
C-7 (12.7d-100c)
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Figure 14: PIRT column N value at shallow depth: unsurcharged and surcharged
0
20
40
60
80
100
120
140
160
180
200
2 4 6 8 10 12
Co
lum
n D
ep
th, d
(mm
)
PIRT Column N Value
PI-6 (6d-150c)
PI-9S (6d-150c)
PI-10 (12d-150c)
PI-14S (12d-150c)
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Figure 15: PIRT column N value with corrected shear strength
Sur
Sur
0
50
100
150
200
250
300
350
400
4 6 8 10 12 14 16 18 20 22 24
Co
rre
cte
d S
he
ar
Str
en
gth
, ccor
(kPa
)
N Value
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Figure 16: PIRT penetrometer at final location in PI-5 with evidence of stabilised sleech adhered to
the penetrometer wings
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Table 1: Published field observed N values for PIRT penetrometers
N
Value:
Soil
Type:
Binder Achieved
Strengths:
Test Details: Reference:
10 Clay Lime cu < 160 kPa 3 month curing period.
400 mm probe (Boman 1979)
11 * Lime
cu ≈ 255 kPa,
(190-320
kPa)
Menard pressuremeter
tests on columns
compared with 400mm
PIRT
(Holm et al. 1981)
12.5-
16.7 * * cu < 255 kPa
375mm PIRT,
comparison with column
vane (85 mm high by 132
mm dia.). 0-320 day
curing periods
(Halkola 1983)
10
(8-11) * * *
Experience based value
Stiffness related
(Axelsson &
Rehnman 1999)
10
10-15
* * *
Sweden
Finland, defined by site
specific column vane
tests
(Halkola 1999)
10 Clay &
gyttja
Cement-Lime
Cement-
GGBS
cu <185 kPa
Field column tests
400mm PIRT
28-134 day curing
periods
(Rogbeck et al.
2000)
10 Clay * Not provided PIRT in unstabilised clay (Axelsson 2001) (in
Swedish)
10-15 Very soft
clay Lime-cement cu < 600 kPa PIRT
(Axelsson 2001) (in
Swedish)
10
15
* * *
600/800mm column,
350-700mm PIRT
Columns under active
loading conditions
Conservative N, for use
with columns in direct
shear with low
confinement
(Axelsson & Larsson
2003). See Axelsson
(2001) for further
details on the tests.
10 Clay Lime-cement cu < 250 kPa. Pre-drilled columns
3-4 day curing period
(Edstam et al. 2004)
(in Swedish)
20 Clayey-
peat *
cu ≈ 100-300
kPa
N = 20 derived from field
PIRT in pre-drilled mass
stabilised clayey peat
(Wiggers & Perzon
2005)
10 Bentonite Cement *
Field calibration tests on
cement-bentonite mixture
with shear vane and
laboratory tests
(Burke et al. 2007)
* No specific data provided in the source literature
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Table 2: Published and recorded results for and for Kinnegar sleech
Parameter Crooks &
Graham (1976)
Bell (1977) McCabe &
Lehane (2006)1
Classification
Testing1,2
Depth 3.0-8.0 3.0-6.0 1.7-6.5 3.0-4.5 m
Moisture Content 60-80 54-80 60±10 48-71 %
Organic Content 3.0-5.0 2.5-5.0 11±1 2.6-5.6 %
Bulk Density 1,500-1,750 1,500-1,780 - 1,620 kg/m3
Peak Vane Strength 3-5 - 22±2 - kPa
Fall Cone Strength - 10-17 - 15 kPa
pH 7.5-8.2 7.5-8.5 - 8.0-8.5 - 1This site comprised approximately 1.0 m of fill not present at the other sites. 2Results from classification tests carried out at time of sampling.
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Table 3: Constructed PIRT column details
PIRT No. PI-4 PI-5 PI-6 PI-7 PI-8 PI-9S PI-10 PI-11 PI-12 PI-13 PI-14S
Test Time, t: 1.9 0.9 5.9 5.9 3.9 5.9 11.9 11.9 11.9 0.9 11.9 days
Binder Content: 150 100 150 100 150 150 150 100 100/ 150* 75 150 kg/m3
Column Height, h: 955 1,025 1,020 1,020 1,000 1,000 1,010 1,010 1,000 1,005 980 mm
Sleech Moisture
Content, wi: 42-48 45-47 46 44-47 48-52 48-54 53-56 48-53 47-53 48-49 48-51 %
Loss on Ignition: 2.6-2.4 2.4-4.6 2.2-3.2 2.8-2.9 2.6-3.6 2.6-3.4 2.4-3.8 3.1-4.1 2.5-4.4 2.6 -3.9 2.9-4.6 %
No. of Samples: 14 16 13 7 13 16 14 10 15 16 22 -
Average Column
UCS, qmeas: 347.5 221.6 474.4 397.4 459.4 463.2 712.5 479.4 539.8 180.5 632.7 kPa
qmeas Standard
Deviation: 60.9 24.8 59 55.5 37.7 78.7 51.8 54.3 54.4 22.5 35.4 kPa
Corrected Average
Column UCS, qcor: 327.8 199.1 469.7 393.8 425.2 458.5 708.6 476.9 535.8 155.7 625.4 kPa
Average Ambient
Temperature, T: 19.2 18.5 18.4 18.7 17.9 18.3 17.8 18.7 17.6 17.5 16.9 °C
Average Push-In
Rate: δ1 & δ2
18.3 31.2 24.9 23.5 27.4 22 34.5 19 19.8 22.1 18.2 mm/sec
21.1 21.3 26.1 29.9 25.3 30.7 26.3 28.9 19.9 20.5 21.6 mm/sec
S designated columns which cured under a surcharge loading
* Column created with 3 no. 150kg/m3 stabilised mixes and 1 no. 100kg/m
3 stabilised mix
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Table 4: PIRT and cone-only tests in unstabilised sleech
Test Ref: PI-13-U PI-14S-U PI-T-U1 PI-T-U2 C-14S-U
Penetrometer: PIRT
No. 2
PIRT
No. 2
PIRT
No. 1
PIRT
No. 2
Cone-
Only
Probing Distance: 820 823 832 858 822 mm
Sleech Moisture
Content, wi: 43-51 43-49 43-53 43-53 43-49 %
Average Shear
Strength cu (Range):
8
(6-12)
9
(7-14)
11.5
(8-16)
10.8
(8.5-13)
9
(7-14) kPa
No. Strength Tests: 12 18 12 12 18 -
cu Standard
Deviation: 1.5 1.7 2.5 1.5 1.7 kPa
Pre-drilled: Yes Yes No No Yes -
Average Push-In
Rate δ1 & δ2:
22.1 20.4 15.1 15.1 16.9 mm/sec
20.5 17.9 14.2 18.7 15.8 mm/sec
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Table 5: Constructed cone-only & sounding bar column details
PIRT Cone Test No. C-1 C-2 C-3 C-4 C-5 C-6 C-7
Test Time, t: 1 2 6 6 12 1.6 12.7 days
Binder Content, mb: 100 150 100 150 150 150 100 kg/m3
Column Length: 750 745 730 747 730 740 740 mm
Sleech Moisture Content,
wi: 45.8 46.0 49.1 49.2 47.0 47.2 45.5 %
Loss on Ignition: 4.1 4.1 4.4 4.8 4.3 4.1 3.1 %
No. Column Samples: 6 7 7 7 6 7 7 -
Average Column UCS,
qmeas: 226.6 389.8 390.9 506.9 788.2 301.0 562.3 kPa
qmeas Standard Deviation: 22.9 49.1 31.3 41.0 29.0 23.7 40.4 kPa
Corrected Average Column
UCS, qcor: 208.0 373.7 387.7 501.3 785.5 285.9 561.4 kPa
Average Ambient
Temperature, T: 16.7 15.7 16.2 16.6 17.1 16.0 14.5 °C
Average Push-In Rate: δ1
& δ2
17.8 19.6 12.9 13.1 16.5 14.2 15.7 mm/sec
19.3 22.6 15.0 15.0 19.4 19.1 18.1 mm/sec
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Table 6: Mixed Model Linear Regression output for PIRT column data
Analysis: 1 2 3 4 5 6 7 8 9
R2 0.868 0.919 0.919 0.92 0.92 0.921 0.921 0.923 0.921
Output q Ln(q) Ln(q) Ln(q) Ln(q) Ln(q) Ln(q) Ln(q) Ln(q)
Covariates: Significances:
Intercept 0.076 0 0 0 0 0.26 0 0.289 0.216
Time 0 - - - - - - - -
Ln(Time) - 0 0 0 0 0 0 0 0
Binder 0 0 0 0 0 0 0 0 0
wi - - 0.914 - 0.869 0.066 0.931 0.08 0.044
Temperature - - - 0.308 0.306 0.64 0.278 0.534 -
Organics - - - - - - 0.81 0.664 -
Density - - - - - 0.069 0.819 0.084 0.046
Depth - - - - - - 0.326 0.464 -
wi - Density - - - - - 0.066 - 0.081 0.043
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