Stone columns remedy failed soil compaction

Journal of Civil Engineering Inter Disciplinaries
Volume 1 | Issue 2
Open Access
https://journalofcivilengg.com
Case Report
Stone Columns as A Remedial Solution to A Compromised Roller
Compaction Activity for A Substantial Thickness - Case Study
Adil Khan1*
, Emmanuel Spyropoulos2
, Junaid Ahmad1
, Hydar Al-Shokur1
1
Soil Improvement Contracting Company, Dammam, Kingdom of Saudi Arabia
2
Saudi Aramco, Dhahran, Kingdom of Saudi Arabia
Received Date: 22 August, 2020; Accepted Date: 30 August, 2020; Published Date: 02 September, 2020
Abstract
The current case study details stone columns construction carried out in response to a geological hazard event instigated
by an incompetent layer-by-layer roller compaction activity. The activity was carried out for substantially thick fill works of +10m
to +11m. The mentioned technique is a considerably conservative approach for fill compaction of such large thickness and the
failure occurred was quite unexpected. An extensive redesign of the location was planned which implied expensive construction
activity. A meticulous geotechnical engineer was able to identify the cause and a cost-effective solution for the area. The current
case study at its core exemplifies the importance of adequate quality control during any activity and the implications of neglecting
the same can have i.e. delays, costs and extensive remediation works. At the same time, the paper presents a case study with a
geological hazard associated with locally present collapsible soils in Riyadh, the risk they pose and the improvement undertaken
to mitigate the risk of future hazards.
*Corresponding author: Adil Khan, Soil Improvement Contracting Company, Dammam, Kingdom of Saudi Arabia.
Email: adil@sic-sa.net
Citation: Adil Khan (2020) Stone Columns as A Remedial Solution to A Compromised Roller Compaction Activity for A Substantial
Thickness - Case Study. J Civil Engg ID 1(2): 17-25.
17
Introduction
The study site is located a few kilometres north of Riyadh (KSA)
within a cement plant. It consists of a heavy load pavement, which
underwent a geo-disaster event. The design load of the pavement
was supposed to be 60 Ton; however, failure occurred under the
self-bearing weight of the constructed platform. Recently placed
fill compacted up to 95% modified proctor dry density (M.D.D.)
experienced excessive settlements following a heavy rainfall
activity. The thickness of placed fill ranged from 10m – 11m and
was compacted using the conventional and conservative roller
compaction in layers of 150mm to 250mm. The method was
chosen to ensure most optimum results and controlled vibrations,
as below the area of concern exists a concrete tunnel at a depth of
approximately 10m. The total area of the project was 611m2.
A complete re-design of the location was planned in an attempt
to prevent a similar geo-disaster following a future heavy rainfall
event. However, realizing the hazard to be of a geotechnical scope,
to ensure the mitigation of risk and understand the actual incident
occurred, the contractor of the project decided to approach a
specialist soil improvement contractor to review the case and
propose remediation works alternate to extensive earthwork and
construction activity.
Background
Reviewing the data and details of the project, it was theorized
the deformation occurred due to the presence of water sensitive
soils in the fill profile imported locally, knowing the plant was
situated in a location of North of Riyadh known for its presence
of collapsible soils [1]. Collapsible soils are bonded soils with an
open structure that where the particles are in metastable condition.
Once collapse agents are active, the bonds break, resulting in a
substantial decrease in shear strength, disintegration of the soil
skeletal structure and the subsequent deformation [2]. As per [3, 4]
collapse instigated requires two agents/conditions, i.e. wetting and
loading in combination, application of either one without the other
will not result in a collapse of the full potential.
Collapsible Soils are majorly of two types, Aeolian Soils
and Residual Soils. In the case of collapsible soils in Riyadh, the
properties exhibited are more in line with those of Aeolian soils
and they are granular soils with little SILT and medium to well
cementation. The cementation found is in varying degrees, but in
more common cases, the soil is well cemented with stiffness in the

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range of dense to very dense state, when in a dry state. The same
soil exhibits substantial collapse upon wetting and a present load.
An example of collapse potential for a site located in Riyadh is given
in (Figure 1) [5, 6].
Figure 1: Example of Collapsible Soil potential observed at a site
in Riyadh (ACES, 2019)
Another way to identify collapsible soils is through their
uncharacteristically low dry density. A preliminary direct test using
field density tests can be a convenient way for early identification
of presence of collapsible material in the geo-profile [7]. produced a
chart to evaluate collapse potential on the basis of 2 parameters, the
dry density and the liquid limit of the sample tested, as a function of
specific gravity [8]. later verified the accuracy of the chart, however
it should be noted that the method does not guarantee collapse nor
the degree of potential collapse; it is primarily used for preliminary
identification. (Figure 2)
Figure 2: Collapsible and Non-Collapsible soil identification Chart
(Holtz and Hilf, 1961)
Geotechnical solutions* to treat collapsible soil include the
following [2, 6],
• Soil Compaction.
• At Natural Moisture Content.
• Compaction with Pre-wetting.
• Soil Replacement.
• Removal, Replacement and Re-compaction with no foreign
material.
• Chemical Stabilization.
(*) The above-mentioned methods are solutions to treat
collapsible soils, alterations to Foundation and Foundation types
are not considered in the current study.
Problem Statement
Following a heavy rainfall event and limited ponding, geological
failure of excessive settlement occurred beneath the already casted
concrete pavement. Substantial settlement of more than 15cm in
most places were observed. At most, the recorded settlement was
17.5cm, indicating a non-uniform mechanism of deformation.
(Figure 3)
Figure 3: Measuring degree of soil deformation at current case
study area
Reviewing remediation measures, geotechnical analysis of the
soil was overlooked initially as the soil was supposedly compacted
to 95% modified proctor dry density. Upon further internal
discussions and desk-study, geotechnical analysis of the soil was
considered.
A concern with any activity that was to be conducted in the
current site, is the presence of an underground concrete tunnel
approximately 10m below the area of concern. (Figure 4).

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Figure 4: Section: Area of concern with approximate location of
concrete tunnel
Hypothesis
Preliminary understanding was that the geo-disaster occurred
as result of heavy rainfall and ponding (see Figure 5). Therefore,
design and earthwork plans were made to prevent ponding and
assist in drainage.
Figure 5: Site Ponding and Deformation
The initial remediation works involved the following steps with
some difficult to achieve criteria as given below.
1. Demolishing of the existing grade slab area
2. Removal and Re-compaction from Existing Level to -18.3m
from Existing Level.
a. -18.3m till -6.30m, roller compaction to ≥ 95% M.D.D.
b. -6.30m till -4.50m, roller compaction to ≥ 98% M.D.D.
c. -4.50m till -2.50m, roller compaction to = 100% M.D.D.
d. -2.50m till 0.00m, reinforced cast insitu concrete (Grade
C30/37)
e. Two base course layers placed and compacted at pooling
area with slope of 2%, to drain water to ditch. (Figure 6)
Figure 6: Proposed Remediation Sketch
Realizing the problem to be of a geotechnical nature. The
Contractor of the project approached a specialist soil improvement
contractor for their input and suggestive action in ensuring a future
heavy rainfall event does not pose a potential hazard.
Method – Ground Improvement
Reviewing data provided and pre-existing knowledge of
collapsible soils in vicinity, the geotechnical contractor suspected
something a-miss, considering the soil had been compacted to a
95% M.D.D. The potential of collapse should be negligible compared
to original collapse in-situ [1], additionally the dry density
would’ve been uncharacteristically low. Soil investigation tests
involving Cone Penetration Tests (CPTs) were carried out to better
understand the geological conditions on site. Light green colour
signifies the presence of SILT layers while dark green signifies CLAY
layers. (Figure 7 and 8)
Figure 7: Common trend in Cone Penetration Tests conducted
(CPT-02)

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.
Figure 8: Common trend in Cone Penetration Tests conducted
(CPT-04)
Of the techniques implemented in the field of compaction,
roller compaction is one of the more established and conservative
technique that exists presently. However, applicability of the
technique is greatly influenced by the material expecting
compaction. Roller compacting cohesive material requires different
roller types, very stringent control of moisture and is a considerably
difficult operation compared to select fill of granular material
(Han, 2015). In a general sense, the works conducted was greatly
incompatible with the material placed, in terms of operation and
equipment employed. Appropriate Quality Assurance and Quality
Control would prevent the occurred geo-disaster, as anomalies
would have been realized at the following stages.
• Testing of Material imported to Site or locally implemented.
• Post Quality Control tests to verify density.
Due to constraints in soil type and allowable vibrations
(underground concrete tunnel), Vibro-Replacement or Stone
Columns was selected as the most applicable technique, due
to its flexibility with soil type, deep depth of improvement and
limited vibrations compared to Dynamic/Impact techniques. More
importantly Stone Columns’ efficiency is exemplified in mitigating
collapse potential as suggested by [9] study on samples collected
from collapsible soils from Borg Al-Arab area, Alexandria, Egypt
[Han, 2015, 10].
Majority of the previous studies reviewed implemented models
or simulations to verify the efficiency of stone columns in treating
collapsible soils. Stone columns can be constructed in two ways, the
dry method or the wet method (the dry method is not applicable
in all locations, due limitations in penetration). As suggested by
[2] and internal analysis by the contractor’s technical team, by
performing the wet method of penetration a better induction of
pre-collapse was predicted via simulation and a large collapse was
not envisioned. Vibro-flotation can greatly enhance pre-collapse
mechanism; however, the operation should be performed vigilantly,
as additional settlement may occur if the layers are in efficiently
saturated [2].
Apart from the induced collapse and partial replacement,
the stone columns created were end bearing i.e. stone columns
executed were until refusal. (Figure 9)
Figure 9: Load Transfer Mechanism for end bearing reinforce-
ments (Kalantari, 2013)
The steps taken in the process of design were as follows (there
is no fixed standard for the design of stone columns in collapsible
soils, researchers may choose their own strategy depending on the
respective site conditions):
• Determining the extent of potential collapseAssessing the
integrity of soil-structure (sufficient lateral shear strength)
• If substantial loss of lateral shear strength and subsequent
column failure is not envisioned
• Design of Ordinary Stone Columns (End-Bearing), else,
• Design of Encased Stone Columns (End-Bearing). (Figure 10)
Figure 10: Applicable Stone Columns Technique Decision Making
Chart
The soil at the site was determined to possess collapsible
potential of the medium category. The decision to validate the
integrity of the soil during stone column construction is based on
both the expected soil behaviour and judgment of the engineer.
Unfortunately, Plaxis does not take into account matric suction

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which is basically one of the governing criteria in the unsaturated
to saturated behaviour of the soil. Implementing the Soil Water
Characteristic Curve, the geotechnical designer modelled the
effect of inundation in different stages, each representing a state
of saturation and respectively adjusted geotechnical properties of
the soil layers. Cavity Expansion method was used to model the
installation of a single column to effectively simulate the generation
of pore pressures in the soil layers and subsequent behaviour.
The created model was made with multiple assumptions so there
was a need for a trial. The decision was of the design engineer to
give a judgment if the generated in-house model and results were
acceptable or a geotechnical failure could be foreseen. Based
on the fact that the soil was of medium collapsible potential and
the favourable simulation results, the final decision was that
the soil would have sufficient lateral shear strength to allow the
construction of ordinary stone columns, subject to a trial column
construction. A trial with the installation of a single column was
made prior to the actual production works.
Designofthe columns were carriedout as perDIN4017-1:2006-
3 [11] for the estimation of Bearing Capacity and [12] for the
calculation of estimated settlement of end-bearing stone columns.
The design was subsequently verified using Elastic Theory [13]. A
summary of the composite geotechnical properties and the results
of preliminary analysis are given below. [Table 1]
Table 1: Composite Soil Geotechnical Properties
Layer Zsup (m) Zinf (m) Es (MPa) νs γs (kN/m3) Ms (MPa) Øc (m) Ac (m) τ (%)
1 0.0 0.3 65.0 0.33 20 97.5 0.90 0.64 12.5
2 0.3 1.3 60.0 0.33 19 90.0 0.90 0.64 12.5
3 1.3 2.3 27.6 10.33 18 41.4 0.90 0.64 12.5
4 2.3 3.3 27.6 0.33 18 41.4 0.90 0.64 12.5
5 3.3 4.3 27.6 0.33 17 41.4 0.90 0.64 12.5
6 4.3 5.6 12.8 0.33 17 19.2 0.90 0.64 12.5
7 5.6 7.0 12.8 0.33 17 19.2 0.90 0.64 12.5
8 7.0 8.3 12.8 0.33 17 19.2 0.90 0.64 12.5
9 8.3 9.7 12.8 0.33 17 19.2 0.90 0.64 12.5
10 9.7 11.0 12.8 0.33 17 19.2 0.90 0.64 12.5
In relation to Table 1, the abbreviations and symbols are
described below,
• Zsup: Top of Layer
• Zinf: Bottom of Layer
• Es: Elastic Modulus
• νs: Poission Ratio
• γs: Unit Weight
• Ms: Oedometric Modulus
• ⌀c: Diameter of Column
• Ac: Area of Column
• τ: Replacement Ratio
(Figure 11) It should be noted, adequate analysis and numerical
modellingshouldbeundertakenpriortoconstruction.Asmentioned
inSection2,ifsubstantiallossoflateralshearstrengthisenvisioned,
alternative techniques may be reviewed, such as geo-synthetic
encased stone columns. Further studies are encouraged involving
implementing Stone Columns in the treatment of collapsible soils,
as presently the topic is to a certain extent unexplored. (Figure 12)
gives an insight on the available research/study that exist presently.
Figure 11: Preliminary Design Analysis Excerpt

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Stone Column were designed on the comparably worse soil conditions as given below. [Table 2]
Figure 12: Percentage-wise available studies recorded in 2017 (Al-Obaidy, 2017)
Soil Type
Elevation M.S.L.(m)
Thickness (m)
Cone Tip Resistance
Average (MPa)
Top of Layer Bottom of Layer
Gravelly SAND to SAND 593.7 592.4 1.3 20.0
Slightly silty SAND to SAND
(with pockets of GRAVEL) 592.4 591.2 1.2 11.0
SAND to silty SAND
(with pockets of SILT)
591.2 589.4 1.8 8.0
Silty SAND to sandy SILT 589.4 587.7 1.7 5.5
SAND to slightly silty SAND 582.7 577.7 5.0 >30.0
Table 2: Soil Profile considered for Improvement
The design criteria to be achieved following construction was as given below,
• Allowable Bearing Capacity > 100kPA
• Allowable Settlement limit < 50mm
In line with the requirement and conditions, Stone Columns of the configuration mentioned in [Table 3] were designed.
A critical aspect of the operation is controlled vibrations with respect to the underground concrete tunnel. British Standards Institution
Structure Applied Pressure
(kPa)
Minimum
Diameter of
Column
(m)
Average
Length of
Column
(m)
Uniform
Square
Grid
(m x m)
Replacement
Ratio
(%)
Calculate
Settlement
(cm)
Allowable
Bearing
Capacity
(kPa)
Concrete
Pavement
100 0.90 9 – 11.5 2.26 x 2.26 12.5 42.0 294
Table 3: Stone Columns’ Design Parameters

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(1993) recommends working limits for allowable peak particle velocity as given in Table 4
Structure Allowable PPV
4Hz to 15Hz >15Hz
Reinforced or framed structures, industrial and heavy
commercial buildings
50mm/s
Un-reinforced or lightly framed structures and resi-
dential or light commercial type buildings
15mm/s at 4Hz increas-
ing to 20mm/s at 15Hz
20mm/s at 15Hz increasing to
50mm/s at 40Hz and above
Table 4: Vibration Monitoring Limits suggested by British Standards Institution (1993)
The maximum allowable limit was set at 20mm/s. Tests were
conducted during Trial Works at 3 different locations as follows.
• Test No. 01 (Inside Tunnel): Performed under the stone
column number SC-004.
• Test No. 02 (Existing Ground): Performed at a distance of
6.0m apart from stone column number SC-018.
• Test No. 03 (Inside Tunnel): Performed under the stone
column number SC-064. (Figure 13)
Figure 13: Vibration Monitoring Testing Plan
Results and Discussion
Prior to actual operations, trial works were carried out for
specific columns with vibration monitoring in parallel at the
locations specified in the previous section. Testing was carried out
using NOMIS Mini graph 7000.
Results of the monitoring were satisfactory, with the highest
recording below the 20mm/s PPV limit with the results as given
in Table 5.
Table 5: Soil Profile considered for Improvement
Description / Location
Peak Particle Velocity
Radial Transverse Vertical
Test No. 01 (Inside
Tunnel)
0.889 0.381 0.1905
Test No. 02 (Existing
Ground)
14.755 12.588 12.779
Test No. 03 (Inside
Tunnel)
0.254 0.254 0.1905
Stone Column construction was carried out using the wet-
method followed by a successful trial as discussed in Section 5.
The entire operation was a time effective solution. The complete
setup, from mobilization until completion, lasted 18 working days,
with the actual works completed in just a total of 7 working days as
shown in (Figure 14).
Figure 14: Stone Columns construction in progress
To put things in perspective, for the conservative approach,
after the demolition of the existing concrete pavement, all the
material would have to be excavated, removed and replaced. The
replaced material would have to be of higher quality i.e. AASHTO
A-1-a, to be compatible with Roller Compaction. Apart from a

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slower production, the conservative approach would also prove as
an expensive alternative.
Following the construction of the columns, quality control Plate
Load Test (PLT) was performed. The maximum applied load was 1.5
times the design load. The results of the test were satisfactory and
in line with the design criteria as illustrated in (Figure 15).
Figure 15: Post-Plate Load Test Results
With the help of a load test, the designer is able to verify the
column has mobilized and was not affected due to the presence
of collapsible soil and in-return reinforced the soil profile and
mitigated the risk of collapse potential.
Future Research
The authors suggest the involvement of geotechnical engineers
involved in Horticulture to provide their valuable input and
expand existing knowledge further regarding collapsible soils.
Horticulture engineers are quite well versed with subject matters
such as unsaturated soil mechanics, soil-water partial saturation
characteristics and behaviour, infiltration and evaporation of
groundwater, etc., all of which are closely related to understanding
collapsible soils.
Another suggestion is a subject for future research, involving
verifying the efficiency of impact-based compaction to highly
cemented collapsible soils. The presence of collapsible soils
with strength that are very high in strength (NSPT > 50), usually
mistaken for a type of rock or intermediate geomaterials, are
usually identified in Riyadh. Compaction is a cost-effective method
in treating collapsible soils; however, most the research available
presently includes soil of medium to medium-well cemented soils,
examples given below.
• Dynamic Compaction: Collection of Case Histories in Treating
Collapsible soils with strengths in the loose to medium dense
category [14].
• Rapid Impact Compaction: Treatment of Collapsible soils in
Karachaganak region, Kazakhstan [15].
The efficiency of compaction decreases significantly with the
presence of stiff geo-material (Han, 2015). The research could
include a comparative analysis of prewetting in combination to
compaction and no pre-treatment compaction, with the level of
efficiency verified for both operations.
Conclusion
The current paper emphasized the vulnerability of inadequate
poor execution and quality control to a well-designed plan,
demonstrated in the case study with the occurrence of a geo-
disaster event. Layer wise roller compaction is the more
conservative approach and in the case of the current project
with fill thicknesses greater than 10m, it was likely even a more
expensive option. Investigations conducted revealed the presence
of largely unsuitable material (very high fine content) for the type
of compaction carried out. The operation was unsuccessful due to
insufficient quality of works.
Apart from the above, the geo-disaster failure mechanism
indicated soils possessing collapsible potential present in the case
study area. The most applicable ground improvement technique
was chosen as Stone Columns (or Vibro-Replacement) due to its
flexibility with applicable material, comparably lower vibration
propagation compared to alternative techniques. Studies regarding
proven efficiency were limited, with a number of researchers
opinionated with varying degrees of success. Taking the
recommendations of most researchers in the type of operation to
be adopted (Wet-Method), the modifications to general operations
in the case of suspected collapsible material (wetting used for
penetration stage to be extended and careful vigilance to ensure
uniform wetting) and the design of the final columns (columns to
be end-bearing in design), ensured mitigating risk of suspected
collapsible soils, usually found in Riyadh (KSA).
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1. Alawaji HA. Model plate-load tests on collapsible soil. Journal of
King Saud University-Engineering Sciences. 1998;10(2):255-
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3. Murthy V. Advanced Foundation Engineering (1st ed.). New
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9. Ali NA. Performance of partially replaced collapsible soil–Field
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