GEO-MECHANICSGEO-MECHANICS(CE2204)(CE2204)

Soil Stress and Pore Water PressureSoil Stress and Pore Water Pressure

Lecture Week No 2

Mdm Nur Syazwani Noor RodiMdm Nur Syazwani Noor Rodi

LINTON UNIVERSITY COLLEGELINTON UNIVERSITY COLLEGESCHOOL OF CIVIL ENGINEERINGSCHOOL OF CIVIL ENGINEERING

TOTAL VERTICAL STRESS

• The total vertical stress (σv) acting at a point below the ground surface is due to the weight of everything lying above i.e. soil, water, and surface loading

• Total vertical stresses are calculated from the unit weight of the soil

• Any change in total vertical stress (σv) may also result in a change in the horizontal total stress (σ h) at the same point

• The relationships between vertical and horizontal stress are complex (Δσv ≠ Δσh)

TOTAL VERTICAL STRESS in homogeneous soil

zv

Ground Level

Depth, z

SOILELEMENT

σv

σv

TOTAL VERTICAL STRESS below a river or lake

wwv zz

Ground Level

z

Water Level

zw

332211 zzzv

Ground Level

z1 Soil1

z2 Soil2

Soil3z3

TOTAL VERTICAL STRESS in multi-layered soil

TOTAL VERTICAL STRESS with a surface surcharge load

qzv

Ground Level

z

Very ‘wide’ surcharge, q (kN/m2)

PORE WATER PRESSURE

• The water in the pores of a soil is called pore water.

• The pressure within this porewater is called pore water pressure (u)

• The magnitude of pore water pressure depends on:

a) the depth below the water table b) the conditions of seepage flow

Ground Level

Water Table

z

Zu w

PORE WATER PRESSURE under hydrostatic conditions (no water flow)

EFFECTIVE STRESS CONCEPT(Terzaghi, 1923)

u '

where

= Total Vertical Stress' = Effective Stress

u = Pore Water Pressure

VERTICAL EFFECTIVE STRESSES

zz w '

Ground Level

z

Water Table

u '

EXAMPLE 1Plot the variation of total and effective vertical stresses, and pore water pressure with depth for the soil profile shown below

Ground Level

4m GRAVELY SAND

SAND

SAND GRAVEL

4m

5m

5.18sat kN/m3

8.17B kN/m3

5.19sat kN/m3

0.19sat kN/m3

Water Table

2m

EXAMPLE 2The soil layers on a site consists of:0 – 4 m Gravel-sand (ρsat= 2038 kg/m3; ρB 1957 kg/m3)4 – 9 m Clay (ρsat= 1835 kg/m3)

Draw an effective stress and total stress profile between 0 – 9m, when the water table is 1m above the top of the clay

EXAMPLE 3On a certain site a surface layer of silty sand is 4m thick and overlies a layer of peaty clay 7m thick, which in turn is underlain by impermeable rock. Draw effective and total stress profiles for the following condition:

a) Water table at the surfaceb) Water table at a depth of 5m, with the silty sand

above the water table saturated with capillary water

Unit weight:Silty Sand = 18.5 kN/m3

Clay = 17.7 kN/m3

EXAMPLE 4A confined aquifer comprises a 5m thick of sand overlain by a 4m thick layer of clay and underlain by impermeable rock. The unit weight of the sand and clay respectively are 19.6 kN/m3 and 18.4 kN/m3. Determine effective overburden stress at the top and bottom of the sand layer, when the levels of the water in a standpipe driven through the clay into the sand layer are:

a) at ground surfaceb) 1.5m below the ground surfacec) 3.0m below the ground surfaced) 1.5m above the ground surfacee) 3.0m above the ground surfaceand hence comment on the effect of changing

water table

EXAMPLE 5A sediment settling lagoon has a depth of water of 4m above the clay base. The clay layer is 3m thick and this overlies 4m of a medium sand, which in turn overlies impermeable rock. Calculate the effective stresses at the top of the clay and at the top and bottom of the second layer under the following condition:

a) Initially, before any sediment is depositedb) After a 3m layer of sediment of silty fine sand

has been depositedc) After draining the lagoon down to base level,

with same thickness (3m) of sediment still in place

Unit weight: Sand = 20 kN/m3; Clay = 18 kN/m3; Sediment = 16 kN/m3

EXAMPLE 6Plot the variation of total and effective vertical stresses, and pore water pressure with depth for the soil profile shown below for the following condition:a)initially before constructionb)immediately after constructionc)few days after constructiond)many years after construction.

Ground Level

4m SAND

CLAY

5.18sat kN/m3

8.17B kN/m3

5.19sat kN/m3

Water Table

2m

Surface surcharge, q (100 kN/m2)

SHORT TERM & LONG TERM STRESSESa) Initially before construction Stress distribution profile at its original stage

b) Immediately after constructionThe immediately effect after the construction is an increasing in the pore

water pressure – loading is too rapid and not allow any significant out flow of pore water and the soils are in an UNDRAINED stage

c) Few days after constructionFew days after the construction, the out flow of pore water takes place at the

Sand layer due to its high permeability and the sand is in DRAINED stage. i.e. excess PWP is dissipated at the Sand layer whereas Clay Layer is in contrast

d) Many years after construction After many years, excess PWP will dissipated in clay layer despite its low

permeability and the soils are in drained stage