preconsolidation stress

(2007) 135–151www.elsevier.com/locate/enggeo

Engineering Geology 91

Preconsolidation stress in the Vega Baja and Media areas of theRiver Segura (SE Spain): Causes and relationship with

piezometric level changes

R. Tomás a,⁎, C. Domenech b,c, A. Mira d, A. Cuenca e, J. Delgado b

a Departamento de Ingeniería de la Construcción, Obras Públicas e Infraestructura Urbana, Escuela Politécnica Superior,Universidad de Alicante, P.O. Box 99, E-03080 Alicante, Spain

b Departamento de Ciencias de la Tierra, Facultad de Ciencias, Universidad de Alicante, P.O. Box 99, E-03080 Alicante, Spainc Instituto Técnico de la Construcción, S.A., Avda de Elche, 164, E-03006, Alicante, Spaind Ceico S.L., Cra. Nacional 301, Km. 397.9, P.O Box 15, 30100 Espinardo, Murcia, Spain

e Laboratorio de Carreteras, Generalitat Valenciana, Ctra. Ocaña s/n, 03005 Alicante, Spain

Received 29 September 2006; received in revised form 20 December 2006; accepted 22 January 2007Available online 31 January 2007

Abstract

Preconsolidation stress (σ′p) is themaximum effective stress that a soil has suffered throughout its life. From a geotechnical point ofview, preconsolidation stress has a great importance because it separates elastic and reversible deformations from inelastic and onlypartially irreversible deformations and marks the starting point of high compressibility. This study calculates the preconsolidationstress for 139 undisturbed soil samples from the Vega Baja andMedia of the Segura river (SE Spain), using the uniaxial consolidationtest and applying the method proposed by Casagrande while using a novel analytical procedure proposed by Gregory et al. [Gregory,A.S., Whalley, W.R., Watts, C.W., Bird, N.R.A., Hallet, P.D., Whitmore, A.P., 2006. Calculation of the compression index andprecompression stress from soil compression test data. Soil and Till. Res., 89, 45–57] to avoid subjective interpretations of maximumcurvature point. The results show overconsolidation ratio (OCR— the ratio of preconsolidation stress to current natural overburdenstress) values for the 10–15 m depth of soil varying from 2 to 14 and maximum preconsolidation stresses above 800 kPa. The maincauses of calculated preconsolidation identified are desiccation due to seasonal drying and wetting cycles that have induced additionalstresses always lower than 42 kPa for the more superficial samples.Water level decline due to the reduction of recharge suffered by theaquifer system during periods of drought and the uncontrolled withdrawal of water is considered to be the second cause of anomalousOCR values. This second cause induces low stresses to the more superficial layers (lower than 41 kPa) that can reach values higherthan 150 kPa for the deeper layers for knownwater level decreases. In consequence, the soils of the Vega Baja andMedia of the Segurariver are highly overconsolidated for the first 5 m, decreasing gradually with depth to 10–15 m deep. For samples located deeper than15 m the soils seem to be underconsolidated, probably due to the existence of confined aquifers that cause deviations from ahydrostatic and linear pore pressure model. This fact has a huge practical significance which implies that deformations affectingsuperficial layers are lower than those expected for deeper layers for the same load.© 2007 Elsevier B.V. All rights reserved.

Keywords: Preconsolidation stress; Piezometric level; Desiccation; Casagrande method

⁎ Corresponding author.E-mail addresses: [email protected] (R. Tomás), [email protected] (C. Domenech), [email protected] (A. Mira),

[email protected] (A. Cuenca), [email protected] (J. Delgado).

0013-7952/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.enggeo.2007.01.006

mailto:[email protected]





http://dx.doi.org/10.1016/j.enggeo.2007.01.006

136 R. Tomás et al. / Engineering Geology 91 (2007) 135–151

1. Introduction

Preconsolidation stress (σ′p), also called precompres-sion stress, precompaction stress or preload stress(Dawidowski and Koolen, 1994), is the maximumeffective stress that the soil has suffered throughout itslife and is used to describe the stress history of cohesivesoils. A soil is said to be overconsolidated when it hasbeen subjected to vertical effective stresses higher thanthe ones acting currently. From a geotechnical point ofview, preconsolidation stress is of great importancebecause it separates elastic and reversible deformationsfrom inelastic and partially irreversible deformations. Inother words, preconsolidation stress marks the start ofhigh compressibility. This fact is of great importancein predicting expected settlement of foundations orembankments because underestimated preconsolidationmay cause overestimation of the magnitude of consol-idation settlement and thus make more expensive andtime-consuming geotechnical solutions necessary. It isalso of major significance for soils suffering subsidencedue to piezometric water level decrease, because pre-consolidation stress indicates the maximum stressesgenerated by the increased stresses caused by a fall inthe water level (Hoffman, 2003). This point marks thepiezometric level position separating non-elastic andirrecoverable subsidence from elastic and recoverablesubsidence (Jorgensen, 1980; Holzer 1981; Hoffman,2003).

Preconsolidation stress is normally calculated by auniaxial confined compressive stress test (AENOR,1994) using an oedometer cell. The results of this testare plotted on a logarithm of the normal effective stressagainst void ratio (e) or unitary strain (ϵ). The resultinggraph shows two different branches. The first is calledthe elastic curve (or elastic recharge curve if it resultsfrom a reload of soil sample) and is characterized by lowdeformations that are recoverable if unloading occurs.The second is called the virgin compression curve andoccurs for higher stresses than the former. It is char-acterized by its linearity and for the strains being ir-recoverable. The point that separates the two branches isthe preconsolidation stress.

Several authors have proposed methods to estimatepreconsolidation stress of a soil sample: Casagrande(1936), Pacheco-Silva (1970), Tavenas et al. (1979),Gregory et al. (2006), among others.

Supposing hydrostatic conditions and rejectingtectonic stresses, a natural soil located at a depth zsupports a stress called natural overburden stress (NOSor σ′o) due to the weight of the soil column above it.The relationship between preconsolidation stress and

natural overburden stress is the overconsolidation ratio(OCR):

OCR ¼ rVp=rVo ð1ÞThis ratio indicates whether the soil is overconsoli-

dated (OCRN1), normally consolidated (OCR=1) orunderconsolidated (OCRb1).

Overconsolidation can be due to several causes(Kenny, 1964; Stapledon, 1970; Jiménez Salas and DeJusto Alpañés, 1976; Hobbs et al., 1976; Feda, 1978;Holzer, 1981; Selby and Lindsay, 1982; Graham andShields, 1985; Stamatopoulos and Kotzias, 1985; Cetin,2000; Arvidsson, 2001; Tovey, 2002; Cetin, 2005):

– Soil erosion at ground surface.– Melting of ice overburden existing in the past.– Changes in groundwater level that cause an increasein effective stresses.

– Water flow through soil that produces a reordering ofsoil particles generating more compacted systems.

– Desiccation of soil due to changes in moisture content.– Diagenesis due to organic or inorganic processes.This includes cementing, changes in ion concentra-tion, oxidation, depositional conditions and mineral-ogical composition.

– Tectonic activity.– Anthropic induced stresses.

This study is part of a project designed to estimateground settlement in the valley of the Segura river (SESpain, Fig. 1), an area where medium to soft soils arepresent and settlement is a geotechnical problem ofprime importance (Delgado et al., 2003). For such astudy, identifying the current stress history of soils in thearea, expressed as the preconsolidation stress and theoverconsolidation ratio (OCR), will help to establish thefuture behaviour of soils when loaded. To this end, weestimated the preconsolidation stress and the over-consolidation ratios for 139 undisturbed soil samples bymeans of the uniaxial consolidation test applying theCasagrande method and using a novel analytical pro-cedure, proposed by Gregory et al. (2006), to avoiddeviations due to subjective interpretations of the pointof maximum curvature.

It is well known that undrained shear strength (su) isclosely related to overconsolidation (Stamatopoulos andKotzias, 1985; Atkinson et al., 1987; Houlsby andWroth,1991; Mesri and Ali, 1999; Larsson and Ahnberg, 2005).As a result, in this study undrained shear strength dataobtained from undrained and unconsolidated (UU)triaxial tests (AENOR, 1998) and from undrained conepenetration tests (CPTU) (AENOR, 1993) was used to

Fig. 1. Geological map of the Vega Baja and Media of the Segura river.

137R. Tomás et al. / Engineering Geology 91 (2007) 135–151

demonstrate overconsolidation. Although not the idealtest devices for estimating undrained shear strength(Devinzenci, 2002), standard penetration tests (SPT)(AENOR, 1992) are also related to undrained shearstrength and, as a consequence, with soil overconsolida-tion. As a result, they are also used to show the over-consolidation affecting the soils being studied. Finally,possible causes of overconsolidation of the Vega Baja andMedia of the Segura river soils were analysed.

2. The study area

2.1. Location and physical geography of the Vega Bajaand Media of the Segura river

The Segura river valley is located in SE Spain (Fig. 1)and covers an area of about 500 km2 corresponding tothe provinces of Alicante and Murcia. It is known as the

Vega Media (VMSR) in the province of Murcia and asthe Vega Baja (VBSR) in the province of Alicante.The main cities in the area are located in the VMSR(Murcia), where the capital city of Murcia and itsmetropolitan area concentrate more than 500,000 in-habitants. On the other hand, the VBSR is characterizedby small, disperse towns, with Orihuela (approx.55,000 inhabitants) being the main urban nucleus inthis sector of the valley.

The study area has aMediterranean type climate, withan average annual precipitation of 280 mm. In summer(June–August) the climate is arid while most of theprecipitations are concentrated in the autumn (October–November). The average annual temperature is 18 °C,but can sometimes exceed 40 °C in summer.

The topography of this valley is flat, decreasing inaltitude from W to E, as the river approaches its mouthin the Mediterranean Sea. The valley is limited by


mountain ranges where old, deformed materials out-crops (the Carrascoy range to the S, and the Crevillenterange to the N). There are also some relief features, theOrihuela and Callosa mountains, in the middle of theflood plain.

2.2. Geological setting of the Vega Baja and Media ofthe Segura river

The VMSR and VBSR are located in the eastern partof the Bajo Segura basin (Montenat, 1977). The valleytrends in an NE–SW to ENE–WSW direction, con-trolled by active faults, particularly the Crevillente faultto the north and the Bajo Segura fault to thesouth (Alfaro, 1995) (Fig. 1). Tectonic activity occurringsince the Late Miocene has folded both the basementrocks (Paleozoic to Mesozoic) and the basin fill (UpperMiocene to Quaternary). The Segura river valley occurswithin one synform formed by this activity.

The nature of the materials outcropping along theboundaries of the valley varies depending on the site(Fig. 1). The southern border of the VMSR consists ofrocks of the basin basement (Permian to Triassic in age),raised to ground surface by the activity of the BajoSegura fault. Meanwhile, the northern border consists ofsedimentary rocks (Upper Miocene to Pliocene) depos-ited in the basin (marls, sandstones and conglomerates).Towards the E, the southern border is made up of marls,sandstones and conglomerates of the basin fill (Plioceneto Pleistocene in age), while the northern border consistsof limestones of the basin basement (Mesozoic) andalluvial fan sediments developed at the base of thismountain front.

The materials found in the valley, corresponding to theaforementioned flat areas are recent (Holocene at groundsurface, Pleistocene at some depth) sediments depositedby the River Segura and, in the eastern zones, the Medi-terranean Sea (littoral and lagoonal sediments). Someanthropic deposits can be also found at certain points in thevalley. Flood plain sediments are principally fine (CL,ML, CL–ML and sometimes OL and CH) with lowcontent of fine sand (SM) and lagoonal sediments arecomposed by fine sediments without sand layers (Rodrí-guez Jurado et al., 2000; Delgado et al., 2003). Finally, thelittoral sediments consist of poorly graded sands andgravels with low fine content (SP). X-ray diffraction testindicated that soils are principally composed by calcite(50–75%) and quartz (20%) with minor contents offeldspars, dolomite and kaolinite (Delgado et al., 2003).

These recent sediments are themost compressible onesin the zone and the most problematic from a geotechnicalpoint of view. Rodríguez Jurado et al. (2000) and Delgado

et al. (2003) made a geotechnical characterization of allthese materials for the VMSR and VBSR, respectively.Their models show that the same sedimentary rocksoutcropping at the valley borders are also found at somedepth in the valley, varying between 0 to 30m towards theW of the valley and 0 to N60 m close to the river mouth(Delgado et al., 2000), and constitute a geotechnicalsubstrate of the area. Above this “basement”, recentsediments are characterized by moderate to highcompressibility.

A detailed description of sediment distributionand geotechnical properties can be found elsewhere(Rodríguez Jurado et al., 2000; Delgado et al., 2003).

2.3. Hydrological setting of the Vega Baja and Media ofthe Segura river

The VMSR and VBSR are part of the so-called“Guadalentín–Segura Quaternary aquifer SystemNo. 47”(IGME, 1986). This aquifer is characterized by two units(Cerón and Pulido, 1996; Aragón et al., 2004): a surfaceunit, an unconfined aquifer, of fine sand and siltsdeposited by the recent activity of the Segura river andcoastal processes (towards the E of the zone). The watertable of this aquifer is found a few meters below groundsurface. Due to the high fine content of sediments, thisaquifer is characterized by low hydraulic conductivity, soit is scarcely exploited in the VMSR, and seldom inthe VBSR, where a high saline content prevents it.Consequently, water level shows small (dm tom) seasonalchanges.

The second unit is formed by the conglomeratespresent in the sedimentary rocks that exist below thesuperficial sediments. These conglomerates are usuallystratified alternating with marls, and constitute a confinedaquifer with greater hydraulic conductivity than thesuperficial aquifer. There are several levels of con-glomerates of hydrological interest. The most exploitedis the most superficial one, located about 30 m deep in theVMSR and 35 to 55 m deep in the VBSR. Its piezometriclevel is found a few meters below ground surface in theVMSR and about 3 m above it in the VBSR (Fig. 2). Thepiezometric levels of this aquifer show notable variationsover time due to overexploitation in periods of drought(Fig. 3). This was especially noticeable during the 1992–95 period, where peaks of −15 m of piezometric leveldescent were recorded (Aragón et al., 2004). As aconsequence, widespread subsidence affected bothVMSR and VBSR, causing damage to structures and agreat public concern (Mulas et al., 2003; Martínez et al.,2004). Tomás et al. (2005, 2006) measured such groundsubsidence in the metropolitan area of Murcia and

Fig. 2. Multilayer piezometer data. Note the different piezometric level existing at different depths. See location of the multipiezometer at Fig. 4.

Fig. 3. Piezometric evolution of several wells of the Vega Media andBaja of the Segura river. See piezometer location in Fig. 4.


Orihuela during this drought period by means of dif-ferential SAR interferometry, detecting maximum move-ments of 6 cm and 4 cm respectively.

Other layers of conglomerates of interest from ahydrological point of view are found at greater depths,but little is known about them.

Generally, no connection between surface and thedeeper unit exists. They are only connected throw theexisting well and in some border areas of the valley(IGME-DPA, 1996).

2.4. Previous data on the preconsolidation stress of thesoils

The preconsolidation stress of the city of Murcia wascalculated using the Casagrande graphical method byVázquez and De Justo (2002). They worked with 12undisturbed samples from geotechnical reports takenbefore 1995 (before the period of piezometric leveldecline). The OCR values of shallow samples (0 to 5 m)varied from 0.6 to 1.9, with an average value of 1.3, withonly two samples underconsolidated. The other 4 samples(5 to 10 m) were underconsolidated, with OCR valuesvarying from 0.6 to 0.9 and an average value of 0.7.Accordingly, soils seem to be slightly overconsolidatedclose to ground surface but becomenormally consolidatedto underconsolidated at shallow depths.

Devinzenci (2003) described a shallow and over-consolidated layer affected by desiccation and pumpingfrom the lower gravel aquifer that constitutes the geo-technical substratum of the city of Murcia (VMSR),using CPTU on-site tests.

3. Methodology used to estimate preconsolidationstress

Preconsolidation stress was calculated for 139 un-disturbed soil samples taken from boreholes drilledfor geotechnical studies performed in the zone (seelocation in Fig. 4). The undisturbed samples were takenusing a 77.5 mm and 89.0 mm diameter Shelby sam-plers following French recommendations (AFNOR,1995) of compulsory application in Spain. This type of


thin wall sampler provides a high quality samples thatminimize the structural properties disturbance of thefine-cohesive soils.

The tests were carried out on samples measuring50 mm (diameter) by 12 mm (height), following UNErecommendations (AENOR, 1994). Anomalous tests(i.e., steps, lags, etc.) were rejected.

A total of 114 samples came from the VBSR area, withtheir sampling depths ranging from 1.3 to 40.4 m. Theremaining samples were taken from the VMSR, at re-search depths varying from 2.2 to 18.5 m. In an importantpart of the VMSR there is a gravel layer at depths varyingfrom 10 to 30 m and normally used as geotechnicalsubstratum for deep foundations. This explains the ab-sence of samples taken at greater depths in this zone.

The method used to estimate preconsolidation stressin this work is that proposed by Casagrande (Fig. 5).This method uses the maximum curvature point, M, (orminimum curvature radius point) of the e-logσ curve todraw a line parallel to logσ axis (h). A tangent line (t) tothe e-logσ curve is also drawn at the same point. Finallythe bisector line (b) of the two lines previously drawn isrepresented. The stress corresponding to the intersection

Fig. 4. Geotechnical boreholes, wells

of the bisector line (b) with the virgin compression curvecorresponds to preconsolidation stress (σp).

In this method, the location of the point M is a keyquestion. It is usually an educated guess by the re-searcher, giving rise to a high degree of subjectivity. Toavoid this problem, we used an analytical-mathematicalprocedure proposed by Gregory et al. (2006). It consistsof fitting a curve to the uniaxial consolidation test data tocalculate its curvature radius function and optimizing itto obtain the maximum curvature pointM needed by theCasagrande method to gain soil compression properties(Fig. 5). Specifically, we fitted an asymmetrical sig-moidal curve, known as a Gompertz curve, whosemathematical expression is:

e ¼ aþ c exp −exp b log10rVð Þ−mð Þð Þð Þ ð2Þ

Where a, b, c and m are constants obtained by means ofleast-square fitting, e is the void ratio and σ′ is thevertical effective stress. This curve was chosen becausethe square regression coefficient was better than thoseobtained by fitting other sigmoidal curves such as fourth-order polynomial and symmetrical logistic sigmoidal.

and piezometer location map.

Fig. 5. Graphical estimation of preconsolidation stress.


The maximum curvature point (M ), necessary toapply the Casagrande method was calculated optimizingthe curvature radius function (κ) of the least-squarefitting Gompertz curve given by the expression:

j ¼ d2e=d log10rVð Þ2

1þ de=d log10rVð Þð Þ2h i3=2 : ð3Þ

Solving this expression by means of a numericalmethod:

djd log10rV

¼ 0 ð4Þ

The different terms of expression (3) are obtained byderiving expression (2), giving the following expressions:

ded log10 rVð Þ ¼ b c exp −exp b log10rVð Þ−mð Þð Þð Þ½ �

� −exp b log10rVð Þ−mð Þð Þ½ � ð5Þd2e

d log10rVð Þ2 ¼ b2 c exp −exp b log10rVð Þ−mð Þð Þð Þ� �

� exp b log10rVð Þ−mð Þð Þ½ �� exp b log10rVð Þ−mð Þð Þ−1½ � ð6Þ

This summary is included here for the sake of com-pleteness. A detailed description of this methodologycan be found in Gregory et al. (2006).

Natural overburden stress (NOS), which correspondsto the vertical effective stress (σ′o) of the soil at a depthz, was calculated as:

rVo ¼ ro−u ð7Þ

Whereσo is the vertical total stress at a depth z,which wascalculated by adding the result of multiplying specificweight (bulk or saturated specific weight, depending onthe positionwith respect towater table) by the thickness ofall the layersmaking up the soil column. The second term,u, represents pore pressure. It was calculated as the stresscaused by the water column acting at depth z, multiplyingwater specific weight, γw (set to 10 KNm−3) by the depthin question by means of the expression:

u ¼ z−hð Þgw ð8Þ

With h being the depth of water table from surface.Note that hydrostatic distribution with depth of pore

pressure has been assumed. This approximation can

Fig. 6. Preconsolidation stress calculated by depth.

Fig. 7. Overconsolidation ratio (OCR) calculated by depth. Theeffective natural overburden stress (σ′o) used to calculate OCR ofevery sample has been calculated using all the available geotechnicalinformation of the considered borehole and not an average value for allsamples.


cause deviations from real effective stress when thepiezometric level is higher in confined aquifer layers.

Finally, overconsolidation ratio (OCR) was calculat-ed by substituting preconsolidation stress, calculated bythe above exposed method, and natural overburdenstress (Eq. (7)) in Eq. (1).

4. Results of preconsolidation stresses

The results obtained are presented in Figs. 6–9 andTable 1. Fig. 7 shows the variation of OCR with depth.Fig. 6 shows the distribution of preconsolidation stresswith depth. Fig. 8 shows the vertical effective stress(σ′o), the preconsolidation stress (σ′p) and the OCR forseveral representative and complete boreholes in thestudy area. Finally, Fig. 9 shows the interpolated OCRvalues for different depths.

The fits of the Gompertz function provided squarecorrelation coefficients that always exceeded 0.9987. Thepreconsolidation stresses vary from 49.1 to 813.5 kPawhile the OCR vary from 0.4 to 12.2 for VBSR samples.

For the VMSR samples, the preconsolidation stresses andOCR values vary from 34.6 to 953.9 kPa and 0.3 to 14.3,respectively.

From Figs. 6, 7 and 9, and Table 1, it is clear that allsamples taken between ground surface and a depth of5 m are highly overconsolidated, with OCR varyingfrom 1.4 to 14.3 (average value of 5.2). From 5 to 10 m,86% of the samples are slightly overconsolidated withOCR values varying from 0.7 to 7.6 (average of 2.0).From 10 to 25 m, the samples are very slightly over-consolidated (55%) and frequently underconsolidated(37%), with OCR values varying from 0.3 to 5.2(average of 1.3). For depths greater than 25 m, most ofthe samples are underconsolidated (84%), due to theexisting high pore pressure that causes deviation fromhydrostatic linear models and only 12% are normallyconsolidated. Only one sample is overconsolidated inthis depth range, with an OCR value of 1.3.

The underconsolidated stresses estimated for depthsgreater than 25 m can be explained by the way inwhich pore pressures were calculated. We considered a

Fig. 8. Natural vertical stress, preconsolidation stress and overconsolidation ratio variation by depth at several geotechnical boreholes of the studyarea. See location of boreholes in Fig. 4.


hydrostatic situation related to the most superficial,unconfined aquifer, with the water table located at theposition below ground surface indicated in the boreholerecord. Nevertheless, a multilayer piezometer recentlyinstalled at Almoradí showed that each aquifer in thezone had its own water table/piezometric level (Fig. 2;see Fig. 4 for location). The piezometer located 25 mdeep showed the position of the water table of theunconfined aquifer associated with the river. Neverthe-less, this figure also showed that aquifer associated to afirst conglomerate layer, located about 50 m deep in thisarea, had a piezometric level more than 3 m aboveground surface. This means that there is a difference of

40 kPa between estimated pore pressures (Eq. (8)) andreal pore pressures. If we take this difference intoaccount, most of the samples taken at depths greaterthan 25 m become normally consolidated.

Finally, the analysis of the spatial distribution ofsamples showed that no significant variations in the OCRvalues and preconsolidation stresses occurred betweensamples taken at the VBSR and at the VMSR (Table 1and Fig. 9). However, as it is noticed at Fig. 9, that showsthe spatial distribution of interpolated OCR values for theentire study zone, OCR values for 0 to 10 m depth areslightly higher for the east sector of the Vega Baja of theSegura river. A part of this sector corresponds to old

Fig. 9. Interpolated OCR maps of the Vega Baja and Media of the Segura river for different depths: (a) from 0 to 5 m; (b) from 5 to 10 m; and (c) from 10 to 25 m.

144R.Tom

áset

al./Engineering

Geology

91(2007)

135–151

Table 1Preconsolidation properties of the VBSR and VMSR soil samples

Preconsolidation stress,kPa (σ′p)

OCR

Max. Min. Avr. Max. Min. Avr.

VBSR0–5 m 531 49 224 12.2 1.4 5.35–10 m 503 50 216 5.5 0.7 2.110–25 m 813 59 240 5.2 0.3 1.3

VMSR0–5 m 534 78 258 14.3 1.5 4.95–10 m 954 92 256 7.6 0.7 2.510–25 m 336 35 157 2.2 0.3 1.0

VBSR and VMSR0–5 m 534 49 236 14.3 1.4 5.25–10 m 954 50 229 7.6 0.7 2.010–25 m 813 35 239 5.2 0.3 1.3

Max.: maximum; Min.: minimum; Avr.: average.Fig. 11. SPT values of fine alluvial sediments.


marshlands that were partially desiccated by humanactivity (Canales and Vera Rebollo, 1985).

5. Variation of shear strength with depth

Soil shear strength is slightly related with preconso-lidation stress and consequently with OCR (Stamato-poulos and Kotzias, 1985; Atkinson et al., 1987;Houlsby andWroth, 1991; Mesri and Ali, 1999; Larsson

Fig. 10. (a) Variation of undrained shear strength obtained fr

and Ahnberg, 2005). Mesri and Ali (1999) haveestablished numerical relationships between OCR andundrained shear strength (su). These have differentforms but all of them establish that undrained shearstrength increases proportionally with OCR and pre-consolidation stress.

The results of undrained and unconsolidated (UU)triaxial tests show a linear increase of shear strengthwith depth (Fig. 10a), which follow a linear model.

om UU triaxial tests (su) and (b) su /σo ratio by depth.


However, for depths lower than 10 m there exist severalanomalous values of su, two or three times higher thanthose theoretically expected according to this linearmodel. Considering that for normally consolidated soil,the ratio of undrained soil shear strength (su) and naturaloverburden stress (σ′o) is near 0.25 (Wood, 1990), valueshigher than that indicate that the soils under study areoverconsolidated for the first 10 m (Fig. 10b).

Robles (personal communication) observed thatuniaxial compressive strength qu (related to undrainedshear strength, su=qu / 2) of the more superficial layersof silt and clay existing at Murcia city significantlyincreased its value after the subsidence associated withwater level decline occurred.

Although it is not the best test to estimate undrainedshear strength, the standard penetration test (SPT) is anindirect way of estimating su variation for fine sediments(Devinzenci, 2003). In the study zone, the results of thistest showed behaviours similar to those detected with theUU triaxial tests. So, the variation of SPTwith depth forthe VBSR and VMSR (Fig. 11) showed a relative linear

Fig. 12. CPTU values. Note the high anomalous value

increase with depth for tests performed between 10 and40 m. However, this tendency was not followed for themost superficial tests, which showed greater values.

The undrained shear strength (su) obtained fromundrained cone penetration tests (CPTU) are superim-posed on preconsolidation stress (σ′p), vertical effectivestress (σ′o) and OCR in order to show the increase of suwith overconsolidation for several boreholes (Fig. 12).

A general overall behaviour can be observed in theabove figures: soils between 0 and 10 m deep showshear strengths greater than those located below them.This can be interpreted as an effect that the preconso-lidation of soils has on their geotechnical properties.

6. Analysis of results

6.1. Causes of preconsolidation stress

An important aspect of the problem analysed is itsorigin. In the study zone, two main superimposed causescould explain the preconsolidation observed. These

s between 0 and 10 m according to OCR values.

Table 2Decomposition of the preconsolidation stress of geotechnical samples of the VBSR and VMSR in different terms

Sample Depth(m)

WLD(m)

Date ofdrilling

Location Water level decreaserange (m)

OCR σ′p(kPa) [1]

σ′o(kPa) [2]

Δσ′MPLD

(kPa) [3]Δσ′D (kPa)[1]–[2]–[3]

ORIPAL 1.9 2.3 1999 VBS 5.7–10.1 5.9 226 38 0 a 188ORI IV 2.7 1.8 1996 VBS 5.7–10.1 3.4 141 41 1 99O1 4.3 1.5 1994 VBS 5.7–10.1 4.6 267 58 27 182ORI AUG 4.3 1.0 1990 VBS 5.7–10.1 2.8 140 49 34 57O2 4.8 1.3 1994 VBS 5.7–10.1 3.7 229 63 36 130BENIEL 2.7 1.3 2006 VMS 17.3–17.7 4.8 190 39 14 137BENIEL 4.9 1.3 2006 VMS 17.3–17.7 2.7 158 58 37 63BENIEL 5.3 1.3 2006 VMS 17.3–17.7 3.6 232 64 41 127CR6 2.8 1.9 2004 VMS 4.8–8.9 3.4 148 43 9 96AT 4.1 4.2 2001 VMS 4.8–8.9 1.5 123 81 0 a 42RONDASUR 5.2 4.4 2005 VMS 4.8–8.9 2.5 235 96 8 131HP 5.7 4.6 2003 VMS 4.8–8.9 2.2 229 102 11 116 b

PROG 5.7 2.4 2005 VMS 4.8–8.9 2.1 153 73 32 48

The water level decrease range was obtained using the nearest available piezometers to the geotechnical borehole.WLD: Water level depth in the borehole during drilling.a Sample located over water level.b Saline veins have been observed between 1.4 and 2.5 m depth.


are: the piezometric level decrease and desiccation dueto wetting and drying cycles with pedological rework-ing. Other causes seem to have a minor effect (tectoniccauses) or are simply impossible in the geological con-text of the study zone (erosion).

We tried to estimate the contribution of each of thesecauses to the preconsolidation of soils. For this purposewe can suppose that preconsolidation stress is the sum ofseveral elements:

rVp ¼ rVo þ DrVMPLD þ DrVD ð9Þ

Where σ′p is the calculated effective preconsolidationstress, σ′o is the effective natural overburden stressexisting at present, Δσ′MPLD is the maximum effectivestress increase due to piezometric level variation in thepast, and Δσ′D is the effective stress increase due todesiccation and other associated causes such aspedological processes.

Preconsolidation stress and effective natural over-burden stress terms of Eq. (9) can be calculated fol-lowing the methodology outlined in Section 3. Theeffective stress increment due to piezometric levelchanges can be calculated by using the known pie-zometric level evolution. Consequently the third term ofEq. (9), corresponding to effective stress due to des-iccation can be isolated and calculated. Table 2 showsthe results calculated for several superficial samples ofthe study area. Note that we have not included samplesof the more easterly sector of the VBSR because nolong-time series of piezometric data are available.

6.1.1. Preconsolidation due to piezometric level declineWhen the water level goes down due to natural or

anthropic causes, both u and σo decrease in Eq. (8),resulting in an increase in the vertical effective verticalstresses (σ′o) acting on soil particles.

Assuming hydrostatic conditions, each meter of waterlevel descent implies that pore pressure reduces by10 kPa and at the same time a change in the terrainloading equivalent to the emerged depth multiplied bythe bulk density, which was previously multiplied bysaturated density. The maximum water level loweringsuffered by the soil throughout its history (Δσ′MPLD)will be recorded by the soil as a “part” of the precon-solidation stress (σ′p). Holzer (1981) calculated pre-consolidation values at several aquifers in the UnitedStates of America, due to water level decline caused bywater withdrawal, ranging from approximately 160 to620 kPa, based on the rate between water decline andsubsidence.

In the VMSR and VBSR, maximum falls in waterlevels were about 17 m and 10 m, respectively, duringthe period 1973–2004. Unfortunately no data forprevious periods exists, nevertheless aquifer exploita-tion is maximum at present (compared with the past) soassuming that current levels are the minimum historicones is not unrealistic. These drops represent maximumeffective stress increases of about 42–153 kPa in theVMSR and 49–87 kPa in the VBSR, depending on thesite for each zone. These increases in effective stressesgave rise to the generalized subsidence in a great part ofthe study zone previously commented.


Maximum possible calculated stress (supposing atotal draining and dissipation of pore pressure) due towater level lowering (Δσ′MPLD) has been calculated forseveral representative samples of the VMSR and theVBSR. These values are always lower than 50 kPa(Table 2).

It is important to note that preconsolidation stressesmeasured with consolidation tests show values higherthan the ones produced by water level decrease forsuperficial samples. Stresses induced by known piezo-metric lowering only explain a part of the preconsolida-tion stresses calculated (less than 40% overconsolidation).A greater water level descent could explain the excessoverconsolidation, but this seems rather unrealistic for thiszone.

6.1.2. Preconsolidation due to desiccationThe second cause of preconsolidation in the VMSR

and the VBSR is desiccation. Significant seasonalmoisture content changes may cause desiccations andreworking by pedological processes such as biologicalactivity and secondary calcite formation (Cetin, 2000).Desiccation, and especially seasonal desiccation, causedby repeated wet and dry cycles induces significantmicroscale stresses. These microscale stresses, referredto as negative pore pressures, are due to an equivalentinternal tension resulting from moisture and capillarywater evaporation. Tschebotarioff (1951) assumed thatvery high stresses can be generated by desiccation invery fine soils establishing values of stress due to capil-larity effects changing from 0.15 kPa to 305 kPa for grainsizes varying from coarse sand to clay respectively.Other authors have noticed preconsolidation pressures inexcess of 400 kPa (Stapledon, 1970; Selby and Lindsay,1982). OCR values from 3 to 7 and from two to threetimes as high due to desiccation have been calculated byO'Neill and Yoon (1995) in pro delta and backswampenvironments respectively. OCR values as high as 8 aresometimes found at the top layers of Singapore clayprobably due to desiccation (Chu et al., 2002).

This phenomenon mainly affects the vadose zone ofthe VBSR and VMSR fill, where water content changesare notable and pedological reworking can be signifi-cant. The pedological processes are principally inducedby plant root activity (observed at more superficiallayers of almost all available geotechnical boreholes)and are normally shown as slightly cemented horizons.Total desiccation due to the emersion of the moresuperficial soils layers is evident in the drained wetlandareas located at east of the Vega Baja (Fig. 9a) werephreatic water can temporally spring up over the surfaceduring wet periods (IGME-DPA, 1996).

The VBSR and VMSR enjoy a typical Mediterraneanclimate with low precipitations and hot summers.Summer temperatures can reach to 43°, when the directinsolation over the surface can favour evaporation ofsoil water that is present in form of humidity or capillaryascent. Seasonal fluctuations of water level are alsocommon in this area due to the reduction of inputs to theaquifer and increased water withdrawals. Consequently,the first 5 m of soil column of the VBSR and VMSRcould be overconsolidated due to these causes.

Using Eq. (9), calculated values of the contribution ofdesiccation to preconsolidation stress (Δσ′D) forrepresentative superficial samples could vary from 42to 188 kPa (Table 2).

6.2. Effect of preconsolidation on the deformationalbehaviour of soils

The preconsolidation stress in soil samples of theVMSR and VBSR was quantified and presented inprevious sections (Figs. 6–9, Tables 1 and 2). Theseresults show that soils are clearly overconsolidated inthe range 0–5 m, slightly overconsolidated between 5and 10 m and normally consolidated between 10 and25 m. For depths higher than 25 m computed OCR islower than 1 (underconsolidated) due to a pore pressureundervaluation because it does not respond to ahydrostatic model.

For the purpose of analysing the deformationalbehaviour of soils when loaded, these results are ofgreat importance. Load can have two main causes:surface loads (due to civil works) or loads due to aquiferoverexploitation and water table descent. Each of thesecauses will have a different consequence.

When soils are loaded at their surface, most ofstresses are supported by the superficial layers of soils.In the case of the VMSR and VBSR, these layers arerather rigid and their deformability will be, mainly, inthe elastic range. This means that ground settlementunder typical structures made in this zone (builds of twoto four stages) will be from small to moderate.

On the contrary, when stresses increase due to loadsaffecting deeper layers of soil, ground surface settlementwill become greater. This has been observed under largeembankments built in the zone (Delgado et al., 2003;Tomás et al., 2000), where settlements of up to 1 m wererecorded, and may be also the cause of widespreaddamage caused during past periods of drought (Mulaset al., 2003; Tomás et al., 2005). In this case, stressincrease affects a thick layer of normally consolidatedsoils, where plastic, irrecoverable, deformations canoccur with even a small increase in effective stresses.


When superficial loads or aquifer discharges due tooverexploitation could affect soil layers located atdepths deeper than 25 m it would be recommended tomeasure carefully pore pressure in order not to cause asettlement prediction overvaluation.

7. Conclusions

Preconsolidation stress is an important geotechnicalparameter that separates the elastic and inelastic strainranges of soils. This is of major importance for geo-technical engineers because it controls ground settlementand, as a consequence, the sizing of the foundations ofstructures.

We calculated such stress for 139 samples taken fromgeotechnical boreholes drilled in the VBSR and VMSR,using oedometer tests using the method of Casagrandecombined with a numerical method proposed byGregory et al. (2006) to avoid subjective interpretationsof the maximum curvature point.

Estimated preconsolidation stresses higher than theones expected in the Quaternary soils of the Vega Bajaand Media of the Segura river have been measured.These soils have recorded the effective stresses causedby water level changes and desiccation. Overconsolida-tion principally affects the upper fringe of the soilcolumn (0 to 5 m depth) with OCR values ranging from14.3 to 1.4. From 5 to 10 m, soil samples are slightlyconsolidated, with OCR values varying from 7.6 to 0.7.For depths greater than 10 m, the soil can be consideredas normally consolidated.

Overconsolidation of shallow layers of soils was alsoshown by the shear strength properties of these soils.Uniaxial compressive strength and undrained shearstrength corroborate the above-mentioned phenomenon,showing anomalous values of undrained shear strengthfor the 0–10 m. On-site tests like SPT also show thisbehaviour for surface tests.

Causes of such overconsolidation may be explainedby the water table changes occurred in the past and thedesiccation processes related with these changes. Forexample, induced stresses by water table descent canexplain an overconsolidation stress increase varyingfrom 0 to 41 kPa for the more superficial samples, andfrom 42 to 153 kPa for deeper layers.

Nevertheless, these increases are only a part of thecalculated overconsolidation stresses. Water table oscil-lations cause an increase in effective stresses of thesaturated fringe affected by water level variations. Fur-thermore, these variations can cause changes in soilmoisture content that favour preconsolidation stressesdue to desiccation processes due to the negative gen-

erated pore pressures. In addition, pedogenic processesaffecting the more superficial soil layers, such as bio-logical processes and secondary calcite precipitation,have been observed at a few points in the area studied.Overconsolidation stresses varying from 42 to 188 kPafor the more superficial samples have been attributed tothis phenomenon in the VBSR and the VMSR.

Finally, this overconsolidation is considered of greatpractical importance when considering the deformation-al behaviour of soils when loaded. Small loads appliedat ground surface (as the ones induced by the spreadfoundation of a single family building) will affect themost surficial layer of soils, those most heavily over-consolidated. Thus, only small scale, elastic and re-coverable strains may be expected. On the contrary, agreat increase in stress (as the ones induced by a roadembankment) can affect deeper, normally consolidated(and more deformable), layers of soils causing importantinelastic and irrecoverable strains.

Acknowledgements

Our thanks to Dr. A.S. Gregory (Rothamsted Re-search, UK) for the preconsolidation calculus Excelspreadsheet and P. Robles (Esfera, S.L.) and P. Alfaro(University of Alicante) for their helpful comments. Thisstudy was partially funded by the Spanish Ministry ofScience and Technology and FEDER (Project TEC-2005–06863), by the Valencia Regional Government(Project GV06/179 and GRUPOS03/085) and by theUniversity of Alicante (Project VIGROB-157). Thecompanies CEICO S.L. and ITC S.A. kindly providedpart of the geotechnical data used. The ExcelentísimaDiputación Provincial de Alicante (DPA) and InstitutoGeológico y Minero de España (IGME) were kindenough to provide piezometric and hydrological data.

References

AENOR, 1992. Geotecnia, UNE 103–800–92. Ensayos in situ.Ensayo de penetración estándar (SPT). Asociación Española deNormalización y Certificación, Madrid. 8 pp.

AENOR, 1993. Geotecnia, UNE 103–804–93. Procedimiento inter-nacional de referencia para el ensayo de penetración con el cono(CPT). Asociación Española de Normalización y Certificación,Madrid. 20 pp.

AENOR, 1994. Geotecnia, UNE 103–405–94. Ensayo de consolida-ción unidimensional de un suelo en edómetro. AsociaciónEspañola de Normalización y Certificación, Madrid. 10 pp.

AFNOR, 1995. XP P94–202. Sols: reconnaissance et essais.Prélévement des sols et des roches. Association Française deNormalisation, Paris. 42 pp.

AENOR, 1998. Geotecnia, UNE 103–402–98. Determinación de losparámetros resistentes de una muestra de suelo en equipo triaxial.


Asociación Española de Normalización y Certificación, Madrid.40 pp.

Alfaro, P., 1995. Neotectónica en la Cuenca del Bajo Segura (extremooriental de la Cordillera Bética). Tesis doctoral. Universidad deAlicante, 219 pp.

Aragón, R., García-Aróstegui, J.L., Lambán, J., Hornero, J.,Fernández-Grillo, A.I., 2004. Impacto de la explotación intensivade aguas subterráneas en la ciudad de Murcia (España). Análisishidrogeológico. Proc. XXXIII Congress of IAH-ALHSUD,Conference on Groundwater Flow Understanding from Local toRegional Scales, Zacatecas, Mexico, pp. 2622–2624.

Arvidsson, J., 2001. Subsoil compaction caused by heavy sugarbeetharvesters in southern Sweden I. Soil physical properties and cropyield in six field experiments. Soil Tillage Res. 60, 65–76.

Atkinson, J.H., Lewin, P.I., Ng, C.L., 1987. Undrained strength andoverconsolidation ratio of clay till. Int. J. Rock Mech. Min. Sci.Geomech. Abstr. 24, 133.

Canales, G., Vera Rebollo, J.F., 1985. Colonización del CardenalBelluga en las tierras donadas por Guardamar del Segura: Creaciónde un paisaje agrario y situación actual. Invest. Geogr. 3, 143–160.

Casagrande, A., 1936. The determination of pre-consolidation loadand its practical significance. Proc. of the First InternationalConference on Soil Mechanics and Foundation Engineering,Cambridge, England, vol. 3, pp. 60–64.

Cerón, J.C., Pulido, A., 1996. Groundwater problems resulting fromCO2 pollution and overexploitation in Alto Guadalentín aquifer(Murcia, Spain). Environ. Geol. 28, 223–228.

Cetin, H., 2000. An experimental study of soil memory andpreconsolidation adjacent to an active tectonic structure: theMeers fault, Oklahoma, USA. Eng. Geol. 57, 169–178.

Cetin, H., 2005. Geologic and geotechnical effects of an impact causedby an airplane crash. Eng. Geol. 80, 260–270.

Chu, J., Myint Win, B., Chang, M.F., Choa, V., 2002. Consolidationand permeability properties of Singapore Marine clay. J. Goetech.Geoenviron. Eng. 128, 724–732.

Dawidowski, J.B., Koolen, A.J., 1994. Computerized determination ofthe precompression stress in compaction testing of field coresamples. Soil Tillage Res. 31, 277–282.

Delgado, J., López Casado, C., Alfaro, P., Estévez, A., Giner, J.,Cuenca, A., Molina, S., 2000. Mapping soft soils in the Segurariver valley (SE Spain): a case of study of microtremors as anexploration tool. J. Appl. Geophys. 45, 19–32.

Delgado, J., Alfaro, P., Andréu, J.M., Cuenca, A., Doménech, C.,Estévez, A., Soria, J.M., Tomás, R., Yébenes, A., 2003.Engineering-geological model of the Segura river flood plain(SE Spain): a case study for engineering planning. Eng. Geol. 68,171–187.

Devinzenci, M.J., 2002. Ensayos geotécnicos in situ. Parte 1: Ensayode penetración estándar SPT. In: López-Jimeno, C. (Ed.),Ingeniería del terreno. INGEOTER, vol. 1, pp. 25–54. E.T.S.I.Minas-U.P.M., Madrid.

Devinzenci, M.J., 2003. Ensayos geotécnicos in situ. Parte 2: Ensayode penetración estática y piezocono. In: López-Jimeno, C. (Ed.),Ingeniería del terreno. INGEOTER, vol. 2, pp. 27–56. E.T.S.I.Minas-U.P.M., Madrid.

Feda, J., 1978. Stress in Subsoil and Methods of Final SettlementCalculation. Elsevier, Amsterdam. 216 pp.

Graham, J., Shields, D.H., 1985. Influence of geology and geologicalprocesses on the geotechnical properties of plastic clays. Eng.Geol. 22, 109–126.

Gregory, A.S., Whalley, W.R., Watts, C.W., Bird, N.R.A., Hallet, P.D.,Whitmore, A.P., 2006. Calculation of the compression index and

precompression stress from soil compression test data. Soil TillageRes. 89, 45–57.

Hobbs, B.E., Means, W.D., Williams, P.F., 1976. An Outline ofStructural Geology. Wiley, New York. 571 pp.

Hoffman, J., 2003. The application of satellite radar interferometry tothe study of land subsidence over developed aquifer systems. PhD.Thesis, University of Stanford, 211 pp.

Holzer, T.L., 1981. Preconsolidation stress of aquifer systems in areasof induced land subsidence. Water Res. 17, 693–704.

Houlsby, G., Wroth, C.P., 1991. Variation of shear modulus of a claywith pressure and overconsolidation ratio. Int. J. Rock Mech. Min.Sci. Geomech. Abstr. 29, A-153.

IGME, 1986. Calidad de las aguas subterráneas en la Cuenca Baja delSegura y costeras de Alicante. Instituto Geológico y Minero deEspaña, Madrid.

IGME-DPA, 1996. Estudio de los recursos de agua salobre en la zonasur de la provincial de Alicante. Previabilidad de la gestión de lasalmuera de rechazo mediante ISP. Instituto Geológico y Minerode España and Diputación Provincial de Alicante, Madrid.

Jiménez Salas, J.A., De Justo Alpañés, J.L., 1976. Geotecnia ycimientos I. Propiedades de los suelos y de las rocas. Ed. Rueda,Madrid, 466 pp.

Jorgensen, D.G., 1980. Relationship between basic soil-engineeringequations and basic ground-water flow equations. U.S. GeologicalSurveyWater Supply Paper, 2064.USGeological Survey,Washington.

Kenny, T.C., 1964. Sea level movements and the geologic histories ofthe Post-Glacial soils at Boston, Nicolet, Otawwa and Oslo.Geotechnique 14, 203–204.

Larsson, R., Ahnberg, H., 2005. On the evaluation of undrained shearstrength and preconsolidation pressure from common field test inclay. Can. Geotech. J. 42, 1221–1231.

Martínez, M., Mulas, J., Herrera, G., Aragón, R., 2004. Efectos de unasubsidencia moderada por extracción de agua subterránea enMurcia, España. Proc. XXXIII Congress of IAH-ALHSUD,Conference on Groundwater Flow Understanding from local toregional scales, Zacatecas, Mexico, pp. 249–252.

Mesri, G., Ali, S., 1999. Undrained shear strength of a glacial clayoverconsolidated by dessication. Geotechnique 49, 181–198.

Montenat, C., 1977. Les basins néogènes et quaternaries du Levantd'Alicante à Murcie (Cordillères Bétiques orientales, Espagne).Stratigraphie, paléontologie et evolution dynamique. Doc. Lab.Geol., Univ. Lyon, vol. 69. 345 pp.

Mulas, J., Aragón, R., Martínez, M., Lambán, J., García-Arostegui, J.L.,Fernández-Grillo, A.I., Hornero, J., Rodríguez, J., Rodríguez, J.M.,2003. Geotechnical and hydrological analysis of land subsidence inMurcia (Spain). Proc. of the 1st International Conference onGroundwater in Geological Engineering, RMZ-M&G, Materialsand Geoenvironment, Bled, Slovenia, vol. 50, pp. 249–252.

O'Neill, M.W., Yoon, G., 1995. Engineering properties of over-consolidated Pleistocene soils of Texas Gulf coast. Transp. Res.Rec. 1479, 81–88.

Pacheco-Silva, F., 1970. A new graphical construction for determina-tion of the preconsolidation stress of a soil sample. Proceedings ofthe 4th Brazilian Conference on Soil Mechanics and foundationEngineering, Rio de Janeiro, Brazil, vol. 2, pp. 225–232.

Rodríguez Jurado, J., Martínez Corbella, M., Mulas, J., RodríguezOrtiz, J.M., 2000. Establecimiento de un modelo geológico para elestudio de la subsidencia por rebajamiento del nivel freático.Geotemas 1, 155–158.

Selby, J., Lindsay, J.M., 1982. Engineering geology of the Adelaidecity area. South Australia Geological Survey Bulletin, vol. 51.94 pp.


Stamatopoulos, A.C., Kotzias, P.C., 1985. Soil Improvement byPreloading. John Wiley and Sons, Inc., New York. 261 pp.

Stapledon, D.H., 1970. Changes and structural defects developed insome south Australian clays, and their engineering consequences.Symp. On Soil and Earth Structures in Arid Climates, Inst. Eng.Aust. Geomech. Soc., Adelaide, pp. 62–71.

Tavenas, F., Des Rosiers, J.P., Leroueil, S., La Rochelle, P., Roy, M.,1979. The use of strain energy as a yield and creep criterion forlightly overconsolidated clays. Geotechnique 29, 285–303.

Tomás, R., Cuenca, A., Delgado, J., Doménech, C., 2000. Ausculta-ción de terraplenes mediante línea continua de asientos en la VegaBaja del Segura (Alicante). Comparación con los asientosprevistos. Carreteras 124, 50–59.

Tomás, R., Márquez, Y., Lopez-Sanchez, J.M., Delgado, J., Blanco, P.,Mallorqui, J.J., Martinez, M., Herrera, G., Mulas, J., 2005.Mapping ground subsidence induced by aquifer overexploitation

using advanced differential SAR interferometry: VegaMedia of theSegura River (SE Spain) case study. Remote Sensing of theEnvironment, vol. 98, pp. 269–283.

Tomás, R., López-Sanchez, J.M., Delgado, J., 2006. Subsidence of theVega Baja of the Segura River, Spain measured by means ofDInSAR. ESA internal report.

Tovey, N.K., 2002. Dessication of Late Quaternary inner shelfsediments: microfabric observations. Quat. Int. 92, 73–87.

Tschebotarioff, G.P., 1951. Soil Mechanics, Foundations and EarthStructures. Mac Graw Hill, New York. 655 pp.

Vázquez, J.N., De Justo, J.L., 2002. La subsidencia en Murcia.Implicaciones y consecuencias en la edificación. COPVT-ASEMAS,Murcia. 262 pp.

Wood, D.M., 1990. Soil Behaviour and Critical State Soil Mechanics.Cambridge University Press. 486 pp.

preconsolidation stress

Documents

deeper layers

depth of soil

themaximum effective

cuenca e

superficial layers lower

undisturbed soil samples

vega baja andmedia

superficial samples