Sand drain -failure case study.pdf

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A young engineer, molasses, and failed sand drainsGeosynthetics | October 2009

By Bob Koerner

Prior to 1959, the chemical industry used sugar cane (mainly from Cuba) for its source of the carbohydra

sucrose used to produce ethyl alcohol for the manufacture of munitions and alcohol.

When Fidel Castro came to power in 1959, this nearby source was abruptly eliminated and the alternative was molasses. Of co

imported molasses required onshore storage tanks for the unloading of molasses tanker ships. Two 37m (120-ft)-diameter,

side-by-side tanks were designed for a site at the Wilmington Port Authority (property owner) at the confluence of the Dela

Christiana rivers in Wilmington, Del. (see Figure 1).

The two tanks were each to be 12.1m (40ft) high, and with the high-viscosity molasses at a specific gravity of 1.5, this is equiva

ground surface loading of 180kPa (3750 psf). The site, however, consisted of approximately 20m (65ft) of saturated organic cl

soil borings (center and at opposite ends of the site) were taken along with numerous undisturbed soil samples. Blow counts

extremely low (0-5 blows/300mm) for 20m and then a firm granular soil layer was encountered. There was some visual evide

horizontal stratification within the organic clay but it was inconclusive.

The water table was at the slag-covered ground surface and site flooding was not unusual. The undisturbed samples were sent

consultants in-house soil testing laboratory and revealed the following average properties for use in the sand drain design:

void ratio, e = 1.72

coefficient of (vertical) hydraulic conductivity, kv

= 0.508 10-8 cm/sec

coefficient of (vertical) consolidation, cv

= 0.0405 cm2/min

assumed coefficient of (horizontal) consolidation, ch= 0.081 cm2/min; i.e., c

h= 2 c

v

The bearing capacity of these geologically recent deposited silty clay soils was extremely low with consolidation times far in ex

the owners requirements. Thus, sand drains, followed by surcharging, were designed by the consulting engineering firm (co

hired by the chemical company (tank owner) involved.

Note in Figure 2 that the proposed site for the molasses storage tanks was directly across from three large two-story warehous

warehouses were supported on steel H-piles driven into the dense granular soil beneath the saturated silty clay. There was a s

road, with a underlying water main providing service to the warehouses. It was located between the proposed storage tanks a

warehouses. The warehouse structures themselves were steel framed with brick facing on all four sides. The first floor was rei

concrete placed directly on the concrete pile caps. There was an asphalt overlay on the concrete floor.

Using standard vertical consolidation theory and the above laboratory-obtained cv

-value, the curve of Figure 3a shows a 90

consolidation time of 41.2 years. This was clearly unacceptable to the tank owner. However, using Barrons theory of sand dra

10 for horizontal flow resulted in slightly more than one year, and for combined flow slightly under one year. There was discu

further reducing the time using anv

= 5 sand drain array (which would have resulted in a four-month timesee Figure 3b). B

year was considered acceptable. In the end, sand drains of 300mm (12in.) diameter at a triangular spacing of 3.05m (10ft) we

installed.

The installation started by placing a 1.0m-thick porous slag blanket over the entire site (Figure 2). This served as both a worki

platform for the large pile-driving crane and subsequently to allow for expulsion of drainage water coming from the sand drai

surcharge was placed. The installation of the sand drains was completed rapidly, with each drain taking approximately five m

oung engineer, molasses, and failed sand drains - Geosynthetics http://geosyntheticsmagazine.com/articles/1009_f3_history.html

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drive, fill, withdraw, and move to the next location.

Upon completion of the sand drain installation, three open-standpipe piezometers were installed. The initial rate of surcharge

placement was approximately 0.5m (1.6ft) per week with no fill placed during weekends. With a target of 10m (33ft) of surcha

would take 20 weeks and, depending on the piezometer readings, surcharge would then be stripped off the site and constructi

two tanks would begin. This timetable met the tank owners schedule since much of the molasses was coming by tanker ships

Africa.

As surcharge fill commenced, the three piezometers responded immediately. Unfortunately, there was almost a direct corresp

of surcharge stress and excess pore water pressure increase. Over weekends there was only a nominal decrease in piezometric

Pore water levels in the standpipe tubes rapidly rose to elevations above the level of the surcharge.

During this time, there were several water main breaks in the service lines connected to the main waterline under the service

Figure 4 (1). After about four weeks, the surcharge rate of filling was decreased to about 0.25m (0.82ft) per week. There was s

essentially no decrease in piezometric water levels and they continued to rise assurcharge fill was placed. Pressure gages were

eventually fitted to the standpipe tubes. Even further, bricks in the warehouse buildings veneer started to come loose and ma

even be removed by handsee Figure 4 (2).

A few weeks later, the warehouse superintendent noted that the floor under his desk was rising upwardsee Figure 4 (3). Cra

asphalt-covered warehouse floor confirmed this upward heave. A futile attempt was made to position large lead ingots on the

they also rose.

Finally, on a late Friday afternoon a large tension crack opened up on the top of the surcharge fillsee Figure 4 (4). The edge

crack closest to the warehouse was approximately 150mm (6in.) lower than the far side and the crack visually lengthened (par

the warehouses) for 4060m (131197ft). It appeared as though it was unzippering. As shown by the arrows in Figure 4 a m

soil foundation shear failure was in progress. It occurred when the surcharge was 5.5m (18ft) high.

[Using minimum shear strength values, along with the piezometer measured pore water pressures, it was actually predicted t

a factor-of-safety of less than one when it was at 4.2m (13.8ft) high, Koerner (1963)].

The removal of surcharge began immediately and continued until the top of the slag drainage blanket was exposed. By the foll

Monday morning, the building floor stabilized (surveying was used throughout the weekend) and apparent equilibrium was r

After discussion among all parties involved, the sand drain/surcharge plan was aborted. An alternative design using two 40m

interconnected steel sheet pile ringwalls were driven to a depth of 12.2m (40ft). The two steel molasses storage tanks were the

constructed on top of the slag working blanket and within each of the two sheet pile containment ringwalls.

Large, flexible connections from ships moored at the Christiana Pier to the storage tanks were installed and small amounts of

pumped into the newly constructed tanks. At this point the molasses load in the tanks was serving as a gradually increasing s

load for the soil mass contained within the ringwalls. It took more than four years for the tanks to be utilized at their full stora

capacity. In the meantime, numerous tanker ships moored in the Delaware River served as temporary molasses storage vessel

great cost to the storage tank owner.

Conclusion

For about the next eight years, lawsuits were filed and the following two judgments were eventually reached in this case:

The property owner sued the tank owner in Delaware court and won a $1.2 million lawsuit for building, street, and utilit

damages and repairs.

The tank owner sued the consultant/designer in out-of-state court and won a $6 million lawsuit, which included the pri

judgment, the alternative sheetpile ringwalls, the rental of numerous tanker ships (called demerage), the loss of capaci

the storage tanks, and maintenance/repair of the storage tanks.

The reason the court found theentire judgment to go against theconsultant was that the consultant did not convey to the tank

that any risk was involved. Indeed, sand drains were still new in the early 1960s, but the consultant felt confident that this wa

correct approach. (It is not known how the field inspectors predicted failure height was addressed in the courtroom deliberati

it probably was a factor.)

Of course, the essential technical question is: Why did the organic silty clay soil not consolidate as designed?

Copyright 2013 Industrial Fabrics Association International. All rights reserved.


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The author (see sidebar starting on pg. 25), who was the inspector on the project, does not know but suspects that smear cre

much lower ch

value than expected was the major item. The issue of smear zone properties adjacent to the sand drain itself i

unanswered today, nearly 50 years after this case history occurred.

A final comment:

This comment has to do with communication between the consultant and the client, usually the owner of a facility or structur

lack of such communication before this sand drain project began was the pivotal point in the large financial judgment rendere

only is such communication required, it must be fully understood and accepted by the client. Only then (in the event of a failu

the consultant avoid a judgment of the type described here.

In short, we must be good teachers as well as good designers!

Robert M. Koerner, Ph.D., P.E., is an emeritus professorDrexel University and the director of the Geosynthetic Institute (GSI). He is a me

Editorial Advisory Committee for Geosyntheticsmagazine.

Figure 1 | Plan view of location of two molasses storage tanks at Wilmington Port Authority site, 1961.

Figure 2 | Cross section through the site from Figure 1 showing project details, adjacent warehouse, and related infrastructure


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Figure 3a | Design curves for consolidation times at Wilmington Port Authority project (after Koerner, 1963).

Figure 3b | Design curves for consolidation times at Wilmington Port Authority project (after Koerner, 1963).

Figure 4 | Progression of events leading to massive circular shear failure: Cross section of sand drain-surcharge fill site and ad


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A J Khan

Oct 4, 201011:11 am CDT

Peter Davies

Nov 1, 20094:36 am CST

warehouse, with approximate shear plane surface encompassing about 300,000m2 of soft organic clay foundation soil.

Comments

Comments are the opinion of individual posters and do not reflect the views of Geosynthetics or Industrial Fabrics Associati

International.

Comment on A young engineer, molasses, and failed sand drains

Perhaps, a stage construction approach would be more appropriate

for the project. With such low shear strength of the subsoil, it is quite

obvious to expect a global stablity failure leading to heaving of a lot

of ground in the surroundings. Also, this global stability failure might

have destroyed continuity of the drains and no wonder they were not

working.

I understand that the drains were driven into the ground. This

definitely caused a lot of smear of the surrounding soils and reduced

Ch value significantly. Alternatively, the amount of smear effect could

be reduced by drilling a shaft using auger and then filling the shaft

with coarse sand.

Thanks indeed for sharing such an invaluable experience.

Comment on "A young engineer, molasses, and failed sand drains

Hi Bob, Thank you for a thought-provoking lesson. I am forwarding

it to a number of acquaintances in the geotechnical field in South

Africa.

In my younger days (I'm 63 now), I worked for around ten years at

Frankipile, and I spent many an hour down pile shafts being

taught the dangers of smear by that doyen of geotechnics in SA - the

late Prof Jere Jennings who studied at MIT under Karl Terzaghi.

With that background, and the fact the company I now work

for manufactures band drains among other geosynthetics, I think thatyour belief that smear may have caused the failure of the sand piles at

Wilmington is well founded. It seems incredible that a thin layer of

smear could cause such a resistance to flow, but it's quite possible.

Thanks again. This sort of practical experience is invaluable and it is

good that Geosynthetics is bringing it to a wider global audience.

Best Regards, Peter Davies: Kaytech South Africa


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Sand drain -failure case study.pdf