Hirokazu Akagi

Soil consolidation associated with grouting during shield tunnelling in soft clayey ground

J. N. Shirlaw, Land Transport Authority, Singapore The authors have provided a very interesting paper, with a case study, laboratory testing and finite element analysis of the effects of grouting during EPB tunnelling. However, little detail is given on the monitoring and tunnelling in the case study, and further information would help to provide a fuller record of their work. For the monitoring, were settlements measured using an array orthogonal to the direction of advance, to measure the width of the consolidation settlement trough? Based on my experience in Singapore with grout injected after tail void closure, I would expect the settlement troughs during consolidation for arrays B and C, and possibly A, to follow an ‘error function’ shape with a width similar to that of the initial settlement trough. Consolidation settlements due to this type of grouting have the effect of delaying the immediate settlement due to tunnelling, but the final effect is the same as immediate settlement. Using an i value of half the depth to tunnel axis, this would imply that the total volume loss due to tunnelling was 5·7%, 7·5% and 7·9% for cases A, B and C. These are relatively high values, but would be consistent with the closure of a gap of about 60 mm all around the tunnel. It would be useful to know the size of the tail void (the diameter cut by the machine minus the external diameter of the lining), to allow comparison with these volume loss figures. The volume loss figures given above are high for an EPB machine operating in a firm clay. The stability number with no support pressure is 3·49. The total settlements recorded are high for such a stability number, even disregarding the support provided by the EPB machine. It would be useful to know the range of face pressures used during tunnelling, and whether the face was over-pressurised, which could lead to remoulding of the very sensitive clay. The volume loss figures given above can also be expressed as volume losses of 0·405–0·558 m=m of tunnel. Trials B and C involved the injection of 3·8 and 6 m of grout. Comparing these figures, I presume that the conventional tail void grouting was discontinued for several metres of the tunnel, but it is not stated over what length of tunnel the tail void grouting was replaced by the alternative grouting. This is important, because it appears that the settlement over Trials B and C was not just related to the effects of the trials. The surface settlement over the two trials would be due to the cumulative effects of tunnelling from a point about 21 m before the settlement point to about 21 m beyond the settlement point. It would therefore appear that the surface settlements over trials B and C would have included effects both from the trial injection and from the more conventional grouting as used in Trial A. I would note that the ‘immediate’ settlement identified in the paper does not allow for the time necessary for the full development of the immediate settlement trough, owing to this threedimensional effect. The conventional tail void grouting appears to have been more effective than the trial injections at controlling total surface settlement. However, the settlement was still large for this size and type of machine operating in firm clay. The authors do not state whether the tail void grouting was carried out using grout pipes laid along the tail skin, to allow grouting simultaneously with machine advance, or through the lining. The former of these has been found to be much more effective at filling the tail void before it closes (in soft clays) than the latter. I was very interested in the high compressibility of the cement/silicate grout (Type I) used, and the significant reduction in grout compression using the Type II grout. This is clearly an area that warrants further investigation. However, I would appreciate some clarification on some of the items in the paper. The authors report a 120 s gel-hardening time for the Type I grout, but for the identical grout used in the tunnelling trial report a 20 s gel time. In the consolidation testing of the grouts, the authors report that Type I lost 30% of its original volume ‘after hardening’. I am not clear whether the 30% loss was measured on hardened grout, or during the interval between injection and hardening. I also note that with Type I grout the laboratory trials show an initial volume expansion of very close to 100% of the grout volume used in all but one case. For the Type II grout the initial volume expansion is well below 100% of the grout volume used in three out of seven cases, implying some other losses in the system. The authors also do not state the mix used for grouting in trial A: it would be useful to know whether this grout was also prone to significant loss of volume. As a final comment, I question the normalisation of the grout volume by initial soil volume in Figs 10 and 12. Within the laboratory trials the soil volume is defined by the size of the container. However, for wider application, this method of presentation is of limited value. Did the authors consider expressing the data in terms of rþ 3 r=r, where r is the radius of the initial injection cavity, and rþ 3 r is the radius of the expanding bulb (assuming expansion as a sphere)?

Destructuration of soft clay during Shield TBM tunnelling and its consequences

It is very important to prevent ground settlement associated with shield TBM tunnelling in soft ground in order to avoid the damage to existing super-structures in urban area. However, consolidation settlement takes place in the surrounding ground under its own weight due to soft clay destructuration with the decrease of effective stress, since it is inevitable that the adjacent clay suffers the undrained shear deformation during shield TBM tunnelling within soft clay ground. Such consolidation phenomena of normally consolidated soft clays due to their destructuration are investigated. It is concluded from experiments and theoretical investigation that the volume decrease of normally consolidated soft clays necessary to predict the ground subsidence behavior is obtained by using the magnitude of shear strain during undrained shear deformation and clay sensitivity and plasticity index, which are related to the susceptibility to soft clay destructuration.

SHIELD TUNNELING ADVANCEMENT SIMULATION USING 3D FEM CONSIDERING DISTANCE FACTOR AND ITS VALIDATION

A new approach of shield tunnel advancement for soil deformation calculation is presented using 3D finite element method. Applying step by step tunneling loads in this method, total soil deformation is calculated by summation of induced displacement from the first loading step to the current step deformation. In this approach, shield machine face distance from monitoring locations is taken as a factor which affects way of calculating soil displacement. If TBM machine face distance from the monitoring location is assumed to be less than a specific distance say “D”, drained condition is used to calculate soil deformation; on the other hand, if the machine face distance from monitoring location is more than “D”, undrained condition is used for soil deformation calculation. Additionally field data of an EPB shield tunneling site were gathered and used to investigate the validity of proposed approach.

A Study on Numerical Simulation during Chemical Grouting

The chemical grouting has the advantages that equipments are small and the direction of boring has flexibility. Therefore, in Japan, it is frequently used for a soil-improvement method in the railway construction site where workspace or working hours is limited. However, it has the possibility to cause a ground displacement around the site. Because there is no method that estimates it quantitatively, the design based on old cases is still used mainly and focuses on safety overly. In this Study, the simplified numerical analysis for ground behavior due to the osmotic pressure during injection is proposed.

Long Term Field Monitoring of Chemically Stabilized Sand with Grouting

Chemical grouting method is employed frequently within an urban area to improve the strength and permeability characteristics of soft ground. In order to ensure the long term performance of chemically stabilized sand with grouting, the systematic field measurement of the mechanical characteristics of the chemical grouted sandy soil has been carried out by the Japanese Grouting Association for the period of five years, from 2000 to 2005. The test field soil condition consisted mainly of comparatively dense sand with gravel and the ground water table was around 6 m below the ground surface. Four types of silicate grouting materials were employed. Standard penetration tests (SPT), lateral loading tests (LLT), in-situ permeability tests and flow meter tests within a bored hole were conducted to obtain the strength and permeability characteristics of the grouted sand. The SPT-N value and LLT deformation modulus of the grouted sand with a silicate material of an organic hardener has increased for the initial two years and then decreased for the next year. However, its N-value was still the triple times the initial one. It has been concluded that silicate grouted materials have demonstrated the satisfactory performance during the period of three years.

PROPOSED VOLUMETRIC MIX DESIGN METHOD FOR HOT MIX ASPHALT BASED ON THE CALCULATED VOIDS IN MINERAL AGGREGATE

The Marshall stability test has been used in Japan as the mix design method for hot mix asphalt since 1961 when this method was adopted in the ‘Manual for Asphalt Pavement’ . Recently, this method has been considered to be an emprical one that can especially design a durable mixture which is rut resistant. We, therefore, propose a rational mix design method whose procedure is to calculate the voids in mineral aggregate from the aggregate gradation then add the necessary asphalt content to get an appropriate air voids ratio of the mixture. The required material properties are checked in both this calculation and in the actual compacted specimens.