Breakout of submerged structures buried to a shallow depth

PETER M. BYRNE A N D W. D. L IAM F I N N Dc~prrrttiretrt o f ' C i ~ d Et~gitreeritrg, Utril~e,:sity ofBritislr Colrrt~hirr, Vrrtrc.orr~.c,r, B . C . , Cntrritlrr V6T I W5

Received September 14, 1977

Accepted December 14, 1977

The results from a testing program designed to simulate the problem of lifting an object embedded in the ocean floor are presented and examined herein. The breakout force, defined as the force in excess of the submerged weight required to dislodge the object, was found to depend on the shear strength of the soil or sediment. The maximum breakout force occurs for undl-ained conditions and can be estimated from bearing capacity equations.

The uplift pressure is transferred to the sediments by a reduction in pore-water pressure. If the reduction in pressure is limited by infiltration of water, or any other process, then a reduction in the breakout pressure will result.

The reduction in breakout pressure that occurs when time for infiltration is allowed can be estimated from available solution curves from theory of consolidation.

Les resultats d'un programme d'essaiscong~ dans le but de simuler le probleme cle levage d'un objet enfonce dans le fond de I'ocian sont presentis et examines. La force dedecrochage. definie comme itant la force requise en excedent du poids submerge pourdetacher I'objet, s'est revelee etre Fonction de la resistance a11 cisaillement du sol ou des sediments. La force maximale cte decrochage correspond A des conditions non drainees et peut &re evaluke au nioyen des equa- tions de capacite portante.

La pression de soulevement est t~msmise aux sediments sous forrne d'une reduction de la pression interstitielle. Si cette reduction de pression est limitee soit parune infiltration d ' e a ~ ~ , soit par tout wtre processus, i l en res~~ltera alors une reduction de la pression de dkcrochage.

Lorsque I'on permet un certain temps pour que I'infiltration se produise, la reduction de la pression de decrochage peut etre evaluee au moyen des courbes fournies par la theorie de - . consolidation.

Can. Geotech. J . , 15, 146- 154 (1978)

Introduction A problem of increasing importance is that of

removing an object from the ocean bottom. The force required to lift an object embedded in the bottom is comprised of the submerged weight of the object, friction forces on its sides and adhesion forces on its base. The breakout force is defined herein as the force in excess of the submerged weight that is required to dislodge the object. If the object is resting on a soil of relatively high permeability, such as sand, then the adhesion force on the base is 0 and the breakout force is comprised of side friction forces only and is small for an object at a shallow depth of embedment. If, however, the object rests on a low permeability cohesive soil, the adhesion forces will be con- siderable and the breakout force required for rapid removal may be quite large.

Vesic (1971) presented a detailed study of the breakout problem. Much of the study was based on the pull-out capacity of terrestrial anchors. He noted that the pull-out capacity for deep anchors in cohesive soil was comparable to their bearing capacity. Meyerhof and Adams (1968) in their study of the uplift capacity of foundations had also suggested this. For anchors buried to a shallow depth Vesic, and Meyerhof and Adams indicated

[Traduit par la revue]

that their pull-out capacity would be less than their bearing capacity.

Finn and Byrne (1972) proposed that an upper limit for the breakout force for an object buried to a shallow depth in a cohesive soil could be obtained by assuming that the failure mechanism is similar to that for a bearing capacity failure with the direction of movement reversed. The concept of a reverse bearing capacity failure implies that a tensile total stress change is applied to the soil at the base of the object. Finn and Byrne hypothesized that this tensile stress change was tmnsmitted essentially by a drop in pore-water pressure and that the ad- hesion force that comprises the major portion of the breakout force is essentially a water 'suction' force.

A number of model tests have now been per- formed in which the water pressure at the base of the object or loading pad has been measured dur- ing breakout, thus allowing the hypothesis to bc tested. The effect of loading rate on both pore pressures and thc breakout force is also examined.

Test Apparatus and Test Procedure The test apparatus consists of a consolidation

ring that holds the soil and a loading pad with a skirt and porous stone to which the uplift force is

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BYRNE AND FINN

applied as shown in Fig. 1. The function of the pad is to prevent water flowing directly to the porous stone. The skirted pad is considered to behave as a foundation embedded to a depth equal to that of the skirt. When the pad is pulled from the soil, pore-water pressures are measured at both the base of the pad and at the base of the con- solidation ring. Water pressure is also applied at the surface of the soil to simulate ocean bottom conditions. This is accomplished by placing the ring and loading cap within a triaxial cell and ap- plying a chamber pressure. The complete test ap- paratus is shown in Fig. 2.

The apparatus is assembled as follows: The soil is placed in the consolidation ring, which is then positioned on the flooded base porous stone and sealed by an O-ring. The plastic cylinder of the triaxial cell is placed in position and half filled with water. The pad porous stone is placed inside the skirt under water and the connecting rod at- tached. The top plate of the triaxial cell is placed in position and the connecting rod attached to the load cell. The load cell in turn is connected to an air piston that is attached to the frame of the testing machine. The cell is completely filled with water and prcssurized to 300 kPa. The triaxial cell is then clamped to the base of the testing machine.

The following testing procedure was used: 1. The skirt was pushed into the soil and the pad

seated on the soil using a pressure of 80 kPa. The drainage line to the pad was kept open for 2 min to allow the water trapped under the skirt to escape. Then the drainage valve was closed and the seating load left in place for a further 8 min to allow the pore-water pressure to come to equi- librium.

2. The seating load was now removed and the sample left to stand for 30 min. The pore pressures

k T z o r e pressure Tronsducer

L o a d i n g P a d ',

i l l I L I

Transducer

FIG. I . B a s i c c o m p o n e n t s of t e s t apparatus.

A i r P iston I I ' ~ i x s d to Testing

Mochine

Lood Cell w Top Plote , [+connecting Rod

1 1 ,I Transducers

E.

'Bose of Testing Mochine

FIG. 2. C o m p l e t e t e s t apparatus.

at both the base and the pad were recorded during this period and it was found that 30 min was suf- ficient time for the pore-water pressure in the soil to return to equilibrium.

3. The air piston was clamped to hold the pad firmly in position, and the base of the testing machine was moved down at a constant displace- ment rate until breakout occurred. Continuous records of load, pore pressure at the pad and pore pressure at the base were obtained using chart recorders. For load controlled tests, the triaxial cell was not clamped to the base and the load was applied by moving down the base of the testing machine and allowing the triaxial cell to dangle from the load cell.

Soil Tested The soil used in the testing program was from

Haney, British Colun~bia. Undisturbed field samples were obtained by cutting blocks from an open pit. The soil is considered to have been deposited in a marine environment and subsequently uplifted and leached. It is a sensitive grey silty clay with the following properties: liquid limit = 44%, plastic limit = 26010, maximum past pressure =

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148 C A N . GEOTECH. J . VOL. 15. 1978

350 kPa, undisturbed shear strength = 50 kPa and sensitivity -- 10.

Ocean bottom sediments may have strengths considerably less than 50 kPa (Finn and Byrne 1971). However, the mechanism of breakout is thought to be independent of the actual strength of the material. The stronger material was used in order to achieve higher breakout forces, which could be more accurately measured.

Characteristic Behavior Test results from a typical rapid displacement

controlled test are shown in Fig. 3a. It may be seen that the applied pull-out stress rises to a maximum of 340 kPa in 10 s, drops to 3 10 kPa at 16 s and then drops abruptly as the loading cap breaks out of the soil. The pore pressure drop at the loading cap follows closely that of the applied stress, although it is smaller as shown in Fig. 3a. The change in effective stress, which is the difference between these curves, is small and negative. This means, therefore, that the pull-out pressure is ap- plied to the soil mainly by pore-water pressure

,-Total U p l ~ f t Pressure

0 5 10 15 20 (a ) T i m e in S e c o n d s

60 Water Pressure -- -\

m d n / -.

( b ) T i m e in S e c o n d s

changes rather than by a change in the effective stress applied to the soil particles.

The drop in pore-water pressure is 310 kPa (-- 3.0 atm). If the initial pore-water pressure is not at least 2.0 atm, cavitation occurs in the pore pres- sure measuring system and breakout occurs at a lower stress level. For this xeason most of the tests were performed using a chamber pressure of 300 kPa. When the chamber pressure was increased to 400 kPa, breakout still occurred at the same stress level.

The total stress and pore-water pressure changes in the soil at the base of the consolidation ring are shown in Fig. 3b. The total stress was calculated on the assumption of zero side friction. The consolida- tion ring was polished and coated with silicone and a very small pressure applied at the base of the soil was sufficient to cause upward movement of the soil as a unit. The pore-water pressure drop is greater than the drop in total stress so that an increase in vertical effective stress occurs at the base of the consolidation ring.

Breakout occurred by a shearing failure within the soil rather than by a breakdown in adhesion between the soil and the loading cap. Typical failure mechanisms are shown in Fig. 4. The

FIG. 3. Stress for a displacement controlled breakout test (displacement rate = 7.6 mm/min): (a) beneath the loading pad, (b) at the base of the soil. FIG. 4. Failure mechanisms for breakout.

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BYRNE AND FINN 149

failure surfaces extended to a depth equal to about the width of the loading cap as shown by the dashed lines. The surface of the soil beneath the loading cap moved up while the surface of the soil outside the cap moved down as shown by the upper dashed lines. In addition, the soil was observed to peel away from the outside of the skirt so that side friction forces were 0 before breakout oc- curred. Failure mechanism b is essentially the same as mechanism a except that horizontal failure planes due to tension stresses developed where the failure surface is closest to the vertical boundaries.

Test results from a typical constant load test are shown in Fig. 5 a and b. The total stress ap- plied to the loading cap was 200 kPa and this was held constant with time. The pore-water pressure drop at the cap also remained constant and equal to 190 kPa as shown in Fig. 5a. This means that the effective stress beneath the cap dropped from 0 to -10 kPa on application of the load and then stayed constant. Failure occurred very abruptly after 70 s. The total stress and pore-water pressure changes at the base of the soil are shown in Fig. 5b and give an indication of why the failure occurred. It may be seen that although the total uplift pres- sure at the base remains constant and equal to 22 kPa, the pore-water pressure, which initially drops

0 20 40 60 80 (0) Time ~n Seconds

0

a .x

v

2 200 + 0 P

e 0

g I O O - '8 '8

e a

0

Water Pressure

E 0 -

0 20 40 60 80

( b ) T i m e in Seconds

Totol Uplift Pressure

FIG. 5. Stresses for constant load breakout test: (a) beneath the loading pad, (b) at the base of the soil.

- - - - ";- - - - - - - - - - - Water Pressure

I I I

to 34 kPa, rises with time. This means that the effective stress at the base of the soil layer, and at points other than at the base of the loading cap, is falling as water is drawn into the soil from the top surface. When sufficient water has been drawn into the soil to drop the strength below that re- quired for equilibrium, an abrupt failure occurs.

These tests indicate that a failure mechanism occurs within the soil mass and therefore the breakout pressure is related to the shear strength of the soil. The maximum breakout pressure oc- curs for undrained conditions. The tests also show that it is the reduction in water pressure beneath the loading cap that transmits the pressure to the soil. If the reduction in water pressure is limited in any way by infiltration of water or cavitation, the breakout pressure is reduced. The value of the maximum breakout pressure together with the re- duction that occurs due to infiltration will now be examined.

Maximum Breakout Pressure A number of rapid breakout tests were per-

formed to determine the relationship between the breakout pressure and the undrained shear strength of the soil. The undrained strength of the soil was determined after each test by means of a vane shear test. Vane shear data were calibrated by com- parison with data from axial compression and ex- tension tests on 36 mm diameter by 71 mm high samples. The results of these calibrations are shown in Table 1. It may be seen that, when failure is brought about by vertical compressive strain, the calibration factor for the vane is 1.39. Whether this compressive strain is caused by an increase in vertical stress or a decrease in horizontal stress has little effect on the results. If failure is brought about by horizontal compressive strain, the calibration factor is 1.1 1. Again, it is the direction of the strain rather than the loading that is important.

The classical failure mechanism for a bearing capacity failure under plane strain conditions is shown in Fig. 6a. Zone 1 beneath the loaded area is a zone of vertical compression in which the cali- bration factor 1.39 would be appropriate whereas zone 3 is a zone of vertical extension in which the calibration factor 1 . I 1 would be appropriate. The Mohr failure circles for these zones are shown in Fig. 6c. Zone 2 is the radial shear zone in which the director of compressive strain changes from vertical to horizontal. An average value of 1.25 will be used in this zone.

If it is assumed that thc failure mechanism on breakout is identical to that for a bearing capacity failure but with the direction of motion reversed,

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CAN. GEOTECH. J. VOL. IJ , 1978

TABLE 1. Comparison of undrained strength from triaxial tests with vane shear tests

Shear strength @Pa)

Direction of Calibration Test Loading compressive Triaxial Vane factor for No. conditions strain test test vane

Vertical 60

Vertical

Vertical 65 45 1

Horizontal 53 45

Horizontal 50 45

Horizontal 48

then vertical extension occurs in zone 1 and vertical compression in zone 3 as shown in Fig. 6b. The failure circles for these zones are shown in Fig. 6c.

If the failure mechanism were as shown in either Fig. 6 a or b then a shear strength given by an average calibration factor of 1.25 applied to the vane values would be appropriate and this value was adopted for this study.

The results of six rapid breakout tests are shown in Table 2 in terms of the breakout pressure qf , the undrained shear strength c, and the factor N,*, which is the ratio of qf to c. N,* is seen to range between 7.41 and 6.48 with an average value of 6.92.

The relationship between the failure pressure qf and the undrained shear strength c for a bearing capacity failure is considered to be most accurately given by Hansen's formula (Bowles 1968) :

in which N, = the bearing capacity factor with respect to cohesion, s, = the shape factor, d, = depth factor and i, = the inclination factor. For the test conditions, N , = 5.14, s, = 1.3, d, = 1.09 and i, = 1, in which case

and the ratio q f / c = N,* = 7.3. The average value of 6.9 obtained from the breakout tests is within 6% of this value.

The test results indicate that the maximum breakout pressure for objects buried to a shallow depth can be predicted with reasonable accuracy from bearing capacity formulae commonly used in foundation engineering.

Effect of Rate of Loading on Breakout Pressure A number of displacement controlled tests were

performed at different rates to determine the effect

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BYRNE AND FINN 151

Z o n e 3

Z o n e 2

B r e a k o u t

Normal S t r e s s , Tens ion Normal Stress,Compression

FIG. 6. (a) Classical bearing capacity failure mech- anism. (b) Assumed breakout mechanism. (c) Stress circles at failure for bearing capacity and breakout, plane strain conditions.

TABLE 2. Relationship between breakout pressure and undrained strength

Breakout pressure q,

Test No. (kPa)

Undrained shear strength (kPa)

Corrected Vane vane, c N,* = qr/c

35 44 7.41 35 44 7.23 37.5 47 6.94 40 50 6.70 40 50 6.48 37.5 47 6.77

NOTE: Average N,* = 6.92.

that the rate of loading or time to failure has on the magnitude of the breakout pressure.

Typical test results for a displacement controlled breakout test performed at a displacement rate of 0.5 mm/min are shown in Fig. 7 a and b. The stress changes beneath the loading pad are shown in Fig. 7a. The maximum uplift pressure is 176 kPa and occurs at 110 s. After this time the uplift pressure drops and breakout occurs at a time of 260 s when the pressure is 95 kPa. At the much faster testing rate of 7.6 mm/min (Fig. 3a), the maximum uplift pressure was 340 kPa and failure occurred after 16 s. The drop in water pressure is

seen to be somewhat less than the uplift pressure so that a decrease in effective stress occurs beneath the pad as was the case for the fast test shown in Fig. 3a. The stress changes at the base of the soil are shown in Fig. 7b. The drop in water pressure is greater than the applied pressure so that an in- crease in effective stress occurs at the base of the soil as in the case of the faster test described in Fig. 3b.

The uplift pressure versus time relationships for displacement controlled tests performed at four different rates are shown in F ~ ~ T 8. In all cases the uplift pressure reaches a maximum and declines prior to breakout. The displacement at breakout is about the same for all four tests, approximately equal to 2.5 mm. Both the maximum uplift pres- sure and the Dressure when breakout occurs de- crease as the displacement rate decreases, as would be expected from the data discussed above. Two ~ressures will be of interest when breakout is caused by a constant rate of displacement: ( 1 ) the maximum uplift pressure and (2 ) the pressure at breakout. When breakout is caused by application of a constant load then of course the uplift pres- sure remains constant with time until breakout occurs as shown by the dashed line in Fig. 8.

The relationship between uplift pressure and the time to breakout is shown in Fig. 9. The solid line gives the maximum value of the uplift pressure that occurred during the test whereas the dashed line gives the value of the uplift pressure at the time breakout occurred. The results of three load controlled tests are also shown and are seen to plot close to the dashed line.

The reduced uplift pressures for long duration tests are caused by water infiltrating into the soil and reducing its strength. The amount of infiltra- tion that occurs will depend on the geometry of the test conditions and the coefficient of consolida- tion of the soil. Davis and Poulos (1972) pre- sented curves for determining the average degree of consolidation U beneath a circular loaded area. which may serve as a useful guide for estimating -the reduction in uplift pressure that occurs with time. Their curves for the boundary conditions that most closely resemble those of the test conditions are shown in Fig. 10. If it is assumed that the reduced breakout pressure is proportional to the average degree of dissipation of pore-water pres- sure, 1 - U , then the reduced breakout pressure as a function of time to failure can be predicted from the theoretical curves presented by Davis and Poulos.

The comparison between the theoretical and

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CAN. GEOTECH. J. VOL. 15. 1978

, T o t o l U p l i f t P r e s s u r e

0 100 20 0 (a 1 T i m e in S e c o n d s

100 200 T i m e in S e c o n d s

FIG. 7. Stresses for a displacement controlled breakout test (displacement rate = 0.51 mm/min): (a) beneath the loading pad, (b) at the base of the soil.

observed reduction in breakout pressure as a func- tion of time to failure is shown in Fig. 11. The comparison is made in dimensionless form. The dimensionless parameters used are the reduction factor, which is the ratio of the breakout pressure q to the maximum breakout pressure q,,,,,,, and the time factor T given by

PI T = clt/h2

in which cl = the coefficient of consolidation, t = the time and h = the height of the soil layer. The test results shown in Fig. 9 are rcplotted in dimensionless form using the measured value of c, = 0.005 c m v s obtained from dissipation tests. The theoretical curve was obtained from Fig. 10 using a value of h/a = 4, which is appropriate for the test conditions. It may be seen that the theoret- ical curve lies between the experimental curves and

thus gives a reasonable approximation of the reduc- tion in breakout pressure that occurs with time.

For field conditions, therefore, an estimate of the reduction in maximum breakout pressure that occurs with time can be made from the equation

where U is obtained from Fig. 10 using the appro- priate field boundary conditions and coefficient of consolidation.

Conclusions The test results and analyses presented lead to

the following conclusions regarding breakout of submerged objects buried to a shallow depth:

1. The maximum uplift pressure required to dislodge the object occurs for undrained conditions and can be predicted from bearing capacity theory.

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0

3 5C

3 0 C

2 5 (

g Y

20( C .- al &

3 UI

15C a

IOC

5 0

0

BYRNE A N D FINN

Time in S e c o n d s

FIG. 8. Uplift pressure versus time.

LEGEND :

o Moximum Uplift Pressure

0 Uplift Pressure ot Breokout

0 I n o d C n n t r n l l ~ d Tnzt

100

T ~ m e to B r e o k o u t in Seconds

FIG. 9. Uplift pressure versus time to failure.

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154 C A N . GEOTECH. J . VOL. 15. 1978

10- 10- 10- lo-' lo0 10' T i m e F o c t o r , T =%

h 2

FIG. 10. Relationship between average degree of consolidation and time factor for circular loaded area (from Davis and Poulos 1972).

o = R o d i u s of

U n d r o i n e d Loaded Area

I

I I Based on Moximurn U p l i f t P ressure

0.8 [ e a s e d on up l i f t Pressure at Breakout

Load Controlled Tests- \

0.6 \

/Theo ry . '/ = I - U 'mar. I

0 I I l l 10-3 10-2 10-1

7 i m e Factor, T = G h2

FIG. 11. Comparison of theoretical and observed reduction in breakout pressure with time.

2. The uplift pressure is transmitted to the soil by a reduction in pore-water pressure. If thc reduc- tion of pressure is limited by infiltration, cavitation or any other process, the breakout pressure will be similarly limited.

3. The reduction in breakout pressure that occurs when time for infiltration is allowed can be estimated from available solution curves from theory of consolidation.

Acknowledgements The research described in the preceding pages

was supported by research grants Nos. A5109 and 1498 of the National Research Council of Canada.

BOWLES, J . E. 1968. Foundation analysis and design. McG~xw-Hill Book Co.. New York. NY. pp. 53-57.

DAVIS. E. H.,arid P o u ~ o s , H. G. 1972. Rate ofsettlement under two- and three-dimensional conditions. Geotechnique, 22(1), pp. 95-1 14.

F I N N , W. D. L..antl BYRNE, P. M. 1971. Engineeringproperties of deep ocean sediments. 3rd Offshore Technology Confer- ence, Houston. TX. pp. 525-532.

1972. The evaluation of the break-out force for n sub- merged ocean platform. 4th Offshore Technology Confer- ence. Houston. TX. OTC 1604.

M E Y ~ R H O F , G. G., and ADAMS, J . I. 1968. The uplift capacity of foundations. CanxJinn Geotechnical Journal, 5, pp. 225-244.

VES~C, S. 1971. Breakout resistance of objects enlbedded in ocean. bpttom. ASCE Journal of the Soil Mechanics and Foundations Division, 97(SM9), pp. 1183-1205.

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