site investigation-week 1-4 -...

Site InvestigationMUHAMMAD AZRIL HEZMI

DefinitionEssential part of the preliminary design work on any important structure in order to provide adequate and reliable geotechnical information and site condition, so that a safe, practical and economical design can be carried out.

Main objectives◦To assess the general suitability of the site for the proposed works, including implications of previous use or contamination.

◦To enable an adequate and economic design to be prepared, including the design of temporary works.

◦To plan the best method of construction, including foreseeing and providing against difficulties and delays that may arise during construction due to ground and other local conditions.

◦To determine the changes that may arise in the ground and environmental conditions, either naturally or as a result of the works.

◦To advise on the relative suitability of differences sites, where alternatives exist.

Stages

◦ Desk study / Site reconnaissance

◦ Ground Investigation

◦ Monitoring

Desk study

◦Involves collecting existing information about the site and put it all together to build a conceptual model of the site.

◦Information gathered at the desk study stage are:

◦Topographical maps Geological maps

◦Soil survey maps Aerial photographs

◦Details of existing development Site history

◦Local authority information Utilities

◦Ownership of adjacent property Existing site investigation reports

Desk study

Site reconnaissance

◦An early examination of the site by geologist, land surveyor and engineer to give an idea of the work that will be required.

◦Information collected on site reconnaissance are:

◦Overall site layout Surface conditions

◦Detail of access and nearest road Surrounding site condition

◦Existing nearest utilities (pipe, electricity)

◦Climates and hazards (rainfall, flood, contamination)

◦Geographical information (plants, river)

◦Obstacles and restrictions

Ground investigation

◦Investigation of detailed geology and subsurface soilconditions.

◦The principal objects of the detailed soil survey are as follows:

◦Determine detail geological structure including thickness and sequence of the strata

◦Determine ground water conditions

◦To carry out in situ testing

◦To obtain disturbed and undisturbed samples for laboratory testing

Ground investigation…Boring

�Site exploration methods:

�Test or trial pits

Simple hole dug in the ground (1m – 3m) that large enough for a ladder to be inserted either dug by hand or mechanical excavator, thus permitting a close examination of the sides.


�Hand auger

The hand auger (attached to drill rods and turned by hand) is often used in soft soils for borings about 6m.


�Boring rig

�Common types of borings are auger borings, wash borings and core borings.

� Auger borings is a screw like tool used to bore a hole in 30 m depth. Removed soil can be used for field classification and laboratory testing.


� Wash borings consist either driving casing or chopping and jetting.

For driving casing method, boring rig consists of dropping the cutter (1.5m – 3m above the soil) with the soil sampler and is largely carried out by hand and when site conditions are suitable the operation is often powered.


For chopping and jetting method, the boring consists simultaneous drilling and jetting action. The soil is loosened by a high pressure water jet from a pipe passing down the borehole. Returning water transports soil to the ground surface, where samples can be collected for examination and classification.


Core borings are commonly used to drill into and through rock formations. Core borings are performed using a core barrel, a hardened steel or steel alloy tube with a hard cutting bit containing tungsten carbide or commercial diamond chips by rotating (speed 600 rpm – 1200 rpm) them with a drill. Core barrels are typically 5 cm – 10 cm in diameter and 60 cm to 300 cm long.

Example 1

A 1.25 m square footing is subjected to a contact pressure of 300 kN/m2. The unit weight of the cohesive soil supporting the footing is 18.5 kN/m2 and groundwater is known to be at a great depth. Determine the minimum depth of boring that should be carried out in the site

Solution

Assume that the minimum depth of boring = z

Overburden pressure at depth z, σ’ = 18.5 x z kN/m2

Using 1:2 stress distribution, the increase in stress at depth z

Table to determine minimum depth of borings based on 10% of the overburden pressure

Example 2Given:

1. An 8ft square footing is subjected to a contact pressure of 4000 lb/ft2.

2. The wet unit weight of the soil supporting the footing is estimated to be 120 lb/ft2.

3. The water table is estimated to be 30 ft beneath the footing

•Required:

The minimim depth of test boring by referring Table Barsdale and Schreiber, 1979.

Solution

Refer figure 3-7;

Minimum depth of test boring is determined to be 22ft

Ground investigation…Sampling

�Disturbed sample� These samples are easy to get and can be taken whenever a change of

soil type observed. The soil is remolded and generally required for identification testing.

� Disturbed sample are usually collected in airtight tins, plastic jars orplastic sampling bags and securely labeled.

�Undisturbed sample� These samples may be collected by several methods. If a test pits is

available in clay soil, an undisturbed sample may be obtained by simply carving a sample very carefully out of the side of the test pit. Such a sample should then be coated with paraffin wax and placed in an airtight container.


� A more common method of obtaining an undisturbed sample is to push a tube into the soil, thereby trapping the sample inside the tube. The soil sampler usually uses 50 mm, 76 mm and 100 mm in diameter with maximum depth of 60 m. The ends of the tube should be sealed with paraffin wax immediately after the sample is brought to the ground surface.

� It is virtually impossible to obtain totally undisturbed samples, but it assumes the soil is undisturbed samples when the area ratio, Ar should not exceed 20%.


Ar = (Dw2 – Dc

2)/ Dc2 x 100

Where;

Ar = Area ratio

Dw2 = External diameter

Dc2 = Internal diameter


�Standpipes

Insert an open ended tube, usually 50 mm in diameter and perforated at its end, into the borehole. The tube is packed around with gravel and sealed with clay and backfilled to prevent access to rainwater. Observations must be taken for several weeks until equilibrium is achieved.


◦Piezometers

The piezometers can exist as an open system (cohesionless soil) or a close system (cohesive soil). The pressure is measured by an electrical transducer.

Ground investigation…Sampling�Seismic refraction

This method involves generating a sound wave using a falling weight or small explosive charge and then recording its reception at a series of geophones located at various distance from the shock point. The time of the first sound arrival at each geophone is recorded.

Insitu testing

Insitu testing is carried out when it’s almost impossible to obtain satisfactory undisturbed samples for laboratory testing. There are several insitu testing:

� Standard penetration test (SPT)

� Vane shear test

� Plate bearing test

� Plate loading test

� Sand replacement test

� Light dynamic cone penetrometer (JKR probe)

Standard penetration test (SPT)

BS 1377: Part 9: 1990

This test is generally use to determine the bearing capacity of sands or gravels with a split barrel sampler, 50 mm in diameter. The sampler is lowered down until it rests on the soil to be tested. It is then driven into the soil for a length of 450 mm by means of a 65 kg hammer free falling 760 mm for each blows. The number of blows, (N value)required to drive the last 300 mmis recorded.

SPLIT SPOON

SAMPLER

HAMMER

SPT

SPT results influenced by overburden pressure.

Methods proposed to correct N-values

1. (Peck et al., 1974) CN , a correction factors to be applied to N-Values:

CN = 0.77 log10 (20/PO) (PO in tons/ft2) (1)

CN = 0.77 log10 (1915/PO) (PO in kN/m2) (2)

1. (Terzaghi et al., 1996 & Liao & Whitman, 1986)

N = N’ X (100/PO)1/2 (3)

Graphic relationship based on Eq. 1 for determining the correction factors to be applied to the N-

value recorded in field

Example 3 (SPT)

An SPT was performed at a depth of 20 ft in sand of unit weight 135 lb/ft3. The blow count was 40. Find the corrected N- value by Peck et al., 1974 & methods presented previously.

Solution (SPT)From Eq. 1:

CN = 0.77 log10 (20/P0)

P0 = (20 ft) (135 lb/ft3) = 2700 lb/ ft2

= 1.35 tons/ ft2

∴CN = 0.77 log10 (20/1.35 tons/ ft2)

= 0.901

N corrected = 40 X 0.901 = 36

* CN can also be obtained from Figure by locating 1.35 tons/ ft2

along the ordinate, moving horizontally to the curved line, and then moving upward to obtain the correction factor , CN.

Example 4 (SPT)

An SPT was performed at a depth of 8.5 m in sand of unit weight 20.04 kN/m3. The blow count was 40. Find the corrected N- value by each of the methods presented previously.

Solution (SPT)From Eq. 1:

CN = 0.77 log10 (1915/P0)

P0 = (8.5 ft) (20.04 kN/m3) = 170.3 kN/m3

∴CN = 0.77 log10 (1915/ 170.3 kN/m3 )

= 0.809

N corrected = 38 X 0.809 = 31

From Eq. 3:

N = N’ X (100/PO)1/2

N = 38 X (100/170.3 kN/m3 )1/2

∴N corrected = 29

Standard Penetration Test

(SPT)

Terzaghi and Peck (1948) give an approximate relationship for clay:

For square footings, qa = 16N (kN/m2)

For continuous footings, qa = 12N (kN/m2)

Standard Penetration Test (SPT)

Peck, Hanson and Thornburn’s relationship between N and the relative density and angle of shearing resistance, f.


� N60 = corrected SPT number

� N = SPT number

� nH = hammer efficiency (%)

� nB = correction for borehole diameter

� nS = sampler correction

� nR = correction for rod length


qu = unconfined compression strength

Pa = atmosphere pressure = 100 kPa

N60 = corrected SPT number

Example 5 (SPT)

A granular soil was subjected to standard penetration test at depths of 3 m. Ground water level occurred at depth of 1.5 m below the surface of the soil which was saturated and had a unit weight of 19.3 kN/m3. The N count was 15. Determine the corrected value N’.

Solution (SPT)� Effective overburden pressure;

= [(3) (19.3)] – [(1.5) (10)]

= 43 kN/m2

� From the chart,

For effective overburden pressure = 43 kN/m2

N’/N = 2.1

� Therefore

N’ = (15) (2.1)

= 31

CORRELATIONS BETWEEN SPT AND SOIL PROPERTIES

Relative Density

Effective Stress Friction Angle

Undrained Shear Strength

Vane shear test� To determine in shear strength for soft clay

soils.

� A vane tester is made up of two thin metal blades (H = 150 mm with D = 75 mm)attached to a vertical shaft.

� The test is carried out by pushing the vane tester into the soil (3 x diameter of vane)and then applying a torque, T (120/minute)to the vertical shaft. The clay’s cohesion can be computed by using the following formula:

+=

62

32DHD

cTu

π

Example 6 (Vane shear test)

A vane, used to test a deposit of soft clay, required a torque of 67.5 Nm. The dimensions of the vane were: D = 75 mm, H = 150 mm. Determine a value for the undrained shear strength of the clay.

cu = 44 kN/m2

+=

6

D

2

HDcT

32

uπ

+=−

6

075.0

2

15.0x075.0c10x5.67

32

u

3 π

Solution (Vane shear test)

Plate bearing test

� To estimate bearing capacity of soil for spread footing by obtain a load/settlement curve. The test consists of application of a compressive stress to soils by a rigid bearing plate.

� A trial pit is excavated to the required level and a steel plate is set on the soil at the bottom of the pit.

� A static load is applied to the plate in successive increments, and the extent and rate of the settlement is recorded. Further increments of load are added and this procedure is repeated until the soil below the plate fails. (Peak load reached or settlement is 10% of plate diameter)

Pile loading test

�Loading tests are usually carried out on one of the piles to be used in the structure installed at an appropriate depth to determine the ultimate load of the pile.

�Test load is applied in equal increments until all observable settlement has ceased. Curves settlement vs. load and settlement vs. time are plotted

Sand replacement test�This test is to determine the soil density at the site. A cylindrical hole approximately of diameter 100 mm and depth 150 mm is dug out and excavated material is then carefully weight. The sand pouring cylinder is placed over the hole and sand run out to fill it, and so determine its volume.

Light dynamic cone penetrometer (JKR probe)

� This testing is widely used in Malaysia to estimate bearing capacity of soil by replacing Mackintosh Probe in 1972. The probe consists of a cased harden steel pointer of 25 mm diameter and 600 cone. The pointer is screwed onto the lower end of the rod. The rods are of 12 mm diameter HY 55C steel, each of length 120 cm.

� Driving is executed with small hammer of 5 kg in weight and falling through a fixed of 280 mm along a guided rod. The total number of blows required for the pointer to penetrate a distance of 300 mm is recorded. Maximum depth of penetration is about 12 m or 400 blows/300 mm.


The correlation between JKR Probe penetration resistance in number of blows/ft and the allowable bearing capacity is given in figure below.

Site investigation report

� The site investigation report is the final product of the exploration programmed. It consists of a summary of the ground conditions encountered, a list of tests carried out and recommendations as to possible foundation arrangements.

� Reports are generally prepared in sections, headed as described in the following section:

� Introduction

� Summary the location of the site, date of investigation, name of the client and the equipment used.

� Description of site

� Here a general description of the site, general geology of the area and map showing site location and borehole locations.

Site investigation report� Description of subsoil conditions encountered

This section should consist of short and readable subsoil conditions refer to borehole journal.

� Borehole journals

A borehole journal is a list of all the materials encountered during the boring and best shown as borehole log.

� Description of laboratory soil tests

This simply a list of the test carried out together with a set of laboratory sheets.

� Recommendations and conclusion

In this section, recommendations as to possible foundation types and modes of construction should be given.

site investigation-week 1-4 -...

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