INVESTIGATION AT SITE

SITE INVESTIGATION

Site Investigation is the gathering of the information about the

proposed location of a project, e.g. highway or buildings.

The Purpose of Site Investigation1. The site investigation is aimed at providing

sufficient reliable subsurface information for most economical, satisfactorily safe foundation for the proposed structure.

2. The site investigation should reveal sufficient subsurface information for the design and construction of a stable foundation safe from both collapse and detrimental movements.

The Scope of Site Investigation

TopographySoil profileGround-water condition

The Stages of Site InvestigationIn general, a site investigation

program should comprise four stages, i.e. :

Desk study and site reconnaissance,

Preliminary ground investigation,

Detailed ground investigation, Monitoring

Desk study and site reconnaissance The desk study is the first stage of the site investigation process which involves researching the site to gain as much information as possible, both geological and historical. A good starting point is to use Ordinance survey maps which allow the selection of the site by obtaining accurate grid reference through the maps. In addition to present maps, old maps are used to gain historical information such as former uses of the site; concealed mine workings; in filled ponds; old pits; disused quarries; changes in potential landslide areas, etc.

The source of information that useful in desk study:

1. Geological map Geological maps are probably most important source

of information as these give and excellent indication of the sort of ground conditions like to be encountered.

2. Aerial photography Aerial photography is another extremely useful source of information on topography and ground conditions.

3. Records of previous investigation Records of previous investigation reports also helpful in a desk study. The many sources of site investigation data include previous company and Public Works Departement.

The reconnaissance phase of a site investigation

This site investigation is done through a site visit or walk-over survey. Important evidences to look for are site lay out, surface condition, climate and hazards water levels, etc. Generally the desk study and reconnaissance is aimed at the

feasibility study of the being planned. If the desk study shows that the site is feasible for the structure, then preliminary investigation should follows.

Preliminary Investigation Preliminary Investigation is aimed at predicting the geological structures, soil profiles and the position of ground water table by geophysical method or by making a few boreholes. The investigation should give information on the existence on ground structures that may need closer examination: for example,

1. The extent of disturbed strata,2. The location and extend of natural cavities and mine workings.3. Fractures and river crossings or alluvial areas that may have buried soft material or pet, their liability to cause subsidence, surface movements or instability4. Information on suitability of soil for fills work, ground water condition and the possibility of flooding should be provided at this stage.

Detailed Investigation

At this stage, the extent of the test, number and depth of boreholes, selection of appropriate equipment for field testing and the choice of laboratory testing are made.

Soil exploration consists of three steps:1. Boring and in-situ testing,2. Sampling,3. Laboratory testing.

Monitoring

Monitoring during construction and maintenance period is required whether the expectations of the proceeding investigation have been realize. No one can ensure that the soil parameters used for design is the most representative of the soil conditions at the site unless the response is observed. Field observation can help for early diagnosis and redemption of any problem that might be encountered during construction. Among the measurement made during the monitoring stage are the settlement, displacement, deformation, inclination, and pore water pressure.

Steps of Soil Exploration

A. BORINGSoil borings are the most common method of subsurface exploration in the field. A bore hole is used to determine the nature of the ground in a qualitative manner and then recover disturbed and undisturbed samples for quantitative examination.Some types of borings are hand/mechanical auger borings, wash borings, percussion drilling, rotary drilling, and core borings. An auger is a screw-like tool used to bore a hole.Some augers are operated by hand: others are power operated

Hand/Mechanical Auger

Hand augers may be used for boring to a depth of about 6 m. Power augers may be used for boring to a depth of about 10 to 30 m. As the hole is bored a short distance, the auger may be lifted to removed soil. The removed soil can be used for field classification and laboratory testing, but it must not be considered as an undisturbed soil sample. Power auger set with a drill rig can be used to obtain samples from deeper strata. Some rigs can be used to drill a hole to 100 m depth.

Wash Boring

Wash borings consists of simultaneous drilling and jetting action. A hole is bored through a casing by using a drilling bit.

Jetting action is accomplished by pumping water downward through the drilling bit to soften the soil. Samples taken using the wash boring methods are disturbed sample.

Percussion Drilling

Percussion Drilling is the process of making boreholes by striking the soil then removing it.

The tools are repeatedly dropped down the borehole while suspended by wire from the power winch.

Water is circulated to bring the soil cuttings to the ground surface.

A casing and a pump are required to circulate the water.

Rotary Drilling

Rotary Drilling uses rotation of the drill bit with the simultaneous application of pressure to advance the hole. This method is the most rapid method of advancing a hole in soil and rock. Drilling mud may be needed to prevent soil cave-in. Sample obtained from drilling by this method is relatively less disturbed as compared to samples obtained by the preceding methods.

Boring tools

17

Auger boring Power drills

SOIL BORING

18

19

Boring LogsBoring Logs

20

B. SAMPLING Sampling refers to the taking of soil

sample from bored hole. There are two types of samples:

1. Disturbed samplesThis sample are usually needed for index properties of soil.

2. Undisturbed samplesThis sample are usually needed for determining the engineering properties such as shear strength and consolidation characteristic of the soil.

The sampling procedures varies according to the type of strata in which the investigation takes place. Undisturbed samples are normally needed for clays at every 1.5 m depth or change of stratum. If undisturbed sample cannot be retrieved at a specific depth, then bulk samples should be taken. Undisturbed sample are not practically for sand and gravel due to the lack of cohesion. Bulk samples to be taken every 1 m or every change of stratum while alternate disturbed and undisturbed samples should be taken for silt layer at 0.75 m intervals. Undisturbed sample may be possible for soft rock such as chalks and marls.

A sampling program should be consistent with the required accuracy of design and the scale of the structures.

Disturbed sample can be obtained from auger boring, core boring, split spoon sampler in standard penetration test (pit and trench, and some types of sampler such as thick walled sampler, displacement sampler, and Beggemann sampler.

Undisturbed sample are generally required during a detail subsurface exploration to provide specimens for laboratory testing.

If a test pit 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 thin tube into the soil, thereby trapping the undisturbed sample inside the tube and then to remove the tube and the intact sample.

The most popular tube is the open drive sampler while the recommended sampler for the soft soil is the piston sampler.

Several types of piston samplers are available, for instance the fixed piston sample, free piston sampler, and restraint sampler.

The term undisturbed is considered relative because the process of extracting the sample from a depth in soil, transporting the samples to laboratory and preparing the specimen for testing my introduce disturbance that can cause the result of laboratory testing will not be representative of in-situ condition.

To ensure the quality of the sample, some step should be taken after obtaining the undisturbed sample appropriate tube.

Immediately after the tube containing the sample is brought to the ground surface, the ends of the tube should be sealed with paraffin wax.

After sealing the tube, the following data should be attached to the sampling tube:

1. Project name,2. Name of drilling operator,3. Date of the sampling,4. Borehole number and sample number,5. Depth of sample.

Care should be taken during shipment and stored of the sealed tube for testing in the laboratory because these processes may result in serious sample disturbance.

On arrival at the laboratory, it is important to check the conditions of the samples and compare them with the states recorded in the field.

The samples should be stored in a room where the temperature and humidity are kept constant and similar to the in situ-conditions.

Visual inspection of undisturbed samples should be made to ensure that there is:

1. no visible distortion of strata in the sample,2. no opening or softening of the material,3. specific recovery ratio (SRR) should not be less than 95%,4. area ratio (Ar) should be less than 15 %.

The SSR and Ar can be defined as follows:

SSR = length of undisturbed sample recovered from the tubeLength of the tube

(2.1)

Where, Di = inside diameter and Do = outside diameter

%10021

21

2

xDDD

A or

(2.2)

IN SITU TESTING

In some cases the data obtained from sampling and laboratory testing is less reliable than those from in-situ testing. Moreover, sampling can be more expensive than in-situ testing or sounding.

Therefore, the program of sampling may be planned in combination with in-situ testing.

Common types of field testing include the standard penetration test (SPT), cone penetration test (CPT), vane shear test (VST), pressure meter test (PMT), and dilatometer test (DMT).

STANDARD PENETRATION TEST (SPT) The standard penetration test (SPT) is a dynamic test and is a measure of the density of the soil. The SPT is carried out in a borehole by lowering the split spoon sampler of about 650 mm length, 50 mm external diameter, and 35 mm internal diameter (Figure 2.5), and driving it using repeated blows by a freely dropped hammer at falling height of 765 mm. There are two types of hammer : automatic trip hammers and slip-rope-hammers but the standard weight of the hammer is 63.5 kg (Figure 2.8). The test procedure is standardized in ASTM D 1586.

The blow count is made in three steps of 150 mm. The strength of the soil is measured by the number of blow count of the last 300 mm penetration denoted as N blows/300 mm.

The blow count (N) may be corrected by field conditions such as,

a) energy used for driving the rod into the soil (Em),b) Variations in the test apparatus (Cs and CR),c) Size of drilling hole (CB)

The values of Em, Cs, CR, and CB depend on the SPT equipment.

Many of the correlations developed based on hammer that have an efficiency of 60%, the results of other hammer should be corrected to this efficiency factor. Thus :

NCCCE

N RSBm

6.060 (2.3)

The SPT data may also be influenced by overburden pressure, thus the N value should be corrected to a standard effective overburden pressure (’o).

For a standard energy and effective overburden pressure of 100 kPa, the corrected N value (Terzaghi et al, 1996, and Liao and Whitman, 1986) is:

5.0

'100'

oN NNCN

(2.2)

The SPT test should be halted when soil shows some refusal i.e. when more than 50 blows are required to penetrate any 150 mm increment or 100 blows are obtained for 30 mm penetration or if 10 successive blow produce no advance in the penetration.

The N values can be correlated with the relative density of the soil, and internal friction angle of cohesionless soil (Table 2.1).

Even though not reliable for cohesive soil, relationship between the N value and the consistency and the undrained shear strength of cohesive soil was also developed (Table 2.2)

Table 2.1SPT

N(blows/300

mm)

Relative Density

(%)

Internal friction angle

State of packing

44 – 10

10 – 3030 – 50

> 50

2020 – 4040 – 6060 – 80

> 80

3030 – 3535 – 4040 – 45

45

Very looseLoose

CompactDenseVery

dense

Table 2.2

SPTN

(blows/300 mm)

Undrained shear strength

Cu(kPa)

Consistency

22 – 44 – 8

8 – 1515 – 30> 30

1010 – 2525 – 50

50 – 100100 – 200

> 200

Very softSoft

MediumStiff

Very stiffHard

CONE PENETRATION TEST (CPT) The CPT is used widely in Europe and other parts of the world because of its versatility. The procedure has been standardized in ASTM D3441. Basic parts of this equipment include a cone to measure the tip resistance and skin friction of soil, some rods, and measuring devices. Two type of cone currently available are mechanical cone and electric cone. Both have two parts , a 35.7 mm diameter cone shaped tip with a 60o apex angle and 35.7 mm diameter and 133.7 mm long cylindrical sleeve. Piezocone is equiped with a pore pressure transducer to measure pore pressure. In recent year, the CPT or CPTU is supplemented by additional sensors, such as seismic cone, lateral stress sensing, and electrical resistivity for estimating in situ porosity or density.

Cone penetration test carried out by mechanically or hydraulically pushing a cone into the ground at a constant speed (20mm/sec) while measuring the tip resistance and friction.

The cone penetration test measures the tip resistance (designated as qc in kgf/cm2) and the friction resistance (fs in kgf/cm).

Friction ratio (Fr) represents the ratio between the friction resistance and the cone resistance in percentage which is very useful in the estimation of soil type.

For piezocone, pore pressure (ub in kgf/cm2) is measured along depth of penetration.

Cone Penetration Test (CPT) Procedures

The parameters obtained from cone penetration test can be correlated with relative density, soil classification, and unconfined compression strength, sensitivity of clay, degree of over-consolidation, pile design parameter, bearing capacity and settlement.

Figure 2.12 shows a commonly used correlation between cone resistance, friction ratio, and the soil classification developed by Robertson and Campanella in 1983.

The cone penetration resistanace can be related to the undrained shear strength (cu) of cohesive soil by the following equation:

In which ’o is the overburden pressure and Nk is the cone factor which ranges from15 to 20 depending on the type cone used.

k

cu N

qc 0'

(2.5)

Another correlation based on CPT data is equal to 2.5 – 3.5 qc.

Other correlations relate the results of cone penetration test with the N value from Standard penetration test.

VANE SHEAR TEST (VST)

Vane shear test is commonly used to measure the shear strength and sensitivity of clay.

The equipment consists of four-bladed rectangular vane, rotating rod, and measuring device.

Vane Shear Test (VST) Procedures The test is carried out in a borehole or

directly pushing the vane into the ground.

The vane rod is then rotated at a rate of 60/min, while the torque is read at interval of 30 seconds.

After maximum torque is achieved, the vane is rotated at a higher rate to obtain the remolded strength of the soils

Measure parameters include the peak torque (Tpeak), and residual torque (Tres).

The theoretical formula for relating the results of vane shear test to the shear strength parameters of the soil is :

Where: cu is the undrained shear strength of soil, T is the maximum torque, d is the diameter of the vane, and h is the height of the vane.

62

32 dhdTcu

(2.6)

The test result may be affected by several factors i.e. the disturbance due to vane insertion, blade thickness, rate of rotation, time lapse between insertion of the vane and the beginning of the test, and possible friction of the rod and surrounding soils. Type of soil and strength anisotropy may also affect the results. Skempton recommended multiplying the vane diameter by 1.05 for interpretation of strength. Bjerrum suggested a correction factor for the shear strength of highly plastic clay obtained from vane shear test (Figure 2.14)

Cohesive soils often lose some of their shear strength if disturbed and most of the soil samples obtained in the field are subject to disturbance.

A parameter known as sensitivity indicates the amount of strength lost by soil as a result of thorough disturbance.

Vane shear test is usually performed to predict the sensitivity of a cohesive soil by repeating the test at the same point after remolding the sample by completely rotating the blade.

The first maximum torque represents the peak strength, while the second maximum torque represent the residual strength of the soil.

Sensitivity of the soil can be calculated from (Equation 2.7).

res

peak

TT

S (2.7)

The Range Of The Sensitivity Of Clays The sensitivity of most clays ranges between 2

and about 4. For sensitive clays, the sensitivity ranges from 4

to 8. For extra sensitive clays, the sensitivity ranges

from 8 to 16. Quick clays, the sensitivity greater than 16.

Pressuremeter Test (PT)

Pressuremeter test is carried out to estimate the soil type, and to measure the undrained shear strength (cu), modulus of horizontal sub-grade reaction (Em), and insitu horizontal stress in the ground (ho). The equipment consists of a probe, measuring unit, and cable (Figure 2.15). The test is performed in a borehole by pushing the probe into the ground and loading it horizontally until it reaches the limit pressure or capacity of the device.

Normally the pressure increments are between 5 and 14 kPa. There are three types of pressure-meter i.e. borehole pressure-meter, self-boring pressure-meter, and push-in pressure-meter. The type of soil, the rate of expansion, membrane stiffness and system compliance, and size of drilling hole may affect the results of the pressure-meter test. Pressure may also be corrected for the resistance of the probe with the pressure volumeter, and hydrostatic effects.

Dilatometer Test

The test is similar to the pressure-meter test, but the measurement is made through a blade with a stainless-steel membrane mounted on one side of the blade.

The test is carried out by pushing or hammering a dilatometer blade into the soil at rate between 10 – 30 mm/seconds, while measuring penetration resistance and then using gas pressure to expand the membrane approximately 1.1 mm into the soil

Various parameters can be measured by , dilatometer; among these is dilatometer modulus as an estimate of elastic Young’s modulus (ED).

Calibration of membrane should be made at ground surface before and after dilatometer test for the gauge pressure necessary to suck membrane against its support, and the pressure necessary to moved it outward to the 1.10 mm position.

The result may be affected by disturbance due to blade insertion, blade thickness, membrane stiffness and thickness, and the soil type.

Observation of Ground Water Information on the groundwater level

and any artesian pressure in particular strata is very important and should be determined carefully during site investigation.

Several problems related to the presence of ground water table:

1. Shear strength of a soil may be reduced below water table.

2. Foundation may be uplifted by the water.3. Possibility of dewatering if the structure

should be constructed in dry conditions, etc.

The location of ground water table is usually determined by measuring the depth of water surface in a borehole after a suitable time lapse because water table in boreholes may take some time to stabilize depending on the permeability of the soil. Common practices is to measure the depth of ground water table after drilling and covering the hole with a small piece of plywood. In soil with high permeability such as sand and gravel, 24 hours is adequate for the water level to stabilize. In soil with low permeability such as silts and clay, it may take several days for the water level to stabilize. In this case, measurement should be made at a regular interval of time until it stabilizes.

For a regular condition, measurement can be made using a tell tale, but if it is desirable to obtain the water pressure in a particular strata, then a piezometer should be utilized.

Ground water sample may be taken for chemical analysis because some chemical may attack structural material such as concrete and steel.

Laboratory Testing

In site investigation program, the determination of soil properties is generally made in soil mechanics laboratory. To get a good quality of testing results, the samples retrieved from the ground should be tested as soon as after arrival at laboratory.

Standard laboratory testing may be grouped based on its purpose as shown in Figure 2.18.

Laboratory Testing for Undisturbed Samples Undisturbed samples are needed

for more sophisticated laboratory test such as;

1. Shear strength, include the unconfined compression test, direct shear or shear box test and Triaxial test under unconsolidated undrained (UU), consolidated undrained (CU), and consolidated drained conditions (CD).

2. Consolidation test. The consolidation test is usually

performed on standard oedometer cell.

Laboratory Testing for Disturbed Samples Disturbed samples are normally used for determining index properties of the soil such as;

1. The unit weight,2. Specific gravity.

The samples also used for classification test such as;1. Sieve and hydrometer analysis to obtained the particle size distribution,2. Atterberg limit tests to find the consistency of cohesive soil.

Soil Exploration Report

Soil exploration report should be presented upon the completion of a soil exploration program.

The report should include the scope of investigation, description of the proposed structure, and general site conditions.

The report should present the general description of soil strata, position of ground water table and other information pertinent to the site.

The detail of field exploration should include the number of borings, lay-out and depth of boring, type of boring and other specifications of field test conducted during the exploration.