Displacement in Clays From Physical Model Tests

8/10/2019 Displacement in Clays From Physical Model Tests

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IPC2008-64186

SUBSCOUR DISPLACEMENT IN CLAYS FROM PHYSICAL MODEL TESTS

Ken Been

Golder Associates IncHouston, TX, USA

Rodolfo B. SancioGolder Associates Inc

Houston, TX, USA

Djavid Ahrabian Agip KCO,

London, United Kingdom

Walther van KesterenDeltares (formerly Delft

Hydraulics)Delft, The Netherlands

Ken Croasdale

K.R.Croasdale & AssociatesCalgary, Canada

Andrew Palmer

National University of Singaporeand Bold Island Engineering,

London, United Kingdom

ABSTRACTA subscour soil displacement model is a key component todetermine the safe burial depth of offshore pipelines in icescoured environments. In order to calibrate numerical modelsand physical tests carried out in geotechnical centrifuges, 3Dice scour tests were carried out in a dredging flume at a scaleapproaching that of observed scour depths in the Caspian sea.Two soil failure mechanisms were observed. For steep keels,the soil is mainly pushed up into a mound in front of the keel.For shallow angle keels most of the scoured soil is forced underthe keel and to the side of the indentor. Each mechanism resultsin a different subscour displacement pattern. Subscourdisplacement equations that include the effect of soil propertiesand keel angle are presented. While these equations areconsidered an improvement on existing sub-scour models, thereremain limitations to their application in 3D subscour untilfurther information relating scour depth and width to soilstrength becomes available.

INTRODUCTIONMobile ice in cold oceans may consist of ice ridges and icerubble features within the floating ice pack, or single icefeatures such as icebergs. In shallow or shoaling water, theseice features drag on the sea floor and create ice scours (orgouges). Ice scours have been mapped on the seabed in theBeaufort Sea (offshore Canada and Alaska), offshore SakhalinIsland in the far east of Russia, and in the North Caspian sea,amongst other regions. Scour marks have in many cases beentracked for several kilometres with sidescan or multibeam sonar.Maximum scour depths up to 2m or 3m are not uncommon insoft soils although most observed scours tend to be less than 1mdeep. Widths of scours range from a few metres for single keelsup to tens of metres for multi-keel features. Figure 1 showsscour marks observed from a helicopter through very clearshallow water in the North Caspian sea.

Figure 1. Scour marks seen through shallow water in the NorthCaspian Sea

Ice scours create hazards for offshore pipelines. While it iseasy to demonstrate that pipes should be buried below thedeepest scours, it is not obvious how much deeper they should

be to avoid excessive displacements of the soil immediately below the scouring ice keel. Development of a sub-scourdisplacement model is a key component to determine the safe

burial depth of offshore pipelines in ice scoured environments.Once soil displacements below a scouring keel have beendetermined, it is relatively straightforward to apply thesedisplacements to a soil-pipe structural model to calculate the

pipe behaviour (e.g. Nixon et al., 1996).

The state-of-practice for sub-scour displacements thatdeveloped during the 1990s was based on the Pressure RidgeIce Scour Experiment (PRISE) joint industry project. During



PRISE, physical model tests of scouring in sands and clays werecarried out in a geotechnical centrifuge and empiricalrelationships between sub-scour displacement and scour widthand depth were developed (Woodworth-Lynas et al., 1996).While these equations are thought to provide conservative(high) estimates of sub-scour displacements, there are

limitations on their range of applicability. Firstly, the soil parameters (strength, modulus) are not included in theformulation whereas soil properties are known to influence

behaviour. Secondly, all of the physical tests on clays involvedscour width to depth ratios greater than 6, for which conditionincreasing width is not expected to affect centerlinedisplacements. However, displacements scale with square rootof width in the empirical equations. Additionally, all tests werecarried out using relatively shallow keel angles (15 and 30 deg).

Finite element analyses have also been attempted to determinesub-scour displacements, e.g. Lach and Clark (1996), Konukand Gracie (2004), Konuk et al. (2005) and the authors(unpublished). Large strain finite element formulations usingadaptive meshing techniques (e.g. an ALE formulation in LSDyna) have shown most promise as a predictive tool but stilllack quality physical data for calibration and verification of thenumerical models for sub-scour displacements.

In order to provide further calibration of the numerical models,and physical test data at a larger scale than in centrifuge tests,model scour tests in clay were carried out in a dredging flume atDelft Hydraulics Laboratory. This work was in support of theExperimental Production pipelines for the Kashagandevelopment in the North Caspian sea. The scale of the tests,scour depth up to 0.3m and width 1.5m, approaches that ofobserved scour depths in the region, for which the mean scourdepth is 0.32m. The wall of the dredging flume includesreinforced glass panels that allow observations of the soildeformation around the scouring ice keel at the centerline(Figure 2).

EXPERIMENTAL PROGRAM

The keel scour tests were designed to model scouring overuniform soil (clay) conditions and to measure the soil forces andsubscour displacements. Complexities such as stratified soils,trench geometry and backfill properties, and the presence of the

pipeline, were avoided so that the forces and displacements

could be compared with those in idealized numerical orempirical models of the scour process. The scour tests were performed in the dredging flume at Delft Hydraulics laboratory.The tests are more accurately described as indenter tests, as arigid indenter was used rather than an ice keel, however, theindenter will be referred to as the keel and they are identifiedsimply as flume tests. The steel indenter was roughened in orderto mobilize maximum adhesion with the clay soil.

Figure 3 shows a sketch of the elevation and plan view of theflume test with relevant keel and scour terminology. Thecoordinate system adopted for the tests and analysis is alsoshown on Figure 3, i.e. x – direction of keel movement, y –lateral, away from glass wall, z – vertically up (right hand rule).The keel was pushed in the direction parallel to a vertical glass

wall in the dredging flume such that the plane of the glass wallis considered to correspond to the centreline of the keel, i.e. thewidth of the indenter (0.75 m) is considered to correspond tohalf of the actual width (1.5 m). The keel was laterally andvertically constrained and pulled at a constant velocity of about15 mm/s. The force and torque required to move the keel whileresisting vertical and lateral motion was measured in threedirections.

a) 15 keel angle

b) 30 keel angle

Figure 2. Ice scour tests in dredging flume – view of deformed soilthrough glass panel on centerline of scour (keel face indicated by

broken lines)



Figure 3 Sketch of indenter test in carried out in dredging flume

The following nomenclature and symbols regarding the flumetests are used throughout this paper:

= keel angle; the angle between the horizontal planeand the plane of the keel face (zero is thehorizontal plane in the direction of movement).

D s = scour depth measured from the seabed/clay bed B = keel width su = undrained shear strength of the soil

The flume bed material was prepared by mixing Illitic E-Ton powder (71.4% dry weight) with Asser sand (28.6% dry weight)for 10 minutes in a large mixer where water was added (22.3%of dry solids). The mixed clay lumps were placed in aluminumcontainers and allowed to cure for 3 to 5 days in a 100%humidity environment. After curing, the clay lumps were

placed in the extruder where clay blocks measuring 0.20 mwide, 0.50 m high and 2 m long were produced. The flume bedwas built by placing an initial layer of clay blocks standing ontheir shortest dimension with the long axis placed perpendicularto the glass wall. The 50 cm high initial layer was compressedwith a wood block that was loaded with equipment of the

dredging flume. The height of the blocks was reduced by 3 cm(6%). A second layer of clay blocks was placed directly overthe first layer to give a total bed thickness of about 1 m.

Vane shear tests were carried out in lumps of clay by DelftHydraulics to confirm that the required properties wereachieved. The peak shear stress in the vane shear tests was

between 35 and 38 kPa and the soil exhibited a sensitivity ofabout 4 (ratio of peak to residual shear stress). ConePenetration Tests (CPT) were performed in the prepared clay

bed with a miniature cone having an apex angle of 60 degreesand a tip area of 3.14 cm 2. The value of su was calculated bydividing the measured tip resistance by a factor N k = 13 andvaries between approximately 32 and 36 kPa. Test bed 3included a pipeline trench with a softer clay to represent

backfill material. An undrained shear strength of 15 to 20 kPa

was achieved by increasing the water content during sample preparation from 22.3% to 25.3%.

Eleven flume keel tests were performed, of which only thoselisted in Table 1 contain a complete set of data. (Other testsincluded scouring over short distances and over previouslyscoured material.) Tests were performed with three differentkeel angles, 45 , 30 , and 15 and the scour depth was varied to

perform two tests for each keel angle. The keel half-width wasconstant.

Table 1 Characteristics of flume tests

Test

ID

KeelAngle

(degrees)

ScourDepth,

Ds (mm)

KeelWidth

B(mm)

KeelLength,L (mm)

B/DsTest

Length*(m)

Test1 45 150 1500 2135 10 4.5

Test3

45 213 1500 2135 7.0 2.3

Test5

15 110 1500 415 13.6 5.2

Test8

15 275 1500 415 5.5 3.0

Test10

30 150 1500 1780 10 7.7

Test11 30 150 1500 1780 10 4.6‡

* the length traveled by the keel over the clay bed ‡ a model trench and steel pipeline were incorporated inTest 11.

OBSERVATIONS

Soil failure mechanisms during scouring

Two distinct scour mechanisms were observed in the tests thatappear to depend on the keel angle. The tests performed with45 and 30 keels produced a mound and a sub scourdisplacement pattern that was significantly different from themound and sub scour displacements produced with the 15angle.

Figure 4 illustrates the scour mechanisms observed in terms ofthe soil particle trajectories in each case. Under the advancing15 keel, the soil in front of the keel is compressed vertically

but also moves outwards from the center line towards the edgeof the keel. Beyond the edge the soil displacement is upwardsinto the mound. By contrast, for 30 and 45 keels, the soil in

Keel Length (L)

Bottom of Flume

KeelScour Depth (D s)

Glass Wall

Keel Displacement (X k )

Keel Half Width

PLAN

ELEVATION

KeelKeelAngle ( )

2.4 m

Concrete Wall

x

zy

x

y



Bed 1, Test 1, Camera #145 degree keel angle, half keel width = 0.75 m, 150 mm scour depth

0

100

200

300

400

500

600

-100 -50 0 50 100 150 200 250

Displacement (mm)

V e r t i c a

l p o s

i t i o n

b e

l o w s c o u r

( m m

)

Horizontal (Measured)

Vertical (Measured)

Figure 4. Soil particle trajectories during ice scouring in clay

front of the keel and within the scour was initially pushed in thedirection of movement and as the keel advances the direction ofsoil movement relative to the keel is upwards into the mound.Thereafter the movement is lateral as the mound clears to theside of the keel.

These mechanisms correspond qualitatively to the slip-lineanalysis by Petryk (1987) for two dimensional indenters insteel. The only variables in Petryk’s analysis are the keel angleand ratio of yield stress on the indenter-steel interface to theyield stress of steel. We assume that the different scourmechanisms observed were solely due to the change in the keelangle, because the interface shear strength was approximatelythe undrained shear strength of the clay. However the scourmechanism may also depend on soil shear strength and keel-soilinterface shear strength, neither of which was varied in this test

program.

Subscour Displacements

Soil displacements in the scoured bed were measured in twoways. At the glass wall of the flume, considered to be the scourcenterline, high resolution video images were recorded duringthe tests and subsequently analyzed. In addition verticalcolumns of steel beads were placed in the soil bed during

preparation at 12 locations and their post-test location was

determined by coring of the verticals and particle tracking onX-ray images of the core.

Figure 5 shows displacements at the centerline of the scour afterthe keel has passed during Test 1. The displacements weredetermined by image processing of the video recordings bymeans of Particle Image Velocimetry (Westerweel 1993, and

Figure 5. Displacements at glass panel (scour centerline) after keelhad passed in Test 1 (45 keel)



Uittenbogaard 1995). As a check the displacements were alsodetermined from two images before and after the test by particletracking.

It is apparent from Figure 5 that some slippage has occurred between the upper and lower clay blocks, at a depth of about340 mm below the keel. This slippage was only observed insome tests and is attributed to somewhat lower shear strength atthe interface between the blocks despite efforts to avoid such acondition. In the subsequent analysis, the horizontaldisplacements above this artificial shear surface have beenadjusted to provide continuity across the interface. (It isrecognized that this may affect the interpretation, but it is

justified based on similarity of the slope of the curve on Figure5 above and below the discontinuity.)

Similar data for horizontal displacement of all of the flume testsis illustrated on Figure 6. Here there is a clear distinction

between the results of the 15 keel and the 30 and 45 degreekeels, consistent with the different observed failure mechanismsfor shallow and steep keels. In the 15 keel case thedisplacement decays approximately linearly with depth belowthe keel, while the decay is curved below the 30 and 45degree keels. This linear decay of displacement has not beenobserved previously in physical model tests (although in theabsence of glass viewing panels this does not mean it did notoccur) and therefore some caution is recommended in usingresults which may have been affected somewhat by the weakerinterface between clay blocks mentioned above.

SUBSCOUR DISPLACEMENT MODEL

The horizontal displacements must be put in non-dimensionalform to develop a predictive sub-scour displacement model.Displacement in uniform clay is expected to depend on theundrained shear strength of the soil ( su), scour depth ( D s), keelangle ( soil stiffness (or shear modulus, G), scour width ( B)and the soil-keel interface shear strength (or adhesion factor, a ).An equation to predict horizontal sub-scour displacements u should thus have the form:

(1) a B D

G s f u

su ,,

)sin(,

1,

In all of the tests to date, the scour width B has been largecompared to the scour depth and therefore it is impossible todetermine the dependence of displacements on B empirically.Woodworth-Lynas et al. (1996) suggest that u is proportional tosquare-root of ( B D s), which is dimensionally correct, but amore general form would have the proportionality to D s B(1- ) where is and exponent expressing the relative importance ofdepth and width on the subscour displacements . = 0.5 whenscour width and depth have equal influence on subscourdisplacements. If it is further assumed that the interface shear

Flume Tests (15 degree keel)

0

100

200

300

400

500

600

0 100 200 300 400 500

Horizontal displacement, u (mm)

z ( m m

)

15 deg keel - 110 mm scour depth


Flume Tests (30 and 45 degree keels)

0

100

200

300

400

500

600

0 100 200 300 400 500

Horizontal displacement, u (mm)

z ( m m

)




Figure 6. Horizontal displacement data from flume tests

strength between an ice keel and soil does not vary ( a ~ 1 in practice for ice soil interaction), and the ratio ( G/s u) is taken to be the rigidity index I r , then the appropriate non-dimensionalgroup for horizontal displacement is:

(2)

r

s

I

B D

u

)sin(

1

The proposed displacement normalization is presented in Figure7, both for the flume test data in the centerline (Figure 6) andfor the PRISE centrifuge test data on clays from Woodworth-Lynas et al. (1996). Since width B is always large for the dataconsidered = 1 has been assumed while for between 0 and45 degrees the value of in radians is similar to sin( ), sowas used instead. The shear modulus used is the small strain



shear modulus Gmax since it is directly measurable (and if it isconsidered that all soils show a similar modulus degradationcurve, it is only necessary that G is determined consistently forthis empirical analysis). Figure 7 indicates that thedimensionless group selected is reasonable but the displacementas a function of depth is different for steep (30 and 45 ) keels

and low angle (15 ) keels. Fitting an exponential decay forsteep keels and straight line for low angle keels on Figure 7, thetentative sub scour displacement equation at the keel centerlinefor wide keels is:

3) sr

s

D z

I D

z u 7.0exp500

)(

For steep (30 and 45 ) keels

(4)r

s

I z D

z u 45200

)(

For low angle (15 ) keels

where z is the depth measured below the keel (positivedownwards).

DISCUSSION

The proposed subscour displacement relationship suggests thatdisplacements will be greater in clays with higher undrainedshear strength, which is slightly counter-intuitive. However, thereason is that for a given scour depth and width, the forcesrequired to create the scour are also proportional to undrained

strength, but the soil stiffness increases more slowly (i.e. I r isless for stiffer soils). The implication is therefore that the scourdepth must be related to soil properties for equations [3] and [4]to be useful. Scour data collection should therefore includemeasurement (or at least estimates) of soil properties andanalysis should consider the relationship between ice strength,soil strength and scour depth (e.g. Croasdale et al., 2005).

It is also apparent that not all of the key variables have beensystematically varied to justify use of the model presented (orany published model, for that matter). In particular it isnecessary to examine the effects of keel width on therelationship, as many of the deepest observed scours arerelatively narrow. Another unstudied variable is the keel length(i.e. the dimension of the keel base behind the front face).However, physical and numerical models show a high degree ofstress relief once the keel face has passed and that subscourdisplacements in the depth of interest reach their maximum asthe keel face passes, so that the results are considered to berelatively insensitive to keel base length. The physical tests onwhich the equations are based are also for a limited range ofsoil properties (undrained shear strength in particular onlyvaried from about 20 kPa to 45 kPa).

Despite these limitations in the data underlying the equations, itis considered that the appropriate parameters are incorporatedin a rational approach, and we therefore considered itappropriate to publish this paper in the interest of stimulatingfurther testing and discussion on the nature of sub-scourdisplacements in clays.

0

1

2

3

4

5

6

0 50 100 150 200 250

z / D

s

Flume 15 deg - 110 mmFlume 15 deg - 275 mmPRISE 03-D1: 0.8 m scour depthPRISE 03-D2: 1.5 m scour depthPRISE 04-D2: 1 m scour depthPRISE 05-D1: 0.81 m scour depthPRISE 05-D2: 1.43 m scour depthPRISE 06-D2: 1.2 m scour depthProposed Equation

a) 15 keels

0

1

2

3

4

5

6

0 100 200 300 400 500 600

z / D s

Flume 45 deg - 150 mm scour depth



PRISE 04-D1: 30 deg, 1.0 m scour depthPRISE 06-D1: 30 deg, 1.19 m scour depth

Proposed Equation

b) 30 and 45 keels

Figure 7 Normalised subscour displacements from flume tests (thisstudy) and PRISE centrifuge tests (Woodworth-Lynas et al., 1996)



ACKNOWLEDGMENTS

We would like to acknowledge the support of Agip KCO andthe North Caspian PSA Joint Venture for permission to publishthis paper. The study was undertaken during engineeringevaluations for buried oil and gas transmission lines from theKashagan field in the northern Caspian to the coast ofKazakhstan.

REFERENCES

Nixon, J.F., Palmer, A. and Phillips, R. (1996) Simulations for buried pipeline deformations beneath ice scour. ProcOMAE, Vol 5, pp 383-392.

Woodworth-Lynas, C.M.L., Nixon, J.D., Phillips, R. andPalmer, A.C. (1996) Subgouge deformations and thesecurity of Arctic marine pipelines. OTC Paper 8222,1996, Vol 4, pp 657-664.

Konuk, I. and R. Gracie (2004). A 3-dimensional Eulerianfinite element model for ice scour. Proc., IPC, IPC 04-0075, 8p.

Konuk, I., S. Yu and R. Gracie (2005). An ALE FEM model ofice scour. Proc., IACMAG, 8p.

Lach, P.R. and J.I. Clark (1996) Numerical simulation of largesoil deformation due to ice scour. Proc. CanadianGeotechnical Conference, 1, pp.189-198

J. Westerweel, 1993, Digital Particle Image Velocimetry, PhDThesis, Delft University of Technology

R.Uittenbogaard, 1995, Particle Image Velocimetry,applications, quality and expertise, DH-report3.4901.10

Croasdale, K.R. Comfort, G. and Been, K. (2005).Investigation of ice limits to ice gouging. POAC 05,Vol 1, pp. 23-32.

Petryk, H., 1987, Slip-line field solutions for sliding contact,Proc. Int. Conf. on Tribology-Friction, Lubrication andWear, vol.II, IMechE, pp. 987-994.



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Displacement in Clays From Physical Model Tests