Deeper excavations

Question 1

Problems faced

Answer 1

-Base heave under water pressure therefore dewatering is used in granular strata
-Design of multi level propping
-Combined base heave and active failure

Question 2

Undrained base heave FoS

Answer 2

Fs = NcSu/(q + gamma.H - p)
-p is the water pressure
-q is the overburden
-Nc varies with excavation H, B and L

-Numerator is the out of balance forces

Question 3

Undrained base heave remediation

Answer 3

-Jet grout
-Fill trench with mud/water
-Unload under the excavation

Question 4

Pecks analysis for multi level prop excavations

Answer 4

Empirical experience from strain gauge monitoring
-Limited to 9-15m
-Applies to clays and dewatered sands

Steps:
1. split H into sub-blocks
2. get F in each strut from the area of a pressure diagram

Pressure diagram:
-Dry sand: Rectangular with a = 0.65Ka.gamma.H
-Clay: 2 shapes depending on the stability number gamma.H/Su
-Integral gives safe strut loads

Question 5

Combined have and active faiure

Answer 5

-Based on upper bound plasticity method for soft clays
-Gives the total strut load by considering a failure mech. and internal/external work