approaches do exist that attempt to extract a single capacity value from test data, the entire load versus displacement response should be noted. Figure 7.8 shows the end bearing response as measured from a load test. A comparison between the measured and predicted values should be prepared so that the original design approach can be cali- brated. The end bearing strength ðb Þ is determined from strain gage data ðtoe Þ using the following expression: q b ¼ P toe =A ¼ toe E toe (7:12) 7.8 Design of Postgrouted Shafts The end bearing component of drilled shafts is only fractionally utilized in virtually all design methods (TCM < 1.0) due to the large displacement required to mobilize ultimate capacity. Consequently, a large portion of the ultimate capacity necessarily goes unused. In an effort to regain some of this unusable capacity, mechanistic procedures to integrate its contribution have been developed using pressure grouting beneath the shaft tip (also called postgrouting or base grouting). Pressure grouting the tips of drilled shafts has been successfully used worldwide to precompress soft debris or loose soil relaxed by excav- ation (Bolognesi and Moretto, 1973; Stocker, 1983; Bruce, 1986; Fleming, 1993; Mullins et al., 2000 a, b; Dapp and Mullins, 2002). The postgrouting process entails: (1) installation of a grout distribution system during conventional cage preparation that provides grout tube-access to the bottom of the shaft reinforcement cage and (2) after the shaft concrete End bearing (tsf) Displacement (in) 0 0 10 20 30 40 0.5 1.0 1.5 2.0 Permissible displacement (e.g., 1.0 in.) Effective ultimate end bearing (28 tsf) FIGURE 7.8 End bearing load test results. Design of Drilled Shafts 315
approaches do exist that attempt to extract a single capacity
value from test data, theentire load versus displacement response
should be noted. Figure 7.8 shows the endbearing response as
measured from a load test. A comparison between the measured
andpredicted values should be prepared so that the original design
approach can be cali-brated. The end bearing strength b is
determined from strain gage data toe using thefollowing
expression:
qb Ptoe=A toeEtoe (7:12)
7.8 Design of Postgrouted Shafts
The end bearing component of drilled shafts is only fractionally
utilized in virtually alldesign methods (TCM < 1.0) due to the
large displacement required to mobilize ultimatecapacity.
Consequently, a large portion of the ultimate capacity necessarily
goes unused.In an effort to regain some of this unusable capacity,
mechanistic procedures to integrateits contribution have been
developed using pressure grouting beneath the shaft tip (alsocalled
postgrouting or base grouting). Pressure grouting the tips of
drilled shafts has beensuccessfully used worldwide to precompress
soft debris or loose soil relaxed by excav-ation (Bolognesi and
Moretto, 1973; Stocker, 1983; Bruce, 1986; Fleming, 1993; Mullinset
al., 2000 a, b; Dapp and Mullins, 2002). The postgrouting process
entails: (1) installationof a grout distribution system during
conventional cage preparation that provides grouttube-access to the
bottom of the shaft reinforcement cage and (2) after the shaft
concrete
End bearing (tsf)
Dis
plac
emen
t (in)
00
10 20 30 40
0.5
1.0
1.5
2.0
Permissibledisplacement(e.g., 1.0 in.)
Effective ultimateend bearing
(28 tsf)
FIGURE 7.8End bearing load test results.
Gunaratne / The Foundation Engineering Handbook 1159_C007 Final
Proof page 315 20.11.2005 10:55am
