IE85299B1 - Ground engineering method - Google Patents
Ground engineering methodInfo
- Publication number
- IE85299B1 IE85299B1 IE2006/0363A IE20060363A IE85299B1 IE 85299 B1 IE85299 B1 IE 85299B1 IE 2006/0363 A IE2006/0363 A IE 2006/0363A IE 20060363 A IE20060363 A IE 20060363A IE 85299 B1 IE85299 B1 IE 85299B1
- Authority
- IE
- Ireland
- Prior art keywords
- soil
- site
- soils
- compaction
- excavated
- Prior art date
Links
- 239000002689 soil Substances 0.000 claims abstract description 136
- 238000005056 compaction Methods 0.000 claims abstract description 59
- 230000003019 stabilising Effects 0.000 claims abstract description 33
- 238000005096 rolling process Methods 0.000 claims abstract description 25
- 238000011068 load Methods 0.000 claims abstract description 22
- ODINCKMPIJJUCX-UHFFFAOYSA-N calcium monoxide Chemical compound [Ca]=O ODINCKMPIJJUCX-UHFFFAOYSA-N 0.000 claims description 14
- 150000001875 compounds Chemical class 0.000 claims description 12
- 239000000463 material Substances 0.000 claims description 12
- 238000009412 basement excavation Methods 0.000 claims description 7
- 239000000292 calcium oxide Substances 0.000 claims description 7
- 239000004927 clay Substances 0.000 claims description 6
- 229910052570 clay Inorganic materials 0.000 claims description 6
- 235000008733 Citrus aurantifolia Nutrition 0.000 claims description 4
- 235000015450 Tilia cordata Nutrition 0.000 claims description 4
- 235000011941 Tilia x europaea Nutrition 0.000 claims description 4
- 239000011230 binding agent Substances 0.000 claims description 4
- 239000010881 fly ash Substances 0.000 claims description 4
- 239000004571 lime Substances 0.000 claims description 4
- 239000000203 mixture Substances 0.000 claims description 4
- 210000001736 Capillaries Anatomy 0.000 claims description 3
- 238000010438 heat treatment Methods 0.000 claims description 3
- 230000004048 modification Effects 0.000 claims description 3
- 238000006011 modification reaction Methods 0.000 claims description 3
- 239000000843 powder Substances 0.000 claims description 3
- 239000004568 cement Substances 0.000 claims description 2
- 238000011065 in-situ storage Methods 0.000 claims description 2
- 238000002386 leaching Methods 0.000 claims description 2
- 239000002893 slag Substances 0.000 claims description 2
- 230000000576 supplementary Effects 0.000 claims description 2
- 230000002522 swelling Effects 0.000 claims description 2
- 238000000034 method Methods 0.000 abstract description 32
- 239000010410 layer Substances 0.000 description 31
- 238000010276 construction Methods 0.000 description 11
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 7
- 230000035882 stress Effects 0.000 description 5
- 230000000694 effects Effects 0.000 description 3
- 230000002195 synergetic Effects 0.000 description 3
- 238000006243 chemical reaction Methods 0.000 description 2
- 238000009472 formulation Methods 0.000 description 2
- 238000002156 mixing Methods 0.000 description 2
- 238000005381 potential energy Methods 0.000 description 2
- 239000011150 reinforced concrete Substances 0.000 description 2
- 210000000481 Breast Anatomy 0.000 description 1
- AXCZMVOFGPJBDE-UHFFFAOYSA-L Calcium hydroxide Chemical compound [OH-].[OH-].[Ca+2] AXCZMVOFGPJBDE-UHFFFAOYSA-L 0.000 description 1
- 239000000920 calcium hydroxide Substances 0.000 description 1
- 235000011116 calcium hydroxide Nutrition 0.000 description 1
- 229910001861 calcium hydroxide Inorganic materials 0.000 description 1
- 238000007906 compression Methods 0.000 description 1
- 239000004567 concrete Substances 0.000 description 1
- 238000007596 consolidation process Methods 0.000 description 1
- 238000001035 drying Methods 0.000 description 1
- 230000002708 enhancing Effects 0.000 description 1
- 238000001704 evaporation Methods 0.000 description 1
- 230000001747 exhibiting Effects 0.000 description 1
- 230000005484 gravity Effects 0.000 description 1
- 230000035876 healing Effects 0.000 description 1
- 230000036571 hydration Effects 0.000 description 1
- 238000006703 hydration reaction Methods 0.000 description 1
- 238000010348 incorporation Methods 0.000 description 1
- 238000009434 installation Methods 0.000 description 1
- 238000009114 investigational therapy Methods 0.000 description 1
- 239000002365 multiple layer Substances 0.000 description 1
- 239000002245 particle Substances 0.000 description 1
- 230000000717 retained Effects 0.000 description 1
- 239000004575 stone Substances 0.000 description 1
- 239000000126 substance Substances 0.000 description 1
- 239000011800 void material Substances 0.000 description 1
Abstract
ABSTRACT A method of modifying geotechnically unsuitable soils (21) at a site (20) so as to render the site (20) capable of bearing a load (30) comprises steps involving soil stabilisation treatment and rolling dynamic compaction (42). A portion (40) of the site (20) is excavated down to a pre-determined depth x. Both the excavated site (40) and the soil excavated there from are subjected to soils stabilisation treatments, before the treated excavated soils is backfilled in layers (43), and subjected to both standard compaction (45) and rolling dynamic compaction (42). The result is a raft (32) of modified soils capable of supporting bearing pressures associated with traditional housing foundations (33, 35). The need to drive piles (25) into deep strata (24) with load-bearing capabilities, or to use other costly or environmentally unsound techniques to address the issue of geotechnically unsuitable or contaminated soils is thus avoided. The use of modified soil (32) to backfill the same site (40) from which it was excavated results in major costs savings and reduced environmental impact due to a substantial reduction in the number of lorry movements required, as compared to conventional ‘dig and dump’ techniques.
Description
Ground Engineerinq Method
This invention relates to a ground engineering method. In particular, it
relates to a method for modifying geotechnicaily unsuitable soils at a site so as
to render the site capable of load bearing.
Traditionally, when undertaking construction work at site with
geotechnicaily unsuitable soils (i.e. soils incapable of bearing substantial loads
or stresses), a number of possible solutions exist, which can be selected to
attempt to overcome the issue. Such conventional solutions include the use of
structural fill (also known as “dig and dump"), by-passing the area of
geotechnicaily unsuitable soils by piling, pre-loading the ground, or designing
the structure to be built so as to minimise the effect on the ground.
In conventional piling techniques, piles are driven into the ground, down
to strata with load-bearing capabilities. The depth of piling required can vary
considerably in depth. as the principle behind this solution is to transfer the load
imparted by a building constructed on the site via the piles to the underlying
strata. The upper layers of weaker soil which are incapable of supporting either
the building load or the pile stresses are therefore effectively by-passed
Piling is however a time consuming, labour intensive, and costly
procedure which moreover does not necessarily alleviate all of the problems
presented by the presence of geotechnicaily unsuitable soils. In particular,
because the weaker upper layers of soil are left unchanged, they continue to
exhibit undesirable properties — most notably in the case of clay soils the
tendency to expand and contract in the presence or absence of water, and in
the case of soils having air pockets or ‘voids’ therein , the tendency to settle.
Because the geotechnicaily unsuitable soil layers are not uniform, such
expansion, contraction and settlement may occur to differing degrees across a
site. This leads to differential settlement of the site, which can ultimately lead to
subsidence in the foundations of the buildings constructed thereon, causing
cracks in masonry, and damage to drains and other subterranean infrastructure.
Where the condition of the soil at a site is marginal, alternatives to piling
have been proposed, directed to modifying the properties of the geotechnically
unsuitable and marginal soils so as to render them capable of bearing a load.
These proposed alternatives centre around two basic principles: consolidation,
which requires the removal of water from the soils; and compaction, which
requires the removal of air from the soils.
Consolidation of marginal soils, has been carried out in one form or
another for many years, and is embodied in the process of soil stabilisation.
Soil stabilisation is primarily used to dry out material which is too wet, and to
modify chemically the make-up of the soils to enhance their weight-bearing
capabilities. This process typically involves treating a hydrated clay soil with an
anhydrous material such as lime, so as to reduce the water content of the soil,
and to initiate a chemical reaction resulting in modification of the chemical
structure of the soil so as to remove its capacity to shrink or heave in the future.
Ultimately, this can enable the soil to be modified so as to exhibit granular
rather than cohesive properties.
Compaction requires the physical application of a load to the ground, so
as to force the soil particles closer together, thereby expelling air. A number of
compaction techniques are available, the type selected being determined by the
depth of influence required.
Standard compaction techniques involve mechanically driving a
cylindrical roller over an area of ground so as continuously to compact the soil
layers therebeneath.
Dynamic compaction (DC) improves the mechanical properties of the soil
by repeated application of very high intensity impacts to the surface, achieved
by dropping a weight across the surface to be compacted. The effective depth
of the treatment will be determined by the magnitude of the weight and the
height of the drop. Dynamic compaction has been found to have an influence
on soils in excess of 20m below ground level. The type of dynamic compaction
selected will depend on the geotechnical conditions to be addressed.
A variation of this technique, known as rolling dynamic compaction
(RDC) has been developed, in which a roller having a non-circular cross-section
is used. RDC rollers have been developed having generally polygonal cross-
sections with 3, 4, or 5 sides. The principle behind rolling dynamic compaction
is that as the non-circular roller is driven across the ground and caused to
rotate, one apex after another will be raised to a zenith, thus effectively gaining
potential energy, before being released by compression springs to fall under
gravity. The potential energy is thus converted into kinetic energy, which in turn
is transferred to the soil when the apex reaches the lowest point of its cycle
upon impact with the surface of the ground.
Rolling dynamic compaction is capable of delivering significantly greater
loads to the soil than dead weight or vibrating compaction, due to the height and
weight multiplier factor which is inherent in its design. As a result, whilst other
compaction methods are capable of delivering a high degree of compaction to
soil layers near the surface of the ground, rolling dynamic compaction has been
found to achieve compaction of soils in excess of 5m below the surface.
Both soil stabilisation and rolling dynamic compaction produce
satisfactory results in modifying marginal soils, though the processes work in
substantially different ways. However, in situations where the soil at a site is
geotechnically unsuitable, neither soil stabilisation nor rolling dynamic
compaction alone can modify the soil properties to such a degree that piling is
no longer required. Instead, so-called "dig and dump" techniques must be
utilised, in which the geotechnically unsuitable soil is excavated, removed from
the site, and disposed of. Dig and dump techniques are undesirable due to
their environmental impact both in terms of lorry movements and use of landfill
sites, as well as being costly, time consuming and labour intensive.
Hitherto, no single method has been developed which is capable of
modifying geotechnically unsuitable soils to such a degree that the need for
piling is disposed of altogether. Furthermore until now, the prevailing
conventional wisdom within the construction industry has held that the effects of
soil stabilisation and rolling dynamic compaction are competing processes
which cannot be utilised in tandem.
The present invention stems from the realisation that, contrary to the
beliefs of many within the construction industry, the techniques of soil
stabilisation and rolling dynamic compaction can be adapted to work together in
synergy. The present invention therefore seeks to combine these two
traditionally disparate techniques in a single ground engineering method,
whereby geotechnically unsuitable soils are modified so as to render them
capable of load bearing. The present invention further seeks substantially to
reduce or eliminate the need for piling and “dig and dump" techniques to be
carried out at sites comprising geotechnically unsuitable soils. This will result in
construction projects benefiting from significant cost savings, shorter
construction times and reduced environmental impact. The present invention
further seeks to deliver a method whereby a geotechnically unsuitable site is
modified such that the risk of differential settlement following construction on the
site is substantially reduced or eliminated.
According to the present invention, there is provided a method of
modifying geotechnically unsuitable soils at a site so as to render the site
capable of load bearing, said method comprising the following steps:
- (a) excavating a volume of soil from the site. to a pre-detemiined depth;
- (b) applying an in situ soil stabilisation treatment to the base of the
excavated site exposed in step (a);
- (c) applying a soil stabilisation treatment to the volume of soil excavated
from the site in step (a);
- (d) applying rolling dynamic compaction to the base of the excavated
site exposed in step (a);
— (e) re-introducing into the excavated site a portion of the treated soil
from step (c) so as to form a layer of pre-deterrnined thickness;
— (f) applying compaction to the layer formed in step (e);
- (g) iterating steps (e) and (f) to fonn a compound layer of pre-
determined thickness;
- (h) applying rolling dynamic compaction to the compound layer formed
in step (g); and
- (j) iterating steps (e) to (h) so as substantially to backfill the site to a
pre—determined level;
and wherein in the soil stabilisation treatment in step (b), the base is over-dried
such that the base layer then acts as a capillary to absorb any moisture
generated during step (d).
The present invention is not limited to the application of any particular
theory or hypothesis. However, it is believed that the synergistic effect
observed when combining soil stabilisation and rolling dynamic compaction
according to the method of the present invention, results from the soil
stabilisation processes breaking down the structure of the soil, thus enabling the
rolling dynamic compaction step(s) to expel air and water, thus causing
compaction and consolidation. It is also believed that soil stabilisation improves
the soil strength, so that more dynamic force can be applied during rolling
dynamic compaction, thereby increasing the compaction and consolidation
effect. In order to achieve this synergistic effect however. the soil stabilisation
process must be adapted from conventional treatments ~ that is to say, the soils
must be modified in excess of normal techniques, and in particular must have a
moisture content of less than the standard optimum moisture content.
The soil stabilisation treatments in steps (b) and (c) preferably involve
treating the soil with one or more powder or binder materials selected from
cement, lime (calcium oxide), pulverised fuel ash (PFA) and ground granulated
blast-furnace slag (GGBS). The powder or binder materials are preferably
selected so as to provide autogenous ‘healing’ properties, to enable the soil to
recover its strength after the application of RDC.
The use of lime is particularly preferred, since anhydrous calcium oxide
reacts with the water of hydration in the soil so as effectively to remove water
from the soil, according to the following exothermic reaction, in which the heat
produced also causes further drying of the soil by evaporation:
CaO + H20 -9 Ca(OH)2
In the in s/tu soil stabilisation treatment in step (b), the calcium oxide is
preferably mixed into the soil at the base of the excavated site by rotavation, to
a depth of substantially 300mm. The soil stabilisation treatment applied to the
cxcavated soil in step (c) also preferably includes a step of mixing the calcium
oxide with the excavated soil.
The soil stabilisation treatments in steps (b) and (c) are preferably
continued until the moisture content of the treated soil is reduced to
substantially 3% less than the standard optimum moisture content for the type
of soil being treated.
The rolling dynamic compaction treatment carried out in steps (d) and (h)
may be performed with any suitable construction of RDC roller, however it is
currently preferred to use a 4-sided, 8 or 12-tonne roller for this treatment.
Rolling dynamic compaction is preferably continued until effective refusal is
achieved (i.e. until no further compaction of the underlying ground is possible).
In practice, this is likely to be achieved after in the range of 20 to 40 passes of
the RDC roller for the base layer in step (d) and after 20 passes for the
compound layers in step (h).
The compaction applied in step (f) need not be rolling dynamic
compaction, since only the individual layers of backfilled material are required to
be compacted in this step, rather than compacting areas deeper below the site
surface, as in steps (d) and (h). The required zone of compaction influence is in
step (f) is therefore typically only in the range of from 300 to 600 mm.
Preferably therefore, compaction with a vibrating cylindrical roller is utilised in
step (f), and is continued until substantially 95% compaction of the layer formed
in step (e) is achieved, as measured by the Proctor dry density test.
The method of the present invention eliminates the need for costly or
environmentally unsound techniques such as piling or ‘dig and dump’ at a site
comprising geotechnically unsuitable soils, by excavating, modifying, backfilling,
compacting and consolidating the soils. The resultant backfilled site then
comprises a system of re-engineered soils, which, in addition to exhibiting load-
bearing capabilities sufficient to allow construction on the site, also effectively
acts as a single mass due to the extensive consolidation and compaction. This
effectively eliminates the risk of differential settlement, and hence subsidence,
at the site.
The reengineering of the site so as to produce a consolidated and
compacted mass makes the method of the present invention particularly
applicable to sites comprising expansive clay soils. in this situation, the soil
stabilisation steps (b) and (c) preferably include soil modification treatment so
as to prevent the subsequent swelling and contraction of the clay soils in the
presence of water.
in a variation of the method of the present invention, an additional step is
included, between steps (d) and (e), whereby there is introduced into the
excavated site an additional layer having pipes for connection to a geothermal
heating system.
in order that the present invention may be more fully understood, a
preferred embodiment thereof will now be discussed in detail, though only by
way of example, with reference to the following drawings in which:
Figure 1 is a schematic, cross—sectional representation of a site
comprising geotechnically unsuitable soils, having a building constructed
thereon using a conventional piling technique;
Figure 2 is a schematic, cross—sectional representation of an equivalent
site comprising geotechnically unsuitable soils, but which has been modified
according to the method of the present invention; and
Figures 3 to 11 form an illustrative sequence depicting a method for
modifying geotechnically unsuitable soils according to the present invention.
Referring first to Figure 1, there is shown a site, generally indicated 20 in
which the upper strata 21, immediately beneath the surface 22 of the ground,
comprises geotechnically unsuitable or weak soils, down to a depth x of around
3m. Beneath the upper strata 21 is a natural ground strata 23, which although
potentially geotechnically superior to the upper strata 21 is similarly incapable of
supporting the stresses incurred in the piling technique illustrated in Figure 1.
Underlying the natural ground strata 23 is a load-bearing strata 24 to which any
load resultant from construction on the site 20 must be transferred in order to
achieve stability.
As can be seen from Figure 1, in conventional piling techniques, piles 25
are driven down through the upper strata of geotechnically unsuitable soils 21,
through the intermediary natural ground strata 23 and into the load-bearing
strata 24. At the upper ends of the piles 25 are formed reinforced concrete
beams 26 upon which is constructed a suspended floor 27 having an integral
void 28 therewithin. A building 30 is then constructed upon the suspended floor
The reinforced concrete beams 26 and piles 25 serve to transfer the load
imparted by the building 30 to the load-bearing strata 24, ettectively by—passing
the upper strata of geotechnically unsuitable soils 21, and the intermediary
natural ground strata 23. However, since drainage and paving 31 is located in
the zone of geotechnically unsuitable soils 21, it must be formed with a flexible
construction so as to account for any differential settlement, expansion or
contraction of the upper strata 21.
Referring now to Figure 2, there is shown an essentially identical basic
site 20, comprising the same three strata as in Figure 1, namely: an upper
strata of geotechnically unsuitable soils 21, an intermediary natural ground
strata 23 and a deep underlying load bearing strata 24. However, in Figure 2,
the site 20 has been re—engineered according to the method of the present
invention, so as to eliminate the need for piling.
As can be seen in Figure 2, a section of the upper strata 21 has been
excavated, modified, backfilled, consolidated and compacted to form a ‘raft’ 32
of re-engineered soils capable oi supporting the required bearing pressure
attributable to traditional foundations 33, such as would be used at a site
comprising geotechnically sound soils. An important factor in the example
shown in Figure 2 is that the intermediary natural ground strata 23 is capable of
supporting the required bearing pressure attributable to the raft 32 of re-
engineered soils, whereas the same strata 23 is incapable of supporting the pile
stresses resultant from conventional piling techniques as illustrated in Figure 1.
This is because the method of the present invention enables the load imparted
by the building 30 to be dissipated over a large area of the site 20, rather than
concentrated at specific points, as with the conventional piling technique
illustrated in Figure 1.
The method of the present invention eliminates the need for re-inforced
concrete beams 26 and piles 25 and instead allows the building 30 to be
constructed on traditional foundations 33 incorporating a stone slab 34 and strip
footings 35 set into the raft 32 of re—engineered soils. Since the drainage and
paving 31 are now located within the raft 32 rather than in the surrounding zone
of geotechnically unsuitable soils 21, they can now be formed with a fixed,
rather than a flexible, construction. The raft 32 of re-engineered soils will exhibit
uniform properties of settlement, expansion and contraction, thus effectively
eliminating the risk of subsidence.
An example of the method of the present invention will now be described
with reference to Figures 3 to 11. Referring first to Figure 3, this shows the site
in its original condition, before being re-engineered according to the method
of the present invention. The site 20 comprises an upper strata of
geotechnically unsuitable soils 21 immediately beneath the surface 22, an
intermediary strata of natural ground 23 incapable of bearing normal stresses
associated with conventional piling techniques, and a deep strata 24 having
load-bearing capabilities.
The method of the present invention begins with the preliminary steps of:
(i) investigating the site to determine the characteristics of the soils in the
various strata 21, 23, 24; and (ii) determining the building load and design
requirements. From the data acquired in these steps a further preliminary step
(iii) is carried out, in which the parameters of the ensuing process are
determined. These parameters included the required excavation depth x, the
required composition of the soil stabilisation treatment materials, the required
individual backfill layer thickness, the required compound layer thickness, and
the required backfill level, as will be described in more detail below.
Referring now to Figure 4, the main part of the method of the present
invention commences with a step (a) of excavating a volume of geotechnically
unsuitable soil from the upper strata 21 of the site 20, down to a depth x as
determined in preliminary step (iii). The excavation depth x is generally around
13m. The excavated soil (not shown) is not removed from the site 20 for
disposal, but rather is retained for soil stabilisation treatment, following which it
will be used to backfill the excavated site 40, as will be described in more detail
below. This aspect of the present invention alone represents a major cost
saving, and a major reduction in environmental impact, due to the reduction in
lorry movements which would normally be required when using a conventional
‘dig and dump’ process.
The excavation of the site 40 in this way also provides a number of
further opportunities which may be incorporated into the method of the present
invention. For example, any contaminated materials identified during the
preliminary site investigation step (i) can be modified to make them safe from
leaching, and then buried at the bottom 41 of the excavated site, away from
possible human contact, and isolated from drainage and other services.
Another option is the incorporation of pipes (not shown) for a geothermal
heating system, which can be incorporated at the base 41 of the excavated site,
i.e. at a depth x of around 3m. This is particularly advantageous since the
depth of installation is key to the efficiency of such systems, whilst the pipes
.13-
would also be protected deep under the building 30, away from other services
and infrastructure.
After each main method step, a supplementary step (iv) is carried out,
wherein the condition of the soil is tested and monitored so as to ascertain and
verify the extent of consolidation and compaction.
Following excavation of the site 40, method steps (b) and (c) are
performed, wherein soil stabilisation treatments are applied, respectively, to the
newly exposed base surface 41 at the bottom of the excavated site 40, and to
the volume of soil excavated from the site 40. Both steps involve treating the
soil with a formulation comprising calcium oxide or other suitable binders, and
mixing said formulation into the soil.
Having applied the soil stabilisation treatment to the exposed base
surface 41 in step (b), the exposed base surface 41 is then subjected to rolling
dynamic compaction (RDC) in step (d), using a four—sided RDC roller 42, as
represented schematically in Figure 4. This ensures that the strata 23
immediately beneath the excavated site 40 is consolidated and compacted to
the required degree. The Application of RDC proves out the base 41 by
identifying any soft spots, and utilises the synergistic properties of stabilisation
and dynamic compaction as the soft spots identified are dug our and replaced
with suitably modified material. To aid the consolidation process, the base 41 is
over-dried such that the base layer 41 then acts as a capillary to absorb any
moisture generated from the RDC process. However, if the base surface 41
deteriorates during the RDC process, then the soil stabilisation step (b) must be
repeated. Following the RDC process, compaction to the top 300mm of the
base layer 41 is carried out using a vibrating cylindrical roller 45.
Referring now to Figure 5, this illustrates the subsequent step (e) of re-
introducing into the excavated site 40 a portion of the soil which was excavated
from the site 40 in step (a) and treated in step (c). The re-introduced treated
soil forms a layer 43, of generally around 200 to 300 mm thickness. The top of
the reintroduced soil layer 43 forms a new exposed surface 44, which is then
subject to standard compaction in step (t) using a cylindrical roller 45, as
represented schematically in Figure 5.
The next step (g) of the method involves repeating steps (e) and (f) of
forming layers 43 of re—introduced treated soil and applying standard
compaction 45 to the newly exposed surface 44. This cycle is repeated until the
total depth of the formed layers 43 reaches a pre-determined thickness y,
generally in the range of from 1.0 to 1.5 m, as shown in Figure 6.
The multiple layers 43 are then subjected to a step (h) of applying rolling
dynamic compaction 42 to the newly formed exposed surface 44 so as to form a
compound layer 46, as can be seen in Figure 7. The RDC process instep (h)
proves out the compound layer 46 in the same way as described above for step
(cl) with reference to Figure 4.
Referring now to Figures 7 to 10, the next method step (j) involves
repeating the previous cycle of method steps (e) to (h): new layers 43 are
added and the newly formed exposed surface 44 compacted under standard
compaction 45 until the total thickness y of newly added layers 43 reaches a
pre-determined value; rolling dynamic compaction 45 is then applied to the
surface 44 of the newly added layers 43 so as to compact them into the
compound layer 46; and this cycle is repeated until the excavated site 40 is
effectively filled, and the level of the formed surface 44 is substantially equal to
the level of the surface 22 of the original site 20, as shown in Figure 10. In
practice, the level of the formed surface 44 is in fact generally 100mm higher
than the surface 22 of the original site 20, to allow for consolidation during the
final compaction steps.
The surface 22/44 of the site 20/40 is then subjected to a final treatment
of rolling dynamic compaction 42 so as to compact the new layers 43 and
compound layer 46 to form a raft 32 of modified soils, with a depth substantially
equal to X as shown in Figure 11. Any excess material is then trimmed back to
the required final surface level 22/44.
Claims (17)
1. A method of modifying geotechnically unsuitable soils at a site so as to render the site capable of load bearing, said method comprising the following steps: - (a) excavating a volume of soil from the site, to a pre—detennined depth; - (b) applying an in situ soil stabilisation treatment to the‘) base of the excavated site exposed in step (a); - (c) applying a soil stabilisation treatment to the volume of soil excavated from the site in step (a); - (d) applying rolling dynamic compaction to the base of the excavated site exposed in step (a); - (e) re—introducing into the excavated site a portion of the treated soil from step (c) so as to form a layer of pre-determined thickness; - (f) applying compaction to the layer formed in step (e); - (g) iterating steps (e) and (f) to fom1 a compound layer of pre- detennined thickness; - (h) applying rolling dynamic compaction to the compound layer formed in step (g); and - (j) iterating steps (e) to (h) so as substantially to backflll the site to a pre-determined level; and wherein in the soil stabilisation treatment in step (b), the base is over-dried such that the base layer then acts as a capillary to absorb any moisture generated during step (d).
2. A method as claimed in claim 1, wherein the soil stabilisation treatments in steps (b) and (c) involve treating said soil with one or more powder or binder materials selected from cement, lime (calcium oxide). pulverised fuel ash (PFA) and ground granulated blast-furnace slag (GGBS).
3. A method as claimed in claim 1 or claim 2, wherein standard compaction is utilised in step (f).
4. A method as claimed in claim 3, wherein the standard compaction in step (f) is continued until substantially 95% compaction of the layer formed in step (e) is achieved.
5. A method as claimed in any of the preceding claims, wherein the rolling dynamic compaction in step (h) is continued until effective refusal is achieved.
6. A method as claimed in any of the preceding claims. wherein the soil stabilisation treatments in steps (b) and (c) are continued until the moisture content of the treated soil is reduced to substantially 3% less than the standard optimum moisture content for the type of soil being treated.
7. A method as claimed in any of the preceding claims, further comprising the preliminary steps of: - (1) investigating the site to determine the soil characteristics; - (ii) determining the building load and design requirements; and - (iii) utilising the data from preliminary steps (I) and (ii) to determine the required excavation depth for step (a), the required composition of the soil stabilisation treatment materials for steps (b) and (c), the required layer thickness for step (e), the required compound layer thickness for step (g), and the required backfill level for step (j).
8. A method as claimed in claim 9, wherein any contaminated materials identified in preliminary step (i) are isolated, modified to prevent leaching, and buried at the base of the site excavated in step (a).
9. A method as claimed in any of the preceding claims. wherein the excavation depth in step (a) is in the range of from 2m to 5m.
10. A method as claimed in any of the preceding claims, wherein the excavation depth in step (a) is substantially 3 m.
11. A method as claimed in any of the preceding claims, wherein the layer thickness in step (e) is in the range of from 200 mm to 300 mm.
12. A method as claimed in any of the preceding claims, wherein the compound layer thickness in step (g) is in the range of from 1.0 m to 1.5 m.
13. A method as claimed in any of the preceding claims. further comprising the supplementary step of: - (iv) testing and monitoring the soil condition following each of steps (a) to (j) so as to ascertain and verify the extent of consolidation and compaction following each method step, and modifying the method appropriately where necessary.
14. A method as claimed in any of the preceding claims, wherein the backfill level in step (j) is substantially 100mm higher than the initial surface level so as to allow for consolidation during subsequent compaction steps.
15. A method as claimed in any of the preceding claims, further comprising the additional step of: - (v) following step (d), and prior to step (e), introducing into the excavated site an additional layer having pipes located therein, for connection to a geothermal heating system.
16. A method as claimed in any of the preceding claims, wherein the soils to be treated include expansive clay soils, and wherein the soil stabilisation step(s) include(s) soil modification treatment to prevent subsequent shrinkage and swelling of said expansive clay soils.
17. A method as claimed in claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.
Priority Applications (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
IE2006/0363A IE85299B1 (en) | 2006-05-08 | Ground engineering method |
Applications Claiming Priority (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
IE2006/0363A IE85299B1 (en) | 2006-05-08 | Ground engineering method |
Publications (2)
Publication Number | Publication Date |
---|---|
IE20060363A1 IE20060363A1 (en) | 2008-02-06 |
IE85299B1 true IE85299B1 (en) | 2009-08-05 |
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