Tunnels excavated in weak rocks need to be properly reinforced for transmitting fluids on safe and healthy. Water conduit tunnels that transfer huge amount of usable water from a source lake or river meet water requirements of big cities. That is why they should be remained stable and isolated from ground effectively. In order to contribute this goal, special steel pipes are inserted into tunnels and spaces between ground and pipes are filled out by cement grouts. After allowing to cure these cement injections, secondary grout application is performed by using holes previously opened on the pipe circumferences by drilling holes enriching weak rock formation behind preliminary grout and support elements. In current study, a conduit tunnel excavated in İstanbul by DSİ, are investigated with respect to geology and amount of the grout application. Geostatistical approximation of Q rock classification of tunnel and grout measures due to face and grout coordinates are performed and compared to each other. Results point out that the less the Q values, the more the amount of the grouts.