ASSESSING THE EFFECTIVENESS OF RIC/RDC TECHNIQUE FOR DEEP FILL

COMPACTION APPLICATION

ABSTRACT

Dynamic compaction (DC) is a ground densification technique whereby loose soils can be effectively and
economically improved, to allow construction of various types of structures directly on the improved
ground rather than using deep foundations. Rapid Impact/Dynamic compaction (RIC/RDC) technique has
been recently developed to offer viable and cost effective alternatives for fill compaction applications.
Many meters of fill can be compacted in one lift/stage using this method, instead of compaction by several
lifts of thin layers, as per conventional method.

The use of Rapid Impact/Dynamic Compaction (RIC/RDC) technique is fast-growing in the Middle East
due to substantial time and cost saving benefits especially in large scale projects. A special trial program
has been developed in the kingdom of Saudi Arabia, in order to identify the potential of RIC/RDDC in
compacting fill material of considerable thickness in a single run, based on different options of locally
available material. This paper discusses results and recommendations obtained from the trial.

Keywords: Rapid Dynamic Compaction, Rapid Impact Compaction, Fill Compaction, CPT

INTRODUCTION

Dynamic compaction is a ground improvement technique that has been used for over 50 years on thousand
projects in the Middle East and worldwide. Dynamic compaction involves dropping heavy steel pounders
repeatedly on the ground at regularly spaced intervals. The weight and height of pounding depends on the
degree of compaction desired and is between 15 tons to 30 tons, with height up to 25m.

Dynamic Compaction (DC) method is applicable for a wide variety of soil conditions, particularly
saturated/unsaturated loose sands, dune sands, non-organic fill, reclaimed soils with variable characteristics
and sizes even with the presence of large sized boulders, landfills deposits and collapsible soils. This method
has been frequently used to densify loose soils to significant depths in order to increase its bearing capacity,
decrease post construction settlements and mitigate liquefaction risks in case of seismic events.

Rapid Impact/Dynamic Compaction (RIC/RDC) is a recently developed ground densification technique

that offers cost-effective alternatives among other conventional compaction methods (i.e roller compaction)
and deep densification techniques (i.e Dynamic Compaction, Vibro Compaction).

RIC/RDC equipment and capacity has been recently enhanced to allow using higher hammer weights and
larger compaction foots (i.e. compaction energy of 16 tons.m versus 9 tons.m for standard RDC/RIC
equipment), which resulted in deeper treatment depths and larger domain of applicability.

The main difference between DC and RIC/RDC results in the rate of application of energy and way of
propagation into the ground. With dynamic compaction, the compaction energy (1 to 2 drops per minute)
is propagated from the surface to the underlying soil by means of Rayleigh (surface) waves and transverse
(shear) waves, whereas with RIC/RDC, the rate can be as high as 40 to 80 drops per minute with more
predominance of the Raleigh waves.

The Rapid Impact/Dynamic Compaction (RIC/RDC) technique is very efficient and economical when used
as a “fill” compaction technique, in order to compact many meters of fill “on a single run” instead of
compacting it by thin layers, as per conventional method.

ADVANTAGES/BENEFITS OF RIC/RDC METHOD

There has been a growing trend in the world (mainly Middle East Area) to backfill undulating sites using
engineering fill and bring them to a more or less uniform level before general construction works. These
activities are in general included within early site preparation contracts. Within such projects filling
operations of variable thicknesses are taking place for attaining the project final grade levels. These
activities are either developed by backfilling in soil layers with subsequent compaction (on-shore areas) or
by dredging and reclamation (off-shore areas).

Within the on-shore areas, backfilling operations are following guidelines provided within various industry
practices (e.g. PIP CVS02100 Site Preparation, Excavation and Backfill Specification) which relates to the
soil placement, compaction, and inspection. More specifically:

 Fill material shall be placed in uniform loose lifts not exceeding 200mm/300mm maximum in loose
depth respectively, provided that the specified compaction is achieved for the full depth.
 Compaction of each lift shall be performed with equipment compatible with soil type. Fill material
shall follow the Relative Density specifications stipulated within various international practices (as
determined by ASTM D4253 and ASTM D4254 for cohesion less granular soils that do not exhibit
well-defined moisture density relationship).
 Quality control of each lift shall be performed through field control tests where the in-place density
and moisture content of soils are determined. Testing shall be performed at certain frequencies for
each lift following industry practices.

The traditional backfilling operations impose various disadvantages in relation to:

 The time required for completion of the project especially at places where thick fills are essential.
For example a fill of 5m thickness requires placement and compaction of 25 different layers of
200mm and associated in situ testing for each layer lift.
 The cost associated since it involves surface rolling activities which are normally charged on square
meter rates. Therefore, area of all lifts shall be accumulated implying a compaction cost which
exceeds the cost of a single compaction effort from the top surface by utilizing a different
compaction approach.
 Handover issues between the site preparation Contractors and the subsequent construction
Contractors which may lead to claim situations. Such matters have been observed within various
projects where construction Contractors questioned the density degree of the soils which have been
placed/compacted through the traditional procedures.
Rapid Impact/Dynamic Compaction (RIC/RDC) has proven to be a viable alternative to traditional roller
compaction method. Considering the size of the infrastructure projects, being developed recently in the
Middle East Area, for areas up to of over few millions square meters, it would not be possible to meet the
project schedule and cost requirements without the use of RIC/RDC for fill compaction.
The usage of the Rapid Impact/Dynamic Compaction (RIC/RDC) within the site preparation earthwork
applications is possible provided, in general, the presence of sandy granular materials.

The benefits acquired by employing the Rapid Impact/Dynamic Compaction (RIC/RDC) method within
areas require various backfilling operations are below summarized:

 The time for project completion can be significantly reduced since a single soil lift of determined
thickness (e.g. 5m) is placed and compacted on a single run.
 The cost of the project scheme by utilizing the RIC/RDC method is reduced since instead of
placement/compaction of thin lifts (200mm) a bulk placement of significant soil thickness is taking
place followed by a compaction undertaken at the formation top.
 Handover issues between the site preparation Contractors and the subsequent Contractors generated
by the density degree of the soils are possible to be removed by including the site preparation works
within the construction Contractors scope due to the limited time required for RIC/RDC.
 Very High productivity (70,000-100,000 sqm/month/rig, double shifts basis)
 Moderate mobilization and operating costs.
 Treatment can be carried out in closer proximity to existing structures and services vulnerable to
vibration damage.
 Flexibility in the backfilling material requirements (material gradation and plasticity requirements);
wide range of material can be compacted using RIC/RDC method, such as poorly graded sands,
fine sands, dredged material and dune sands,
 Ensures proper compaction at the soil surface, since the energy is more efficiently transferred
through the compaction foot which remains in contact with the ground.
 RDC technique can be used a standalone ground improvement or fill compaction technique, as
applicable to granular soils down to 5-6 meters.
 Due to less amount of equipment involved this method requires less supervision and eliminates
major safety concerns in traffic management during compaction, while also giving more flexibility
to the Contractor.

Fig. 1. Rapid Impact/Dynamic Compaction (RIC/RDC) work

The RIC/RDC device consists of an excavator-mounted hydraulic hammer striking a circular compaction
foot that rests on the ground. The tamper typically strikes the foot at the rate of 40 to 80 blows per minute.

The hammer (ranged from 7 tons to 16 tons weight) is hydraulically raised to a maximum height of 0.5-
1.2m and then allowed to free-fall, resulting in a maximum energy of about of 20 tons.m per blow.

Fig. 2. Equipment Details

The maximum number of blows for each point is generally linked to the capacity of equipment used and
site conditions. Stopping criteria is always defined as: maximum foot travel of 80-120cm and/or 50-60
blows per point per pass.

An automated record is essential for RIC/RDC procedure to allow proper monitoring and adjustment of all
execution parameters (height, number of phases/blows, foot dimeter, final penetration, grid spacing) in
accordance with the ground response during compaction and based on a defined stopping criteria. A
systematic assessment of the achieved results using Cone Penetration testing (CPT) is also necessary to
make sure that the appropriate amount of energy is transferred to the ground and project criteria area
reached.

Fig. 3: RDC’s monitoring software

TRIAL AREAS

A large scale trial area has been developed in the Kingdom of Saudi Arabia, within the framework of large
scale infrastructure project involving embankment works for several millions of cubic meters, in order to
identify the potential of Rapid Impact/Dynamic Compaction (RIC/RDC) technique in compacting fill
material of considerable thickness in one go as a cost effective alternative to traditional method by Roller
Compaction.

The intended objectives from the trial areas includes also the confirmation of the effectiveness depth and
maximum achievable criteria according to different types of filling material.
Trial areas have been divided into two (2) locations according to the type of material to be used:
1- Trial A : Dune Sands, with fines content as high as 8%,

Fig. 4: Sieve Analysis for material in trial Area

2- Trial B : Dune Sands, with fines content as high as 15%,

Fig. 5: Sieve Analysis for material in trial Area B

A pre-compaction testing campaign was carried out at site in order to check the quality of filling material,
confirm the depth of improvement and define the initial compaction parameters, including cone penetration
tests (CPT) and boreholes with soil sampling for sieve analysis tests.

Fig. 6. Work Sequence in Trial Areas A and B

COMPACTION PROCEDURE

Applying RIC/RDC shall include compaction at different phases (grids) and passes (number of compaction
times on each impact point), with use of different compaction foot and different diameters, in order to
effectively reach the deepest depth of improvement of influence and maximum compaction degree.

The compaction was performed repetitively following three grids, starting from a large grid of 6x6 m
spacing, by using small dimeter foot, then ending to a closer grid with 3x3m spacing, with larger foot, as
shown in the following sketch:

Fig. 7. RDC compaction Procedure

The main execution parameters used for the trial areas, can be summarized herein below:

 Phase 1 (primary grid): Total number of 40 to 80 blows (height ranging between 0.5 and 1m), using
1.5m dia foot and 16tons-Hammer.
 Phase 2 (secondary grid): Total number of 40 to 80 blows (height ranging between 0.5 and 1m), ,
using foots of 1.5-2m diameter and 16tons-Hammer.
 Phase 3 (tertiary grid): Total Number of 20 to 60 blows (height ranging between 0.5 and 0.7m),
using foot of 2.5m diameter and 16tons-Hammer.

ASSESSMENT OF RDC EFFECTIVENESS BASED ON CPT RESULTS

Following RIC/RDC works, a set of three (3) CPT tests per trial was carried out in order to quantify the
increase in cone tip resistance with depth and confirm the maximum achievable compaction degree. The
comparison between pre and post compaction test results shows a massive improvement of the cone
resistance and sleeve friction values varying between 100% to 500%, comparing to the pre-compaction
results, with achievement of relative density greater than 70% over 5-6m depth.

Trial A : fines content (passing sieve # 200) as Trial area B : fines content (passing sieve # 200)
high as 8% as high as 15%

Fig. 8. Comparison between Pre and Post CPT results in Trial Area A and B

PERFORMANCE LINES CRITERIA

The targeted acceptance criteria for the Rapid Impact/Dynamic Compaction (RIC/RDC) works are mainly
based on the Relative Density (Dr) specifications as described within various industry practices.
Performance Lines have been created by back calculating the required degree of Relative Density to cone
tip resistance values of Cone Penetration Tests (CPT) by using different correlations. The average of the
Performance Lines are compared to the post soil improvement Cone Penetration Tests (e.g. cone resistance
values, qc) for identifying compliance to the acceptance criteria. Post Cone Penetration Tests (CPT) are
undertaken between the compaction prints.
Specifically for this project, averaged values from performance lines (corresponding to Dr = 70%) created
using the three methods mentioned below. The average performance line has been considered the targeted
criteria for deep fill compaction works.
Baldi et al(1986)modified by
Jamiolkowski et al.(1985) Schmertmann et al. (1978)
Lune(2006)
1
= ln = −98 + 66. log .

=157, =0.55, =2.41

A correction factor for the relative

density equal to 1.2, 1.4 and 1.5 should
be used with estimated relative densities
of smaller than 40%, 40 to 60%, and
greater than 60%, respectively.

Relationship between cone resistance

(qc) and relative density (Dr) as a
functions of vertical effective stress.
= measures tip resistance and initial effective vertical stress

Trial A Trial B

Fig. 9. Comparison between Post CPT results and Performance Lines

 Area A: Relative Density Criteria Achievement

Results of the post soil treatment tests have revealed the following:
o A minimum Relative Density of 70% has been attained down to the depth of 6m by Post
CPT 1 (Print) and Post CPT 2 (Intermediate soil).
o Minimum Relative Density of 70% till depth 5.75m was achieved by Post CPT 3
(Intermediate soil).
 o Although a substantial growth in stiffness was detected between 6m to 7m depth, the
Relative Density criterion of 70% was not achieved.
o An increase of the soil stiffness is also found at the bottom of the loose soil formations
(from 7m to 8m depth).

 Area B: Relative Density Criteria Achievement

Results of the post soil treatment tests have revealed the following:
o A minimum Relative Density of 70% has been attained down to the depth of almost 5.5m.
o The consequence of compaction is also shown at depth 5.5 to 6m however, it is not
adequate to achieve a degree of 70% Relative Density.
o An increase of the soil stiffness is also found at the bottom of the loose soil formations
(from 7m to 8m depth).

OTHER SOIL STIFFNESS PARAMETERS

CPT data can be used to estimate different parameters of soils for subsequent use in estimating foundation
bearing capacity and settlement:

 Elastic Young Modulus : !" = # − × 0,015 × 10 , () * ,+,

 Peak friction angle : - = 17,60 + 11 log /#0

Where:
- 2 Cone resistance
-Qtn Normalized Cone resistance
-Ic Soil behavior type Index
-SBTn Normalized CPT soil behavior type
- 3 Effective overburden pressure
Based on the achieved results, as shown in the following graphs, it can be concluded that an average Young
Modulus of about 70 to 100 MPa and friction angle of 40 to 45degres, have been achieved as a results of
applying RDC/RIC fill for compaction.

Trial A Trial B

Fig. 10. Young modulus and friction angle results correlated from post CPT
CONCLUSION

Technical evaluation and benefits of the Rapid Impact/Dynamic Compaction (RIC/RDC) and its ability to
compact subsurface soils to various depths based on respective soil compositions has been presented. The
goal of this study concentrates to the adoption of the Rapid Impact/Dynamic Compaction (RIC/RDC) as
alternate to the traditional backfilling operations within the on-shore areas. RIC/RDC provides compaction
- from the top surface - of a single soil lift to the specified tolerances as provided within various industry
practices. The targeted acceptance criteria for the Rapid Impact/Dynamic Compaction (RIC/RDC) works
are mainly based on the Relative Density specifications.

The usage of the RIC/RDC within the site preparation earthwork applications is possible provided the
presence of sandy granular materials, and that particles finer than No. 200 sieve shall not exceed 15%. The
RIC/RDC method has successfully piloted at a special trial area in Saudi Arabia.

This method (RIC/RDC) considerably cuts down on time and schedule, by around 50%, allowing the early
start of construction activities. Furthermore, the net cost per meter for this activity reduces dramatically
when compared to the net cost per meter applicable to the traditional backfilling operations, a net reduction
of around 50%, making this alternative not only practical, but effective.

In addition, due to less amount of equipment involved this method requires less supervision and eliminates
major safety concerns in traffic management during compaction, while also giving more flexibility to the
Contractor.

Concluding, Rapid Impact/Dynamic Compaction (RIC/RDC) technique can be safely, sustainably and
efficiently used for mitigating loose granular fill materials of considerable thickness in a single lift bulk
placement as a cost and time effective alternative to the traditional method by Roller Compaction.

REFERENCES

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