Earthwork and Ground Technology

Site Improvement

Slides adapted and upgraded from original presentation slide by College of Engineering, University of Washington.

Methods of site improvement

• Removal and replacement• Preloading• Vertical drains• In-situ densification• Grouting• Stabilization using admixtures• Reinforcement

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

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 10-25 feet thick and generally produces settlement of 1 to 3 feet.

• Most effective in clay soil

Advantages of preloading

• Requires only conventional earthmoving equipment

• Any grading contractor can perform the work

• Long track record of success

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

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

Vertical Drains

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

Vertical Drain Installation

Photo from: http://www.joostdevree.nl/bouwkunde/vertical_drain_2_www_imtek_com_tr.jpg

Photo adapted from vertical drain supplier

Typical installation of PVD

• Typically spaced 3 m on centers

Prefabricated Drains Available in Malaysia

•Nylex

•Emaskiara

• etc

Principle of Effective Overburden Pressure

When load is applied to soil, the deformation which occurs will depend on the grain to grain contacts (inter-granular 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 PranginMall)

Only the inter-granular forces is effective in resisting shear and limiting compression. Therefore it is called effective stress. It is the controlling factor.

Overburden pressureThe 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.

ExampleFind 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 17.62 = 52.86 kPa (Po above water table)

Step 2: 3 m x (17.62 – 10) = 22.86 kPa (Po below water table)

Po = 52.86 + 22.86 = 75.72 kPa

75.72

3

6

0

52.86

0

Exercise

Elv: 60m

Elv: 45 m

Elv: 51 m

Water table 6 m below ground

Ground level 60 m

Clay γ = 20kN/m3

Sand γ = 17 kN/m3

Draw Po diagram and determine the overburden pressure at 9, 12 and 15 m below ground.

Pressure Diagram

40

45

50

55

60

65

0 102 123 183

Pressure (KPa)

Elev

atio

n (m

)

Road Settlement Problem

Existing Ground level 60 m

Clay γ = 20kN/m3

Sand γ = 17 kN/m3

Elv: 45 m

Elv: 51 m

Elv: 60m

Water table 6 m below ground

FRL 66m

Granular Fill γ = 18 kN/m3

0

108

210231

291

0 0

102123

183

0

50

100

150

200

250

300

350

66 60 54 51 45

Elevation (m)

Pres

sure

(KPa

)

PfPo

Primary Settlement

o

f

o PP

eCcH log

)1( +×=Δ

H = thickness of Clay layer

Cc =coefficient of consolidation

Eo = initial void ratio

Pf = pressure after fill

Po = existing overburden pressure