Content uploaded by Dimcho Evstatiev
Author content
All content in this area was uploaded by Dimcho Evstatiev on Nov 28, 2016
Content may be subject to copyright.
Hydrogeology, Engineering Geology and Geotechnics
115
GEOGRID USE IN LOESS FOUNDATION WORK
Prof. Dimcho Evstatiev
1
Assist. Prof. Dr. Vanushka Petrova
2
Eng. Borislav Kalchev
3
1
Geological Institute of Bulgarian Academy of Sciences, Bulgaria
2
Geological Institute of Bulgarian Academy of Sciences, Bulgaria
3
GeosynMat-VB Ltd., Bulgaria
ABSTRACT
The article considers the application of geogrids in the foundation work in collapsible
loess of the Workshop for Volume Reduction and Deactivation of materials from the
decommissioning of four power units of the Kozloduy NPP. In conformity with the
design the loess base improvement had to be realized by a loess-cement cushion, built
under the foundations. Foundation work by means of such a cushion in loess is a
Bulgarian method, applied in the construction of dozens of buildings and facilities,
including the facilities of the six power units of the Kozloduy NPP. In the case of the
Workshop its implementation was seriously hampered due to soil over-moistening at the
excavation bottom. The over-moistened soil was with soft consistency in some places
and it was impossible to spread and compact on it the first soil layers of the cushion.
This obstacle was overcome by placing a 30-50 cm thick compacted layer of river
gravel, reinforced with a geogrid along the entire excavation bottom. The reinforcement
was made by a layer of a monolithic hexagonal triaxial Tensar TriAx TX160 geogrid.
The technology of constructing the strengthened gravel layer and the soil-cement
cushion and the results of the plate tests are described in the article. Other possibilities
for geogrid uses are considered for foundation in loess with a soil-cement cushion, for
example between some of its layers.
Keywords: loess, foundation work, loess-cement cushion, geogrid
INTRODUCTION
Loess soils occupy about 11 % of the territory of Bulgaria. Their origin is Aeolian, the
structure is not compact and their most characteristic feature is collapse upon
moistening. Loess is a major problem for construction, solved by applying methods for
soil improvement. One of these methods includes the construction of a soil-cement
cushion (SCC), which is primarily used for foundation in loess, when collapsing is due
to the additional load of the facility, the so-called loaded collapsibility [1].
Since the mid-sixties of the last century till the end of the nineties SCC was applied in
the foundation in loess of more than a hundred industrial and residential buildings,
including the six power units of the Kozloduy NPP [2]. Over the past three decades
SCC construction has been reduced but it is still used even for responsible structures. In
2014-2015 SCC was implemented in combination with ballast (river gravel) reinforced
with a geogrid in the construction of the Workshop for Volume Reduction and
Deactivation of materials from the decommissioning of the first four power units of the
16
th
International Multidisciplinary Scientific GeoConference SGEM 2016
116
Kozloduy NPP. This case is considered in the present article. The National repository in
the vicinity of the Kozloduy NPP for conditioned short-lived radioactive waste will be
also built on such a cushion.
Essence of foundation with soil-cement cushion
The soil-cement cushion represents a strengthened layer of the soil base, situated
immediately under the foundation. It is built with the aim of preventing the risk of loess
collapse, increasing the bearing capacity of the soil base and reducing the hazard of
water penetration from the facility. The cushion is built under the entire area of the
structure and is from 1,5 to 5 m thick. It is built in layers according to a technology
applied in road construction and the mixture can be prepared by in-situ blending or in a
stationary mixer. The amount of cement is increased from the lower to upper layers,
ranging from 3 % to 7 %. The design of SCC is realized in compliance with the
guidelines of the Ministry of Construction and Architecture [3] and recently computer
software is applied for the purpose. In practice there are two principally different
approaches to the application of SCC.
The first approach is used in the foundation work of lighter buildings with single steps
and strip foundations. It is regulated by [3]. In this case the cushion, as part of a two-
layer base with an upper stronger layer (Fig. 1), redistributes the stresses from the
foundation to a larger area at the boundary with natural loess, reducing them to values
lower than the initial stress of loess collapse. This approach is applied in the case,
considered in the present article.
Fig. 1. Principal scheme of stress redistribution by the soil-cement cushion at the
boundary with natural loess
Hydrogeology, Engineering Geology and Geotechnics
117
Two conditions are necessary, so that the cushion can redistribute the stresses at the
boundary with natural loess. The first condition is that SCC has to be with sufficient
thickness h in order to obtain a ratio between its thickness and foundation diameter d:
h/d > 0,2 (1)
The second condition is that the ratio between the deformation (plate) moduli of SCC
and the soil underneath should be:
Е
0sc
/Е
0 l
> 5 (2)
In the most common cases the modulus of loess is Е
0l
=15,0 – 20,0 MPa and of SCC –
Е
0sc
>110,0 MPa, so that the ratio Е
0sc
/Е
0l
is higher than 5.
The second approach is used in the foundation work with a common reinforced
concrete plate of heavy buildings and facilities. In this case SCC is applied alone, in
combination with compaction with a heavy tamper or with river gravel cushion, for
complete or partial replacement of the collapsible loess. The foundation of the 180-m
high TV tower in Ruse is an example of such application, where after deepening of the
excavation, compaction with a 6-ton tamper has taken place, followed by the
construction of a 4,5-m thick SCC [4]. Another example is the designed in the
proximity of the Kozloduy NPP National Disposal Facility Radiana for conditioned
short-lived radioactive waste (RAW), where collapsible loess is entirely removed and
replaced by SCC with a thickness of 5,0 m [5].
Loess-cement cushion of the Workshop for Processing of materials in the Kozloduy
NPP
The Workshop represents a spacious hall with a frame structure with dimensions
69,7х36,4 m, founded with single step and strip foundations. The single footings are
rectangular with sizes 2,5х2,8 m and 1,4х2,8 m and the width of the strip foundations is
2b=1,0 m. The load from the foundations amounts to 25,0 t/m
2
. The elevation of
foundation is at 2,0 m from the surface.
The workshop is built on the first non-flood plain terrace of the Danube River, covered
with 10,0-11,0 m thick loess, which is collapsible upon moistening due to the additional
load of the facility (loaded collapsibility) to a depth of 6,0 m, i.e. 4,0 m below the
elevation of foundation. The groundwater level is at about 7,0 m from the surface. The
soil base is seriously disturbed and over-moistened by the underground communications
of the nearby facilities of power unit I of the Kozloduy NPP, as well as by the
excavation, left in the open for a long time.
After comparing the possible methods of foundation work, the choice was made for
foundation with 1,8-m thick soil-cement cushion, located below the elevation of
foundation. For this thickness, with design plate modulus of the cushion E
0
=90,0-
110,0 MPa, the requirement is met that the load at the boundary with natural loess
should not exceed 10,0 t/m
2
, which is in this case the initial load of loess collapse.
Construction of the soil-cement cushion and of the reinforced river gravel
After reaching the excavation depth of 3,8 m the bottom was graded and its surface
layer was disintegrated with a frezno, followed by spreading the cement and mixing it
16
th
International Multidisciplinary Scientific GeoConference SGEM 2016
118
with the soil also with a frezno. If necessary, the water content w of the mix was
corrected with lime powder to correspond to the design. The mixture was leveled again
and compacted with a road roller. The other layers of the cushion were built in the same
way, spreading an 18,0-20,0 cm thick loess layer for each of them.
The construction of the lower layers of the cushion was seriously hampered by the
presence of sections with over-moistened soil, where it was impossible to carry out the
technological operations. The consistency of the natural soil in them was soft, the water
content was w=30-32 % instead of w
opt
=15-16 %, i.e. the optimum water content for
maximum compaction, and the plate modulus was with very low values – Е
0
<5,0 MPa.
Under these conditions the soil-cement mixture could not be compacted and its surface
was strongly cracked. The construction took place in the rainy period of the year and it
was very difficult to choose the method for strengthening of natural soil in the over-
moistened areas.
After juxtaposition of the possible solutions (removal of part of the over-moistened
layer, treatment with quick lime, etc.), the decision was made to realize the
strengthening with river gravel, reinforced with a geogrid. The average thickness of the
reinforced river gravel was t=0,3 m as determined by the TensarPave software, for plate
modulus of the loess base Е
0
=15,0 MPa. This modulus was obtained from the second
(reloading) test, corresponding to a plate modulus from the first loading Е
0
=5,0-
7,0 MPa. As already mentioned earlier, in some areas loess was over-moistened and its
plate modulus was Е
0
<5,0 MPa. However, it has to be taken into account that reinforced
river gravel in this case is intended mainly for technological reasons. It was used not so
much to increase the bearing capacity of the soil base under the foundations, but for
better compaction of the first two layers of the cushion. To this end the thickness of the
river gravel in the some over-moistened areas was increased to 50 cm.
The reinforced layer with the geogrid was built of river gravel with predominant
fraction to 50 mm and uniformity coefficient u=d
60
/d
10
>20. The reinforcement was
made using a layer of the monolithic hexagonal Tensar TriAx TX160 triaxial geogrid.
The geogrid is with strong triangular openings and radial stiffness at low deformation
exceeding 400,0 kN/m.
The reinforced layer was built in the following manner:
The excavation bottom was leveled and cleaned of coarse objects and the surface layer
was removed from the muddy areas. The compaction was done using a static roller
without vibrations. A lighter roller or bulldozer was used in the sections with very wet
soil. The geogrids, in rolls with dimensions of 4,0х7,5 m, were placed on the so
prepared base. The geogrid was unrolled in overlapping strips. It was laid flat on the soil
base without folding and passing over holes at the soil surface. The gravel was delivered
by trucks and dumped in the end of the section, covered with the geogrid. The vehicle
wheels were not allowed to disturb the geogrid. The material was then spread by a
bulldozer. It was inadmissible that heavy machines should pass directly on the geogrid.
Then compaction with static rollers without vibrations started until thickness of 30 cm
was reached (Fig. 2). Eight to ten roller runs were made along each strip with
overlapping of tracks at least 30 cm.
Hydrogeology, Engineering Geology and Geotechnics
119
Fig. 2. View of the geogrid covered with river gravel, spreading of the river gravel
layer and its compaction
After compaction of the reinforced gravel the construction of the loess-cement cushion
started. The compaction of the lower layers of the cushion was also done by static
rollers without vibrations. Taking under consideration the climate specificities the
reinforced layer was not built entirely along the whole bottom but only in the sections,
where soil-cement had to be prepared in the next 2-3 days. The average thickness of
each layer of the cushion was 0,15 m in the compacted state. The content of Portland
cement of the lower half of the cushion layers was 5 % and of the upper half – 7 %.
The surface of the SCC layers on the reinforced river gravel was smooth in contrast to
these built on over-moistened loess (Fig. 3). Furthermore, the design values of the
density and unconfined compressive strength of the soil-cement were achieved, which is
impossible without building the substrate of reinforced river gravel.
Fig. 3. View of the surface of the soil-cement layer, built on reinforced gravel
16
th
International Multidisciplinary Scientific GeoConference SGEM 2016
120
The control works include plate tests with a stamp diameter of 30 cm, part of them
being performed on reinforced river gravel, built on top of a soil-cement layer with 5 %
of Portland cement and plate modulus of reloading E
0
=64,0-65,0 MPa. This makes it
possible to determine E
0
of the reinforced river gravel itself. Other plate tests have been
carried out on reinforced river gravel, placed at the excavation bottom, as well as on
loess itself. The tests have proved the following:
1. For loess with w close to natural content the modulus E
0
=11,8 MPa has
been determined in reloading, which is a normal value under the test
conditions.
2. For reinforced river gravel on heavily over-moistened loess the modulus
E
0
=11,2 MPa has been obtained from the reloading test. This means that due to
the reinforced gravel the weak loess base behaves as a normal loess base.
However, the thickness of the river gravel layer in the weakest section is
increased up to 40-50 cm.
3. Tests on reinforced river gravel on a soil-cement layer with 5 % of
cement. Similar results have been obtained from two plate tests: E
0
=63,1 and
E
0
=63,2 MPa for reloading. The “loading-settlement” relationship for one of
the tests is shown in Fig. 4.
4. Tests on the first soil-cement layer. The obtained values from reloading are
E
0
=64,3 and 64,8 MPa.
5. Tests at the surface of the soil-cement cushion. The plate loading has been
conducted prior to placing the substrate concrete. The average value of the
plate modulus from ten tests of the first loading is E
0
=95,6 MPa, and of the
second (reloading) tests – E
0
=221,5 MPa. These values are valid for the last
four layers of the soil-cement cushion, made with 7 % of cement.
0,00
1,00
2,00
3,00
4,00
5,00
0 0,05 0,1 0,15 0,2 0,25 0,3 0,35 0,4 0,45 0,5
σ
, МРа
S, mm
Fig. 4. “Loading-settlement” relationship from plate loading of reinforced river
gravel
Hydrogeology, Engineering Geology and Geotechnics
121
Possibilities for other applications of geosynthetic materials for loess strengthening
with Portland cement
In Bulgaria loess base strengthening with Portland cement is applied mainly in
foundation and hydro-melioration construction. It is also used in landfills for household
waste. It is envisaged to build the National Disposal Facility for short-lived radioactive
waste on a soil-cement cushion. In some future applications it is appropriate to use
geosynthetic materials with the aim of increasing the water tightness of the soil-cement
screen and the bearing capacity of the soil-cement cushion.
More than 20 water compensating basins of irrigation systems have been built in loess
in Bulgaria with a 15-20-cm thick soil-cement screen at the bottom, covered by a 15-cm
thick compacted soil layer. After a failure of one of these compensating basins, a
geomembrane (polyethylene foil) was placed on the soil-cement and the soil cover was
restored. A combination of a soil-cement screen and a geomembrane can be also used in
other types of screens for restricting water influx towards underground facilities, when
utilizing the ground space in loess. Two big urban landfills with a soil-cement bottom
screen were built in Bulgaria. The effectiveness of this screen can be increased if it is
combined with a geomembrane.
The loads of the single step and strip foundations on the soil-cement cushion are
restricted due to the danger of emerging large tensile stresses in the lower end of the
cushion that may cause its cracking. The placement of a geogrid between the lower
layers when the soil-cement mixture is prepared in a stationary manner could contribute
to overcoming this hazard and increasing the load from the foundation.
CONCLUSION
The experience from the foundation of the Workshop for Volume Reduction and
Deactivation of materials in the Kozloduy NPP proves that the over-moistened soil at
the excavation bottom can be strengthened with reinforced gravel, so that the
construction of the soil-cement cushion can be launched even under not very favorable
climatic conditions.
The reinforcement of the lower layers of the cushion with a geogrid increases their
tensile strength and makes it possible to increase the loading from single step and strip
foundations.
The combination of soil-cement and geomembranes can increase the water tightness of
the soil-cement screen, which is used in Bulgaria in hydro-melioration construction and
in building of urban landfills.
REFERENCES
[1] Minkov, M., D. Evstatiev. A method for foundation of buildings and facilities.
Authors’ license No 16276 with priority from March 17, 1970, 8 p., 1970 (in
Bulgarian).
16
th
International Multidisciplinary Scientific GeoConference SGEM 2016
122
[2] Evstatiev, D., D. Karastanev. Investigation and application of loess-cement in
Bulgaria, Engineering Geology and Hydrogeology, Bulgarian Academy of Sciences, 85-
112, 2013 (in Bulgarian).
[3] Guidelines for Foundation of Buildings and Facilities in Collapsible Loess Soils
Using a Soil-cement Cushion, Bulletin of Construction and Architecture, Ministry of
Construction and Architecture, ХХ, 1-2, 4-21, 1976 (in Bulgarian).
[4] Еvstatiev, D., J. Evlogiev, M. Nedelcheva. Foundation Work of a High TV Tower in
Collapsible Loess, In: Sixth International Conference on Case Histories in Geotechnical
Engineering and Symposium in honor of Professor James К. Mitchell, Arlington, VA
(USA) August 11-16, 2008.
[5] Technical design of the National Repositoty for short-lived radioactive wastes.
Consortium Westinghouse, DВЕ, Enresa Unpublished report, 2014.
