COMPUTATIONAL METHODS IN ENGINEERING AND SCIENCE
EPMESC X, Aug. 21-23, 2006, Sanya, Hainan, China
©2006 Tsinghua University Press & Springer
Soil Additionally Affected by Non Force Loading and Its Influence on
Upper Structure
P. Kuklík*, M. Brouček
Department of Mechanics, Faculty of Civil Engineering, CTU in Prague, Czech Republic
Email: [email protected]
Abstract It is an experimentally confirmed fact that a soil substantially changes its material properties when
subjected to external loading. Apart from that, the soil, when subjected to a certain loading history, has the ability to
memorize the highest level of loading mathematically represented by over-consolidation ratio, and so called initial
void ratio. In virgin state the soil deformability is relatively high. On the contrary, following the unloading/reloading
path shows almost negligible deformation until the highest stress state the soil has experienced ever before is reached
[1, 2]. The paper presents basic resources for the evaluation of the numerical codes for soils affected by floods. One
of them is research project FLAMIS describing the flood in 2002 in the region of the South Bohemia [11, 12]. All
substantial influences, such as suction, piping etc., can change the over consolidation and porosity inside the subsoil.
This affects the upper structure due to collapses of the soil skeleton following by the stress redistribution. Time
dependent progress of influence zone can be used for the fast estimation of this phenomenon in the subsoil [3, 13].
Several numerical examples are carried out using professional code GEO 5 FEM from FINE Ltd. The results are
compared with real damages in situ.
Key words: floods, influence zone, over consolidation, modified Cam clay model,critical state model, layered
subsoil, finite element method
INTRODUCTION
During the last decade the Czech Republic was severely affected by a series of flood events. The last major crisis
occurred in August 2002 and the number of buildings damaged by the flood showed the necessity of numerous
actions to be taken to improve general preparedness and prevention. The differences between the predicted numbers
of buildings damages during the flood and the real number of buildings damaged as a result of flood have forced us
to doubt the existing methodologies for evaluating the flood danger for buildings which are based only on the water
depth and flow velocity (Fig. 1). The sudden rise of the groundwater table that usually accompany the flood event
results in additional changes in subsoil which being large enough could result in significant deformation of the upper
structure.
Figure 1: Flood hazard classification for houses
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PROCESSES INVOLVED
The processes described further don’t occur only as a result of flood but they can be observed under certain
circumstances far away from the flood plains. The piping effect in backfill in trenches for sewage or waterlines can
serve as an example. For the purpose of this paper the following processes are to be understood under the name of
additional non force loading. These are neglecting of suction, piping and changes of influence zone. Every one of
them is described and its effect on the subsoil discussed separately.
FLOOD HAZARD METHODOLOGY
The FLAMIS project (Flood Analysis and Mitigation on the Lužnice River in South Bohemia) presents the classical
approaches for the flood modeling and hazard evaluation. 2D flood modeling based on the very complex 3D terrain
digital model solving partial differential equation system, which includes vertically integrated Raynold’s equations,
continuity equation and two conveyance equation for turbulence, results in knowledge of water level, depth, flow
velocity and streamlines in every computation cell of the model.
Based on this knowledge the flood hazard maps for building can be created using DHCG “Downstream Hazard
Classification Guidelines” methodology which separates the flood hazard for different subjects (buildings, cars, adult
people, children etc.). Example of the flood hazard maps produced is shown below (Fig. 2).
Figure 2: Flood Hazard for buildings according to DHCG [10] in the town of Veselí nad Lužnicí of the flood in August 2002
As was suggested in the introduction the areas with highest danger do not accurately correspond to the areas where
most damaged buildings are located. The future progress in evaluating the non force loading effect should lead to
development of a methodology involving the type of subsoil and the type of foundation as a variable.
PIPING – SMALL PARTICLE LOSS DUE TO THE GROUNDWATER FLOW
Under the name piping this paper understand internal erosion due to force impact of the groundwater flow.
The erosion starts from the finest particles and as the preferential flow path are crated the force impact of the
flux increases and the erosion can continue with larger particles. As a result of piping process without any
changes in load the void ration becomes higher, the over consolidation ratio decreases as well as structural
strength. It has been well described in the case of dispersive clays where the spaces created have the shape of
the pipes. The piping led to several collapse in cases of homogeneous earth filled dams containing the
dispersive soils. Apart from flood events the piping effect takes part wherever the circumstances are suitable
to start it. For every type of soil the critical seepage velocity and critical hydraulic gradient could be
evaluated and when one of them is exceeded the water begins to wash away the particles of an appropriate
size. The typical places endangered are backfill in trenches for sewage or waterlines and places where the
cracked tube of the drip enters the ground. The last example of piping was many times confirmed as a main
cause due to its action the upper structure was heavily damaged. The void ratio and structural strength
dependence can be seen on the Fig. 5 below. More details are introduced in [1, 2, 4, 7].
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Figure 3: Veselí nad Lužnicí visualization of the calculation of the flood in August 2002
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Figure 4: Void ratio and structural strength dependence
In case of river structures the piping prevention is done by earth filters and geotextiles. For other endangered
structures geotextiles, plastic fibers spread in the soil and finally well done compaction are the possible ways for the
piping prevention.
SUCTION AND OVER CONSOLIDATION
Under the term suction the negative pore pressure which is mainly moisture dependant is understand. As the
groundwater level rises the soil is becoming saturated and the effect of suction vanishes. That leads to the decrease
in shear strength. This phenomenon has been studied in the past for cases of slope stability during the long term
rainfall periods. The evaluation of the suction varies for different clays and different authors from 1 MPa to 1000
MPa and as an example the measured and predicted shear strength for Guadalix Red silty Clay is shown in Fig. 5.
Figure 5: Shear strength and suction dependence for the Guadalix Red silty Clay
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Figure 6: Behavior of highly over consolidated soils; drained triaxial compression test with modified Cam clay model
Anulment of suction due to wetting proces causes high over consolidation. The collaps of soil due to high over
consolidation was investigated in [9]. Behavior of soil sample under deviatoric loading conditions is introduced in
Fig. 7. The ability of the program to describe the loosing of overconsolidation is seen.
CHANGES OF INFLUENCE ZONE
The change of groundwater table level is directly proportional to the change of the depth of influence zone. As the
soil increases its level of saturation the over consolidation ratio decreases and part of the load is carried by pore
pressure. The over consolidation ratio dependence on the effective stress describes the virgin consolidation line, well
known from the Terzagi’s solution. The task of the shear becomes smaller and the depth of the influence zone
decreases. The subsoil behavior is getting close to the Winkler’s model. This description does not involve the
suction effect and it is more likely valid for gravel than for clay soil. On contrary in soils with a significant suction
effect the rising level of saturation first cancel this effect. So at first the influence zone is getting deeper because of
the lack of the pore pressure. The soil structure collapses, and porosity decrease which inevitably increase the level
of saturation. As soon as the soil becomes fully saturated the pore pressure rises and the soil begins to consolidate.
During the consolidation the structural strength grows and the influence zone becomes smaller.
Figure 7: Changes in depth of the influence zone due to changes of groundwater table level
Generally is valid that decrease in the depth of the influence zone results in higher uniformity of the subsoil reaction.
In the case of zero value of influence zone the reaction of subsoil is uniform.
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Figure 8: Calculation of the influence zone depth due to changes of groundwater table level
The type of subsoil can be considered as the Winkler subsoil. Increase in the depth of the influence zone then results
in high concentration of the stress on the edges and corners of the foundations. The depression zone comes into
being. If during this process no plastic strains arise then after the groundwater table comes back to the usual level,
the soil will return to state in which it was before the quasi-steady state was breached. Unfortunately, during this
process plastic strains arise in the upper construction and the piping usually takes effect. These cause irreversible
changes of the foundation conditions. The described process is demonstrated on the upper picture (Fig. 6). The way,
how to calculate the depth of influence zone, is depicted in the graph (Fig. 7). In the figure, there are introduced
following symbols: γ , γ w are the specific weights of soil and water, f z is the uniform load bellow circle footing, ν
is the Poisson ratio, r is the diameter and H is the depth of influence zone. Details are presented in [3], [10].
Figure 9: Tension crack in the carrying wall due to flooding
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CONCLUSIONS
Any changes in quasi–steady state in groundwater table level or water flow result the changes of the internal forces
in the upper structure subsoil boundary. During the floods this process is unprompted. In general there is no chance
to describe this process in time. So there is known only situation of the building before and after floods. Small
period during the floods should to be prophesied. Nevertheless relevant information from the hydro geological
research can be used. Now a day geotechnical and hydro geological information have been still mutually
independent. Although data of ground water table and subsequently the depth of influence zone can be accepted. The
numerical model working with the change of influence zone seems to be simple and cogent. When solving an
interaction between the upper structure and subsoil, the shear depression comes into being in the subsoil. The shear
depression bearing capacity is very high. It depends on the type and shape of footing. The capacity can be up to
60%. The shear depression causes the grate concentrations of the internal forces in the edges and corners of the
footing. The stress concentration is accompanied by plastic deformations. Flooding abolishes the concentration of
stresses and reaction of foundation becomes uniform. That induces additional tensions inside the upper construction
and tensions cracks as it is seen in the buildings after floods. Also the deformations caused by piping or cancellation
of suction are irreversible and the sanation redevelopment of the failed site is economically very demanding and
expensive. No a day the numerical examples and comparisons with real damge are carried out. The results will be
presented in the conference.
Acknowledgements
The support of GAČR 103/04/1134 is gratefully acknowledged.
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