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Dynamic Replacement Ground Improvement – Field

Performance Versus Design Predictions for the Alexandria City Centre Project in Egypt

P.K. Wong, Coffey Geosciences Pty Ltd, Sydney, Australia

M. Lacazedieu, Menard Soltraitement, Nozay, France

AbstractThis paper describes the use of a partially penetrating Dynamic Replacement (DR) ground

improvement solution at a site in Egypt. It presents conventional and numerical analyses

used in the design. Field monitoring results are compared to the analysis results, and material parameters back analysed for calibration of the design model for later stages of the project.

An interesting finding on the ground improvement at this site was that significantly higher

than predicted strength gain was achieved on the soft soils. This beneficial effect is postulated

to be caused by the high lateral stresses produced by the DR installation process.

Project BackgroundA very large shopping centre was constructed on a 220,000m

2 site in Alexandria, Egypt. The

initial earthworks contract required reclamation of part of a lake. Very soft, compressible

organic clay deposits existed up to 9m in places beneath the lakebed. The specificationrequired the site to be raised by 2m above the lake water level. The design criteria was for

post-construction settlement under the specified loads to not cause the site to drop below thedesign level, and for differential settlements to be within design tolerance. In particular,

proposed tiled floors required stringent differential settlement limits of 1 in 1000.

The design column load was 700kN, and columns were to be supported on shallow footings

founded at 1.5m depth below bulk earthworks level. However, as the layout of the buildings

was not finalised at the time of the earthworks design, the challenge was to come up with an

economical earthworks/ground treatment strategy to enable shallow footings to be adopted at

the site, irrespective of the building column locations.

Site ConditionsThe site is situated east of Alexandria on the Cairo Desert Road, on the edge of Lake

Maryout, in the Western Nile deltaic zone of Egypt. A significant part of the site is below the

existing lake level, with an average water depth of 1.5m.

The subsurface profile at the site is characterised by three main units as summarised below:

Unit 1 Very soft clay with organic matter 4m to 9m thick (typically 7m)

Unit 2 Stiff silty clay and clayey silt 5m to 9m thick

Unit 3 Very dense silty sand not penetrated




A typical piezocone test result is shown in Figure 1.

Figure 1 – Typical Piezocone Test Results

It was obvious that Unit 1 will control site settlement and will govern the design of ground

improvement works. Based on the laboratory testing results, the following soil properties

were adopted for Unit 1:

Moisture content, Wn 76% to 130%

Liquid Limit, WL 102% to 146%

Plastic Limit, WP 35% to 43%

Plastic Index, IP 67% to 106%

Bulk Unit Weight, γ b 14.5 kN/m3

Vertical Coefficient of Consolidation, cv 2.0 m2/year

Horizontal Coefficient of Consolidation, ch 10.0 m2/year

Modified Compression Index, Cc /(1 + e0) 0.3

Modified Recompression Index, Cr /(1 + e0) 0.03Modified Creep Coefficient, Cα /(1+ e0) 0.015

Design of Dynamic ReplacementFollowing preliminary assessment of a number of ground improvement options which

included vacuum consolidation and rigid inclusion methods, it was decided to adopt Dynamic

Replacement as the ground improvement solution for Phases 1 and 2 of the project (72,000m2

of building area and 50,000m2 of on-grade car parking area) due to its relative speed of

construction and economy.

The Dynamic Replacement (DR) technique, pioneered by Menard, is a “marriage” of

Dynamic Compaction (dropping of a heavy weight from a substantial height to cause deep

compaction of the ground) and Stone Columns (gravel columns placed in the ground using a

vibrating probe to increase the stiffness and strength of the ground). In Dynamic

Replacement, however, stone columns are introduced into the ground by a heavy weight

dropped repeatedly onto a gravel layer while the craters created by the impact of the heavy

weight are backfilled with gravel during the process as shown in Figure 2. The resulting

stone columns are significantly larger in diameter, have higher load carrying capacity, more




rapid to install, and hence more economical compared with the conventional Stone Column

ground treatment method. The disadvantage of Dynamic Replacement, however, is that there

is a limiting depth to which the DR stone columns can be installed, and at which the gravel

near the top of the columns will tend to heave rather than being pushed downwards by the

falling weight. Some previous usage of Dynamic Replacement have been reported by Juillie

and Sherwood (1983), Lee and Lo (1985), and Varaksin et al (1994).

Figure 2 – Dynamic Replacement Installation

To meet the stringent post-construction settlement criteria, it was also necessary to preload

the site. And to meet the limited time programme, prefabricated wick drains were installed to

increase the rate of consolidation even though the DR columns would already facilitate radial

drainage to occur in the soft clay.

The preload and wick drain spacing were designed using conventional one-dimensional andradial drainage theory (Schmertmann (1955), Barron (1948), and as described in Fell, Wong

& Stone (1987)). The design of wick drain spacing took into account soil disturbance and

discharge capacity of the drains using the procedures described by Hansbo et al (1981). The

design solutions are summarised in Table 1 below:

Table 1 – Summary of Design Solutions

Proposed Development Phase 1

Buildings

Phase 2

Car park

Approximate Area 70,012 m2 41,370 m

2

Settlement Limit 13.5mm under uniform live load

of 20kPa;

20mm under 700kN column load;34mm creep over 50 years;

Differential settlement 1:1000

100mm over

50 years

DR Spacing 5.5m 7m

DR Diameter 2.5m at surface 2.5m at surface

Wick Drain Spacing (square grid) 1.1 1.25




An important aspect of the design was that the DR columns would not be fully penetrating.

After placement of a 1.7m thick working platform to provide access, the maximum depth of

penetration of the DR columns was assessed to be 6.5m, thereby leaving about 2.2m

thickness of the soft clay layer (for a design soft clay thickness of 7m) untreated.

One Dimensional Settlement Analysis

Initial settlement predictions were made using conventional one-dimensional consolidationtheory. Due to the relatively large anticipated settlement, an iterative approach was need to

assess the amount of fill required to bring the site to the required design level, and with

sufficient preload to ensure that the post-construction settlement criteria would be met.

One complication is that the effective stress induced in the soft clay layer due to the filling is

also a function of the settlement, with the final submerged part of the fill exerting only the

buoyant weight. If not taken into account, this effect can produce over-estimation of

settlement. To assess the amount of fill required to bring the site to the required grade level

taking into account settlement and the buoyancy effect below the water level, a spreadsheet

was developed, from which calculated settlement is plotted against applied effective stress,

and the total fill thickness can be read off the diagonal lines that are governed by the unit

weight of the fill as shown in Figure 3.

Figure 3 – Graphical Assessment of Settlement and Required Fill Thickness

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

0 50 100 150

Applied Pressure (kPa)

E m b a n k m e n t H e i g h t ( m )

T o t a l F i l l T h i c k n e s s R e q u i r e d t o A c h i e v e A

P a r t i c u l a r H e i g h t A b o v e G

r o u n d L e v e l

Water Level for this Example

Assessed Settlement

Diagonal lines representingeffective stresses imposed by thefill depending on unit weight of fill(note “bent” lines below watertable due to buoyancy effect)




The next problem was the assessment of the likely settlement reduction that could be

achieved with the proposed DR columns. Most of the published design solutions on stone

columns are based on relatively small diameter columns that penetrate the full depth of the

soft clay layer. Balaam et al (1977) presented some numerical analysis results for partially

penetrating stone columns. They concluded that significant settlement reductions can only be

achieved if the columns are closely spaced with a diameter to spacing ratio of 5 or less, and

usually if the columns are installed to the full depth of the consolidating layer.

For this project, it was decided that the soft clay layer may be divided into two sub-layers

representing the treated and untreated zones. From compatibility of strain, the elastic

modulus of the treated zone may be expressed as follows:

Eeq = Ec {ar 2 +Es/Ec(1 – ar

2)} [Eq. 1]

where: Eeq = equivalent modulus of the treated soil mass

Ec = modulus of the DR columns

Es = modulus of the soft soil

ar = DR replacement ratio = 0.1623 in this case for 2.5m dia.

DR columns at 5.5m square grid spacing.

As settlement is inversely proportional to soil stiffness, the settlement ratio of the treated to

untreated soil layer may be expressed as Es/Eeq . The soil modulus can be assessed from the

one-dimensional consolidation parameters described earlier by considering the appropriate

stress level, and was found to be approximately 0.45MPa in this case. An initial assumption

was then made that the DR columns would be constructed to give a column modulus of

50MPa. Applying these to the above equation, Eeq was computed to be 1.8MPa and Es/Eeq

was computed to be 0.28. For the initial settlement assessment, we adopted a settlement

reduction factor of 0.3 for the upper 4.8m of treated zone and 1.0 for the remaining untreated

2.2m thickness of the soft clay layer.

Ignoring the effect of the DR columns, a consolidation settlement of 1.1m in the soft clay

layer was estimated, comprising 0.78m in the upper 4.8m and 0.32m in the lower 2.2m.

Applying the reduction factors with the DR columns introduced, the computed settlementswere 0.23m and 0.32m in the upper and lower sub-layers, giving a total estimated

consolidation settlement of 0.55m or an overall settlement reduction factor of 0.5 for the

partially penetrating DR solution proposed. Together with an estimated immediate settlement

of 0.2m, the total settlement due to placement of about 6.2m (including preload) was

predicted to be 0.75m.

Three-Dimensional Numerical Analysis

To assess the effectiveness of the DR ground improvement strategy to meet the differential

settlement criteria, three-dimensional numerical analysis was carried out using the

commercially available software package 3D FLAC (Fast Lagrangian Analysis of Continua,

ITASCA (1999)). A 3D FLAC model was set up based on a building column spacing of

7.3m as shown in Figure 4. For the soft clay layer, one-dimensional consolidation parameterswere converted to three-dimensional elastic parameters according to the appropriate

settlement relationship with stress level and preload history. An unload/reload modulus of 10

times the first time loading modulus was adopted for the soft clay. The various soil units

were subdivided and the adopted analysis parameters are shown in Table 2.




Figure 4 – 3D FLAC Model

Table 2 – Soil Parameters Adopted for 3D FLAC Analysis

UnitLayer Thickness

(m)

Drained Young’s

Modulus

E (MPa)

Unit Weight

b (kN/m3)

Compacted Fill 2.8 to 4.5 30.0 20

Working Platform 1.7 5.0 18

Unit 1A 2.40.44

4.4 (unload/reload)14.5

Unit 1B 2.40.475


Unit 1C 2.20.475


Unit 2A 2.5 16.0 18.0

Unit 2B 2.5 7.6 18.0

Unit 2C 3.0 14.0 18.0

Unit 3 - Rigid Base -

DR Columns - 50.0 17.0

Unit 2a, 2B, 2c

Unit 1a, 1B, 1c

DynamicReplacementColumns

Stage 1 Fill

Pad Footings

Stage 2 and Stage 3 Fill

5.5m

7.3m




A Poisson’s Ratio of 0.3 was adopted for all soil layers.

All vertical boundaries on this model were modelled as symmetrical boundaries to model

repetition of this column loading pattern over an infinite plan area. The DR column and

footing pattern adopted in the model was selected to enable differential settlement to be

assessed, for column loads occurring either directly over the DR columns or between DR

columns.

The results of the 3D analysis indicate the following:

• Calculated settlements from initial filling matched those calculated using one-

dimensional consolidation theory, which confirmed that the equivalent elastic

parameters were appropriately converted from the 1D parameters.

• The design post-construction settlement and differential settlement criteria were met.

• Irrespective of the footing locations relative to the DR columns, the vertical effectivestress increase within any given horizontal plane in the soft clay layer was relatively

uniform laterally, except near the edge of the DR columns, and confirmed the ability

of the DR columns and the overlying compacted fill layer to spread the load and

settlement uniformly to meet the stringent differential settlement criteria.

• At the maximum fill thickness (under maximum preload) of 6.2m including theworking platform, the vertical stress increase in the DR columns was 250kPa

compared to 50kPa in the soft clay sub-layer 1A, indicating a load carrying ratio of

5:1 for the 2.5m diameter DR column at 5.5m spacing.

Sensitivity analyses were also carried out to assess the potential situation where the soft clay

layer was greater than 7m, which would result in a greater thickness of untreated soft clay

(Sub-layer 1C) below the DR columns. The results of the sensitivity analyses indicated that

the post-construction settlement would exceed the design criteria if the soft clay layer is more

than 8.3m for the design preload. To overcome this problem, a detailed program of friction

cone tests was conducted prior to construction, and contigency plans devised for higher

preload and/or closer DR column spacing to be adopted in areas found to have greater clay

thickness than the design value. This contingency plan was to be put into place if the

settlement monitoring results indicated it to be necessary, but was subsequently found during

construction to be not required.

Dynamic Replacement ConstructionDynamic Replacement columns were driven into the soil by pounding with a 15 tonne

pounder dropped from a height of 20m. Each DR column was prepared by a pre-excavation

which was partially filled by crushed limestone. The pounding then forced the material to the

desired depth. Alternative phases of filling and pounding were performed until completion of

DR columns.

At the beginning of the works, preliminary heave tests were carried out to determine the

optimal procedure for Dynamic Replacement; this considered the following aspects:

• Number of blows and impact energy• Stages for filling DR columns

• Suitability of grid and phases of filling and pounding

Heave tests were realised by executing a DR column while recording the following

parameters: (a) diameter of print after each blow; (b) depth of print after each blow; (c) levels

Deleted: {Marc: The paragraph

below is just a sample of what I

have in mind. Please amend as

required.}




of all benchmarks during the test after two consecutive blows; (d) penetration of the pounder.

Utilising these parameters, the crater volume was computed and corrected considering the

heave volume by plotting effective penetration volume versus the number of blows. The

optimal parameters were those immediately before which the pounding resulted in heave or

volume displacement rather than in effective penetration of the material in the soil.

Two specialised 80 tonne cranes were used for the project. The average production rate

reached was about 15 DR columns per shift per rig, which corresponded to a treated arearanging from 450 m

2 per shift per rig for Phase 1 to 700 m

2 per shift per rig for Phase 2.

Instrumentation and MonitoringTo confirm the design assumptions and to enable decisions to be made on when the preloads

could be removed, detailed testing, instrumentation, and monitoring were carried out as

summarised in Table 3 below:

Table 3 – Summary of Testing and Instrumentation

Type of Testing and Instrumentation Phase 1 Buildings Phase 2 Car Park

Cone Penetration Tests Before Construction 17 7

Vane Shear Tests Before Construction 9 6Boreholes 3 3

Inclinometers 2 -

Surface Settlement Plates 33 15

Downhole Extensometers 6 sets 2 sets

Downhole Pore Pressure Gauges 10 3

Cone Penetration Tests Through DR Columns

After Construction 20 11

Vane Shear Tests After Consolidation 5 4

The testing and monitoring results are discussed in the following subsections.

Cone Penetrometer Testing

Cone penetrometer testing was carried out to better identify the soft clay thickness across the

site. The testing showed the following range of clay thickness:

Phase 1 – 4.8m to 7.4m with an average of 6.2m


As the testing was carried out after placement of the 1.7m thick working platform, the actual

thickness of the soft clay is probably 0.2m to 0.3m more than that shown by the CPT results

due to punching in of the fill material and/or immediate settlement under the weight of the

working platform. In any case, the assessed soft clay thickness was well within the critical

thickness found during the sensitivity analyses as discussed above.

Laboratory Testing

A number of laboratory consolidation tests were carried out from thin wall tube samples

recovered from the boreholes, and the test results confirmed the earlier findings and adopted

design parameters as summarised in Table 4.

Deleted: Three

Deleted: ???? linear metres of

Deleted: week/day etc




Table 4 – Results of Laboratory Consolidation Testing

Phase 1 Phase 2

Cc /(1+eo) Cr /(1+eo) Cc /(1+eo) Cr /(1+eo)

No. of Tests 10 (1 unusual low result ignored) 9

Range 0.25 to 0.376 0.017 to 0.065 0.287 to 0.432 0.018 to 0.046

Mean 0.286 0.029 0.348 0.03

Standard Deviation 0.042 0.013 0.057 0.01

These test results compared reasonably well with the originally adopted design values.

DR Stone Column Depth Constructed

The CPT carried out through the DR columns constructed indicated the following column

depths were achieved:



The results indicate that the average as-constructed depth of the DR columns was only 0.1m

less than the design depth of 6.5m.

However, an interesting finding of the CPT was that the cone resistance through the

limestone DR columns was only 8MPa to 13MPa, and no refusal of the cone occurred as may

have been expected for compacted rock fill. This indicated that there was a significant degree

of particle breakdown due to the dynamic pounding process, and the design modulus of

50MPa used for the original design may have been optimistic. This aspect is further

discussed in the back-analysis of the monitoring results presented below.

Settlement Monitoring Results

The settlement versus time plots of the Phase 1 preload were compared with the design

prediction as shown in Figure 5. It is worth noting at this point that the adopted design cv and

ch values of 2m2

/year and 10m2

/year were used with the following assumptions in the originaldesign of the wick drains for radial drainage:

• Ratio of smeared zone to in-situ permeability = 0.25

• Ratio of smeared zone to wick drain radius = 3• Equivalent wick drain radius = 0.025m• Drainage via the DR columns was ignored. This is considered reasonable given

the significantly closer wick drain spacing compared to the DR column spacing.

The following comparison between the predicted and measured settlement were observed

from Figure 5:

• Settlement were generally underestimated by about 200mm.

• The time for 90% consolidation (from the last lift) according to the design was 3.5

months which compared well with actual performance except statistically there

was a variation of about ±0.5 months. Such variation due to uncertainties in

design parameters and soil variability is an important factor to bear in mind in

design, particularly if the construction programme has to be tightly controlled.




From the results of the downhole extensometer results, it was observed that the actual

settlements within the DR column treated soil layers (i.e. Units 1A & 1B) were higher than

those predicted, and it was concluded from the post-construction CPT results that the elastic

modulus of the DR columns was over-estimated during the design. Back-analysis of the

results using Equation 1 indicated that the use of an elastic modulus of 25MPa would have

provided a closer match with the measured results. This value is consistent with the CPT

resistance measured through the DR columns.

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

0.800

0.900

1.000

0 50 100 150 200 250 300 350

Time (days)

S e t t l e m e n t ( m ) Legend:

Measured Settlement

Predicted Settlement

Initial settlement of about 0.2m (estimated by survey) due to placement of

1.7m of working platform above lakebed before instrumentation installed.

Figure 5 – Settlement Monitoring Results (Phase 1)

It would appear that had a harder gravel been used for the DR columns compared to the soft

limestone which broke down under the DR process, a stiffer reinforced soil mass could have

been produced and resulting settlement would be expected to be lower than those measured.

The back-analysis provided a useful calibration of the design, and the lower DR columnmodulus was subsequently used for Phase 2 of the project with close agreement between

predicted and measured settlement.

To allow for variability of soil permeability, lower cv and ch values of 1.5m2/year and

7.5m2/year were also adopted for subsequent stages of the project (i.e. a 25% reduction from

the original design values).

There is also another possible alternative explanation for the additional settlement observed.

That is, the compaction of the large diameter DR columns is likely to have had significant

impact on increasing the lateral effective stress of the soil as discussed below.

Undrained Shear Strength IncreaseA total of 6.3m (compared to 6.2m adopted in the original design) of fill (including preload)

was placed in Phase 1, resulting in an estimated vertical effective stress increase of about

100kPa after accounting for settlement and buoyant weight effects below the water level. For

the soft clay at this site, an expected undrained shear strength increase corresponding to this

Approximatetime of last lift




vertical effective stress increase would have been about 30kPa using a ratio of su/σv’ = 0.3 for

normally consolidated clay.

However, five field vane tests carried out after the preloading gave surprisingly high shear

strength increases ranging from about 70kPa to 85kPa near the top of the soft clay layer to

about 40kPa at depth. The surprisingly high shear strength increase was discounted during

construction and design refinement for subsequent stages as it was not a critical issue at the

time. On reflection, it is considered that such increase may be due to high lateral effectivestress increase caused by creation of the large diameter DR columns. We believe that there is

a strong possibility that the DR process caused an over-consolidation effect by increasing the

bulk effective stress of the soft soil.

From a theoretical viewpoint, we can assess the equivalent over-consolidation ratio (OCR)

using the following equation developed by Jamiolkowski et al (1985):

(su/σv’)oc = OCR m

. (su/σv’)nc [Eq. 2]

where: (su/σv’)oc = shear strength ratio for over-consolidated soil

(su/σv’)nc = shear strength ratio for normally-consolidated soil

OCR = over-consolidation ratiom = an exponent found by Jamiolkowski et al (1985) to be about 0.8

Using the limited field vane test results and adopting the ratio (su/σv’)nc = 0.3, the equivalent

OCR values for the soil clay at this site after preloading would range from about 2.1 to 3.4.

These are considered to be plausible values and are consistent with the small creep settlement

observed following primary consolidation under the preload.

It should be pointed out that the field measurements were rather limited in this respect, and

one would also expect that, if the above lateral stress increase mechanism were true, then the

stress increase would reduce with distance away from the DR columns. We consider this

aspect to be worthy of further research by both theoretical and field studies, as there are

obvious benefits with respect to higher strength increase and over-consolidation effects in

reducing post-construction settlement.

ConclusionThe building and car park for Phase 1 and Phase 2 of the project were completed successfully

following the Dynamic Replacement (DR) and preload ground treatment at the site. The

shopping centre has extensive tiled floors that are reported by the developer of the project to

be performing to expectation with no obvious signs of settlement or differential settlement.

The following conclusions are drawn from the field performance versus design predictions

for this project:

• Provided appropriate elastic modulus values are chosen at the correct stress levels,

elastic analysis may be used to assess complex soil-structure interaction problems on

soft ground.• The three-dimensional numerical analyses carried out provided the confidence that the

Dynamic Replacement design solution will meet the stringent tolerance on differential

settlement irrespective of the building column locations relative to the DR columns.




• Good match was found between field performance and the wick drain design

regarding the rate of consolidation, although it was later found to be prudent to allow

for possible soil permeability variations.

• The overall magnitude of settlement was under-predicted by about 25%. The higher

observed settlement compared to those predicted is assessed to be caused by (i) lower

stiffness of the DR column material than originally assumed, and (ii) likely higher

bulk effective stress increase due to installation of the large diameter DR columns.

• The calibration carried out using the field data in Phase 1 was an important success

factor for later stages of the project.

• Dynamic Replacement can be used effectively to strengthen soft grounds to enable

rapid construction of fill platform and preloads.

• It is possible that higher consolidation related strength increase could be achieved in

soft clays using the DR process compared with conventional preload type solutions.

This additional strength increase is thought to be attributable to a significant lateral

effective stress increase associated with the installation of the large diameter DR

columns. This process is also thought to increase the over-consolidation ratio of the

soft soil and has the beneficial effect of reducing post-construction settlement. This

aspect is considered to be worthy of further theoretical and field research.

References:

Balaam, N.P., Booker, J.R. and Poulos, H.G. (1976) Analysis of granular pile behaviour

using finite elements. Proc. Int. Conf. Finite Elements in Eng., 1-13, Adelaide, Australia

Barron, R.A. (1948) Consolidation of fine-grained soils by drain wells. Trans. ASCE, vol

113, 718-754.

Fell, R., Wong, P.K., & Stone, P (1987) Slope instability in soft ground. Proceedings of an

extension course on soil slope instability and stabilisation. (Ed.), Walker, B., and Fell, R.

(Pub.), Balkema 231-278.

Hansbo, S., Jamiolkowski, M. and Kok, L. (1981) Consolidation by vertical drains.

Institution of Civil Engineers, Symp. On Vertical Drains. Geotechnique 31, 45-66.

ITASCA (1999) 3D FLAC – Fast Lagrangian Analysis of Continua. User Manual. ITASCAGroup, USA.

Jamiolkowski, M., Ladd, C.C., Germaine, J. and Lancellotta, R. (1985) New developments in

field and laboratory testing of soils. 11th

ICSMFE.

Juillie, I. and Sherwood, D.E. (1983) Improvement of Sabkhas soil of the Arabian Gulf coast.

Proceedings of the Eighth European Conference on Soil Mechanics and Foundation

Engineering, Helsinki, (Pub.), A.A. Balkema 781 – 785.

Lee, S.L. and Lo, K.W. (1985) Ground improvement by dynamic replacement and mixing.

Third International Geotechnical Seminar on Soil Improvement Methods, Singapore 27-29

November 1985, 19 – 30.

Schmertmann, J.M. (1955) The undisturbed consolidation of clay. Trans. ASCE, Vol. 120,

1201.Varaksin, S., Liausu, P., Berger, P., & Spaulding, C. (1994) Optimisation of dynamic

consolidation and dynamic replacement pillars to limit surface deformations of man made

fills overlaying heterogeneous soft subsoil. Proceedings of seminar organised by the

Geotechnical Division of the Hong Kong Institution of Engineers on “Ground Improvement

Methods” 103 – 116.

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