10.4.2 Chemical grouting
The development of grout in chemical grouting is not easily controlled and any small incaution might lead to the unpredictable flowing of grouts，which would cause damage to structures and pipes and reduce the effects of grouting.A detailed plan of the grouting pressure，the arrangement of injecting points，the amount of injection，and measures for preventing fugacious flowing of grouts should be made before implementing chemical grouting to rectify a building.Generally speaking，the grouting pressure，the arrangement of injection points，and the amount of injection are all determined on the basis of empirical experience and adjusted according to the monitoring of the heave of the building.No quantitative methods are available so far.Besides，referring to past case histories is recommended.For the prevention of grout flows going where they are not wanted，there are two methods：installing sealing piles（such as sheetpiles）within the grouting range or reducing setting time of grouts.
The cost of chemical grouting to correct a slanting building is relatively low.Nevertheless，the operation of carrying out a chemical grouting for building rectification is highly technical and ingenious，any incaution easily leading to damage to structures and pipes.
We will take a case history from the construction of the Taipei rapid transit system for illustration.The slanting building had 12 floors of superstructure and one floor of basement with a mat foundation located 4.9 m below the ground surface（Wong et al.，1996）.The building was influenced by the construction of the Taipei rapid transit system so that it settled 100 mm at the southeastern comer.The building tilted about 1／100 eastward and 1／200 southward.Figure 10.15（a）diagrams the plan of the building and the arrangement of the grouting holes for the building rectification operation.The grouting holes on row A and B were set at angles of 22.5° and 31.5°with respect to the vertical line below the building，as shown in Figure 10.15（b）.The geological conditions are as shown in Figure 10.15（b）.To prevent the grouts from fleeing，the contractor installed a row of continuous jet grouting piles for sealing（called the JSG method）and limited the setting time to 60 seconds.
The grouts consisted of cement of 250 kg，No.3 sodium silicate solution of 0.25 m3，and water of 0.671 m3 per cubic meter.The congealing time was between 34 and 60 seconds.
Figure 10.15 Building rectification by chemical method
To set the grouting pressure uniformly on the base of the foundation，three to four grouting machines were used simultaneously，each set to pump between 0.02 and 0.04 m3／min of grouting.First row A was grouted and then row B after row A was totally grouted.The grouting depth of each row was between GL⁃13⁃GL⁃9 m，starting from the bottom and lifting the drilling rod gradually up.With the clayey soil below the depth of GL⁃13.8 m［see Figure 10.15（b）］and the sandy soil above，the grouting operation took the sandy soil below the depth of GL⁃13 m as the bearing stratum where a larger amount of pressurized grouts was injected to constitute a solid ground in preparation for uplifting the building.With the lifting of the drilling rod，the grouting rate and pressure were lowered gradually.According to the above principles and experience，the control values of the grouting pressure and flow were determined.After 11 days’building rectification，the building was uplifted.