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Embankment stability during grouting Stabilité de barrage en remblai pendant l'injection

Y. J. CHEN, Senior Engineer, Institute of Water Conservancy and Hydroelectric Power Research, Beijing, China

SYNOPSIS The hydraulic fracturing induced by grouti ng in cohesive fill results in an increase of the minor principal stress while the ma jor principal stress remains nearly unchanged, thus the maximum shear stress will decrease and the mean normal stress as as the of soil will increase. Both of these effects can in crease the stability of dam. The stress and movement analysis of a homogeneous dam has confirme d the above postulations. Finally, the possibi­ lity of improving the behavior of dam by grouting h as been discussed.

I NTRODUCTI ON grouting may be harmful to the dam, after learning that the dam will be splitted into two The grouting procedures for sealing the leakage blocks during grouting, and that injecting water of dam are more simple than those into slope for a long time would induce used for penetration grouting, and have been slip. modified constantly since the hydraulic fracturing induced by grouting was recognized (Chen, 1982). holes along the dam axis are drilled from the dam crest to near the I NDUCI NG FAI LURE OR I MPROVI NG STABI LI TY bottom of the fill. The spacing between the holes as large as 20 m has proved satisfactory. A soil element at the centerline of a homogeneous The grout, being a of local clay and dam is subjected to the major and minor principal water, and having a density up to 15 kN/m3, is total stresses (Toi and 0"o 3, and pore pressure u0. directly led to the hole from a grouting pump The total and effective stresses of this element with no return line. The hole generally does can be expressed by stress circules 1 and 2 not accept grout until the grouting pressure ap respectively in Fig. 1. If one drills a hole (indicates the pressure applied at the top of down to near the element, and pours water into the hole) rises to a value causing hydraulic the hole, the pore pressure will increase fracturing along the minor principal plan; at gradually with time. The this time, the rate of grout take increases circle thus moves towards the strength envelope, abruptly, and the grouting pressure will drop as soon as it touches the envelope, as shown by simultaneously to achieve the balance between circle 3 in Fig. 1, shear failure occurs. the take and the supply of the pump. The full depth of the hole is grouted in single stage, so the hydraulic fracturing usually starts near the middle height of the dam, where the minor principal stress is much smaller than the grout pressure p 0 (indicates the pressure caused by the weight of grout). As the grouting work goes on, the vertical crack extends both laterally and vertically, and finally it appears on the dam crest. At that time, the pressure exerted on the crack wall is controlled by the grout pressure rather than by the grouting pressure. The appearance of longitudinal crack on the dam crest is considered to be thé signal for ending grouting, and was formerly considered to be the most dangerous condition of dam Fig. 1 Stress Changes Caused by Grouting stability during grouting.

If the dam is grouted in this way, the grout Sometimes, there may be sudden loss of water from from holes made from the crest will travels the hole due to hydraulic fracturing before along the vertical plane parallel to the dam shear failure of the element occurs. The crac.c axis. The overlapping grout sheets (or mud wall) induced by fracturing is parallel to the minor as well as grout intrusions into the channels principal plane, so the water pressure pw and cavities can off the leakage paths exerted on the crack wall must be the minor through the dam. The above explanation seems principal stress U 3 and must exceed (To3- If the acceptable, however, some people worry that water elevation in the hole after fracturing is

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maintained constant for a long time, the water weight of grout having a unit weight of L3.7 has a better chance of flowing into the soil kN/m3. This loading condition has been voids through the large area of crack wall. As considered to be the most critical. a result, the shear failure of a huge mass of soil occurs. TABLE I However, the situation c hanges when the thick grout is used instead of water. Firstly, the Property Indices of the Fill grout is incapable of en tering voids of cohesive soil, so the pore pressu re adjacent to crack has only got a little increa se owing to the migration Material category 1 2 3 of water from grout to s oil; besides, the Unit wei ght kN/ m3 21.2 21.2 21.2 duration of grouting is usually much shorter Young's modul us kN/ m2 925 925 9250 than that of injecting w ater for the purpose of Poi sson's ratio 0. 33 0. 40 0. 33 inducing slope failure i n the mining industry, Cohes i on kN/ m 2 19. 6 19. 6 19. 6 therefore the pore press ure increase due to angle degree 30 30 30 water migration is small enough to be neglected, But the change of total stress AO 3 = (J3 - (T03 may cause pore pressure buildup, which will dissipate with time even the total stress U 3 The maj o r and minor principa 1 stresses in the itself does not decrease Secondly, Fig. 2 dam, hav ing proporty indices of ca tegory 1 , shows the stresses ffol, O 03, Po and Pw along the before and during grouting are shown in Fig. 3, centerline of a dam, it can be seen that p 0- Cr03 is much greater than pv/- ( To3, thus ¿03 caused by grouting is much larger than that caused by (a) Major principal stress, kPa injecting water. Before grouting --- Duri ng grout i ng —

Stress, kPa 0 200 400 600

(b) Minor principal stress, Before grouting --- Duri ng grout i ng —

However the major principal total stress remains Fig. 3 Stress Changes due to Grouting in a Dam nearly unchanged, as a result, the total stress circle 1 in Fig. 1 changes to circle 4. The effective stress circle at this moment should which shows that the major principal stress near be circle 5 in Fig. 1, because the pore pressure the centerline almost remains unchanged, while change Au will be less than A 0*3. Consequently the minor principal stress has a considerable the circle 5 will shift to 6 as Au dissipates increase. Therefore the shear stress level to zero. It can be seen in Fig. 1, both the decreases in this area, as shown in Fig. 4. On maximum shear stress and the shear stress level the contrary, the slight increase of major of either circle 5 or 6 are smaller than those principal stress in conjunction with the nearly of circle 2, hence the stability increases rather unchanged minor principal stress in the area far than decreases, provided that p 0 > away from the centerline results in small increase of shear stress level. By comparing the shear stress level in the dam before and during grouting, as shown in Fig. 4, one can STRESSES AND MOVEMENTS ANALYSI S OF A DAM conclude that the stress condition in the area on the right side of line 1 in Fig. 5a is A linear elastic analysis by using FEADAM improved, while the stress condition becomes computer program of the University of California worse on the left. The line 2 in the same is carried out to study the stresses and figure refers to the dam having property indices movements in a dam during grouting in comparison of category 2. Although the area improved is with those before grouting. The example is a smaller than that worsened, the overall stability 30 m high homogeneous dam having side slope 1 on of dam still increases, because the degree of 2.5, the is imcompressible. Three improvement is much high than that of worsening. categories of the fill property indices are Fig. 4 also shows that not only the mean value shown in Table I. In analysing the stresses of of shear stress level decreases, but also the the dam during grouting, it is assumed that the value of shear stress level becomes more uniform horizontal pressure acting along the centerline along the potential sliding surface. The safety was equal to the grout pressure p 0 caused by the factor K expressed in terms of the reciprocal of

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near the middle height than at both the top and the bottom as found in the field. It can be seen in the figure that the horizontal displacement reduces with the increase of Young's modulus. The maximum thickness of the wall, which should be twice the displacement shown in Fig. 5, ranges from 0.7 m to 0.07 m, the range covers the values usually seen in the field.

The analysis also shows that the increase of Poisson's ratio from 0.33 to 0.40 results in the increase in safety factor (see Table II), the reduction in horizontal displacement of the centerline (see Fig. 5b), and the decrease in shear stress level during grouting. All these results are caused by samller value of Affj in the case of higher Poisson's ratio.

The above results are compatible with the field evidence that the slope failure has never occurred during the grouting of hundreds of in China. The most severe condition appeared in the 70 m high Chengbihe dam with a central core while a diaphragm wall was being constructed along the dam axis. During excavating for placing , it was found that the mud pressure itself was large enough to force the split apart, but the splitting soon ceased when the mud level, and thus the mud pressure, was lowered because I mproved of flowing away of mud from the trench to the zone crack. The width of the crack appeared on the dam crest was as large as 6 cm, which did not cause any indiction of slope failure, and it decreased after completion of the diaphragm wall. All these results do not contradict to the fact that slips are usually induced by infiltrating rain water into sliding surface, because sliding Fig. 5 Improved Zone and Displacement surface is a shear plane but not a minor principal plane. mean value of shear stress level along a deep seated circle 1AB changes from 1.84 before grouting to 2.08 during grouting. If comparison I MPROVEMENT OF STRESS CONDI TI ON AFTER GROUTI NG is made between the most critical sliding surface IB before grouting and 1A during grouting, K The above analysis shows that grouting is even increases from 1.54 for IB to 1.72 for 1A. possible to improve the stress condition and These values of safety factor and the others thus the behavior of dam, provided that p 0 or AO '3 for the cases having property indices of is maintained unchanged. However, AO '3 in fact category 2 are shown in Table II, which shows decreases in the course of consolidation of mud that the stability of the dam is generally wall, but it will not reduce to zero, unless increased during grouting. the thickness of mud wall will be zero, therefore, improving stress condition to a certain degree is not impossible. If the mud TABLE II wall is thickened by the intermittent and successive grouting in each hole, the degree of Safety Factor before and during Grouting improvement will increase further. The field evidence that the fracturing pressure of a hole increases and grout take decreases with the Material Sliding surface K before K during number of times of grouting is likely to support category for comparison grouting grouting the idea postulated. In addition to this evidence, the results of field measurement in a Same surface 1.84 2. 08 16 m high Xibu dam (Bai, et al., 1982) is also Critical surface 1. 54 1. 72 worth mentioning. The available way of installing cell in the dam was excavating a cavity on the L Same surface 2.14 2.42 shaft wall and pushing a cell into the cavity Critical surface 1.72 1.89 having a size slightly smaller than that of the cell. Hence the measurement is only a qualitative indication. They reported that there was no any Fig. 5b shows the horizontal displacements AX measurable increase of vertical stress both during of the centerline during grouting, the lines 1,2 and after grouting, but the horizontal stress and 3 refer to the property indices 1,2 and 3 near the crack increased substantially; the stress respectively as shown in Table I. These lines did decrease with time but not vanish 52 days are in fact the indications of the mud wall after ending grouting as shown in Fig. 6. thickness, they show that the wall is thicker Therefore it seems likely that the grouting can

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because of the high restraint provided by the abutment. The mud walls formed in the cracks were excavated 10 years later, they bonded tightly to their adjacent fills, such a high bond was caused by the closed up action of the dam. Laboratory test results showed that the tensile and shear strengths at the contact surface between the wall and the fill were even greater than those of the fill itself, and the permeability of the mud wall was much lower than that of the fill. Therefore the risk of piping may not increase, if a mud wall is formed parallel to the river flow. Distance from the wall of crack, m

Fig. 6 Horizontal Stress (After Bai, 1982) Reduction of Arch Action in the Central Core It can be inferred that fracturing grouting may form a mud cushion in the central core where the arch action is strong enough to change the minor improve the stress condition and behavior of a principal plane to horizontal. This cushion is dam in the following ways. capable of increasing the principal stress considerably, and thus preventing hydraulic fracturing and piping due to reservoir water. Stabilization of Dam Slope

The stabilization of railroad fills by pressure grouting has already been successfully carried out in the United States (Simth § Peck, 1955). CONCLUSI ON They reported that grouting was usually started along the toe of the slope before the next The stability of the embankment dam during higher series of injection was made, the grouting may increase but not decrease as usually injection near the toe sometimes broke out on expected. Grouting not only can seal the the top of , and the grout take was usually leakage through dam but also may improve the high. Now it can be concluded that both the stress condition and thus the behavior of dam. long distance travelling of grout as well as the high grout take are the signs of typical fracturing grouting, and that the improvement of stress condition by fracturing grouting is one ACKNOWLEDGMENTS of the main factors in achieving stabilization of cohesive railroad fill. A number of dams in This study was conducted under the direction China were grouted through the holes which were of Prof. W.X. Huang of Qinghua University, arranged on a grid pattern over the entire Beijing. The writer also wishes to acknowledge upstream face of dam, those dams kept stable the support provided by his colleagues W.Z.Zhang, during grouting. Z. Chen, Y.X. Song and R.X. Zhu.

Elimination of Transverse Cracks

There were several dams on record where the REFERENCES transverse cracks near the abutment were remedied by grouting. A typical example was the Administrative Bureau of Fenhe Reservoir (1977). 60 m high homogeneous Fenhe Dam (Administrative Technical review of grouting for cracks of Bureau of Fenhe Reservoir, 1977), constructed by Fenhe Dam. Shanxi Journal of Hydraulic dumping loessial soil into ponded water, and Engineering, 2, pp. 43 - 58, in Chinese. completed in 1960. Several transverse cracks, having the width of 2 cm and the depth of more Bai, Y.N., et al. (1982). Study on the than 5 m, appeared on the surface of the dam fracturing grouting of Xibu earth dam. near the right abutment in 1963. Later on 110 Chinese Journal of Geotechnical Engineering, holes were drilled in the area of 50 X 66 m (4), 4, pp. 114 - 127, in Chinese. where cracks occurred, grout containing 150 t of soil was accepted. The average depth of the Chen, Y.J. (1982). Grouting for sealing the holes was 22 m, and the spacings between the cohesive fills of embankment dams. Trans. holes were 2.5 to 7 m. In the second period, 14th ICOLD, (4) pp. 785 - 797. 132 holes were further drilled within the same area, the total take amounted only about one Smith, R. and Peck, R.B. (1955). Stabilization fourth of that accepted in the first period. by pressure grouting on American Railroads. Hydraulic fracturing took place during grouting Geotechnique, (5), 3, pp. 243 - 252. as expected, but the cracks thus formed were vertical and perpendicular to the dam axis, because the minor principal plane near the abutment changed to that direction. But the mud wall formed in the later stage became parallel to the dam axis, however such change of direction did not occur in the other section of the dam. This phenomenon showed that the increase of minor principal stress near the abutment was larger than that far away from the abutment

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