4.4 Dynamic Compaction, Consolidation & Replacement
4.4.1 Introduction
Dynamic compaction is one of the oldest forms of ground improvements in existence. The Romans
reportedly utilized a variation of this technique and it was used in the United States as early as 1871.
Although the dropping of weight on the soil had probably been used for centuries, in 1970 Louis
Menard patented the technique in France and reintroduced to the profession under its present form,
into sporadic use in the United States in the early 1970's. The use of dynamic methods for the
densification of granular fill is well documented in the literature, particularly the technique of dynamic
compaction. Useful information on techniques and equipment employed and ground response to
dynamic compaction may be found in Mayne, et al. (1984), Varaksin (1981), Liausu (1984) and
Findlay and Sheevood (1986).
As the availability of suitable construction sites decreases due to developments of the urban areas,
the need to utilize the sites with poor bearing and settlement characteristics for foundation support
increase. Dynamic compaction has proven to be an economical alternative to other available methods
such as excavation and replacement, surcharging, compaction grouting and other soil improvement
techniques. One of the most common and effective uses of dynamic compaction is to compact man
made deposits of waste and rubble fills which are frequently placed in the old quarries or clay pits,
mine spoil, and landfills for both old and recent sites.
Dynamic compaction was developed and successfully used for densification of loose, saturated,
cohesionless soils and has proven to be particularly effective for liquefaction potential reduction. The
densification process is similar to that of vibro-compaction. Although used also in fine cohesive soils,
its success in these soils is uncertain and requires special attention to the generation and dissipation of
pore pressures. On occasion, other ground improvement techniques such as stone columns are used in
conjunction with dynamic compaction (Bayuk and Walker, 1994).
Although developed for densification for loose natural soils, the majority of the dynamic compaction
work in the US has recently been performed at sites of solid waste, questionable or uncontrolled old
fills and mine spoils. Another common application in recent years has been the stabilization of
collapsible soils which are stiff and dry in their natural state, but lose strength and experience
significant settlement when they become wet (Rollins and Kim, 1994).
Table 6 illustrates soil classification based on the mechanism of compaction.
The method of dynamic compaction and replacement has been reported by Liausu (1984),
and Findlay and Sherwood, (1986) leading to the following definitions:
a) Dynamic Compaction: the compaction by heavy tamping of unsaturated or highly
permeable saturated granular materials. The response to tamping is immediate.
b) Dynamic Consolidation: the improvement by heavy tamping of saturated materials in
which the response to tamping is largely time dependent. Excess pore water pressures are
established as a result of tamping and dissipate over several hours or days after each tamping
pass.
c) Dynamic Replacement: the formation by heavy tamping of large pillars of imported
granular soil within the body of the soft saturated soil to be improved. The original soil is
highly compressed and consolidated between the pillars and the excess pore pressure
generated requires several hours to dissipate. The pillars are used both for soil replacement
and drainage.
Figure 35 illustrates grouping of soils by sieve analysis for dynamic compaction (Lukas,
1992). It is worth noting that the method is suitable for pervious and semi-pervious soils with
fine contents less than 20% (Zone I, and Zone II). Zone III is relatively impervious soils such
as clays and organic deposits. When these deposits are saturated, excess pore pressures
develop quickly but because of the low permeability, long periods of time are required for
dissipation, which make dynamic compaction impractical. Dynamic compaction has been
successful in zone II deposits, but the construction procedure has to be carefully planned so as
to allow excess pore pressures to dissipate between impacts. As a guide to categorize the soil
deposits into one of the three categories described above, either field permeability tests or
laboratory grain size gradation tests could be undertaken.
4.5 Deep Dynamic Compaction
4.5.1 Basic procedure
The American Society of Civil Engineers (ASCE) ten years updates on deep dynamic compaction
(ASCE, 1997) provides a review of the state of practice of dynamic compaction and its engineering
applications.
Dynamic compaction is applied in a systematically controlled pattern of drops on a coordinate grid
layout The initial impacts are spaced at a distance dictated by the depth of the compressible layer,
depth to groundwater, and grain size distribution. Initial grid spacing generally approximates the
thickness of the compressible layer. Typically, 5 to 15 blows per grid point are applied.
Often, the proximity of groundwater or excessive crater depth limits the number of blows applied to
each grid to avoid getting the tamper stuck, or to allow for pore water pressure dissipation. Standard
practice is to curtail energy application when crater depth exceeds one and a half to two times the
height of the tamper, or when the groundwater surface rises into the crater. When this occurs,
additional passes after ground leveling, or backfilling the crater are required to complete the required
number of drops.
This first phase of treatment is designed to improve the deeper layers. Incorrect spacing and energy
level at this stage could create a dense upper layer making it difficult or impossible to treat loose
material below. The initial phase is also called the "high energy phase" because the compaction energy
is concentrated on a wider grid. Completion of the high energy phase is usually followed by a low
energy phase, called "ironing," to densify the surficial layers in the upper 1.5 m (5 ft). Here, the tamper
is only raised from 5 to 6 m (15 to 20 ft), and is dropped on an overlapping grid.
After each pass, the imprints are either backfilled with the surrounding materials or with
off-site material. In a situation where groundwater is at shallow depth, the craters should be
backfilled with imported materials to insure staying above the water table. At least 1.5 m (5
ft) is generally required between the tamping surface and groundwater.
In saturated fine-grained soils, the process is complicated by the creation of excess
porewater pressures during compaction, a phenomenon which reduces the effectiveness of the
subsequent compaction passes unless the pore pressure is adequately dissipated. For clayey
soils, dynamic compaction is generally not recommended unless the craters are backfilled
with crushed stone and repounded, creating large diameter columns of compacted stone
(dynamic replacement).
4.5.2 Type of soil improved
The single most determinative factor in the suitability of a soil type to be improved by dynamic
compaction is its ability to dissipate the excess pore pressure generated by the DC process, During
dynamic compaction, soil particles are displaced into a tighter configuration or a tighter state of
packing. If water is present in the soil voids, an instant rise in pore water pressure occurs. It is
necessary for this pressure to dissipate before additional densification can occur under repeated high
energy drops. If this isn't allowed to happen, then repeated drops from the tamper only cause
displacement of the ground, and not densification.
As with the increase in applications of dynamic compaction over the last decade, the types of
materials treated by dynamic compaction have also increased dramatically. Originally, the
predominant soil types considered for dynamic compaction included only granular natural or fill soils.
But because of the inherent economic advantages involved with the use of dynamic compaction, a
multitude of materials have been improved. They include;
Uncontrolled fills: Soil types within old fills can include the entire spectrum of natural soils, manmade
debris, byproducts, and any combination of the three. Dynamic compaction works best, however, on
dry granular fills, including sand, gravel, ash, brickbats, rock, shot rock, and steel slag.
Dynamic compaction in granular fills is similar to a Proctor compaction test, in that there is a
physical displacement of particles into a denser configuration. Dynamic compaction produces a low
frequency vibration, in the range of four to ten cycles per second, and it is this low frequency
excitation along with this input of impact energy that reduces void ratio and increases relative density
resulting in improved bearing capacity and enhanced settlement characteristics
For deposits below the water table, the vibrations cause an increase in pore pressure, and after a
sufficient number of surface impacts, cause a sufficient rise in pore pressure as to induce liquefaction,
very similar to the process occurring during earthquakes. Once this occurs, additional energy
application is ineffective until the pore pressure dissipates. Additional pounding following pore
pressure dissipation produces more low frequency vibrations that reorganize the particles into a denser
configuration.
Dynamic compaction has been used more recently to improve fine-grained fills as well. These Oils
are much more difficult to improve, and require much tighter field control and experience. Clays and
silts tend to "heave" after repeated pounding, and if additional pounding continues, can have a
detrimental effect on compaction. If heaving occurs, pounding at that point should stop, and the
number of passes should be increased with either a reduced drop height or fewer drops per point.
A more common technique that has been increasingly employed in the US over the past decade to
improve fine-grained sites is dynamic "replacement" technique. This technique consists of producing a
crater by conventional heavy tamping, and filling the craters with a "boney" or granular backfill
material to create in-situ highly compacted large diameters granular pillars, which is either floating or
driven to a firm strata.
This boney material can be gravel, shot rock, brick bats, reprocessed concrete, or anything that will
lock together under additional heavy tamping. Because of the higher permeability of this backfill, pore
wafer pressure from the underlying and adjacent fine-grained soils will dissipate more quickly. This
process is repeated until a noticeable decrease in crater formation occurs. This technique essentially
results in large diameter columns of compacted stone underlying a site or individual column locations
Dynamic compaction is often used in conjunction with other ground improvement techniques. A
retail site in New Jersey was constructed over an old fill which was underlain by organic soils is an
example. Here, a vibroflot was used to install stone columns at each interior footing location, and then
the surface deposits at each of these column locations was dynamically compacted (Bayuk and Walker
1994).
There have been several old steel mill sites that have been underlain by steel slag (Troy, NY;
Youngstown, OH; Trenton, NJ, St. Louis, East Chicago, IN) Steel slag is generally quite granular, and
responds very well to dynamic compaction.
Municipal Solid Waste (MSW): Post-construction settlements of sanitary and rubble landfills under
embankments are difficult to predict. Without site improvement, settlements can sometimes range
from 1.5- 4.6 m (5- 15 ft). The main causes of settlement in landfill deposits are due to:
•
•
•
•
Mechanical compression due to distortion, reorientation of the materials under self-weight,
Biological decomposition of organic wastes,
Physio-chemical change such as oxidation, corrosion, and combustion,
Ravelling of fines into larger voids
Dynamic compaction has been used extensively on MSW to remediate the above causes and for a
multitude of reasons. Again, experience is essential in improving MSW, in that grid spacing, weight
contact pressure, and number of passes are crucial in achieving the desired results. Highway
embankments, roadways, parking lots, and even retail structures have increasingly been constructed on
dynamically compacted MSW.
In sanitary landfills, settlements are caused either by compression of the voids or decaying of the
trash material over time, Dynamic compaction is effective in reducing the void ratio, and therefore
reducing the amount of immediate and long-term settlements after construction. It is also effective in
reducing the decaying problem, since collapse of voids means less available oxygen for decaying
process. Future settlements, however, can still be expected due to a secondary consolidation process,
and future decaying of the trash material.
A distinction must be made between older landfills and more recent landfills when considering the
long-term settlement of the landfill after improvement with dynamic compaction. Organic
decomposition has generally already taken place in older landfills, and the land5.1l usually consists of
a dark-colored soil matrix containing varying amounts of bottles, metal fragments, wood: and debris.
Decomposition generally takes more than 25 to 30 years to occur.
For deposits where biological decomposition is complete, dynamic compaction has its greatest
benefit. Densification results in higher unit weight and reduction incompressibility under load with
little long-term subsidence under load.
For recent landfills where organic decomposition is still underway, dynamic compaction increases
the unit weight of the soil mass by collapsing voids and decreasing the voids ratio. It will not however,
stop the biological decomposition, which may result in a loosening of the soil structure followed by
long term settlement.
Coal Mine Spoil: Drumheller and Shaffer (1997) discussed 19 coal spoil sites in the US that have
been improved by dynamic compaction. Dynamic compaction methodology in coal spoils varies with
the consistency of coal spoil. Some spoil are predominately shotrock with minor amount of cohesive
material, whereas some spoils have much higher concentration of fines.
Contact pressure of tamper, size of weight, and grid spacing are generally considered important
factors in coal spoils. Depending upon the nature of the proposed structure, dynamic "replacement" is
sometimes used following the DC area pass at column locations to further reduce the risk of
intolerable settlement.
Collapsible Soils: Rollins and Kim (1994), Drumheller and Shaffer (1996), and Davis (1996)
discussed eleven sites in Western States where dynamic compaction was used to improve collapsible
soils. Settlement associated with collapsible soils can lead to expensive repairs, either in highway or
structure construction.
In 1982, FHWA conducted an extensive field test program of various ground improvement
techniques to improve collapsible soils in New Mexico. The various techniques included
vibroflotation, deep mixing, pre-wetting, and dynamic compaction. Dynamic compaction was found to
be the most cost effective, and was selected to improve three separate sections of I-25 and I-40 around
Albuquerque.
Liquefiable Soils: dynamic compaction is a useful ground improvement tool to reduce liquefaction
potential as it increases the relative density as well as lateral earth pressure. Disc et al., (1994) with the
US Bureau of Reclamation, discussed three large projects where dynamic compaction was used to
remedial liquefiable soils and improve the seismic stability of several embankment dams.
20 to 30 tone tampers were employed at all of these locations. Wick drains and surface drainage
were installed in conjunction with the dynamic compactian at the sites.
4.5.3 Dynamic Consolidation
For soft cohesive soils, the densification of soil following heavy tamping is attributed to;
(a) Compressibility of saturated soil due to the presence of micro-bubbles;
(b) The gradual transition to liquefaction under repeated impacts;
(c) The rapid dissipation of pore pressures due to high permeability after soil fissuring;
(d) Thixotropic recovery.
With successive tamping, energy is imparted to the soil, a certain amount of immediate volumetric
strain is mobilized, and excess pore pressure is generated. The level of energy input into the system is
called the ‘saturation energy’ when the pore pressures equal 100% liquefaction pressure. No further
volume change can be achieved by imparting additional energy to the soil. Dissipation of pore
pressures with time leads to consolidation and gain in strength of the soil. The process of densification
under a number of passes with time delays between each pass can be visualized from Fig. 3. The
background of the analysis of the heavy tamping mechanism has been described in detail by Van Impe
(1992).
For low-velocity impacts on soft cohesive soils the impact energy is used efficiently to improve the
soil only in a thin layer. If the impact energy is very high, as in the case of common dynamic
consolidation of normally consolidated soils, the depth of influence and the final compacted density
are greater, although the energy partly dissipates due to radiated longitudinal stress waves.
Beneficial effects such as inhibiting heave and greatly increasing the impact efficiency have been
obtained recently in Belgium by the Soils company patented impact block, capable of extending the
duration of the pulse on the soil being treated, towards a more ‘plastic collision', and allowing
implementation of variable block stiffness by prestressing the anchors (Van Impe, 1992). The extent to
which heavy tamping improves the in situ soil is one of the primary parameters studied.
4.5.4 Design and Analysis Considerations:
The design of a dynamic compaction project involves determination of tamper weights, grid pattern,
drop heights, and depth of influence. The following section briefly discuss theses design parameters
(Lukas, 1986; Menard and Boris, 1975; Van Impe et al, 1997; Mayne et al, 1984).
Depth of Improvement, D, Prediction of the depth of influence and the level of improvements are the
primary concern when using the dynamic compaction method. These, however, depend on several
other factors which include: the soil conditions, energy per drop, the contact pressure of the tamper,
grid spacing, number of passes and the time lag between each pass.
Impact Energy, E, The energy induced by the dropping of the tamper is simply the weight of the
tamper times the height of the drop. These represent the main design parameters in determining the
depth of improvement when using dynamic compaction. Menard and Broise, (1975) proposed that
depth of influence was simply proportional to the square root of the energy per blow, the equations
was modified later by Lukas, 1986.
D= n (WH)0.5
Where,
D = Depth of Influence (meters)
W = Weight of Tamper (tonnes)
H = Height of Drop ( meters )
n = empirical coefficient that depends on the type of soil (0.3 to 0.6)
This equation is based on the free falling of the weights.
(1)
The factor n, is to account for the applied energy, tamper contact pressure, influence of cable drag,
presence of energy absorbing layers and ground water table. Table 7 lists the proposed values of n, for
applied energy with the range of 34 to 100 ton.ft/ft2. Figure 37 graphically shows the range for various
case histories (Leonard et al, 1980).
The grid spacing is related to the impact energy by the following equation,
E = (NWHP)/S2
(2)
Where, E is the average applied energy over the treated area, N, is the number of drops, P is
the number of passes, and S is the grid spacing. Lukas (1986) ves typical impact energy
values per unit volume of treated soils. These values can be multiplied by the thickness of the
treated soil to estimate the required applied energy at the surface. The estimated energy is
used in the above equation to determine either the number of drops for a specific spacing or
the minimum spacing for a particular number of drops. The grid spacing usually used is about
1.5 to 2.5 times the dimensions of the tamper (Munfakh, 1997).
Influence of Cable Drug, since dynamic compaction is a repetitive process, substantial
amount of time is required to manually rehook the weight after each drop. As a result, drops
are conducted with the cable attached to the tamper This however poses another problem due
to the influence of cable drag which is due to friction of the cable unwinding over the spool
drum and reduction in tamper velocity due to air resistance. Lukas (1992) bases his
observations on five separate dynamic compaction projects indicating that whenever tampers
are reused and dropped with a single cable with a free spool, the measured velocity was found
to range from 0.88 to 0.93 of the theoretical velocity. Results were, however, encouraging
since the influence of the cable drag on the energy applied was relatively constant and, hence,
does not have to be measured for each equipment or tamper weight.
Equipment Limitations, The type of equipment used will also have an effect on depth of
influence. Conventional crawler cranes with a rated capacity of 136 tons are commonly used
for dynamic compaction for drop heights up to 24 meters. The usual mass of tamper used
however is in the order of 10 to 20 tons with drop heights usually ranging from 10 to 20
meters. Higher drop energies have been achieved with tamper masses of up to 150 tons and
drop heights as high as 40 meters with special cranes or tripods.
Influence of Tamper Size, Tamper size is instrumental in controlling the contact pressure at
impact. Contact pressure which is defined as the weight of tamper divided by the contact area
is commonly used in the range from 30 to 75 kN/m2. Low contact pressure could develop a
crust of soil and prohibit any soil improvements below this crust Conversely contact pressure
higher than those indicated above could result in the temper punching into the ground upon
impact, which reduces energy efficiency.
Grid Spacing, The print spacing (the spacing between the compaction points) used in
dynamic compaction has a significant effect on the soil improvement within the grid (Chow,
et al, 1994). The first pass is designed to improve the deeper level, and is dependent on the
thickness of the compressible layer, grain size distribution and depth of the groundwater.
Initial grid spacing is usually at least equal to the thickness of the compressible layer. Other
passes that follow are aimed at densifying shallower level, which may also require lesser
applied energy. Finally, an "ironing" pass to densify the top layer of the ground is conducted
by dropping of a square or rectangular tamper over the entire surface area with relatively low
drop energy.
Time Delay between Passes, Where pore-water pressure can develop, the timing between
each pass must be such that it will allow for the pore-water pressure to dissipate Piezometers
can be installed to monitor the dissipation of pore-water pressure following each pass
Soil Conditions, As described before, dynamic compaction is best suited for densification of
deposits grouped as pervious and semi-pervious (Lukas 1986). In addition, the position of the
water table and the amounts of fine contents generally influence the effectiveness of dynamic
compaction. Presence of clay content greater than I5% fines by weight, generally renders this
method less effective (Luongo, 1992)
Degree of Improvement, Main factor controlling the degree of improvement is the applied
energy. Menard and Broise (1975) stated that the applied energy used for Dynamic
Compaction, should produce a minimum static load of 2 - to 3 t/m2 at the depth corresponding
to the water table level. Other suggestions were also made for magnitude of the applied
energy. Lukas (1992) based his recommendations on different types of soil conditions as
shown in Table 7.
Although increasing the number of drops in each pass and ice number of passes can be used
to achieve a greater depth of influence, there is a point for which the further application of
energy produces only minimal gains. This threshold is called the "saturation energy". Lukas,
(1992) presented a typical graph of depth of improvement versus the number of drops.
Depending on the soil type, increase in the number of drops will have very little gain in
improvement of the lower levels.
For cohesive soils, Charles et al. (1981) proposed an influence depth which takes into
consideration other parameters such as soil type, surface area and shape of the pounder.
D = 0.4 (EdB/Apcu)0.5
(3)
where B is the width or diameter of the pounder, Ed/Ap is the total impact energy applied
per unit area of the pounder and cu is the undrained shear strength of the soil.
An exhaustive compilation of data from over l20 sites was presented by Mayne et al.
(1984). Moreover, useful correlation for normalized crater depth, D/(WH)0.5, overall
subsidence of the ground, peak particle velocity, and maximum depth of influence, D, have
been presented. The normalized crater depth increases with the number of passes (Fig. 5a), the
trend showing a limit for this parameter. The overall ground subsidence increases with
applied energy (Fig. 54), while peak particle velocity decreases (Fig. 5c) with scaled distance,
d. The maximum depth of influence is proportional to the energy per blow.
Site Preparations, The site to be consolidated must first be prepared to support the weight of
the tamping machine (60- 200t). Occasionally 1 to 2 meters of granular materials are applied
to the ground surface, particularly in landfills and other soft ground conditions, to provide
bearing surface for the machine. It must also be safeguarded against bad weather. If sensitive
to rain water (alluvia and clays ), and removal of water rising to the surface during the
consolidation process must be facilitated by means of peripheral trenches, drains, and so on.
4.5.5 Environmental Considerations
One major concern with heavy tamping is the high impact energy that generates damaging
ground vibrations The use of such methods of ground improvement is therefore much more
limited in the urban surroundings. Ground vibrations caused by dynamic compaction not oniy
can be damaging to the nearby structures, under ground utilities and electrical or mechanical
equipment, and are also disturbing to people.
Vibrations are normally quantified in terms of the Peak Particle Velocity (PPV). Peak particle
generally used to define the damage criteria for buildings and the annoyance tolerance levels
to people. Figure 39 shows the relationship between particle velocity is and scaled energy
factor. It illustrates that well-constructed buildings can tolerate a PPV of 50 mm/sec, however,
a limit of12.5 mm/sec is often used as a maximum value for safety margin. To facilitate
comparison between various projects, the peak particle velocity has been plotted against the
inverse scaled distance as shown in Figure 39. The inverse scale distance is the square root of
compaction energy, (WH)0.5, divided by distance from the impact point.
4.5.6 Other Design Considerations
Many dynamic compaction sites have irregular subsurface conditions particularly boulder
and rubble deposit where it is difficult to interpret verification test results and assess soil
properties after dynamic compaction improvement. Based on extensive laboratory model
study of dynamic compaction of dry sand with measurements included tamper acceleration
and soil pressure during impact, a procedure was presented by Poran et al (1992) to use
Dynamic Settlement Modulus (DSM) to determine the degree of improvement during
construction. DSM was defined as the slope of the tangent of the loading portion of impact
stress-relative settlement curve where relative settlement, ej, is defined as the tamper
settlement dt, divided by its diameter, D.
DSM = pt / (dt/D)
(4)
where, pt, is the impact stress.
Also, DSM values is a function of tamper drops, As it can be observed, for most of the
tests, the rate of DSM increase was significantly reduced from the 12' drop on This relative
change in the DSM values was found to be proportional to the rate of densification (density
increase as a function of Number of drops).
Although not widely used, other design methods have been developed in order to make a
better prediction of the effect of factors affecting dynamic compaction and the required or the
desired level of ground improvement Lo, et al (1990) introduced a DC design method based on their findings that a relationship
exists between the saturation energy and enforced settlement. The authors presented a plot
(Figure 40) of enforced settlement versus total applied energy intensity, pointing out that for a
given initial soil consistency and energy per blow of pounder, a hyperbolic curve may
reasonably be fitted to the field results Thus, a saturation energy can be defined beyond which
further enforcement of settlements would be relatively insignificant. Furthermore, saturation
energy intensities were plotted against the ratio of energy per blow of tamper Ey (energy per
blow) to the initial pressuremeter limit pressure PL of each site. As shown in Figure 41, a
unique relationship may reasonably be constructed between parameters Is (saturation energy)
and EB/PL.
The authors also emphasized that in view of scarcity of well-documented tamping projects
to draw upon, the assumed trends for IS in Fig 8 have been inferred on the basis of rather
limited data. Additional research would no doubt refine the results, establishing more reliable
design curves.
Further, since IS is uniquely determined by EB/PL it should in principle, be reasonable to
expect that a collective term of these parameters relating to the operational requirements of
each tamping project would similarly determine the maximum degree of ground improvement
uniquely. This collective term or operational factor maybe expressed as
 = IS (EB/PL)
(5)
where,
IS = Saturation Energy (t-m / m‘)
EB = Energy per Blow (t-m)
PL = Pressure meter Limit Pressure (t/m‘)
The maximum degree of ground improvement maybe expressed in terms of enforced
settlement per unit thickness of treated soil deposits as follows:
= SE/Ht
(6)
where, Ht = total initial thickness of soil deposits requiring treatment According to Figure
41, the relationship would then be applicable.
= /(30+3)
(7)
Thus, given the initial ground conditions, it may in principle be possible to specify a priori,
with the characteristics of Figure 41, energy per below of pounder print spacing, and number
of below per print to achieve the required degree of improvement, and thereby rationalize
performance design Furthermore, due to the subsequent thixotropic recovery and dissipation
of pore-water pressure, long terms effects of heavy tamping might result in greater ground
improvement than inferred herein.
4.5. 7 Dynamic Replacement
Dynamic replacement is based on the same mechanism of densification as dynamic
compaction and utilizes similar plant, essentially a tall rig with a drop weight. It further
incorporates ground replacement techniques by progressively filling the crater, or "prints"
formed by the drop weight with rock fill to create columns of strong, heavily compacted
material. The columns greatly improve the average stiffness of near surface zone, as well as
better transmitting the impact forces from the drop weight to greater depths within the fill.
The basic processes involved in dynamic replacement are indicated in Figure 42.
The main design parameters for this technique could be summarized as (Barksdale and Bachus,
1983), i) shear strength h of the composite mass of vertical compacted granular material within the
existing soft soils; ii) stress ratio deemed as loads transferred to columns to that transferred to adjacent
soils, which will be a function of the soft material stiffness (cohesion), and the volume of the replaced
material per unit volume of the soft soils, iii) vertical capacity of the columns, governed by the
diameter and allowable confining pressures provided by the surrounding soils; iv) drainage properties
of the replaced soils and their influence on the consolidation and strength gain of the soft soils.
The main draw back of dynamic replacement is that the improved soil may experience 40 % of the
estimated settlement without improvement due to the fact that soils between columns, while improved
through the compaction induced lateral compression are not treated in the same manner.
As reported by Bevan, 1997 a major dynamic replacement project for a multi-story building was
carried out in Iran for ground improvement to enhance settlement and strength characteristics of soft
sandy clayey soils. Boreholes revealed either very silty collapsing sandy soils above ground water or
soft compressible sandy clay below ground water table overlying a marl bedrock at a depth between
zero to 12 meters below finished facility ground level. Low standard penetration test ($PT) blow
counts are observed just above ground table. Ground table varied between about 3 to 4 meters below
finished floor level. There appeared generally to be a stiff surface crust across the whole site with
loose to very loose horizons just above and below the ground water table. The initial proposal was to
improve the alluvial deposits by Dynamic Compaction. However, due to the high content of fines,
mainly the clayey fraction, the process becomes dynamic replacement. In this particular site, plugs of
stone, consisting of imported backfill, which is repeatedly filled into the craters by the pounder, were
driven into the soil. These plugs were reinforcing the existing soil to a certain extent as load
supporting elements. They reduced the settlement, increased the bearing capacity and accelerate the
consolidation process.
To test the effectiveness of the spread treatment from the tamping points, tests were carried out with
various number of drops and CPT's made at the center of the print and at 1,2,3,4, and 5 meter Rom the
center of the print. Figure 43 illustrates that the effect is concentrated in the layer above the water table
and at the center of the tamping print area in the plan. Disruption of the original surface crust between
0 and 2 meter can be observed, which would need careful restoration in ironing pass.