Chapter 3 The formation of soil and its engineering properties

第 3 章 土的形成和土的工程性质

1 the importance of the study of soils

Soils are the results of natural geological history, at the same time

soils are also the most common materials distributed on the ground

surface of the earth crust. The engineering properties of soils have

closely relations with the engineering activities of human being.

Because of complexities of soils, many engineers have been paid

much attention to the study on soils. In some sense, the complexities of

soils are mainly from the complexity of formation process of soils and

the complexities of soils constitution. In this chapter, We are mainly

concerned with the formation, constitution of soils, the classification of

soils based on the different formation, since others aspects about soils

will be discussed in soils mechanics.

§3-1 Soil formation

2 the soils definition

Soils are porous natural materials composed of

inorganic and organic matter, soils are the end product

from weathering rocks.(Also we can merely take it as a

stage in the gigantic cycle of mineral recycling by the

movement of tectonic plates)

3 the influencing factors of soils formation

parent material

time

climate atmospheric composition

topography

organisms

§3-2 The main types of soils based on the formation process

Generally speaking, the engineering properties of

soils are much influenced by the varieties of formation

process. So in this aspect, it is important for a civil

engineer to know the soils types based on formation

process.

1 Eluvial soils( 残积土 )

The eluvial soils （ residual soils ） are the direct

products of weathering, they are formed in the place

where their mother rocks are located.

Features:

● soluble materials have be

en carried away

● the composition has the relation with the mother rock

● different particle size

● obvious edge and corner

● thickness change greatly

lower psephicity [si’fisiti]

mother rock

eluvial soils

2 Slope deposits( 坡积物 ) Qdl

The slope deposits are formed from erosion of surface

flow along the mountain slope surface . after being carri

ed to the foot of hill by surface flow( 面流 ), where the sp

eed of flow （流速） will slow down and the slack sedi

ments will deposit there.

冲沟形成过程图

Features:

●slack condition

● the composition has no re

lation with the bad rock

● fine particle size

● obvious edge and corner

● thin thickness

● lower psephicity [si’fisiti]

The pluvial deposits are more commonly distributed

along the connection zones between the mountainous

areas and plain areas. The flood water caused by

heavy rain or snow dissolved when temperature rise up

is the dynamic resource.

3 Pluvial deposits ( 洪积物 ) Qpl

AB C

A the zone for groundwater level deeply buried

◆large grain size

◆ lower compressive factor

◆ Higher bearing capacity

◆ lower psephicity

B Zone for ground water overflowing

◆ sandy soils and clay are deposited alternatively

◆ the groundwater shallowly buried

C Zone for periodic dry and damp condition ◆clay ◆engineering properties depends on damp condition

洪积扇地形

4. Alluvial soil 冲积物 (Qal)

（ 1 ） the definition of Alluvial soil

Alluvial soils are the results of river geological Action. (including river erosion ;carrying;sedimentation)

Cut erosionV 字型河谷

（上游）

Side erosion and sedimentation U 字型河谷

（中游）

Sedimentation成形型河谷地上河

（下游）

（ 2 ） river geological action

河流侧蚀作用 牛轭湖

的形成

The result of side erosion

侵蚀阶地

阶座基地（侵蚀堆

积）

堆积阶地（内迭阶

地）

The formation of river terrace 阶地的形成

河床

河漫滩

河谷斜坡

一级阶地二级

阶地三级阶地

（ 3 ） Faces of River deposits

◈river bed faces （河床相） :

coarse sand, gravel, boulder

well psephicity

◈ terrace faces （阶地相） :

upper part→fine sand

lower part→river bed faces

◈ oxbow lake faces （牛轭湖相） :

swamp d<0.005mm

ooze: high water content

high porosity, lower strength, high compressive

5. Lake deposits (QL)

6 Sea deposits (Qm) 海积物

7 Wind deposits (Qeol) 风积物

wind sand （风积砂）wind-loess （风积黄土） collapsible soil uniform, homogeneous porous, scattered structure

8 Glacial deposits (Qgl) 冰积物

河谷横断面及各部位名称

§3-3 Mineral composition in soils

1 Primary minerals （原生矿物）

2 Secondary minerals (unsolvable with water)

次生矿物

3 Soluble salt and decomposed minerals

可溶盐类

4 Organic materials （有机物质）

阶地的类型1 Primary minerals （原生矿物）

◈ Primary minerals are from magma rock, they are mainly quartz, feldspar, ice, amphibole

◈ general feature of Primary minerals : --Stable in chemical properties --weathering resistance --Good performance of engineering Properties

河谷的形态阶地的类型2 secondary minerals （次生矿物）

◈ Primary minerals are from the process of soils

formation , they are

① clay minerals （黏土矿物） Kaolinite ， montomorillonite ， water mica

② SiO2, Al2O3, Fe2O3: colloidal state

③ Oxidized material （）

The mineral described above is very important to the engineering properties of soil. Reason are ： mineral particle surface→surface power absorbing water affinity( 亲和力 )

3 Soluble salt and decomposed minerals

可溶盐类

soluble salt: NaCl, Caso4.2H2O( 石膏 ),

CaCO3(calcium carbonate)

salinized soil ( 盐 土 )

Usually, soluble salt in soil don’t have much influence to

soil engineering properties, because its little content (in drou

ght area, salt can be accumulated in the upper parts of groun

d surface), But if soil containing much salt react with water, t

he connection between soil grain will disappear, so the soil’s

strength and stability will be decreased.

4 Organic materials （有机物质）

Which usually exits in the ooze. When content of

ooze is more than 5%. The influence will be obvious.

§3-4 The structure and tectonics of soils ( 土的结构与构造 )

一、 soils structure type( 土的结构类型 )

Which mean the characteristics of soil grain and the relationships among grains.

◇ the size of grain ◇ Shape and roughness of grain ◇ arrangement of grainand ◇ connection between different grain

1. Single Particle Structure (SPS)

(crushed stone, gravel, sand) --d>0.074mm --no cohesion, no capillary water --permeability is obvious --high strength and low deformatio

Single Particle Structure

2 honeycomb structure

d<0.075mm

--silt grain ( 粉粒 d=0.075-0.005mm)

--clay grain ( 粘粒 d<0.005mm)

--sand grain ( 砂粒 d=2-0.074mm)

honeycomb structure

3 cotton structure

which in some sense like honeycomb structure The clay usually have the honeycomb and cotton structure and have the engineering properties as below ： d<0.005mm  big porosity (50%-98%)      easy to be deformed     water content (w) >75%    unstable when encounter water

cotton structure

二 .The tectonics of soil （土的构造）

soil’s tectonics mean the space shape of soil layer.

♦ In north-west area loess—vertical cracks

♦ In coast area, soft clay have usually distributed, stratified structure, isotropic soil

§3-5 The index of physical properties of soils ( 土的物理性质指标 )

soils’s three-phase composition

liquid

solid

air

V ： total volume of soils W: total weight(KN)Vs: solid volume Ws: solid weightVw: water volume Ww: water weightVa: air volumeVv: pore volume

VW

V=Vs+Vw+Va=Vv+VsW=Ws+Ww

(1)  relative unit weight(G) 土的相对密度

ws

s

w

s

s

w

s

rv

w

r

v

w

r

rG

unit weight of solid mineral particle unit weight of water(10KN/m3)sr

wr

（ 2 ） unit weight 土的重度

)/( 3mKNv

wr

（ 3 ） dry unit weight 土的干重度

)/( 3mKNv

wr sd

which indicate the density of soils grain

（ 4 ） saturated unit weight 饱和重度

）（ 3/mkNv

rvwr wvssat

（ 5 ） effectve unit weight 土的有效重度

（ 6 ） water content (w) 土的含水量

)/(' 3mKNv

rvwr wss

s

w

w

ww

The water content in soils, especially in cohesive

soils, has much influence on the engineering

properties of soils. So the determination of water

content is necessary

(7) saturation degree 饱和度

%100v

wr v

vs

（ 8 ） porosity(n) 空隙度

v

vn n

(9) relative porosity(void ratio) 孔隙比

s

v

v

ve