VII

Slurry trench practice for diaphragm walls and cut-offs

A L LITTLE, BSc FICE (Geotechnical Consultant, Binnie and Partners)

1. T h e P r o g r a m m e conveniently divides the subjects of this section into three:

1) Precast concrete diaphragm walls. 2) Slurry trench cut offs 3) Thin wall cut offs.

P R E C A S T C O N C R E T E D I A P H R A G M W A L L S

2. O v e r about the past 10 years, the use of precast units in slurry trenches has developed rapidly. O n e contractor estimates that he has constructed about 153, 000 s q . m , of precast panels in slurry trench with a further 12,000 s q . m , under construction, constituting about 1 0 % of his total diaphragm wall output (Le Sciellour in discussion on Nash, 1974). P r e s u m a b l y , the percentage is rising.

3 # W h e n the slurry trench m e t h o d w a s first applied to support trenches for the construction of concrete walls which w e r e subsequently exposed by excavation to f o r m permanently excavated areas, b a s e m e n t s , underpasses and so on, the finish of the exposed concrete s o m e t i m e s left something to be desired. Apart f r o m honeycombing, partial collapse of the sides of the trench during excavation left corresponding excrescences of concrete which had to be r e m o v e d to produce an acceptable line of wall. T h e use of precast units has been an important step forward in eliminating these causes for complaint. Another important benefit of the use of precast units, as well as close control over the concrete which can be manufactured under factory rather than site conditions, is that the reinforcement is not contaminated by bentonite and a better bond is obtained between reinforcement and concrete, with a consequent greater strength of the completed unit. H o w e v e r , formation of satisfactory joints between individual precast sections, always a p r o b l e m in slurry trench construction, requires particularly careful consideration in precast w o r k . This has been solved by using a self-setting slurry, containing cement, which, on completion of the work, sets to f o r m an erosion resistant i m p e r m e a b l e layer behind the precast units, sealing the joints between them. In one m e t h o d the self setting slurry is introduced as a fresh material after excavation under conventional slurry.

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Where the same slurry is used throughout the operation, a suitable retarder is added to the slurry to enable placement of the precast units before the cement/bentonite slurry mixture sets. Once it has set, the slurry provides an impermeable layer behind the precast unit wall (surplus set slurry can easily be removed from the front of the wall because the slurry even when set is always so much softer than the precast unit; if necessary, the precast panels can be coated with a suitable preparation to facilitate the process). This self-setting slurry has a wider and possibly more important application (see below in section 2) which was probably not foreseen by the originators of it. 4. Many different forms of precast units have been divised by various contractors. It is understood that a proposal has now been put forward for a precast unit forming not only the walls but also the roof of a cut and cover type of tunnel. Whether this proposal has been used anywhere is not known but perhaps some members of the panel can say more about it. 5. It is clear that the connections between precast units remain of concern to the users who have more or less ingenious ways of connecting the units together. In the Prefasif method (SLF-Bachy) for example, a latch on one panel, locks with a projection on the next one. The Panosol method (Soletanche) relies on various forms of joggle joint between the panels. 6. The wall formed by the precast units may be additionally supported with ground anchors, thus leaving a clear space in the excavation, free of strutting. I leave it to the various experts on the panel and to others to describe, if they wish, different systems in more detail. SLURRY T R E N C H CUT OFFS 7. The U. S Corps, of Engineers in September, 1945, began a series of experiments on constructing slurry trench cut offs in Mississippi River levees. Using a dragline, a depth of 35 ft was attained without sloughing of trench walls although the ground water level was at 14 ft down. Brig. General H. Kramer (Kramer, 1946) foresaw the use of the method in the construction of cut offs & core walls for new earth dams and levees of any height. Ensuing events have amply justified his forecast. 8. One of the earliest applications to dam construction of which this writer is aware was at Wanapum on the Columbia River, U. S. A. , in I960. A 10 ft wide trench using bentonite slurry was excavated by dragline to a maximum depth of about 80 ft. Excavated gravel was backfilled into the trench to form a cut off against a maximum impounded head of 88 ft (La Russo, 1963). Gravel backfills impregnated with slurry are now less used in permanent works than other types of backfill.

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LITTLE SLURRY TRENCH PRACTICE VII 9# In 1964 a slurry trench subsequently backfilled with concrete was excavated to a depth of more than 100 ft. to form a cut-off for the ZOO ft high Allegheny dam in Pennsylvania, USA. A rather elaborate construction system was employed. After constructing a guide trench at ground level, holes were drilled down to rock by percussion method at either end of the panel to be excavated. The intervening material was then removed with a clam shell. After excavation was completed, the panel was backfilled by tremie with plasticised cement concrete having a Z8 day strength of 3, 800 psi. Connecting spaces between panels were excavated using a special expanding chisel, shaped to fit round the ends of the panels already constructed. After excavation, this connecting element was concreted. This rather involved method was adopted to ensure verticality of the cut-off. In fact, only two of the guide holes exceeded the tolerance of 6 inches and had to be corrected. 10. This method contrasts with the simpler approach which is usually adopted nowadays. It would be interesting to have comments on the general accuracy of slurry trench work. 11. Another early use of the slurry trench technique was in repair of erosion damage to Balderhead dam in 1968. This has been described more fully elsewhere (Vaughan et al, 1970). Briefly, a 0. 6m thick slurry trench was excavated and backfilled with a cement/bentonite ("plastic") concrete in the rolled clay core of the embankment which had been previously grouted with clay/ cement to minimise slurry losses. In spite of the depth of 46.4m, the contractors were able to maintain verticality of the slurry trench and land the bottom of it on a 60cm wide flat top of a concrete cut off wall with a maximum error (which occurred at one panel only) of about 150mm. 1Z. The same method was used to repair two older dams (c80 yrs old), Lluest Wen and Withens Clough, where considerable seepage had been taking place through their puddle clay cores (Little 1975). In both cases a so-called plastic concrete mix with about 1. 3% bentonite added, was used for backfill; additionally, at Withens Clough only, 16% of flyash was used because of the highly corrosive nature of the reservoir water (Arah, 1975). 13. Another use for the self setting slurry of the previous section has already been mentioned. It was used, to form the cut off for a newly constructed dam in Singapore (Little, 1975). The dam 90 ft. high above its foundation level had to be constructed within an existing water supply reservoir which could not be taken out of use. Because of water lowering problems, it was not considered feasible to excavate the soft layered alluvium beneath the dam. Consequently a cut off had to be formed through this alluvium. At the same time, large settlements of the dam and its foundations were anticipated so that a cut off was required of

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sufficient flexibility to accommodate the resulting strains. It was decided to use a self-setting slurry type of cut off as possessing the necessary properties and at the same time providing an economic solution. The average axial strain at peak stress of the set slurry at 90 days was about 3j%; the strength, depending on^the ambient triaxial cell pressure, lay between 1, 800 and 2, 300 kN/m . The dam at Singapore was completed in 1975 and to date its performance as evidenced by extensive instrumentation has been satisfactory. 14. A further development of the technique of the use of self setting slurry has been described by Hetherington, Nelder and Puller (1975). At the Alton Water Scheme, very long cut off wing walls were required in the two abutments. Conventional methods of forming the wing walls (including concrete backfilled slurry trenches) were found to be prohibitively expensive and a cheaper method had to be devised. Accordingly, a self setting slurry with excavated gravel backfilled into it was used; this is reminiscent of the Wanapum operation, except that cement is added to the slurry, 15. Cut offs have applications other than to embankment dams. Land cofferdams, for example, frequently require cut offs. Here, the concrete backfilled slurry trench can be used, not only to provide a cut off to seal off ground water but also to provide support for the excavation. At the Ullswater water pumping station, for example, in 1969, a 0. 9m thick diaphragm wall circular in nJLan, 45m in dia., was excavated to a maximum depth of 26, 8m. The wall was constructed as a series of straight 4m long panels. After concreting, the ground inside the diaphragm wall was excavated to 20m and the pump house constructed. In this daring design, no ring beams or other supports were provided (Pawulski, 1975), For environmental reasons, the pump house is entirely below ground and the top of it is soiled and seeded to blend in with the landscape.

THIN W A L L CUT OFFS 16. In this technique, a mandrel, usually a steel joist section, is used to penetrate the groundand a low pressure injection of clay/cement is made into the space so formed. The result is a thin diaphragm which nevertheless, usually has a minimum thickness two or three times that of the forming mandrel. 17. The method was described by Maillard and Serota in 1963 in Grouts and Drilling Muds in Engineering Practice. So far as this author knows, the method was first used in this country in 1963 to form the cut off round the "Pound" or intake pond for the River Ouse intake at the Diddington scheme (Hammond and Winder 1967) Eight H-piles were used

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LITTLE SLURRY TRENCH PRACTICE VII as the forming mandrels, leapfrogging each other to progress round the pond perimeter. A total length of 1, 638 ft was driven though an average of 13 ft of gravel and 4 ft into underlying clay. The operation was sucessful and excavation of the gravel to form the pond was done with very little pumping despite its proximity to the river. 18. At Diddington, the clay/cement injection was made as the mandrel was withdrawn. In other methods, injections are made both during driving of the mandrel and during its withdrawal. 19. Like many other European developed construction methods, the thin wall cut off system has been subsequently adopted in U. S. A. with improvements. A recent article (ENR, 1975) describes driving a 33 inch wide flange H beam at 190 lb/ft with a vibrating hammer. Injection is done during driving and withdrawal. It is claimed that the injected slurry has been improved by the addition of soda ash and a phosphate to prevent cracking. A 14 inch fin welded to the beam flange forms a tongue and grove joint between each section as the beam is driven with a 4 inch overlap to the previous section. About 4, 200 sq ft of cut off is formed per 24 hour shift. The bid price was $US3 per sq ft compared with $US4 to 5 for a slurry trench operation. 20. Since the technique described in this section produces only a relatively thin membrane, it is clearly unsuitable for high hydraulic head differences or under severely corrosive conditions. It would be helpful to have some indication of extreme conditions under which the technique has been successful. 4

CONCLUSIONS 21. The slurry trench technique can be used in ground where sheet piling would fail; in some forms the slurry trench is cheaper. Its introduction into civil engineering has conferred large benefits on the industry. The ingenuity of engineers in marrying the technique with others, such as precasting or ground anchors has greatly extended the range of usefulness of the possible applications. 22. The success of the slurry trench method is largely due to the remarkable supporting power of fluids. Even in a soft clay, measurements in a 28m deep trench have shown negligible immediate deformation (10mm or less); even after 31 days, the reduction in trench width was no more than 55mm. Indeed, the investigators were bold enough to suggest that the trench could probably have been excavated with water as the supporting fluid, although clearly this would not work in more permeable ground (Dibiagio and Myrvoll, 1972). 23. Costs are being reduced by competition and by improvements in technique. As a guide, this author uses the rough rule of thumb:

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Unit Cost Thin wall cut off 1 Self setting slurry in trench 2 Steel sheet piling 4 Plastic concrete backfilled in

slurry trench 8 Comments on these ratios and on the value to be attached to the unit (a difficult matter in these inflationary times) would be welcomed. 24. On the debit side, this writer feels that the problem of joints has not been completely solved. Considerable ingenuity has been shown by various contractors in devising different means of forming joints, although the stop end tube probably remains the commonest method, in spite of its drawbacks. 25. Another problem which might merit consideration is the keying of trenches into bedrock, especially when it is of varying degrees of hardness or where it is steeply sloping. Particularly in the last case, fissures might be present which could lead to mud losses and even erosion after completion of the work, although this last is perhaps rather outside the scope of this discussion. 26. Di sposal of used bentonite also poses problems. With the continually mounting pressure from environmentalists, the problem is increasing, particularly as the use of slurry trenches becomes more widespread. At the last conference, only one participant seemed to have discussed this problem. Are we now in a position to make further pronouncements ?

R E F E R E N C E S Arah, R. M. , 1975

Investigations, Problems and Remedial Works at Withens Clough. Symposuim: Inspection Operation and Improvement of Existing Dams. Newcastle-upon-Tyne Paper 5 . 5 . Dibiagio, E. and Myrvoll, F. 1972.

Full Scale Field Tests of a Slurry Trench Excavation in Soft Clay. Fifth European Conf. on Structures Subject to Lateral Forces. Madrid. Paper IV-3, p 461. Fuquay, G. A. , 1966

Foundation Cutoff Wall - Allegheny Reservoir Dam ASCE. Hammond, T. G. and Winder, A. J. H. , 1967

Problems affecting the Design and Construction of the Great Ouse Water Supply Scheme. Journal Inst. Water Eng. Vol. 21 p. 15

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LITTLE SLURRY TRENCH PRACTICE VII Hetherington, J. S. , Nelder, B. , and Puller, MJ. , 1975

A Development in Slurry Trench Technique for Cut-Off Construction. Ground Engineering November, 1975, p 16. Kramer, H. , 1946

Deep Cut Off Trench of Puddled Clay for Earth Dam and Levee Protection. Engineering News-record, 27 June 1946, p 76. La Russo, R. S. , 1963

Wanapum Development - Slurry Trench and Grouted Cut Off. Grouts and Drilling Muds in Engineering Practice, p 196. Butterwaths. Little, A 0 L 0 1975

In Situ Diaphragm Walls for Embankment Dams. Diaphragm Walls and Anchorages, I. C. E. , London, p 23. Nash, K., 1974

Stability of Trenches filled with Fluids and Diaphragm Wall Construction Techniques. ASCE, Construction Division, Vol. 100, C04, (Dec), pp 533, 605. Discussions by S. C. Doughty, A. Le Sciellour, A. L. Little, Vol. 101, C02, C03. Pawulski, M. , 1975

Discussion on pages 1 to 4. Diaphragm Walls and Anchorages, ICE, London, p 29. Vaughan et al, 1970

Cracking and Erosion of the Rolled Clay Core of Balderhead Dam and the Remedial Works adopted for its repair. Tenth Int. Congr. on Large Dams. Montreal Q 36 R 5 p.

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PRECAST CONCRETE DIAPHRAGM WALLSSLURRY TRENCH CUT OFFSTHIN WALL CUT OFFSCONCLUSIONSREFERENCES