12blz. Failure of a Trial Embankment on Peat in Booneschans; The Netherlands - Zwanenburg

8/12/2019 12blz. Failure of a Trial Embankment on Peat in Booneschans; The Netherlands - Zwanenburg

http://slidepdf.com/reader/full/12blz-failure-of-a-trial-embankment-on-peat-in-booneschans-the-netherlands 1/12

Zwanenburg, C. et al. (2012). Ge otechnique 62, No. 6, 479–490 [http://dx.doi.org/10.1680/geot.9.P.094]

479

Failure of a trial embankment on peat in Booneschans, the Netherlands

C . Z W A N E N B U R G , E . J . D E N H A A N , G . A . M . K R U S E a n d A . R . K O E L E WI J N

A trial embankment 6 m high was built on peat in theNetherlands, and brought to failure. The aim was to testwhether innovative sensor technology could detect incipi-ent failure in time. However, the data generated alsomade it possible to conduct a geotechnical assessment of stability. The paper discusses the relation between theparameters derived from laboratory tests, field measure-ments and behaviour observed in the field. Problemsencountered in the standard triaxial testing of peatsamples are discussed, as the samples tend to fail intension. A trial pit through the failed embankmentshowed that there were wide tension cracks in the peatlayer, indicating that the peat layer failed in tension, atleast locally. However, horizontal movement dominatesthe overall failure mechanism. As an alternative to tri-axial testing, direct simple shear (DSS) testing for theassessment of peat parameters is considered. The resultsof the DSS tests correspond well with field measurementsand the back-analysis of the failure.

KEYWORDS: embankments; failure; full-scale tests; laboratorytests; organic soils

On a construit, aux Pays-Bas, une digue d’essai de 6 mde haut sur de la tourbe, jusqu’a sa rupture. L’objet etaitd’etablir si la technologie innovante de detection etait enmesure de detecter a temps l’amorcage de la rupture.Toutefois, les donnees produites ont egalement permisd’effectuer une evaluation geotechnique de la stabilite.La communication examine le rapport entre les para-metres derives d’essais en laboratoire, de mesures effec-tuees sur le terrain, et comportements releves a piedd’oeuvre. On discute des problemes rencontres dans lesessais triaxiaux standards d’echantillons de tourbe, lesechantillons ayant tendance a se rompre lorsqu’ils sontsoumis a tension. Une fosse d’essai pratiquee dans ladigue defaillante a montre la presence de larges fissuresde contraintes dans la couche de tourbe, en indiquantainsi la rupture de la couche de tourbe sous l’effet de latension, tout au moins a l’echelon local. Toutefois, lemecanisme de rupture general est domine par un mouve-ment horizontal. En alternative aux essais triaxiaux, onexamine egalement des essais a cisaillement simple pourl’evaluation des parametres de la tourbe. Les resultatsdes essais de cisaillement simple correspondent parfaite-ment aux mesures effectuees sur place ainsi qu’aux retroanalyses de la rupture.

INTRODUCTIONDespite the considerable experience of water authorities,

occasional dyke failures cannot be excluded. An example isthe failure of a dyke near Wilnis in the Netherlands in 2003(Van Baars, 2005; Van et al., 2008). After the Wilnisincident, methods were sought to improve the assessment of dyke condition, including the application of modern sensor technology.

The IJkdijk (‘calibration dyke’) project was initiated totest the viability of modern sensor technology for the assess-ment of actual dyke strength. A full-scale trial embankmentwas built and brought to failure, as discussed by Abdoun et al. (2010). The results of this test and other studies will lead to an early warning system for dyke stability in the near future. The test provides the opportunity to study the stabi-lity of an embankment in relation to peat behaviour. This

paper describes the induced failure of the trial embankment,and compares the mobilised shear strength and failure modewith the laboratory tests and field measurements.

The subsurface underneath the trial embankment includesa peat layer, the strength of which is a major factor control-ling the stability of the embankment. As shown by, amongothers, Yamaguchi et al. (1985) and Den Haan & Kruse(2006), parameter assessment using standard triaxial tests on peat samples provides unrealistically high strength para-meters. This holds especially for the friction angle, 9. Thiscomplication was also encountered in the design of the trialembankment.

The second and third sections of this paper describe thetest and subsurface conditions, followed by a description of

the failure in the fourth section. The fifth and sixth sectionsset out the laboratory test results and field measurements.The seventh section discusses the mobilised shear strengthalong the failure plane, and makes a comparison withlaboratory and field measurements. The final section presentsconclusions.

TEST SET-UPFigure 1 shows an aerial photograph of the trial embank-

ment. A small strip of land, 100 m wide and 750 m long,situated alongside a canal (which can be seen in the top left-hand corner of Fig. 1), was available for construction of the

Manuscript received 24 July 2009; revised manuscript accepted 22

September 2011.Discussion on this paper closes on 1 November 2012; for further details see p. ii. Deltares, Delft, The Netherlands.

Fig. 1. Aerial photograph of trial embankment during test, andlocation of site. Canal at top left-hand corner



test embankment. A small dyke borders the canal, resultingin an additional boundary condition for the tests, since itshould not be damaged. The area has been in agriculturaluse, and has been a nature reserve for the past 15 years.

Figure 2 shows a cross-section of the trial embankment.The embankment consisted of a sand core with a clay liner,which is typical for dykes constructed since the 1960s in the Netherlands, and a crest 6 m above ground level, representa-

tive for a larger river defence. To facilitate failure, thelandward slope was made steeper (1:1.5) than permitted bycurrent standards.

The design of the test included five stages, shown in Fig.3. Stage 1 consisted of raising the water level between thecanal dyke and the trial embankment (see Fig. 1). Themaximum water level was limited by the height of the dyketo 3 m above ground level. A stepwise excavation of a ditchat the toe of the landward slope reduced stability further. Instage 2, the ditch was excavated down to 1 m below ground level. In stage 3 (shown in Fig. 1), excavation continued until the ditch reached a sand layer about 2– 3 m belowground level. During and after excavation, groundwater flowinto the ditch was low.

Stage 4 consisted of pumping water into the sand coreuntil it reached a height of two thirds of the core. Thisincreased the weight of the embankment and reduced thestrength of the sand core. The trial embankment failed instage 4, as anticipated. However, to ensure the failure of theembankment, an additional step (stage 5) was optional inwhich the sand core was fully saturated and containers ontop of the embankment were filled with water. Fig. 1 showsthe empty containers on the crest of the embankment. Sincethe trial embankment failed in stage 4, stage 5 was notrequired.

Before construction, the grass cover and top 10 cm of thesubsurface were removed in order to ensure good contact between the embankment and the subsurface. The embank-ment was erected in steps of 0.5–1 m height.

Table 1 presents the characteristics of sand and the state parameters for the sand in the embankment 1.5 m abovedatum level. The sand was not densified, except by dumptrucks in the initial stages of construction up to +2 m abovedatum, after which the sand was applied by hydrauliccranes.

Table 2 gives the characteristics of the locally derived clayfor the liner. The clay liner had to be compacted in order to permit water filling of the sand core The clay was compacted by a roller in 0.3– 0.5 m layers. A constant-volumecore container was used for compaction control; Table 2gives the average of eight measurements taken from differentlevels along the clay cover.

The strength of the sand was derived from triaxial compression tests. The samples were prepared for three differentlevels of relative density, Re ¼ 0.44, 0.49 and 0.56. For eachlevel, three samples were tested. The value for the frictionangle, 9, ranges from 30.98 for Re ¼ 0.44 to 31.78 for Re ¼ 0.49 and 31.98 for Re ¼ 0.56. It should be noted thatthe tested samples have a lower density, indicated by a lower value for Re than found in the trial embankment, as given byTable 1. Extrapolation of the test data to Re ¼ 0.71 gives9 ¼ 33.28, which is used in design and analysis calcula-tions.

The strength of the clay cover was determined by directsimple shear (DSS) tests. Samples of the original clay weredensified until the volume weight and water content corre-sponded to the field values after densification, as given byTable 2. In the DSS apparatus, the samples were consoli-dated under a vertical load of 10 kN/m2 and sheared tofailure. For four tests an average undrained shear strength of su ¼ 31.2 kN/m2 was found.

SUBSOIL PROPERTIESA total of 33 cone penetration tests (CPTs) were con-

ducted and 22 boreholes were made using the Begemannsampling system. Fig. 4 shows a typical CPT at the test

6·07 17·33 4·32 1·6

27·7

3·7

3·25

6

2–3

.

Sand core

Clay cover

Datum level

Slope1:1·5

Slope 1:2·5

Infiltration system

Fig. 2. Cross-section of trial embankment

1

2

3

4

5

5

Stage 1: raising water level to 3 m above ground level

Stage 2 excavation to 1 m below ground level:

Stage 3 complete excavation soft soil layers:

Stage 4 raising water level in sand core to 2/3 sand core height:

Stage 5 filling sand core completely and filling containers on top:

Fig. 3. Stages of the experiment

Table 1. Properties of construction material: sand

Property Value

General properties

Sand median d 50: mm 0.22Uniformity coefficient, d 60/d 10 2.57Maximum void ratio, emax 0.86

Minimum void ratio, emin 0.44

After construction

Volume weight, ª : kN/m3 18.74Void ratio, e 0.56Relative density, Re ¼ (emax e)=(emax emin) 0.71Water content, W 0.125

Table 2. Properties of construction material: clay

Property Value

Original

Volume weight, ª : kN/m3 15.75Water content, W 0.60Liquid limit, wl 0.86Plasticity index, I p 0.57Void ratio, e 1.66

After densification

Volume weight, ª : kN/m3 17.3Water content, W 0.44Void ratio, e 1.17

480 ZWANENBURG, DEN HAAN, KRUSE AND KOELEWIJN



location. The stated depth is related to datum level, theDutch NAP reference, which corresponds to approximatelymean sea level. The ground level at the location ranges from NAP 0.9 m to NAP 1.1 m. Fig. 4 also shows CPTinterpretation, together with data from an adjacent borehole.The original soil build-up at the site consisted of Pleistocene

aeolian fine sand at the base, followed by Holocene peatdominated by sedge and wood, with a thin intercalated clayinterval in most of the area. At the centre of the trialembankment the thickness of the peat is approximately3.0 m, diminishing to 1.5 m to the north and 2 m to thesouth. The surface consists of a 0.2 m to locally 1.0 m thick clay layer, overlying the peat.

Just below the top of the Pleistocene sand an Allerød soilhorizon occurs, and is covered by less than 0.2 m loose sand.This soil horizon hampers vertical drainage in the sand.

Figure 5(a) shows the unit weight ª for the samples taken

from a typical borehole. Values lower than the water densityare found in the lower half of the peat layer. This might becaused by the loss of some pore water from large poresduring sampling, or the presence of gas bubbles, which iscommon in this type of peat. Fig. 5 shows increasing water content (W) and loss on ignition ( N) with increasing depthin the peat layer, while unit weight falls slightly with depth.

Before and during construction of the trial embankment,

the hydraulic head in the sand layer and pore pressures inthe peat layer were measured. Fig. 6 shows the initial profiles for total stress, pore pressure and effective stress before construction started.

The measurements, which were conducted in the summer season before the start of construction, indicate a phreaticlevel of NAP 1.9 m, which is below the top of the peatlayer. The hydraulic head in the sand layer was measured for a longer period, and a seasonal fluctuation was found be-tween NAP 1.4 and 1.7 m. Measurements did not show acorrelation between the water table in the nearby canal and pore pressure in the sand and peat layers. The low unitweight of the peat and non-hydrostatic pore pressuredevelopment in depth meant that there was a minor fall withdepth of the effective stress in the peat layer.

Figure 6 also shows the vertical pre-consolidation stress( 9vc) derived from six constant rate of strain (CRS) teststaken from one borehole. Fig. 7 clarifies the distribution of 9vc in the peat layer, showing the results of all the CRSand oedometer tests. Because of variations in the thicknessof the top clay, the vertical effective stress level may differ for samples taken from the same depth in different bore-holes. To account for this effect, Fig. 7(b) shows the over-consolidation ratio (OCR) for each of the tests. Here, theOCR is defined as 9vc= 9v, with 9v representing the verticaleffective stress in the field, estimated on the basis of theunit weight of the soil in the borehole from which thesample was taken, and the measured hydraulic head inthe sand. Fig. 7 shows a scatter of 9vc at about the phreatic

level in the range 22–38 kN/m2, with an OCR in the range2.5–4 and an extreme value around 8. At a lower level inthe peat layer, a small increase in 9vc is found with depth,with an OCR in the range 1.8–1.9.

THE OBSERVED FAILUREThe pumping of water into the sand core resulted in the

failure of the trail embankment (Fig. 8). Infiltration started on 27 September 2008 at 12.00 noon. The infiltration systemconsisted of six infiltration tubes with a diameter of 15 cm,

8 6 4

0 20 40 60 80

2 0 2 4 6 8 10 12 14

qc: MPa

R f : %

1·0

2·0

3·0

4·0

5·0

6·0

7·0

Clay

Peat

Sand

qc

U 2

Depth:NAPml

U 2: kPa

Fig. 4. Typical subsoil profile at test location CPT S37

10 12 14 16 18 20

γ: kN/m3

1

2

3

4

5

Depth:NAPm

0 2·0 4·0 6·0

W

0·2 0·4 0·6 0·8 1·0

N

Clay

Peat

Sand

Fig. 5. Soil characterisation for borehole 12: (a) unit weight, ª; (b) water content, W ; (c) loss on ignition, N

FAILURE OF A TRIAL EMBANKMENT ON PEAT IN BOONESCHANS, THE NETHERLANDS 481



interconnected by a manifold and placed in the sand core

50 cm above ground level. There was also a tube in the topsoil layer, close to the centreline of the embankment.Infiltration started through this lower tube. The six infiltra-tion tubes were then activated until failure occurred. There

was a pause in filling from 15.02 to 15.41 hours and another at 15.58 hours to alleviate the rapid rise in infiltration pressures.

Failure occurred at 16.02 hours. The observed failure wasessentially translational, with the peat layer and overlyingsoil moving into the ditch. The maximum horizontal displacement, perpendicular to the embankment, was approxi-mately 5 m. The core sand subsided into the soft clay and peat layers, and moved outward. The displacement of thesand core caused slumping of the clay liner. The far side of the ditch halted the sliding mass. The width of the sliding plane, parallel to the embankment, was 40 m.

Figure 9 shows a cross-section of the failed section of the

embankment by reference to the original section. The infor-mation in Fig. 9 follows from the inspection of the walls of the trial pit, and CPT and borehole data. Fig. 10 shows thelocation of the trial pit. Most of the lateral deformationwitnessed in the trial pit was found at the contact of the peatand underlying sand layers. The peat and top clay units had slid, largely intact, into the excavated ditch upon failure. Thehorizontal part of the sliding plane runs along the sand/peatintersection. Since the sand/peat intersection is not a per-fectly flat level, contact between the peat and underlyingsand was found to be locally intact, whereas elsewhere inthe pit the sand/peat intersection was, at least at the top of the original sand layer, cut off. There was extensive localfragmentation of the base of the peat layer locally; elsewhere

it was intact up to the contact with the sand. Furthermore, itwas found that the top of the sand layer was loose and saturated, and appeared liquefied. A thin shear zone, or shear plane, involving both sand and peat, had apparently devel-oped upon failure.

The trial pit showed that wide vertical cracks had devel-oped in the peat and top clay layer. Tension cracks, 40 cmwide, were found, filled with clay liner: see Fig. 11. Thetension cracks resolved at the base in sub-horizontal shear planes inclining in a downslide direction, just above thecontact with the sand. The location of the tension cracks isconsistent with a movement pattern seen in video recordingsof the failure. Steeply dipping and opposing shear planes inthe embankment core underneath and landward of the origi-

nal crest of the embankment were found to extend into theclay and peat underneath. Cumulative offset along the shear planes extended several decimetres. The shear planes demar-cated a zone where the core had squeezed into the clay and

10·0 20·0 30·0 40·0 50·0 60·0

σ : kN/m2

1

2

3

4

5

D e p t h :

N A P m

Total stressEffective stress

∗ Vertical pre-consolidation stress, σ vc

Location of p pressure transducersore

Clay

Peat

Sand

Pore pressure

Fig. 6. Soil stresses at borehole 12

σ σ vc v/

.

10·0 20·0 30·0 40·0

σ vc2: kN/m

1

2

3

4

5

Depth:NAPm

0 2·0 4·0 6·0 8·0

(a) (b)

Fig. 7. (a) Pre-consolidation stress; (b) OCR of tested peatsamples

Fig. 8. The embankment after failure




C l a y

l i n e r

S a n

d

P e a

t

I n i t i a l m o v e m e n

t :

I n i t i a l d o w n w a r d m o v e m e

n t o

f a w e

d g e o

f d y

k e m a

t e r i a

l i n t o t h e s u

b s u r f a c e

b y

t r a n s

l a t i o n a

l o n g

f a u

l t 1 a n

d 2

, w

i t h l a t e r a

l t r a n s

l a t i o n a

l o n g a

b a s a

l s

h e a r

p l a n e .

T h e s p a c e c r e a

t e d b e

l o w

t h e

i n t r u d i n g w e

d g e a

l o n g

f a u

l t 1 i s a c c o m m o

d a

t e d

b y m

i n o r s q u e e z

i n g o

f p e

a t .

( A r r o w s

i n d i c a

t e t r a n s

l a t i o n o

f b l o c

k s ,

i n d i c a

t e s e n s e o

f f a u

l t m o v e

m e n

t )

1

2

S a n

d f i l l

B l o c k t r a n s

l a t i o n :

T h e

i n t r u d i n g w e

d g e p u s

h e s

t h e s u

b s u r f a c e a n

d o v e r l y

i n g

d y

k e s e g m e n

t

a s a

c o

h e r e n

t b l o c

k t o w a r d s

t h e

d i t c h

.

S a n

d P e a

t

S a n

d f i l l

( a )

( b )

( d )

C o

l l a p s e o

f t h e

d y

k e m a t

e r i a

l a n

d s

t r e t c h i n g o

f t h e u n

d e r l y

i n g p e a

t :

T h e c o

h e r e n

t b l o c

k c o

l l a p s e s w

i t h s

t r e t c h i

c o

i n c

i d e

n t w

i t h l o s s o

f s u p p o r

t f r o m

t h e s

i d e w a

l l s

( a f t e r v

i d e o r e c o n s

t r u c

t i o n

) .

C l a y

l i n e r c

l a y e n

t e r s

t h e

e x

t e n s

i o n c r a c

k s

i n t h e p e a

t , a n

d i s s u

b s e q u e n t

l y

s q u e e z e

d b y

i n e r t i a o

f t h e m o v

i n g m a s s .

n g a n

d m

i n o r s q u e e z

i n g o

f t h e

u n

d e r l y

i n g p e a

t ,

S a n

d

S a n

d f i l l

( c )

C l a

y l i n e r , p r e - f a

i l u r e

F a

i l e d c

l a y

l i n e r

T o p c

l a y

P e a

t

S a n

d

P r e - f a

i l u r e

t e r r a

i n s u r f a c e

A p p r o x .

2 0 m

B o u n

d a r y s a n

d / p e a

t

p r e - f a

i l u r e

2 m

2 m

D r a

i n ( r e m n

a n

t s )

P e a

t

S a n

d f i l l

F a u

l t s

I n i t i a l a n

d f i n a

l t r a n s

l a t i o n s u r f a c e

I n i t i a l t r a n s

l a t i o n s u r f a c e

C l a y - f i l l e

d e x

t e n s

i o n c

r a c

k s

E x c a v a

t e d a r e a

2

D a

t u m

E x c a v a

t e d a r e a

1

F i n a

l c o n

f i g u r a

t i o n a n d

p o s

i t i o n o

f t h e e x p o s u r e s

5 m

F i g

. 9

. S k

e t c h o

f f a i l u r e m e c

h a n

i s m :

( a ) i n i t i a l m o v e m e n t ; ( b ) s u

b s e q u e n t m o v e m e n t ; ( c ) fi

n a

l c o

l l a p s e ;

( d ) fi n a

l c o n

fi g u r a t i o n




zero radial effective stress condition – the ‘ tension cut-off’with inclination 3:1 through the origin – and are then forced to remain there, making failure ill defined. This phenomenonis typical for peat samples (see Yamaguchi et al., 1985; DenHaan & Kruse, 2006; Mathijssen et al., 2009). This phenom-enon could be a result of tensile strength provided by the plant remain fibres in the peat (Cola & Cortelazzo, 2005;Landva, 2007). However, the fibre tensioning does notexplain the similar behaviour of organic clays, lacking sucha fibre framework, stressing the need for more thoroughunderstanding.

The very high triaxial compression strength is not likelyto be representative of bulk behaviour in a peat layer with-standing embankment loading. The inherent anisotropy of

the peat, the initial stress state anisotropy, and the rotationof the principal stress states during embankment loading arelikely to produce effects that combine to reduce the averagestrength to a value well below the triaxial compressivestrength.

Den Haan & Kruse (2006) describe a graphical procedureto reduce the measured undrained shear strength of peat intriaxial compression. The initial part of the effective stress path, in which pore pressures are still increasing, extends tothe tension cut-off line, resulting in an undrained strength, su, that disregards the effects of dilation during the latter part of the test.

Figure 14 shows the results of 10 direct simple shear (DSS) tests conducted on peat samples. The samples shear at

a constant height, representing undrained behaviour. Fig. 14shows the 9v – combination at failure. Horizontal sliding planes are assumed at failure. Then, using the simple conven-tional interpretation, a line through the data points givesdrained parameters c9 and 9. Using the least-squares method,the best fit is found for c9 ¼ 2.3 kN/m2 and 9 ¼ 32.38, or 9 ¼ 33.98 if the 9v – curve is forced through the origin.

Table 3 shows the interpretation in terms of undrained shear strength. The undrained shear strength is normalised by the vertical consolidation stress applied in the test, vy.Comparison of ( su/ vy) peak found for triaxial compressionwith the value found for DSS tests indicates strong aniso-tropy in the strength of peat. The fibrous nature of peat and organic clay may explain the anisotropy. Mesri & Ajlouni

(2007) give an overview of the ( su/ vy) peak ratios for fibrous peat presented in the literature. The values( su/ vy) peak ¼ 0.61 for triaxial compression and 0.46 for DSSdetermined here correspond well to this overview. Den Haan

& Kruse (2006) report similar values for Dutch peat samples.

Leroueil et al. (1990) advocate the use of the undrained shear strength at large strain – the USALS approach, inwhich large strain is defined as an axial strain of 15% intriaxial testing. Fig. 13 shows that, after reaching the TCOline, each triaxial test indicates a further increase in mobi-lised shear strength. This means that no realistic values for ( su/ vy )USALS can be achieved. The analysis in this paper isfocused on peak strengths.

FIELD MEASUREMENTSThe failure of the trial embankment was induced shortly

after its construction. The presence of excess pore water pressure therefore plays an important role in determining

subsoil strength. In addition to measurement of the porewater pressure, the interpretation of CPTs gives an impres-sion of how strength developed during the construction of the trial embankment. For safety reasons, CPTs could not be

0 20·0 40·0 60·0 80·0 100·0 120·0 p: kN/m

2

0

20

40

60

80

100

120

140

160

180

200

q :kN/m

2

K 0 0·26

q p3

Fig. 13. Effective stress paths of CIU and CAU tests on peatsamples

γ

(b)

(a)

0 20 40 60 80 100σ v

2: kN/m

0

20

40

60

80

τ h:kN/m

2

τ h:kN/m

2

τ σ h v0·63 2·30

τ σ h v0·67

0 0·1 0·2 0·3 0·4 0·5 0·6

0

20

40

60

80

Fig. 14. Results of DSS tests on peat samples

Table 3. Undrained shear strength ratio su / vy found in labora-tory measurements

su/ vy Peak

Triaxial 0.61DSS 0.46




carried out before the end of the test. It was only 4 daysafter the failure took place that CPTs could be conducted through the embankment. This causes some uncertaintyabout the actual subsoil strength during the test.

Figure 15 shows an interpretation of CPT data before and after the test. The su values are derived from the CPT datausing

qt ¼ qc þ 1 að Þu2,

su ¼ q t v

N kt

(1)

where qc is the cone resistance, u2 is the water pressuremeasured above the tip of the cone, a ¼ 0.86 for the conetype used and N kt is the bearing factor. From experience,

N kt ¼ 12.6 is used.Figure 10 shows the location of the CPTs. CPT S37, also

shown in Fig. 4, took place before the construction of thetrial embankment. After the completion of the test, CPT S40was conducted through the embankment at a distance of 19.4 m from S37. Fig. 15 also shows laboratory resultscombining the su/ vy ratios discussed earlier with the pre-consolidation stresses presented by Fig. 6. The laboratoryresults correspond closely to CPT S37, which represents theinitial conditions. The DSS test results seem to agree slightly better than the triaxial test results. CPT S40 shows thatstrength was low at the transition between the peat and sand layer. This corresponds to the location of the failure plane.The difference in strength between CPT S37 (prior to theconstruction of the trial embankment) and CPT S40 (after completion of the test) is considerable.

The increase in strength observed in the CPTs before and after construction is explained by the consolidation of thesubsoil. CPT S40 was made four days after the failure, and the presence of the failure plane may have further acceler-ated consolidation. Fig. 16 shows the hydraulic head meas-urements from transducer W51 in the lower part of the peatlayer at a depth of 2.87 m below datum, near the locationof CPT S37 (see Figs 10 and 15 for the location). Fig. 16shows the pore pressure reaction in each construction phase,with a rapid increase in hydraulic head. Between the con-struction phases some reduction in hydraulic head wasrecorded, which is attributable to consolidation. The meas-urements indicate considerable pore pressure development

during construction, leading to a high excess pore water pressure at the start of the test. Fig. 16 zooms in onmeasurements during the test. It is found that step 1 haslittle or no influence on the hydraulic head measured bytransducer W51. Both the initial excavation (step 2) and thefurther excavation (step 3) lead to a hydraulic head increasethat stabilises after some time. During step 4 (filling thesand core), the hydraulic head initially decreases until theinfiltration discharge is increased, leading to a rapid increasein hydraulic head. The initial hydraulic head decrease could be related to visually observed water outflow from thesubsoil to the excavation. After increasing the infiltrationdischarge, the outflow stopped. An instantaneous drop inhydraulic head indicates the failure in Fig. 16.

Den Haan & Feddema (2012) discuss finite-element simu-lations of the failure of the IJkdijk trial embankment. Thefinite-element analysis, in an estimate of the effective stresslevel at the location of CPT S40 at the depth of the sliding plane, gives 9v ¼ 40 kN/m2: Assuming OCR ¼ 1 at the end of construction, and applying the su/ vy values earlier, leadsto an estimate of the undrained shear strength of peat at thestart of the test: su ¼ 18.4 kN/m2 for su/ vy ¼ 0.46 (simpleshear strength) and 24.4 kN/m2 for su/ vy ¼ 0.61 (triaxialcompression strength). Fig. 15 compares these values withthe CPT data.

It should be noted that S40 was made through the failed embankment, approximately at the location of the originaltoe. With stress conditions in the subsoil changing from

underneath the crest to the excavated ditch, S40 is assumed to represent average conditions along the sliding plane.

It follows from CPT S40 that su ¼ 19.6 kN/m2 at theshear plane location. The soil is strongly remoulded in the

* 0·61,su

σ p

0·61,su

σ p

0·46su

σ p

0·46su

σ p

(a)

(b)

Clay

Peat

Sand

Original soilprofile

0·4 0·8 1·2 1·6

qc: MPa

Location transducer W51

Depth:m

NAP

0·5

0

0·5

1·0

1·5

2·0

2·5

3·0

3·5

4·0

S37

S40

4·0

3·5

3·0

2·5

2·0

1·5

1·0

0·5

0

0·520 40 60 80 100

su: kPa

Depth:m

NAP

S37, before construction

S40, through failure plane

From laboratory tests:

From estimated after construction:σ vc

Fig. 15. su profiles calculated from CPT data: S37 conductedbefore construction of embankment; S40 after finishing testthrough failure plane




failure zone, and, owing to the time interval between thefailure and CPT S40, some consolidation has occurred. The su value cannot therefore be compared directly with the peak values found in the laboratory tests.

STABILITY ANALYSIS

From the field and laboratory measurements, several dif-ferent values for the undrained shear strength of the peatlayer are found. Table 4 summarises results from five differ-ent approaches, as discussed in the previous sections.

An extended finite-element method (FEM) analysis of theIJkdijk failure is given by Den Haan & Feddema (2012). Inthis paper the validity of the values presented in Table 4 ischecked by a limit equilibrium analysis used to calculate thestability of the trial embankment for the different su values.The stability is calculated for stage 3, without failure, and for stage 4 when failure did occur.

The configuration of the failure indicates that failure did involve more than a single continuous failure zone. Ladd (1969) suggests a working method for stability assessment of

embankments, in which the sliding plane is divided into anactive part, a direct shear part and a passive part (abbrevia-tion ADP). Parameter assessment in this method is based onthe different conditions for the material in each section, withtriaxial compression tests for the active part, shear tests for the direct shear part and triaxial extension tests for the passive part. This method is generally accepted for the

design of embankments on soft soil (Leroueil et al., 1990).Here an adapted approach is given for the observed failureof the test embankment.

The active part of the observed failure was largely in thesand of the embankment core, and constituted an assemblyof shear zones. The direct shear part of the failure was ahorizontal plane at the base of the peat layer. The passive part of the ADP sliding plane was absent in the configura-

tion, since the geometry did include an excavation at a shortdistance from the toe. Such a ditch is common for Dutchwater-retaining embankments. Following Ladd’s method, thestrength of the embankment is best derived from triaxialcompression tests, while strength parameters for the under-lying peat layer follow from DSS tests.

Figure 17 shows a two-dimensional schematisation of horizontal sliding. Equilibrium is found when the horizontalforce F h, which is the summation of the total horizontalearth pressure and water pressure, exceeds the shear resis-

0 10 20 30 40 50 60

Time since start of construction: days

- 4

- 3

- 2

- 1

0

1

2

3

4

5

6

Hydraulichead:NAPm

Construction period Test

0 10 20 30 40 50 60Time since start of test: h

5·0

5·5

6·0

6·5

Raising water level at the front(stage 1)

Start excavation(stage 2)

Further excav(stage 3)

ation

Moment of failure

Start filling core(stage 4)

Accelerate filling

Fig. 16. Hydraulic head development at base of peat layer recorded by transducer W51

Table 4. Results for five different cases for assessment of undrained shear strength for peat layer

Case Description su: kN/m2

1 ( su/ vy) peak from triaxial testing in combination with pre-construction 9vc 15.22 ( su/ vy) peak from triaxial testing in combination with actual 9vc estimated from FEM calculations 24.43 ( su/ vy) peak from DSS testing in combination with pre-construction 9vc 11.54 ( su/ vy) peak from DSS testing in combination with actual 9vc estimated from FEM calculations 18.45 su from field testing 19.6

2–3F h

S

∼14 m

Sand core

Clay cover

Fig. 17. Squeezing approach for trial embankment




tance S at the potential sliding plane. Since the width of thefailure, parallel to the trial embankment, B, is relativelysmall ( B ¼ 40 m), a two-dimensional approach (plane strain)is not sufficient. Friction along the failed mass, perpendicu-lar to the trial embankment, has a considerable contributionto the forces working on the failed mass. The horizontalequilibrium, based on the total forces working on the failed mass, is described by the equations

F h ¼ B 3

X ºa 3 9v þ wð Þ˜h½ (2)

ºa ¼ tan2 45 9

2

(3)

S bottom ¼ B 3 L 3 su (4)

S sc ¼

ð Asc

ºn 9v tan 9d Asc, ºn ¼ 1 sin 9ð Þ (5)

S pþtc ¼ A pþtc 3 su, S cc ¼ Acc 3 su (6)

S ¼ S bottom þ 2 S sc þ S cc þ S pþtcð Þ (7)

FS ¼ S

F h(8)

where F h is the total earth pressure; ºa is the active earth pressure coefficient; B is the width, parallel to the trialembankment ( B ¼ 40 m); ˜h is the height of an individualsoil layer (see Fig. 2); S bottom is the shear resistance alongthe bottom failed soil mass; L is the length of the sliding plane, perpendicular to the trial embankment (see Fig. 17); ºn is the neutral earth pressure coefficient; S sc, S pþtc and S cc

are the shear resistance along the side perpendicular to thetrial embankment for the sand core, peat and top clay, and clay cover respectively; Asc, A pþtc and Acc are the superficialarea perpendicular to the trial embankment of the sand core, peat and top clay, and clay cover respectively; S is the totalshear resistance; and FS is the factor of safety.

The horizontal force F h is found in equation (2) bysummation of the active earth pressure and water pressure ineach layer. Table 5 summarises the required soil parameters,as discussed in the previous sections. The value for ºa isfound from equation (3). Table 5 gives the value for 9applied in equation (3) in brackets. These values originatefrom the DSS tests on the peat and clay samples. For thesand of the embankment itself, 9 from triaxial testing isused. Since limited tests on the top clay layer are available,and the top clay layer is a very thin layer that makes only asmall contribution to the stability, the value for su top clay istaken to be equal to su peat: Interpolation of w given inTable 6 gives the water pressure needed for equation (2).

The maximum shear resistance, S , is found by summationof the shear resistance along the base and the sides of thefailed mass. The resistance of the clay and peat layers isfound by multiplying the contact surface area of the slidingmass by the relevant su values, as in equation (7). Thefriction along the contact areas in the sand of the embank-ment itself is found by integrating equation (5) over the

area. The factor of safety is defined in equation (8) as theratio of the resisting shear force S and the driving horizontalforce F h:

Table 7 shows the calculation results. Cases 1 and 3underestimate the stability of the trial embankment, sincethey predict failure in stage 3, whereas in this stage nofailure was observed in the test. Case 2 overestimates thestability, since it predicts SF . 1.0 for both stages, whereas

the trial embankment did fail in stage 4 in the test. Cases 4and 5 fit the observed failure.The calculation results indicate that the consolidation

during construction led to a relevant increase in su, resultingin underestimation of the stability by cases 1 and 3. Onlythe stability calculated using the DSS test results (case 4)fits the observed failure if the effect of consolidation on thedevelopment of su is included. Using the triaxial test results(case 2) results in overestimation of the stability conditionof the embankment in the field test. The shear strengthderived from the field tests (case 5) compares well with thevalue found by the DSS tests (case 4).

A sensitivity analysis was carried out to check the validityof the calculation results. Fig. 18 shows the results of thesensitivity of the four most relevant parameters. These four parameters are related to the embankment properties( su clay cover , 9sand , ªsand ) and the active earth pressure coeffi-cient for the peat layer, ºa peat: For each parameter, thestability is calculated for the five cases for stages 3 and 4.Fig. 18(a) shows the sensitivity of the analysis for su clay cover , which contributes to the shear resistance alongthe sides. As is to be expected, an increase in su clay cover

leads to an increase in SF. Fig. 18(b) shows that an increasein ªsand leads to a minor reduction in SF; an increase inªsand causes an increase in F h as well as an increase in S sc:Fig. 18(c) shows the influence of 9sand : An increase in 9sand

leads to a reduction in F h ( ºa decreases for increasing 9according to equation (5)) as well as in an increase in S sc:For the range for which 9sand i s to be expected

(308 , 9sand , 358) Fig. 18(c) shows that its influence on thecalculation result is small. The same holds for the para-meters su clay cover and ªsand :

Figure 18(d) shows the influence of the active earth pressure coefficient for peat, ºa peat: Equation (3) gives theexpression for ºa as a function of 9: It should be noted that ºa ¼ 1 is found for 9 ¼ 08, and ºa ¼ 0 is found for

Table 5. Material properties for the different layers

Layer ª: kN/m3 su: kN/m2 9: degrees ºa (given by equation (3))

Clay cover 17.30 31.19 – 0.376 (9¼ 27.08)Sand core 18.74 – 33.2 0.292 (9 ¼ 33.28)Top clay 15.93 y – 0.309 (9 ¼ 31.98)Peat 10.09 { – 0.284 (9 ¼ 33.98)

For sand below the water table in stage 4, ª ¼ 20.1 kN/m3 is applied.y For the thin top clay layer, s u peat is applied.{ For s u peat, the five different cases from Table 4 are applied.

Table 6. Relevant water pressure in stages 3 and 4

Depth: m above datum level w : kN/m2

Stage 3 Stage 4

2 0 01 0 23.91.84 42.3 43.62.87 87.2 90.0




9 ¼ 908. For a decrease in ºa ( increase in 9) F h decreases,leading to an increase in SF. The calculations shown inTable 7 are based on ºa peat ¼ 0.284, 9 peat ¼ 33.98, which isfound in the DSS tests. As discussed earlier, five largevalues for 9 peat and therefore low values for ºa peat are found by triaxial testing. Fig. 18(d) shows that for low values of

ºa peat the conclusions drawn from Table 3 are still valid;only cases 4 and 5 predict stability in stage 3 and failure instage 4, as observed in the test. Case 2 still overestimatesthe stability, whereas cases 1 and 3 underestimate the

stability. When 9 peat , 208, ºa peat . 0.49, the conclusionsdrawn from Table 5 are no longer valid. Then only case 2 predicts a stable situation for stage 3 and failure in stage 4,as observed in the test. The laboratory tests discussed earlier indicate that values as low as 9 peat ¼ 208 are not expected.

CONCLUSIONSA 6 m high trial embankment was constructed on peat

and brought to failure. Horizontal displacement of a 2.5 m

0 10 20 30 40 50 60

0·2

0·4

0·6

0·8

1·0

1·2

1·4

1·6

1·8

suclay cover 2

: kN/m

SF

(a)

0 10 20 30 40 50 60

0·2

0·4

0·6

0·8

1·0

1·2

1·4

1·6

1·8

φsand: degrees

SF

(c)

14 16 18 20 22

0·2

0·4

0·6

0·8

1·0

1·2

1·4

1·6

1·8

γsand3

: kN/m

SF

(b)

0 0·2 0·4 0·6 0·8 1·0 1·2

0·2

0·4

0·6

0·8

1·0

1·2

1·4

1·6

1·8

λa peat

SF

(d)

Case 1, stage 3

Case 2, stage 3

Case 3, stage 3

Case 4, stage 3

Case 5, stage 3

Case 1, stage 4

Case 2, stage 4

Case 3, stage 4

Case 4, stage 4

Case 5, stage 4

Fig. 18. Results of the sensitivity analysis for: (a) su clay cover; (b) ª sand; (c) 9sand; (d) ºa peat

Table 7. Stability calculation for different su values for stages 3 and 4

Case su: kN/m2 Before filling (stage 3) After filling sand core (stage 4)

F h: kN S : kN FS F h: kN S : kN FS

1 15.2 12 958 10 793 0.83 15 724 10 722 0.682 24.4 12 958 16 573 1.28 15 724 16 502 1.053 11.5 12 958 8425 0.65 15 724 8353 0.53

4 18.4 12 958 12 783 0.99 15 724 12 712 0.815 19.6 12 958 13 541 1.04 15 724 13 470 0.86




thick peat layer underneath the trial embankment dominated the mainly translational failure, with a failure plane at the boundary of the base of the peat and the underlying sand layer at about 3 m below ground level.

The stability of embankments is often based on theassumption of a single continuous (circular or similar) slid-ing plane comprising both the active and passive zones, asin the Bishop analysis. For practical applications, triaxial

compression tests are used for parameter assessment, includ-ing those for peat. The full-scale test, however, shows thatstability assessment of an embankment on a peat substratumwould be improved by incorporating distinct horizontal slid-ing. The analysis of the observed failure shows that a clear differentiation between active and shear sections, withadapted parameter derivation, is a good and relevant approx-imation for failure on soft soil, as suggested by Ladd (1969). Such an analytical stability analysis has been givenand discussed.

The observed failure of the embankment corresponds withfailure calculated with the parameter values derived fromDSS test measurements for peat. These DSS parameter values also correspond with the values derived from in situmeasurements. The embankment failure test shows that peak strengths derived from triaxial compression tests overesti-mate the mobilised strength along the sliding plane in peat.It was found that all peat samples fail in tension in triaxialcompression tests, and it is therefore unclear whether thefailure mechanism in that test is sufficiently representativefor determination of parameters for deformation behaviour atthe application scale. The DSS-derived parameter values perform better, which may well be due to the nature of thedeformation induced in DSS tests, which corresponds better with the shear deformation found to be dominant in theembankment failure. The strong anisotropy in the undrained shear strength ratio su/ vy of most peat also favours the useof DSS tests on vertically cored samples.

ACKNOWLEDGEMENTSThe authors would like to thank the Dutch Ministry of

Public Works, Rijkswaterstaat, the landowner, Staatsbosbeh-eer, and the IJkdijk foundation for making the full-scale tests possible.

NOTATION A superficial area B width of failed soil mass, parallel to trial embankmentc9 cohesion

d 50 sand mediand 60/d 10 uniformity coefficient

e void ratioemax, emin maximum and minimum void ratios

F h horizontal earth pressure (equation (2)) I p plasticity index

K 0 ratio of horizontal effective stress to vertical effectivestress (¼ 9h= 9v)

L length of sliding plane, perpendicular to trialembankment

M s mass of solids M w mass of water

m1 mass of solids after heating to 5008C for 4 h N loss on ignition (¼ ( M s m1)/ M s)

N kt bearing factor (see equation (1)) NAP reference datum level, approximately mean sea levelOCR over-consolidation ratio, 9vc= 9v

p9 isotropic effective stress for triaxial conditions(¼ ( 9v þ 2 9h)=3)

qc cone resistanceqt corrected cone resistance (see equation (1))q deviatoric stress for triaxial conditions (¼ 9v 9h)

Rf friction ratio, ratio of sleeve friction to cone resistanceS shear force (equation (2))

su undrained shear stressu2 pore pressure measured above tip of cone

W water content (¼ M w/ M s)wl liquid limitª volume weighta vertical strain

ºa , ºn active horizontal earth pressure coefficient, neutral earth pressure coefficient

stress 9h horizontal effective stress 9v vertical effective stress 9vc vertical pre-consolidation stress, field value vy vertical consolidation stress applied in laboratory test w water pressure shear stress (kN/m2)

9 friction angle

REFERENCESAbdoun, T., Bennet, V., Dobry, R., Koelewijn, A. R. & Thevanaya-

gam, S. (2010). Real-time monitoring of full-scale levee testing. Proc. 7th Int. Conf. on Physical Modelling in Geotechnics,Zurich 2, 1711–1176

Cola, S. & Cortelazzo, G. (2005). The shear strength behavior of two peaty soils. Geotech. Geol. Engng 23, No. 6, 679–695.

Den Haan, E. J. & Feddema, A. (2012). Deformation and strengthof embankments on soft Dutch soil. Proc. Inst. Civil Engrs,Geotech. Engng (in press).

Den Haan, E. J. & Kamao, S. (2003). Obtaining isotache para-meters from a CRS K 0-oedometer. Soils Found . 43, No. 4, 203– 214.

Den Haan, E. J. & Kruse, G. A. M. (2006). Characterisation and

engineering properties of Dutch peats. Proc. 2nd Int. Workshopon Characterisation and Engineering of Natural Soils, Singapore3, 2101– 2133.

Ladd, C. C. (1969). The prediction of in situ stress strain behaviour of soft saturated clay during undrained shear. Proceedings of the

Bolkesjo symposium on shear strength and consolidation of normally consolidated clays, pp. 14–29. Norwegian Geo-technical Institute.

Landva, A. O. (2007). Characterization of Escuminac peat and construction on peatland. Proc. 2nd Int. Workshop on Character-isation and Engineering Properties of Natural Soils, Singapore,2135–2191.

Leroueil, S., Magnan, J.-P. & Tavenas, F. (1990). Embankments on soft clay (Muir Wood, D. (transl.)). Chichester: Ellis Horwood.

Mathijssen, F. A. J. M., Boylan, N., Molenkamp, F., Leroueil, S. &Long, M. (2009). Material behaviour and constitutive modellingof organic soils. Proc. CEDA Dredging Days. Dredging Tools

for the Future, Rotterdam. Delft: Central Dredging Association.Mesri, G. & Ajlouni, M. (2007). Engineering properties of fibrous

peats. J. Geotech. Geoenviron. Engng 133, No. 7, 850–866.Van, M. A., Sharp, M. K., Zwanenburg, C., Mosher, R. L. & van

Esch, J. M. (2008). Horizontal translational failures of leveesdue to water filled gaps. Proc. 6th Int. Conf. on Case Historiesin Geotechnical Engineering, Arlington, VA, 7 pp.

Van Baars, S. (2005). The horizontal failure mechanism of theWilnis peat dyke. Ge otechnique 55, No. 4, 319–323, http://dx.doi.org/10.1680/geot.2005.55.4.319.

Yamaguchi, H., Ohira, Y., Kogure, K. & Mori, S. (1985). Undrained shear characteristics of normally consolidated peat under triaxialcompression and extension. Soils Found . 25, No. 3, 1 –18.


12blz. Failure of a Trial Embankment on Peat in Booneschans; The Netherlands - Zwanenburg

Documents

Transcript of 12blz. Failure of a Trial Embankment on Peat in Booneschans; The Netherlands - Zwanenburg