8_engineering geology and soil mechanics_chapter 9_site investigation and laboratory testings (to...

7/25/2019 8_Engineering Geology and Soil Mechanics_Chapter 9_Site Investigation and Laboratory Testings (To Student).pdf

http://slidepdf.com/reader/full/8engineering-geology-and-soil-mechanicschapter-9site-investigation-and-laboratory 1/45

Topic 1 Site Investigation

1. Primary Objective of Site Investigation :

1.1 Suitability

To assess the general suitability of the site and environs for the

proposed works.

1.2 Design

To enable an adequate and economic design to be prepared.

1.3 Construction

To plan the best method of construction;

to foresee and provide against difficulties and delays that may

arise during construction due to ground and/or other local

conditions;

to explore sources of indigenous materials for use in

construction;

to select sites for the disposal of waste or surplus materials.

1.4 Effect of changes

To determine the changes that may arise in the ground and the

1/34

ENGINEERING GEOLOGY AND SOIL MECHANICS

CHAPTER 9

of 45

Ground Investigation and Laboratory Testing of soil



environmental conditions, either naturally or as a result of the

works,

and the effect of such changes on the works, on adjacent works,

and on the environment in general.

1.5 Choice of site

Where alternatives exist, to advise on the relative suitability of

different sites, or different parts of the same site.

In addition, site investigations may be necessary in reporting upon

the safety of existing features and works, for the design of

extensions, vertical or horizontal, to existing works, and for

investigating cases where failure has occurred.

2. Stages of Investigation

A site investigation will normally proceed in stages, as follows :

desk study;

site reconnaissance;

detailed examination and special studies;

review during construction.

2/34

of 45

CHAPTER 9



1.1-

/34

Source: GeoGuide 2

Page 3 of 45

CHAPTER 9



2.1 Desk Study

A desk study involves the collection and review of information

required for the planning of the project and of the site

investigation, which includes :

2.1.1 Maps, Plans and Charts

Topographic maps - for study the general features of the site

Tidal information - necessary for marine works

Old maps - to check the location of an old seawalls or a buried

stream course, etc.

Archaeological maps - may be required to establish the

boundaries of an archaeological site.

2.1.2 Ground Conditions

The following information is useful for studying the detail of the

ground conditions:

Aerial photographs

Geological maps and memoir

Past site investigation record

4/34

of 45



Fig 2 A typical aerial photograph and a topographic map (source: Survey & Mapping Office)

2.1.3 Meteorological and Hydrological Information

Local rainfall records are useful for slope and drainage design.

Hydrological information is useful in drainage studies, and the

assessment of flooding risk.

Data on ambient temperatures (including air and ground

temperatures) and solar radiation are useful in the design

temperature sensitive structures.

2.1.4 Past Records

Site formation works such as construction of slopes, retaining

structures and basements.

5/34 of 45

CHAPTER 9



Foundation works such as piling.

Details of preventive or remedial works.

Tunnels and disused tunnels, including details of linings and

ground support.

2.1.5 Services and Utilities

Information on gas, electricity, telephone, cable television, water

supply mains, public stormwater drains, foul sewers and similar

services should be sought from the private companies or

government departments supplying these services for the

following purposes:

Assessment of the effect of the proposed works (including

ground investigation works) on the existing services and

utilities.

Provision of services and utilities for the project

Provision of temporary electricity and water supplies for the

ground investigation.

2.1.6 Lease and Engineering Conditions

These conditions govern the use of the site. They also set down

the requirement and restrictions on development, and define the

6/34 of 45



responsibilities of the related parties and authorities, for examples:

restrictions on height of structures, and non-building areas

responsibilities of maintaining the stability of the land

2.1.7 Other Considerations before Ground Investigation

Before ground investigation works commence, a lot of factors

need to be studied or considered, for examples:

The effect of the ground investigation works on the ground, on

adjacent properties and structures and on existing services and

utilities.

Requirements and restrictions imposed by local statutes. e.g.

restriction of the use of powered mechanical equipment

between 7 p.m. to 7 a.m..

Land matters such as land ownership and permission to enter

or transport through adjacent land.

Matters relating to 'fung shui' and burial ground.

Permission of felling or removing trees.

The approval of the Building Ordinance Office if the work falls

within the Mid-levels Scheduled Area.

Information on the as-built alignment of the Mass Transit

7/34 of 45

CHAPTER 9



Railway and its "protection boundary" if the work falls with

the protection boundary.

Road excavation permits from Highways Department where

the work may disrupt the use of public roads.

2.2 Site Reconnaissance

The purpose of the site reconnaissance is to confirm and

supplement the information collected during the desk study. The

general procedure of site reconnaissance:

Traverse the whole area, preferably on foot, and take

photographs of the site and its surroundings.

Set-out the proposed location of work.

Record differences and omissions on plans and maps.

Inspect and record the details of all existing structures.

Record potential obstructions (e.g. transmission lines,

telephone lines, historical features, large trees, gas and water

pipes, electricity cables and sewers).

Check access, including the effects of construction loads on

existing roads, bridges and services.

Note water levels, direction and rate of flow in nullahs and

8/34

of 45



streams and also flood levels and tidal and other fluctuation.

Record features of the adjacent property.

Inspect and record old structures, and any other features.

Interview local inhabitants about the history of the site.

2.3 Detailed Examination and Special Studies

Detailed ground investigation will be discussed in section 3.

Other special studies may include:

detailed topographic survey

hydrographic survey

influences of weather - e.g. wind speed

sources of materials for construction

sites for disposal of wastes or surplus materials

2.4 Review during construction

It is difficult to forecasting the ground conditions very

accurately since only a small proportion of the ground is

examined.

Sometimes amendment of the design or the construction

procedures is necessary and further investigation will be

carried out.

9/34 of 45

CHAPTER 9



3. Ground Investigation

Methods chosen will depend on several factors :

1. size of contract.

2. size of proposed foundation.

3. type of sample required.

4. type of subsoils which may be encountered.

3.1 Methods of Ground Investigation

3.1.1 Trial Pits

usually dug by hand,

square in cross-section of about 1.2 x 1.2 m and up to 4m deep

can stand unsupported if less than 2m deep and in firm soil

in soft ground or deeper than 2m, timber support is required

Functions:

to permit the ground condition to be examined both

laterally and vertically

to provide access to take undisturbed samples

to provide access to carry out insitu tests

to ascertain the exact position of buried utilities and

services

10/34 of 45

1.5 x 1.5 3

timber shoring is required when the trial pit is excavated to 1.2m deep



1.1a-

Page 8

of 45

CHAPTER 9



Fig.3 Trial Pit Hand auger Hole Hand auger

(source: R. Chudley)

3.1.2 Hand Auger Boring

auger driven into the ground by hand

up to 200mm diameter and 5m deep

no casing is used

used in self-supporting ground with no gravel or boulder

used for ground water observation and

to obtain disturbed samples and open tube samples

3.1.3 Mechanical Augers

comprising a continuous-flight auger and a hollow stem driven

by a machine

of limited use in soils with boulders or corestones (therefore

seldom used in Hong Kong)

11/34 of 45

1.5

1.5

3 .

0



b .

P a g e

1 3

o f 4 5

C H

A P T E R

9



3.1.4 Rotary Drilling

the most common method of subsurface exploration used in

Hong Kong.

drill bits or casing shoes are rotated on the bottom of the

boreholes,

drilling fluid is pumped down to the bit through hollow drill

rods, lubricates the bit and flushes the drill debris up the

borehole

Fig. 4 Typical Configuration of a Rotary Drilling Rig (source: Geoguide 2)

12/34 of 45



two basic types of rotary drilling :

open hole drilling (or full hole drilling): the drill bit cuts all

the material within the diameter of the borehole, samples

may be obtained between drill runs by open tube samplers.

core drilling: an annular bit fixed to the outer rotating tube

of a core-barrel cuts a core that is returned within the inner

stationary tube of the core-barrel and brought to the

surface for examination and testing.

Drill casing is normally used to support unstable ground.

Fig. 5 Drill bit Fig. 6 Casing Shoe

(Source: GeoGuide 2)

13/34 of 45

CHAPTER 9



Extent of The Ground Investigation

Depth of Exploration

for shallow foundations: the depth should be at least one and a

half times the width of the load area measured below the base

of the footing or raft.

for piled foundations: the length of pile usually cannot be

decided until an advanced stage of the project, but it may be

possible to gain an early indication from standard penetration

test (SPT) blow counts.

for end bearing piles in strong rock: boreholes should be

penetrated into bedrock of at least 5 m, or two and a half times

the diameter of the pile, whichever is larger.

Bore hole data

The information obtained from trial pits or bore holes can be

recorded on a pro forma sheet or on a drawing showing the

position and data from each bore hole.

14/34 of 45



Fig. 7 Location plan of bore holes (source: R. Chudley)

Bore holes can be taken on a 15 to 20 m grid covering the whole

site or in isolated positions relevant to the proposed foundations.

Fig. 8 Bore hole data (source: R. Chudley)

15/34 of 45

CHAPTER 9



Figure 1.11 -

of 45



Figure 1.11 -

of 45

CHAPTER 9



Figure 1.12 -

of 45



Sampling The Ground

Sample Quality

Disturbed soil samples

The method of extraction disturbs the natural structure of the

subsoil.

suitable for visual grading, establishing the moisture content

and soil classification tests.

Disturbed soil samples should be stored in labelled air tight

jars.

Undisturbed soil samples

The method of obtaining preserve the natural structure and

properties of the subsoil.

suitable for soil strength tests, e.g. triaxial test.

The extracted undisturbed soil samples should be labelled and

laid in wooden boxes for dispatch to a laboratory for testing.

16/34 of 45

CHAPTER 9

stored in secured location for dispatch to a laboratory for testing.



Undisturbed soil samples

• The method of obtaining preserve the natural structure and

propertied of the subsoil.

• Suitable for soil strength tests, e.g. triaxial test.

• The extracted undisturbed soil samples should be labelled and

stored in secured location for dispatch to a laboratory testing.

of 45



Table 1.2-

of 45

CHAPTER 9



of 45



Techniques for Obtaining Samples

There are four main techniques for obtaining samples :

1. taking disturbed samples from the drill tools or from excavating

equipment in the course of boring or excavation,

2. drive sampling, in which a sampling tube having a sharp

cutting edge at its lower end is forced into the ground either by

a static thrust or by dynamic impact,

Fig. 9 General purpose

100mm diameter open-tube

sampler (U100 sampler)

(source: GeoGuide 2)

Fig. 10 A non-retractable triple-tube

core-barrel (source: GeoGuide 2)

18/34 of 45

CHAPTER 9



3. rotary sampling, in which a tube with a cutter at its lower end is

rotated into the ground, thereby producing a core sample,

4. taking block samples specially cut by hand from a trial pit,

shaft or heading.

Fig. 11 Block sampling (source: GeoGuide 2)

Insitu Soil Tests

Insitu tests to obtain the density or shear strength of soils are very

valuable since they can be carried out without disturbing the soil.

Standard Penetration Test (SPT)

This test measures the resistance of a soil to the penetration of

a split barrel sampler driven into the bottom of a bore hole.

19/34

of 45



The sampler is driven into the soil to a depth of 150 mm.

The sampler is further driven through 300mm by free falling a

standard weight of 65 kg through a distance of 760 mm.

SPT = the number of blows counted.

The test gives an index of density of the soil

Sands Clays

No. of Blows Relative Density No. of Blows Relative Density

0 - 4 very loose 0 - 2 very soft4 - 10 loose 2 - 4 soft

10 -30 medium 4 - 8 medium

30 - 50 dense 8 - 15 stiff

over 50 very dense 15 - 30 very stiff

over 30 hard

Fig. 12 Standard Penetration Test Setup Fig. 13 A Split Barrel SPT Sampler

20/34(source: GeoGuide 2) []

Cohesionless Materials

of 45

CHAPTER 9



Figure 1.8b

of 45



Vane test

This test measures the shear strength

of soft cohesive soils.

The vane is pushed into the soil and

rotate by hand at a constant rate.

The amount of torque necessary for

rotation is measured and the soil shear

strength can be calculated.

Fig. 14 Vane Apparatus

(source: http://www.ele.com)

Fig. 15 Vane Test Setup

(source: GeoGuide 2)

21/34

A test used to evaluate soil

strength and consistency.

After a hole is drilled into

the earth, a spinning four-

bladed shaft is inserted andthe soil's resistance to the

shaft's vanes is measured.

00mm if Diamater of

borehole is 100mm

To test the soil

remoulded cohesive

strength

of 45

CHAPTER 9



Plate bearing test

This test determines the bearing capacity of the soil for shallow

foundation.

Procedure:

Excavating a pit to the level of the proposed foundation

Loading a steel plate (600 x 600 mm) on the bottom of the pit

by a hydraulic jack bearing against a kentledge.

The load is applied in increments and each increment would be

1/5 of design load

Failure load is attained when settlement reaches 10% of breath

of the plate

The safe load should be taken as one-third of the failure load.

Fig. 16 Plate bearing test (source: R. Holmes)

22/34

of 45

breadth



limitation

The bulb pressure from this test is usually far smaller than the

bulb pressure from the actual foundation.

This could lead to error in detecting settlement of a weak

stratum

Fig. 17 Showing difference in bulb pressure between plate test and actual foundation.

(source: R. Holmes)

23/34 of 45

CHAPTER 9



Unconfined Compression Test

A 75 mm long x 38 mm diameter soil sample is loaded in the

apparatus until failure occurs.

This test can be used to establish the shear strength of a

non-fissured cohesive soil sample.

Fig. 18 Unconfinded compression test apparatus

(source: R. Chudley)

24/34 of 45



Dynamic Probing

In H.K., G.E.O. Probe is commonly used

The apparatus comprises a sectional rod fitted at the end with a

cone.

It is driven into the ground by a standard mass falling through

a fixed distance

The number of blows per each 100 mm penetration is recorded.

Probe results are useful for assessing the depth and degree of

compaction of buried fill

Fig. 19 G.E.O. Probe (source: GeoGuide 2)

25/34 of 45

CHAPTER 9

C.



Fig. 20 G.E.O. Probe Record (source: Geoguide 2)

26/34 of 45



Insitu Density Tests

Two methods are commonly used for determining insitu density:

Sand Replacement Method

A hole of about 150mm x 100mm is dug and the soil is

collected for weight and moisture content determination.

The exact volume of the hole is measured by filling standard

sand of known bulk density into it.

Dry Density of the soil can be then determined

Commonly carried out for earth filling works

Fig. 21 Insitu Density Test - Sand Replacement Method

27/34 of 45

CHAPTER 9



Nuclear Method

The nuclear density gauge comprises radioactive materials in a

sealed capsule which emit gamma rays and neutrons

The degree of absorption of gamma ray / neutron is measured

and correlated to density / moisture content of the soil

The method is very fast but safety precaution must be

followed.

Fig.22 Nuclear Density Gauge

28/34

(source: ELE)

of 45



Laboratory Testing

Laboratory testing is undertaken to establish the following

characteristics of soils :

1. identification and classification

2. measurement of their engineering properties

3. chemical content

Identification and classification

Visual examinations

Visual examinations are made to note the

colour

texture

consistency

29/34 of 45

CHAPTER 9



1.6.7.2

1.6.7.1

1.6.7

of 45



of 45

CHAPTER 9



1.6.7.3

of 45



Particle Size Distribution

This is the percentage of the

various particle sizes present in a

soil sample.

It is determined by sieving and/or

sedimentation method

The grading of a soil helps to

estimate its engineering properties

Fig. 23 Sieves and Shaker (source: ELE)

Fig. 24 Particle size distribution chart (source: ELE)

30/34 of 45

CHAPTER 9



Measurement of Engineering Properties of Soils

Shear Box Test

The apparatus comprises two bottomless boxes which are filled

with the soil sample.

A horizontal shearing force is applied against a vertical load

causing the soil sample to shear along a line between the two

boxes.

This test determines the shear strength of soils (C, values).

Fig. 25 Shear Box Test and typical test result (source: R. Chudley)

31/34 of 45



Triaxial Test

An undisturbed soil sample (150mm x 75 mm ) is placed in a

plastic cell and confined by lateral hydrostatic pressure.

The sample is then loaded vertically

Usually three tests are made with different confined pressure

and the results are plotted in the form of Mohr's circles.

It is the most popular method to establish the shear strength of

soils (C, values)

Fig. 26 Triaxial test apparatus and Mohr's circle (source: R. Chudley)

32/34 of 45

CHAPTER 9



Consolidation

The apparatus is called Oedometer

A soil sample is placed in a metal ring and loaded vertically.

The load is increased in stages and the time-settlement curve is

plotted

It is used to estimate the movement of soil under foundations

Fig. 27 Oedometer

(source: ELE)

Fig. 28

Consolidation Test

(source: ELE)

33/34 of 45



Chemical Content

Some chemicals exist in the soil may have some harmful effects

on the materials used in the proposed works.

It is necessary to find out their quantities:

sulphate content - sulphate attack of concrete

chloride content - corrosion of steel

pH value - acids attack steel and concrete

organic matter - settlement and hygiene

Reference

GeoGuide 2 - Guide to site Investigation (1993), Geotechnical Engineering Office, Hong Kong

Government

R.Chudley, Building Construction Handbook (1992) BH Newnes

Roy Holemes, Introduction to Civil Engineering Construction (1983), College of Estate

Management

Civil and Environmental Engineering Test Equipment 9th Edition Catalogue, ELE International

http://www. ele.com/index.html

CHAPTER 9

8_engineering geology and soil mechanics_chapter 9_site investigation and laboratory testings (to...

Documents