unsaturated soil mechanic

UNSATURATED SOIL UNSATURATED SOIL MECHANICMECHANIC

TEAM MATESTEAM MATESMOHD FARIZ ISMAILMOHD FARIZ ISMAIL

MOHD FIRHAN ANUARMOHD FIRHAN ANUARZULKIFLI ABASZULKIFLI ABAS

AFHAM ZULHUSMI AHMADAFHAM ZULHUSMI AHMADMUHD HAZMAN FAKHRIMUHD HAZMAN FAKHRI

INTRODUCTIONINTRODUCTION

Dedicated to the study of unsaturated soil behavior.Dedicated to the study of unsaturated soil behavior.

To describe the interaction among the three phases that To describe the interaction among the three phases that form an unsaturated soil (solid skeleton, water, and air) form an unsaturated soil (solid skeleton, water, and air) plays a dominant role.plays a dominant role.

Application for the classical fields of soil mechanics.Application for the classical fields of soil mechanics.

The essential for many practical problems to understand The essential for many practical problems to understand their interaction. their interaction.

The role of pore water in the mechanical behavior of The role of pore water in the mechanical behavior of unsaturated soilsunsaturated soils

Stress components in unsaturated soilsStress components in unsaturated soils •

In saturated soil,Terzaghi’s effective stress concept is applicable:In saturated soil,Terzaghi’s effective stress concept is applicable: σσ = = σ´σ´ + + uw uw (1a) (1a) thereforetherefore σ´ σ´ = = σ σ - - uw uw (1b) (1b)

where, where, σ σ = total stress,= total stress, σ´σ´ =effective stress and =effective stress and

uw uw = pore-water pressure= pore-water pressure

• uw uw is generated so as to balance directly a part of the applied load (total stress) is generated so as to balance directly a part of the applied load (total stress) • The effective stress is a component of stress which is transmitted through the soil skeleton from one The effective stress is a component of stress which is transmitted through the soil skeleton from one

boundary surface to another.boundary surface to another. When the air exists as a continuous phase in the soil pore space, the pore-water pressure When the air exists as a continuous phase in the soil pore space, the pore-water pressure uw uw in equation in equation

(1a) and (1b) should be replaced by the pore air pressure (1a) and (1b) should be replaced by the pore air pressure uaua

σ σ = = σnet σnet + + ua ua (2a) (2a)

thereforetherefore

σnet σnet = = σ σ + + uaua (2b) (2b)

• Terzaghi’s effective stress concept becomes inapplicable. It can be understood from Terzaghi’s effective stress concept becomes inapplicable. It can be understood from the above consideration that these stress factor, (ua – uw), χb and χm should be the above consideration that these stress factor, (ua – uw), χb and χm should be taken into account in formulating the stress-strain relationships for unsaturated soils.taken into account in formulating the stress-strain relationships for unsaturated soils.

degree of saturation degree of saturation

Sr = Sr = ( ( ew / eew / e ) x 100 = ) x 100 = w . Gsw . Gs

ew ew = relative void ratio introduced by Toll(1995), = relative void ratio introduced by Toll(1995),

w = w = water content in percent, andwater content in percent, and

Gs = Gs = specific gravity of soil particles.specific gravity of soil particles.

• χm = gm χm = gm ((SrSr) = ) = gm gm [[h(s)h(s)]]

• χb = gbχb = gb((Sr) = gb Sr) = gb [[h(s)h(s)]]

• Sr = h(s)Sr = h(s)]]

Figure 1. Figure 1. Equilibrium of stress components at surface and inside of soil Equilibrium of stress components at surface and inside of soil element.element.

Figure 2. Figure 2. Wavy section of general unsaturated soil Wavy section of general unsaturated soil

Application of unsaturated soil.Application of unsaturated soil.

The blocking effect of piles on ground water level in slopes under The blocking effect of piles on ground water level in slopes under rainfall.rainfall.

- - Piles are effective measure to stabilize dangerous slopes, but they may also Piles are effective measure to stabilize dangerous slopes, but they may also

cause cause

higher groundwater level in the slope under rainfall due to the pile’s blocking higher groundwater level in the slope under rainfall due to the pile’s blocking effect. effect.

- The pile’s blocking effect on the ground water level.- The pile’s blocking effect on the ground water level.

- Influences of the spacing and position of piles on the ground water level in a - Influences of the spacing and position of piles on the ground water level in a typical typical

slope under rainfall are numerically analyzed for three set of hydraulic slope under rainfall are numerically analyzed for three set of hydraulic characteristics of characteristics of

soils.soils.

- The numerical result show that the pile’s blocking effect on the ground water - The numerical result show that the pile’s blocking effect on the ground water level in the level in the

slope under rainfall is small.slope under rainfall is small.

- The used of piles to stabilize active landslides, and as a preventive measure in - The used of piles to stabilize active landslides, and as a preventive measure in stable stable

slope. slope.

Stress in the ground.Stress in the ground.

Total stressTotal stress Pore pressure Pore pressure Effective stress Effective stress Calculating vertical stress in the groundCalculating vertical stress in the ground

--When a load is applied to soil, it is carried by the water in the pores as well as the When a load is applied to soil, it is carried by the water in the pores as well as the solid grains. solid grains.

The increase in pressure within the pore water causes The increase in pressure within the pore water causes drainagedrainage (flow out of the (flow out of the soil), and the soil), and the

load is transferred to the solid grains. The rate of drainage depends on the load is transferred to the solid grains. The rate of drainage depends on the permeability of the permeability of the

soil. The strength and compressibility of the soil depend on the stresses within the soil. The strength and compressibility of the soil depend on the stresses within the solid granular solid granular

fabric. These are called effective stresses. fabric. These are called effective stresses.

Total stressTotal stress

In a homogeneous soil massIn a homogeneous soil mass In a soil mass below a river or lake In a soil mass below a river or lake In a multi-layered soil mass In a multi-layered soil mass In a soil mass which is unsaturated In a soil mass which is unsaturated In a soil mass with a surface surcharge load In a soil mass with a surface surcharge load

-The -The total total vertical stress acting at a point below the ground surface vertical stress acting at a point below the ground surface is due to the weight of is due to the weight of everything everything lying above: soil, water, and lying above: soil, water, and surface loading. Total stresses are calculated from the unit weight surface loading. Total stresses are calculated from the unit weight of the soil. of the soil.

Unit weight ranges are: Unit weight ranges are: dry soil γd14 - 20 kN/m³ (average 17kN/m³)dry soil γd14 - 20 kN/m³ (average 17kN/m³) saturated soilγg18 - 23 kN/m³ (average 20kN/m³) saturated soilγg18 - 23 kN/m³ (average 20kN/m³) Water γw 9.81 kN/m³  (= 10 kN/m³)Water γw 9.81 kN/m³  (= 10 kN/m³)

-Any change in vertical total stress (σv) may also result in a -Any change in vertical total stress (σv) may also result in a change in the change in the horizontal horizontal total stress (σh) at the same point. The total stress (σh) at the same point. The relationships between vertical and horizontal stress are complex. relationships between vertical and horizontal stress are complex.

Total stress in homogeneous soilTotal stress in homogeneous soil

Total stress increases with depth and with unit weight: Vertical total stress at depth Total stress increases with depth and with unit weight: Vertical total stress at depth z, z,

σv = γ.zσv = γ.z

The symbol for total stress may also be written γz, The symbol for total stress may also be written γz,

i.e. related to depth z.i.e. related to depth z.

The unit weight, The unit weight, γγ, will vary with the water content of the soil. , will vary with the water content of the soil. gg

Total stress below a river or lakeTotal stress below a river or lake

The total stress at depth z is the sum of the weights of soil in each layer The total stress at depth z is the sum of the weights of soil in each layer thickness above. thickness above. Vertical total stress at depth z, Vertical total stress at depth z,

σv = γ1d1 + γ2d2 + γ3(z - d1 - d2) σv = γ1d1 + γ2d2 + γ3(z - d1 - d2) wherewhere γγ1, γ2, γ3, etc. = unit weights of soil layers 1, 2 , 3, etc. respectively1, γ2, γ3, etc. = unit weights of soil layers 1, 2 , 3, etc. respectively

Total stress in unsaturated soilTotal stress in unsaturated soil

Just above the water table the soil will remain saturated due to capillarity, Just above the water table the soil will remain saturated due to capillarity, but at some distance above the water table the soil will become but at some distance above the water table the soil will become unsaturated, with a consequent reduction in unit weight (unsaturated unit unsaturated, with a consequent reduction in unit weight (unsaturated unit weight = γu) weight = γu)

σ = γw . zw + γg(z - zw)σ = γw . zw + γg(z - zw)

The height above the water table up to which the soil will remain saturated The height above the water table up to which the soil will remain saturated depends on the grain size. depends on the grain size.

Total stress with a surface surcharge loadTotal stress with a surface surcharge load

The addition of a surface surcharge load will increase the total stresses The addition of a surface surcharge load will increase the total stresses below it. If the surcharge loading is extensively wide, the increase in below it. If the surcharge loading is extensively wide, the increase in vertical total stress below it may be considered constant with depth and vertical total stress below it may be considered constant with depth and equal to the magnitude of the surcharge. equal to the magnitude of the surcharge. Vertical total stress at depth z, Vertical total stress at depth z,

σv = γ .z + qσv = γ .z + q

For narrow surcharges, e.g. under strip and pad foundations, the induced For narrow surcharges, e.g. under strip and pad foundations, the induced vertical total stresses will decrease both with depth and horizontal distance vertical total stresses will decrease both with depth and horizontal distance from the load. In such cases, it is necessary to use a suitable stress from the load. In such cases, it is necessary to use a suitable stress distribution theory - an example is Boussinesq's theory. distribution theory - an example is Boussinesq's theory.

Pore pressurePore pressure

Groundwater and hydrostatic pressureGroundwater and hydrostatic pressure

Water table, phreatic surface Water table, phreatic surface

Negative pore pressure (suction) Negative pore pressure (suction)

Pore water and pore air pressure Pore water and pore air pressure

The water in the pores of a soil is called The water in the pores of a soil is called pore waterpore water. The pressure within . The pressure within this pore water is called this pore water is called pore pressure (u).pore pressure (u). The magnitude of pore The magnitude of pore pressure depends on: pressure depends on:

-the depth below the water table -the depth below the water table

- the conditions of seepage flow - the conditions of seepage flow

Groundwater and hydrostatic pressureGroundwater and hydrostatic pressure

Under hydrostatic conditions (no water flow) the pore pressure at a given point is Under hydrostatic conditions (no water flow) the pore pressure at a given point is given by the given by the hydrostatic pressure:hydrostatic pressure:

u = γw .hw u = γw .hw

where where hw = depth below water table or overlying water surface hw = depth below water table or overlying water surface

It is convenient to think of pore pressure represented by the column of water in an It is convenient to think of pore pressure represented by the column of water in an imaginary standpipe; the pressure just outside being equal to that inside. imaginary standpipe; the pressure just outside being equal to that inside.

Water table, phreatic surfaceWater table, phreatic surface

The natural static level of water in the ground is called the The natural static level of water in the ground is called the water tablewater table or or the the phreatic surfacephreatic surface (or sometimes the (or sometimes the groundwater levelgroundwater level). Under ). Under conditions of no seepage flow, the water table will be horizontal, as in the conditions of no seepage flow, the water table will be horizontal, as in the surface of a lake. The magnitude of the pore pressure at the water table is surface of a lake. The magnitude of the pore pressure at the water table is zero. Below the water table, pore pressures are positive.zero. Below the water table, pore pressures are positive.

u = γw .hw u = γw .hw

In conditions of steady-state or variable seepage flow, the calculation of In conditions of steady-state or variable seepage flow, the calculation of pore pressures becomes more complex. pore pressures becomes more complex. See Groundwater See Groundwater

Negative pore pressure (suction)Negative pore pressure (suction)

Below the water table, pore pressures are Below the water table, pore pressures are positivepositive. In dry soil, the pore . In dry soil, the pore pressure is pressure is zerozero. Above the water table, when the soil is saturated, pore . Above the water table, when the soil is saturated, pore pressure will be pressure will be negativenegative. .

u = - γw .hw u = - γw .hw

The height above the water table to which the soil is saturated is called the The height above the water table to which the soil is saturated is called the capillary risecapillary rise, and this depends on the grain size and type (and thus the , and this depends on the grain size and type (and thus the size of pores): size of pores):

· in coarse soils capillary rise is very small · in coarse soils capillary rise is very small

· in silts it may be up to 2m · in silts it may be up to 2m

· in clays it can be over 20m · in clays it can be over 20m

Pore water and pore air pressurePore water and pore air pressure

Between the ground surface and the top of the saturated zone, the soil will Between the ground surface and the top of the saturated zone, the soil will often be partially saturated, i.e. the pores contain a mixture of water and often be partially saturated, i.e. the pores contain a mixture of water and air. The pore pressure in a partially saturated soil consists of two air. The pore pressure in a partially saturated soil consists of two components: components:

· pore water pressure = uw · pore water pressure = uw

· pore-air pressure = ua · pore-air pressure = ua

Note that water is incompressible, but air is compressible. The combined Note that water is incompressible, but air is compressible. The combined effect is a complex relationship involving partial pressures and the degree effect is a complex relationship involving partial pressures and the degree of saturation of the soil. In Europe and other temperate climate countries of saturation of the soil. In Europe and other temperate climate countries most design states are associated with saturated conditions, and the study most design states are associated with saturated conditions, and the study of partially saturated soils is considered to be a specialist subject. of partially saturated soils is considered to be a specialist subject.

Pore pressure in steady state seepage conditionsPore pressure in steady state seepage conditions

In conditions of seepage in the ground there is a change in pore In conditions of seepage in the ground there is a change in pore pressure. Consider seepage occurring between two points P and pressure. Consider seepage occurring between two points P and Q. Q.

The hydraulic gradient, The hydraulic gradient, ii, between two points is the head drop per , between two points is the head drop per unit length between these points. It can be thougth of as the unit length between these points. It can be thougth of as the "potential" driving the water flow. "potential" driving the water flow.

i = constanti = constant

Effective stressEffective stress

Terzaghi's principle and equation Terzaghi's principle and equation Mohr circles for total and effective stress Mohr circles for total and effective stress Importance of effective stress Importance of effective stress Changes in effective stress Changes in effective stress -Ground movements and instabilities can be caused by changes in -Ground movements and instabilities can be caused by changes in

total stress (such as loading due to foundations or unloading due total stress (such as loading due to foundations or unloading due to excavations), but they can also be caused by changes in pore to excavations), but they can also be caused by changes in pore pressures (slopes can fail after rainfall increases the pore pressures (slopes can fail after rainfall increases the pore pressures). In fact, it is the combined effect of total stress and pressures). In fact, it is the combined effect of total stress and pore pressure that controls soil behaviour such as shear strength, pore pressure that controls soil behaviour such as shear strength, compression and distortion. The difference between the total compression and distortion. The difference between the total stress and the pore pressure is called the effective stress: stress and the pore pressure is called the effective stress:

effective stress = total stress - pore pressureeffective stress = total stress - pore pressure or σ´ = σ - u or σ´ = σ - u

Note that the prime (dash mark ´ ) indicates effective stress. Note that the prime (dash mark ´ ) indicates effective stress.