Earthwork and Ground Technology. Site Improvement

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1 Earthwork and Ground Technology Site Improvement Slides adapted and upgraded from original presentation slide by College of Engineering, University of Washington.

2 Methods of site improvement Removal and replacement Preloading Vertical drains In-situ densification Grouting Stabilization using admixtures Reinforcement

3 Removal and replacement One of oldest and simplest methods is simply to remove and replace the soil Soils that will have to be replaced include contaminated soils or organic soils Method is usually practical only above the groundwater table

4 Preloading Simply place a surcharge fill on top of the soil that requires consolidation Once sufficient consolidation has taken place, the fill can be removed and construction takes place Surcharge fills are typically feet thick and generally produces settlement of 1 to 3 feet. Most effective in clay soil

5 Advantages of preloading Requires only conventional earthmoving equipment Any grading contractor can perform the work Long track record of success

6 Disadvantages of preloading Surcharge fill must extend horizontally at least 10 m beyond the perimeter of the planned construction, which may not be possible at confined sites Transport of large quantities of soil required Surcharge must remain in place for months or years, thus delaying construction

7 Vertical Drains Vertical drains are installed under a surcharge load to accelerate the drainage of impervious soils and thus speed up consolidation These drains provide a shorter path for the water to flow through to get away from the soil Time to drain clay layers can be reduced from years to a couple of months

8 Vertical Drains

9 PVD (Prefabricated Vertical Drain) Geosynthetics used as a substitute to sand columns Installed by being pushed or vibrated into the ground Most are about 100 mm wide and 5 mm thick

10 Vertical Drain Installation Photo from:

11 Photo adapted from vertical drain supplier

13 Typical installation of PVD Typically spaced 3 m on centers Prefabricated Drains Available in Malaysia Nylex Emaskiara etc

14 Principle of Effective Overburden Pressure When load is applied to soil, the deformation which occurs will depend on the grain to grain contacts (intergranular forces) and the amount of water in the voids (pore water). For saturated soil, the more permeable a soil is, the faster the deformation under load will occur. (remember Prangin Mall) Only the inter-granular forces is effective in resisting shear and limiting compression. Therefore it is called effective stress. It is the controlling factor.

15 Overburden pressure The effective overburden pressure (Po) at any depth is determined by accumulating the weights of all layers above that depth as follows: 1) Soil above water table multiply the total unit wt by the thickness of each respective soil layer above the level. 2) Soils below the water table reduce the total unit wt by the wt of water (1000 kg/m3) i.e. use the effective unit wt.

16 Example Find Po at 6 meter below ground level in a sandy deposit with total unit wt of 1762 kg/m3 (17.62 kn/m3). Water table is 3 m below ground. Step 1: 3 m x = kpa (Po above water table) Step 2: 3 m x ( ) = kpa (Po below water table) Po = = kpa

17 Exercise Elv: 60m Elv: 51 m Elv: 45 m Ground level 60 m Clay γ = 20kN/m3 Sand γ = 17 kn/m3 Water table 6 m below ground Draw Po diagram and determine the overburden pressure at 9, 12 and 15 m below ground.

18 Pressure Diagram Elevation (m) Pressure (KPa)

19 Road Settlement Problem FRL 66m Elv: 60m Elv: 51 m Elv: 45 m Granular Fill γ = 18 kn/m3 Clay γ = 20kN/m3 Sand γ = 17 kn/m3 Existing Ground level 60 m Water table 6 m below ground

20 Pressure (KPa) Pf Po Elevation (m)

21 Primary Settlement Δ = H Cc log P f ( 1+ e o ) P o H = thickness of Clay layer Cc =coefficient of consolidation Eo = initial void ratio Pf = pressure after fill Po = existing overburden pressure

GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE

GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE Prof. J. N. Mandal Department of civil engineering, IIT Bombay, Powai, Mumbai 400076, India. Tel.022-25767328 email: cejnm@civil.iitb.ac.in Module - 9