INVESTIGATION OF PERFORMANCE AND EFFECTIVENESS OF GROUND
IMPROVEMENT USING VIBRO-DENSIFICATION
by
Balasingam Muhunthan and Rafik Itani
Department of Civil and Environmental Engineering
Washington State University
Pullman, WA 99164-4870
and
Washington State Transportation Center (TRAC)
101 Sloan Hall
Washington State University
Pullman, WA 99164-2910
A proposal prepared for
Research Office
Planning and Programming Service Center
Washington State Department of Transportation
Transportation Building, MS: 7370
Olympia, Washington 98504-7370
June 2002
i
CONTENTS
PROBLEM STATEMENT ......................................................................................1
BACKGROUND .....................................................................................................3
OBJECTIVES ..........................................................................................................7
BENEFITS ...............................................................................................................7
PRODUCTS.............................................................................................................8
IMPLEMENTATION ..............................................................................................9
WORK PLAN ..........................................................................................................9
Phase I: Literature and State of the Practice Review ..................................9
Phase II: Field Program and Laboratory Testing ........................................9
Phase III: Analysis of Data ......................................................................10
STAFFING PLAN .................................................................................................12
LEVEL OF EFFORT .............................................................................................13
FACILITIES ..........................................................................................................13
WORK SCHEDULE .............................................................................................13
REFERENCES ......................................................................................................14
BUDGET ...............................................................................................................15
ii
PROBLEM STATEMENT
Deep foundations support a vast majority of the nations bridges. Their
performance plays a significant role on maintaining the structural integrity of highway
bridges. Unfortunately, most foundation systems in many existing bridges are old and
often founded on weak soils. Most have been found to be inadequate to meet the everincreasing demand of larger static loads or seismic loads. Therefore, there is a need for
deep foundation remediation.
Geotechnical practice for retrofit of deep foundations has advanced considerably
in the past few decades. The methods of retrofit include a variety of ground improvement
techniques. Site constraints such as access, overhead clearance, plan view obstructions,
proximity to sensitive facilities or important utilities, and presence of hazardous materials
in the soils strongly influence the selection of a specific ground improvement method.
Vibro-densification and vibro replacement also referred to as installation of stone
columns are effective ground improvement techniques often used to improve foundation
performance in different situations (Ashford et al. 2000, Allen et al. 1991).
Regardless of the ground improvement method used, its effectiveness is generally
estimated empirically based on pre and post penetration tests (SPT, CPT or DMT).
However, these methods cannot accurately determine the increase in soil stiffness,
bearing capacity, and stability of the overall soil mass. Prediction of the performance of
the ground improvement is vital for the design process and adequacy of the stability of
the superstructure. Further, the database on such empirical methods is limited to certain
soils especially on quartzitic sands. Extension of these methods to soils with other
mineralogies especially weak soils is not straightforward.
1
Increase of compaction and soil density alone do not accurately provide a true
measure of the increase in soil strength/stiffness and bearing capacity of the global soil
mass and the foundation. Therefore, there is a need for a comprehensive method to
examine the effectiveness of ground improvement methods around deep foundations.
Recent advances in “state based soil mechanics” provide a fundamental
framework to determine the effectiveness of soil improvements, in terms of normalized
shear strength, bearing capacity/stiffness and other mechanical properties.
Such a
framework is useful to study the mechanics of the understanding of soil improvement
using vibro-densification methods. It will lead to advance in the state of practice with the
use of ground improvement methods in the industry and by FHWA
The proposed research will build upon the state of practice and focus on vibrodensification (vibro-flotation or vibro-replacement/stone columns) as an effective retrofit
methodology for deep foundations. These methods can increase the in situ density
radially with decreasing intensity. The increase in density and the effectiveness of
ground improvement decrease away from the “nucleus” and the global improvement of
the foundation varies from point to point and could only be an “empirical estimate” at
best. The role of increase in density in combination with confining stress should be
addressed in the degree of ground improvement. Such understanding is not available in
the state-of-practice.
The researchers propose to develop a theory to translate such density increase
around the nucleus to parameters such as normalized base resistance, Nq, and normalized
shaft friction of piles () (Klotz and Coop, 2001) and predict better the performance of
the “improved ground”. The research will focus on the understanding of the mechanics
2
of the improvement techniques and on the development of design models/charts. The
models will be validated using field data on selected sites with vibro-densification,
preferably using the vibro-flotation method.
BACKGROUND
Ground Improvement Methods using Vibro Densification
Under normal
conditions,
increase in
soil
density is
achieved
through
consolidation/compression by an applied stress, which could be static or dynamic. The
methods for deep compaction of cohesionless soils are characterized by the insertion of a
cylindrical shaped probe into the ground followed by compaction by vibration during
withdrawal. Usually a granular backfill is added so that a sand or gravel column is left
behind within a volume of sand compacted by vibration. Therefore, the vibrodensification/compaction method involves loose soil particles rearranging to denser
packing through vibration with little change in the confining or overburden stress.
Depending on the fines content of the soils, vibro-densification can be classified into two
categories as follows:
(i)
Vibro-Compaction (Vibroflotation)
Vibroflotation is a deep compaction method using vibro-technique generally used for
coarse cohesionless soils. This technique achieves good results in clean granular soils
with less than about 15 –20 % fines. The action of the vibrator, usually accompanied by
water jetting, reduces the intergranular forces between the soil particles allowing them to
move into a more open configuration. After a certain time, the optimum configuration
has been reached, the vibrator is raised a short distance and the procedure is repeated.
3
This increase in density is accompanied by backfilling the annulus around the vibrator
with sand as it is withdrawn.
(ii)
Vibro-replacement (Stone-Column)
Using similar vibrotechniques, vibro-replacement is a deep densification method
generally used for fine-grained cohesionless soils. Vibro-replacement is used in soils
with a higher fine content (>15-20%) than can be densified by vibroflotation, or even in
clay soils where the strata do not respond satisfactorily to vibrations.
Most stone
columns do not respond satisfactorily to vibrations. Most stone column installations are
made using vibro-replacement method in a manner similar to vibrocompaction.
Although, liquefiable silts and clayey silts cannot be significantly densified by vibration,
the stone columns help to confine and reinforce the soil and act as vertical drains.
The vibro-replacement techniques not only increase the relative density of the
coarser soils that are susceptible to liquefaction but also allow rapid dissipation of excess
pore water pressure induced by earthquake loading. Also the increased stiffness and shear
resistance provided by the columns themselves create additional reinforcement to the soil
mass.
As the vibro-densification methods may produce significant water with silts and
fines, it will be necessary to dispose it in a satisfactory manner that is environmentally
acceptable at each site.
There are considerable qualitative data showing that vibrodensification is an
effective means of ground improvement especially for mitigating liquefaction hazards. A
comprehensive report by Mitchell et al. (1995) gives case histories of the performance of
improved ground during earthquakes for more than 30 sites. Five of these sites from the
4
Loma Prieta earthquake and the 1994 Northridge earthquake, were treated with stone
columns.
In each case, good performance was observed following the earthquake.
Priebe (1990) describes additional case histories of improved sites that performed well in
earthquakes. Other researchers have discussed the design of stone columns ground
improvement and the extent of improvement that can be expected from their use (Refs).
Although qualitative data from past performance is valuable in confirming that
vibro-densification can be an effective means of ground improvement, more deterministic
methods are needed for cost-effective design that could predict performance. In the
current state-of-practice, such ground improvements are generally measured qualitatively
by comparing the blow counts profiles based on SPT or CPT. However, the increase in
blow counts alone is not an adequate measure of the effectiveness of the ground
improvement locally or globally. Such empirical approach can only mislead the designer
and the owner with respect to the true stability of the structure, performance and the costeffectiveness of the ground improvement. Therefore there is a great need to advance the
technology in the state-of-practice to develop a better fundamental method(s) to
determining the true improvement of the treated ground.
Recent soil mechanics research has shown that density alone does not fully
govern the stress-deformation behavior and the performance and stability of foundation
soils. The true behavior of soils and foundation structures are governed by their state by
a coupled parameter based on density and confining stress. This parameter called state
parameter is usually expressed as Rs = pa/pe (Fig.1).
5
Fig. 1: State parameter for sand
Pillai and Muhunthan (2001) have shown that the normalized shear strength under
undrained monotonic loading and cyclic loading is a function of the state parameter, Rs.
Klotz and Coop (2001) have shown that the normalized base resistance, Nq, and
normalized shaft friction of piles () in sands are functions Rs as compared with the
relative density, Dr, used in the conventional practice. Further, Jovicic and Coop (1997)
have shown the soil stiffness to be dependent on Rs. Therefore, Rs is a unifying parameter
to effectively quantify soil strength and deformation performance.
This research will focus on determining accurately the effectiveness of such
improvements and performance of the treated ground based on the state parameter. Once
the scientific relationships are established and simplified for the practicing engineers,
6
such method(s) could be utilized by the FHWA and the industry at large to effectively
quantify ground improvements and subsequent performance in the field.
OBJECTIVES
The main objectives of the proposed study are to:
1) perform a comprehensive literature review on the State of the Practice of vibrodensification (vibro-flotation/stone columns) for retrofit of deep foundations in
the United States and in other parts of the world.
2) select a suitable site and carry out a field program. Develop ground-models for in
situ densification by vibro-compaction (possibly vibroflotation). Carryout drilling,
sampling and testing; all necessary laboratory testing required to map the soil
states on e - ln p space.
3) use state parameter concept to analyze field experiment results and laboratory
tests to validate the theoretical/ analytical models of the ground before and after
vibro-densification and predict performance.
4) develop design guidelines based on the results of the research.
BENEFITS
Ground improvement using vibro-densification has been increasingly utilized to
enhance the existing foundations or new structures. However, because of the lack of
understanding of the mechanics of the improvement on the mechanical properties of soils
and reliable methodology to predict performance hampers the use of such relatively
inexpensive improvement.
The terms, “stone-columns” and “sand-columns” give a wrong impression as they
may be portrayed as load bearing elements or they themselves have improved the ground.
But the fact of the matter is, it is the vibro-process that provides most of the benefit by
way of densifying soils or changing “the soil state” to a new one, which will have a
7
significant increase in benefits such as soil capacity/stiffness and bearing capacity. Such
increase in benefits and demonstrating the mechanisms of such benefits and in terms of
predictable performance are the focus and essence of this research program. The
development of such methodologies will provide WSDOT wider flexibility of employing
the modern improvement technology for their applications.
PRODUCTS
This research will be focused to develop a leading edge “methodology” that will
be based on “state based soil mechanics”. The research will follow systematic steps from
concept, theoretical framework, field program, laboratory testing, analysis and final
results. The final results will be interpreted and developed into a set of performance
guidelines. The content and the format of the final report will be in concordance with
WSDOT guidelines. A list of the products and tentative dates for submission of draft
reports is given in Table 1.
Table 1. Research Products
Product
Tentative draft submission
Performance guidelines for vibro- September 2004
compaction
Final Report
December 2004
8
IMPLEMENTATION
Implementation of the research plan will follow predefined steps and schedule
and cover all the components in the plan. The components will include developing the
conceptual model, field program of drilling, sampling, and testing of soils before and
after finishing a column of vibro densification holes, analysis and interpretation of data
and dissemination of results.
WORK PLAN
The proposed research will be conducted in three phases. A comprehensive
review of the current literature and state of practice and the development of the
“conceptual model” will form the first phase. Phase II will include the field program and
laboratory testing and data collection. Phase III will include analysis of data,
interpretation of results, and development of design guidelines.
Phase I: Literature and State of the Practice Review
A literature review will be made to determine the state-of-the-art for the design
and analysis of vibro densification both under static and seismic loading.
It will
encompass a review of published work and a compilation of experimental results. The
literature review will include a comprehensive assessment of recent experimental and
analytical work performed in the US, Japan, and Europe. In addition to a review of
published literature, this phase will include a compilation of field tests performed by
engineering contractors.
Phase II: Field Program and Laboratory Testing
For selected sites of uniform sand deposits, tube samples will be obtained using
fixed piston (FP) and/or equivalent (Christensen) sampling methods. These samples will
9
be used to determine the grain size and other index properties of the soil profile. several
triaxial compression tests will also be performed to establish the critical state line, in the
v-ln p space (Fig. 1). In the FP holes it is possible to carry out a gamma-gamma in -situ
density profile.
A series of CPT holes should be established at the test site and
normalized tip penetration data should be profiled.
Phase III: Analysis of Data
Task 1: Methodology
Densification using vibro flotation method generally increases the density of the
soil mass in the zone of vibration without increasing the confining stress significantly.
This will have a different implication on the state of soil and the soil capacity as
compared with compaction using vibro-replacement method (stone columns). In vibroreplacement method, the soil state is changed partly by vibration and partly due to
drainage and consolidation.
For example in vibroflotation compaction, densification takes place radially from
the borehole and peters out within a short distance. Confining stresses do not change but
the density changes and so is the state parameter, Rs.
Effectiveness of compaction and increase in density should be viewed as “a
coupled property” relative to the critical state line (Fig. 1). Such a coupled-property
(state parameter) would provide a better measure of soil behavior, normalized strength/
stiffness and soil capacity and bearing capacity factors (Klotz and Coop, 2001, Pillai and
Muhunthan, 2001). This new parameter would enable the accurate determination of the
effectiveness
or
increase
in
the
soil
capacity
at
depths
due
vibro-
compaction/densification.
10
The investigators propose to explore this new concept and its application to soil
capacity/stiffness/strength of the treated ground and the effectiveness of vibrocompaction/densification.
Task 2: Analysis of field and laboratory data
This task will involve analyzing the field and laboratory data in the perspective of
the proposed concepts and theoretical models. Engineering correlations will be made
between various parameters based on the field and laboratory data. For the site
investigated correlations will be made of the changes in the densities /mechanical
properties /soil capacities in 3-D space. The densification is expected to decrease radially
and may peter out one to one to four to seven feet away from the densification hole and
varying degree in a parabolic manner. The mechanisms of such decrease should be
determined and possibly an integrated total densification effect can be determined.
Task 3: Development of design guidelines.
From the various data correlations and an integrated model of total effect of
vibro-densification at each hole will be made for the site investigated. The 3-D
densification model and the parabolic decrease of the densification will be correlated to
standard parameters such as shear modulus, bearing capacity factors, Nq and . Based on
such correlations and model, design guidelines will be developed for use by others. The
final report will include all of the analysis and recommendations for future
implementation
11
STAFFING PLAN
Dr. Balasingam Muhunthan will direct the technical aspects of this project.
Professor Rafik Itani, Director of the Transportation Center (TRAC), will oversee the
overall administration of the project and ensure a smooth progression and transmission of
knowledge. Mr. V. S. Pillai, a senior geotechnical engineer with a major utility firm in
Canada, will assist the PI with the research on a part-time basis. Dr. Muhunthan has
research expertise in the areas of critical state soil mechanics, liquefaction, soil structure,
and energy dissipation. Mr. Pillai has over 30 years of practical experience in design,
construction, and dam safety performance evaluation of many hydro-electric dams and
the application of CSSM to a wide variety of field problems including earthquake
liquefaction assessment and remediation. The remediation includes ground improvement
for high towers using vibro-densification (stone columns) in deep river alluvium. He has
authored many papers on soil mechanics aspects of dam remediation against earthquake.
His work on dams and liquefaction correction factors using critical state concepts has
received wide recognition (Rankine Lecture-1993). The K correction proposed by Pillai
and the U.S. National Committee on Earthquake Engineering Research has adopted
Byrne (1994) with minor modification at higher stress levels for use in the current state of
practice.
Professor Itani has considerable experience in structural mechanics and
earthquake engineering. He has conducted research into the response of wood framed
structures under seismic loading. One graduate research assistant will assist the
investigators in all phases of the project.
12
LEVEL OF EFFORT
Phase I
B. Muhunthan
V. S. Pillai
Graduate Student I
Phase III
Phase II
40
200
20
100
300
500
Task 1
40
250
Task 2
40
Task 3
200
20
100
250
600
TOTAL
720
360
2300
FACILITIES
All of the analysis will be done on desktop microcomputers belonging to the Civil and
Environmental Engineering department at Washington State University.
The field
programs will require the services of outside contractors. The geotechnical engineering
laboratory at Washington State University is fully equipped with state of the art
equipment including triaxial equipment (monotonic and cyclic), cyclic simple shear
equipment, and a resonant column device.
WORK SCHEDULE
Activity
2002
JulSep
OctDec
2003
JanMar
AprJun
JulSep
2004
OctDec
JanMar
AprJun
Phase I
Phase II:
Phase III: Task 1
Phase III: Task 2
Phase III: Task 3
13
REFERENCES
Allen, T. M., Harrison, T. L., Strada, J. R., and Kilian, A. P. (1991). Use of stone
columns to support I-90 cut and cover tunnel, In Deep Foundation Improvments: Design,
Construction, and Testing (Eds. Esrig, M and Bachus, R), ASTM, STP 1089, pp. 101115.
Ashford, S. A., Rollins, K. M., Case Bradford V, S., Weaver, T. J., and Baez, J. I. (2000).
Liquefaction mitigation using stone columns around deep foundations: Full-scale test
results., Transporation Research Record 1738, pp. 110-118.
Klotz, E.U. and Coop, M.R. (2001). An investigation of the effect of soil state on the
capacity of driven piles in sands, Geotechnique, Vol. 51, No.9: 733-751.
Mitchell, J. K., Christopher, K. D., Baxter, P., and Munson, T. C. (1995). Performance of
improved ground during earthquakes. In Soil Improvement for Earthquake Hazard
Mitigation, Geotechnical Special Publication No. 49, ASCE, pp. 1-36.
Pillai, V.S., and Muhunthan, B. (2002). Discussion of An investigation of the effect of
soil state on the capacity of driven piles in sands Geotechnique, in press.
Pillai, V.S., and Muhunthan, B. (2001). A review of the influence of initial static shear
(K) and confining stress (K) on failure mechanisms and earthquake liquefaction of
soils. Paper No. 1.51, Proc. 4th International Conference on Recent Advances in
Geotechnical Earthquake Engineering and Soil Dynamics, San Diego, CA.
Priebe, H. J. (1991). The prevention of Liquefaction by vibro replacement. In Proc.
International Conference on Earthquake Resistant Construction and Design (S. A.
Savidis, ed.,), A. A. Balkema Publishers, pp. 211-219.
14
BUDGET:
AGENCY:
FHWA - Retrofit P.I. NAME:
B. Muhunthan
Year 1
Year 2
Requested Date:
Year 3
8/15/02-8/14/05
TOTAL
PERSONNEL
NAME
Salary
Benefits
Muhunthan
2 Months Total
GRA
RA-II
12/12 mo
Salary
QTR
Health
1.5% Med
OTHER
Consultant Svcs.
-
7,829
2,114
8,142
2,198
15,971
4,312
-
15,070
6,437
1,187
226
15,673
6,759
1,234
235
30,743
13,196
2,421
461
5,000
4,000
27,899
9,964
37,863
27,815
10,426
38,241
9,000
55,714
20,390
76,104
Wages
Benefits
TOTAL SALARY/WAGES
TOTAL BENEFITS
TOTAL PERSONNEL
-
EQUIPMENT
Field Test
20,000
Total Equipment
20,000
-
-
20,000
20,000
3,000
200
3,200
3,000
200
3,200
3,000
200
3,200
9,000
600
9,600
1,500
1,500
1,655
3,155
1,500
1,500
4,500
1,655
6,155
MATERIALS & SUPPLIES
Supplies (Lab tests)
Publication Costs
Total Supplies
TRAVEL
Domestic
Foreign
Total Travel
OTHER COSTS
Computer Use
Phone, postage
GRA
Total Other Direct Costs
1,500
QTR
TOTAL DIRECT COSTS
INDIRECT COSTS
Exclusions
Rate 46.8%
QTR
Equipment
Total Indirect Costs
TOTAL PROJECTS COSTS
300
300
300
300
300
300
900
900
25,000
44,518
43,241
112,759
-
6,437
6,759
13,196
20,000
-
-
20,000
2,340
17,822
17,074
37,236
27,340
62,340
60,315
149,995
15