case 2: Tunnel bore stable, tunnel heading not stable
11.3 Percolating water through dam rock foundations;
11.3.2 Classical grouting techniques for concrete dam
(1) The grout curtains. Glossop (1960, 1961) established that the first grouting was carried out in 1802 in Dieppe (France) by Charles Berigny.
The method was a remarkable success and was well known in France long before the 1832 publication by Berigny.
In 1933 Lugeon set down rules for grouting of concrete dam rock founda- tions with a cement grout, still used today on many dam sites. He considered three different systems.
(a) Low-pressure grouting under the whole area of concrete dam founda- tions, binding concrete to rock and consolidating foundation rock shattered by excavations (contact grouting).
(b) High-pressure deep grout curtain along the heel of the dam.
(c) A grout curtain at large (French: voile au large) supposed to cut water percolation round the dam.
An excellent example of Lugeon's technique is his work on the Sautet arch dam site (France) (fig. 11.8).
Fig. 11.8 Sautet dam. (Jb) Main grout curtain following the dam heel; (c) the curtain was grouted from adits 1, 2, 3 and 4 and from 5, the diversion tunnel (after Lugeon, 1933).
Low-pressure grouting under the concrete dam is carried out after the rock has been loaded by a reasonable thickness of concrete, in order to avoid lifting of rock masses by the grout pressure. The main grout curtain is usually done before the dam foundations are concreted. Lugeon suggests drilling boreholes 5 m deep under the foundation level. The borehole is sealed at 3 m under this level and the lower 2 m tested under water pressure and then grouted. The borehole is then redrilled through the hardened grout and bored down to —10 m, water tested and grouted from — 5 m to = —10m below foundation level. The drilling, water testing and grouting proceeds 5 m at a time downwards to the required depth. The alternative technique consists of drilling the boreholes downwards by 15-m steps and grouting them upwards by 5-m steps, redrilling and then boring another 15-m length.
(2) The Lugeon unit and the water tests. One Lugeon unit corresponds to a water test where one litre of water per minute is absorbed by the rock by a one-metre-borehole under a test pressure of 10 kg/cm2. According to Lugeon a rock absorbing less than one Lugeon unit is considered to be reasonably watertight and no grouting or further grouting is required. One Lugeon unit is approximately equivalent to K — 10~5 cm/s (Darcy's law).
Tests are often carried out on 5-m-long borehole sections, but shorter test
334 Rock mechanics and dam foundations
lengths are sometimes required for locating isolated fissures. The effective water pressure to be included in Lugeon's formula is, according to fig. 11.9:
water tablevV r
water table
II seal
\
ttested borehole -length
Fig. 11.9 Borehole section for locating isolated fissures.
where pm is the manometer pressure, H (in metres) the static head above the water-table and Ap the pressure losses in pipes and valves.
Assuming a borehole with diameter d—2r feeding a fissure (e = width of fissure) over an area TTR2, the discharge q through the fissure is then (fig.
11.10):
6rj loge R/rpe6
Fig. 11.10 Water discharge through a fissure.
(rj = viscosity of the water, loge = natural log, q varies little with Rjr).
For a 5-m test length of the borehole:
1 Lugeon unit corresponds to q = 5 1/min and e = 0-1 mm 10 Lugeon units correspond to q = 501/min and e = 0-2 mm 100 Lugeon units correspond to q = 5001/min and e = 0-5 mm.
When E is the modulus of elasticity of the rock a pressure p causes an increase Ae of the width of the fissure (Sabarly, 1968):
A, = ôÊ,
and
The discharge q increases rapidly with the pressure and (for e ^ 0) is proportional to /?4. This sharp rise of q is not really due to internal rupture (as sometimes assumed) but to elastic rock compression. The discharge q is inversely proportional to E and larger in deformable rock. (French claquage.) (3) Grout mixture and pressure \ borehole density. Lugeon recommends the use of normal cement and grout pressues of 35 kg/cm2, grout mixture, with a cement to water ratio C\W = \ to -fa. If it is not possible to plug a fissure within twelve hours, he recommends the use of thicker grouts, with CjW = i or J. He suggests grouting should stop when the grout losses are not higher than 251/min and meter under a pressure/? = 45 kg/cm2.
Today, special cements are being used, with no grain larger than 45 pm.
Lower pressures (p ^ 10 kg/cm2) are often efficient with fine cement.
American authors recommend that grout pressure should be related to the thickness of the overburden. Their recommendation is 1 lb/in2 per foot over- burden (1 kg/cm2 per 4*2 m). But European experts accept pressures as high as 1 kg/cm2 per metre overburden thickness, or about four times as much.
For example, for a rock depth of 45 m the American rule would limit the pressures to 11 kg/cm2 whereas the European would go as high as 45 kg/cm2. The American Task Committee (ASCE) accepts higher pressures than the 1 lb/in2 per ft rule. For stratified rock they recommend 22 kg/cm2 for 45 m depth and 45 kg/cm2 for a depth of 40 m in massive rock.
(This can be discussed on more precise assumptions along the lines developed in section 10.4, figs. 10.22 to 10.30, for uplift forces caused by tunnel grouting.)
In fig. 11.10 pressure p is supposed to be applied over an area 7rR2 causing an uplift force U = pirR2 on the fissure. How dangerous is this uplift? Some rough figures are available. Tests have been carried out for the Allt na Lairige dam (Jaeger, 1961c), which attempted to rupture the anchorage rock.
An uplift force U varying from 1000 tonnes to 4400 tonnes was applied to an area of 4-3 m2 (fig. 11.11), using flat jacks. The pressures increased to p = 23 kg/cm2 and then top = 102 kg/cm2 when the flat jacks burst before the rock was severely damaged. Only minor rock movements, possibly caused by internal cracks, had been noticed. The depth at which the flat jacks were anchored was only 18 ft (5-5 m) under the rock surface (Banks, 1955).
336 Rock mechanics and dam foundations
^2.35 m
Fig. 11.11 Test anchorage for Allt na Lairige dam: (a) six Freyssinet flat jacks each 870 mm in diameter; (Jb) hand pump; (c) deflectometer; (d) steel tube duct;
(e) staffs for vertical displacement measurement.
Grouting pressures as high as these would never be applied at such shallow depths and the experiment indirectly proves the resistance capacity of com- petent rock to grouting pressures; with poor rock, results would probably have been different.
Another approach is to choose the grout pressure on the basis of the potential hydrostatic pressure transmitted to the rock by the storage reservoir when filled, the grout pressure being larger than the hydrostatic interstitial water pressure. Grouting can then be carried out from deep-lying galleries rather than from the surface of the rock foundations. As an example, the grout curtain of the Mauvoisin dam (fig. 11.12) was injected from a deep gallery, at 237 m below dam crest.
520 m (along arch)
237 m
Fig. 11.12 The 237-m-high Mauvoisin arch dam grouting scheme: (1) grouting gallery level 1858; (2) deep grouting gallery, level 1724; (3) inspection galleries;
(4) lift; (5) power tunnel to Fionnay; (6) discharge gallery.
The distance between boreholes is chosen on the site according to test results. In competent rock the first boreholes are tentatively drilled 12 m or 6 m apart. A water test log is established along the whole borehole (fig. 11.13).
8.07 12.4'9 >
22.33
31.18-
37.19J
501/min 1 kg/cm2 17 1/min 12 kg/cm2 9 1/min 10 kg/cm2
30 1/min 10 kg/cm2 0.85 1/min 15 kg/cm2
Fig. 11.13 Water loss diagram, Spitallamm dam (after Lugeon 1933).
A similar diagram is traced for the grout absorption along the same borehole.
Depending on results, intermediary boreholes are drilled, sometimes the distance between them has to be reduced to 1 m or 0-5 m, or even two rows are drilled. For the Kariba dam, the boreholes through the pervious right rock abutment were arranged in a hexagonal pattern.
A few cases of rock masses being lifted by grout pressure are mentioned in the literature (Zaruba, 1962) with diagrams relating rock deformations to grout pressures and time. Deformations of 0-05 to 0-06 m under pressures of 3 to 6 kg/cm2 were measured. Other diagrams by Zaruba mention defor- mation of 160 mm. He suggests the following empirical values (metric system).
For competent rock, nearly vertical main system of fissures and fractures:
For weaker rock masses, horizontal stratifications:
p = 0-2577 + 0-005772
(4) Other types of grouting. Bitumen, mixtures of cement and silicates, chemicals (silicates), sand, sawdust, ashes, even gravel have been used for grouting. These are specialized techniques extensively dealt with in textbooks (Lugeon, 1933; Cambefort, 1964).