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Downloaded from the Online Library of the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE) INTERNATIONAL SOCIETY FOR SOIL MECHANICS AND GEOTECHNICAL ENGINEERING This paper was downloaded from the Online Library of the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE). The library is available here: https://www.issmge.org/publications/online-library This is an open-access database that archives thousands of papers published under the Auspices of the ISSMGE and maintained by the Innovation and Development Committee of ISSMGE. 8 /A /4 Embankment dam 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 well as the shear strength 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 embankment dam are more simple than those into clay 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). Grout 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 mixture 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 effective stress 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 cut 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 1 9 7 7 8 /A /4 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 Friction 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 foundation 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 1 9 7 8 8 /A /4 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.
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