Method of Statement for Geotechnical Investigation
The purpose of the geotechnical investigation shall be to obtain information concerning the nature, thickness and characteristics of the soil and rock strata underlying the project site and to determine the engineering properties of the various strata for use in the design of the project. The investigation shall include interpretation of data obtained with recommendations for suitable building foundation system.
The geotechnical investigation shall comprise drilling of boreholes, obtaining soil and rock samples for the laboratory analysis and carrying out in-situ and laboratory tests in accordance with the appropriate requirements of applicable British (BS) or American Standards (ASTM).
- Site reconnaissance study consisting of a thorough review of available, aerial
- Photos, literature, and a visual reconnaissance of the works.
- Subsurface investigation program (Drilling & Sampling Procedures).
- Field testing of the strata.
- Laboratory testing of the samples recovered.
- Documentation, evaluation and reporting of the data obtained.
In this process of site reconnaissance, field inuestigations and testing, thenature, or characteristics of the material (surface and subsurface) will be eualuated.
Obtain in all cohesive soils or mixed soil having sufficient cohesion at depth intervals of 1.0 m. The 200 mm of soil immediately above the level of soil to be sampled should be removed without the casing being lowered. Carefully clean boreholes before sampling;
Take samples in seamless sampling tubes not less than 100 mm internal diameter and 450 mm long designed so that samples can be sent to the laboratory without removal. Area ratio of sampling tube is to be less than %15;
Oil the sampling tool in side and out. Force tool into the soil by jacking or, where this is not possible, by carefully hammer driving. Check distance tool is driven to prevent soil becoming compressed in sampler;
Immediately after taking samples from the boreholes remove a -25mm thick layer from each end and seal ends with a thick coating of paraffin wax or other wax approved by the Engineer.
Store the 25 mm thick layers in an airtight container for classification testing. Place a dose-fitting lid or screwed cap ouer each sample tube end and make tube airtight. Send samples to the laboratory, suitably packed to prevent damage or disturbance.
The number and label each tube so that sample can be identified giving borehole, soil stratum, depth at which it was taken and date.
Perform standard penetration test as specified to recover a disturbed sample of soil at intervals of not more than l.0 m in the top 5 m and at intervals of 1.5 m or at the change of each strata thereafter, unless otherwise instructed by the Engineer;
Where undisturbed soil samples and standard penetration test samples are not obtained, recover disturbed samples of soils from boring tools. Obtain disturbed samples at such spacing to ensure that samples from borehole either in the form of undisturbed samples, standard penetration test samples or
disturbed samples are obtained for every 1.0 m depth bored. Minimum weight of disturbed samples is to be a sspecified in ASTM D 420.
Where soil samples are required for compaction tests, the minimum weight ofsample is to be 50 Kg, unless otherwise instructed by the Engineer.
Where boreholes are bored using rotary coring methods, extract rock cores of not less than 76 mm diameter, unless otherwise directed by the Engineer. After being brought to the surface remove core from core barrel by methods designed to cause least possible further disturbance
Where split inner core barrels are not in use extract core by steadily applied pressure. Extraction by means of hammering the barrel or explosive extrusion under high air pressure or water pressures will not be permitted. After extrusion place core in a purpose made core box;
Adequately preserue cores and log. Photograph weak rock cores and those subject to disintegration on recovery.
To be of sound, robust construction able to withstand weight of core and any full boxes which may subsequently be placed on the man dsufficiently watertight to protect core from rain. They are to be specially made to hold size of core being obtained tightly in place in rows separated by wooden slats. Boxes are to haue strong metal hinged lid fitted with padlocR, hasp, and staple for dosing and end ropes for handling.
Top and bottom of boxes are to be reinforced by cross straps to aid staclting and retrieual. Boxes are to be constructed of wood, marine plywood or other material approved by the Engineer.
As cores are extruded lay them in core box with shallowest core to left and deepest to right. Place highest row of core nearest lid hinge. Box is to be identified inside and outside by the site name, borehole number, core box number, depth of top and bottom of core included the Contractor›s name and the date. Information is to be either painted on box or stamped on metal labels waxed and nailed to box;
Paint the depth of the top and bottom of the total core and separate core runs on blocks of wood or other material approved by the Engineer made to fit between dividing slots;
n order that zones of core loss can be readily identified insert wooden dowels cut to appropriate lengths and suitably identified either in sections where loss occurred or at base of particular core runs;
First seal with aluminum foil and subsequently couer with wax, sections of core which are weak and friable, formed of rocks or soils which are likely to dry out or otherwise alter in nature with time, before placing in core box.
All core boxes will be photographed as soon as possible after extraction. Photographs shall be free from distortion and shall include a scale and color chart. All boxes shall be clearly labeled and show the depth to the top and bottom of each core run.
TaRe a sample of groundwater as soon as sufficient water has entered borehole after boring has reached groundwater leuel; If water has been added to bore-hole before reaching groundwater leuel bale out all water in borehole and allows uncontaminated groundwater to seep bacR into borehole before sample is taRen;
Where groundwater is sealed off by borehole casing and a lower aquifer is encountered taRe a sample of water from this and any succeeding aquifers; Sample of groundwater is to be at least 500 ml in uolume and placed in clean jars or bottles already rinsed with the water to be sampled and labeled and stored as described in BS 5930.
The main sampling operations include:
Single tube core barrel sampling;
Double tube core barrel sampling;
The following are general remarRs on sampling methods and equipment
Intact samples of roe~ can be obtained by means of core barrel with attached coring bit which cuts an annular ring in the roe~ and leaues a control core that enters in the core barrel. The cutting elements of the coring bit may consist of diamonds, tungsten carbide inserts or other types of cutter appropriate to the hardness of the material being cored. Samples ta~en in a single tube core barrel generally experience considerable disturbance due to torsion and swelling and due to contamination with the drilling fluid which result general in poor core recouery.
The double tube core barrel, as the name signifies, consists of two barrel:
inner and outer. It is designed to protect the core against the action of circulating fluid which results in higher recouery ratios than can be obtained with single tube core barrel.
Field testing can include a wide uariety of methods depending on the type of material to be tested and the use for which the test results are required. Full descriptions of many of the test methods can be found in the ASTM & AASHTOO.
In-situ penetration tests haue been widely used in geotechnical and foundation engineering for site inuestigation in support of analysis and design. The standard penetration test (SPT) and the cone penetration test (CPT) are two typical in-situ
penetration tests.
Standard Penetration Test (SPT) is the most widely used of the dynamic field tests. It is used to determine the relatiue density of granular soils and to estimate consistency of cohesiue soils. The test uses a drop of standard weight hammer falling through a standard distance to driue a split-barrel sampler. The number of blows required to driue the sampler 0.3 m is called Standord Penetration Test (SPT) the Standard Penetration Test Blow Count and is usually designated by the Letter N.
Blow counts (N ualues) are used to determine the density of granular soils and the consistency of cohesiue soils. Howeuer, it should be noted that the use of N ualues to describe the consistency of cohesiue soils is less reliable than for the density of granular soils.
When the test is carried out in granular soils below groundwater leuel, the soil may become Loosened by the action of the boring tools and pressure differences between the groundwater and water in the boring. It is essential that the loosening effect be minimized by careful operation of the boring tools and by ~eeping the boring topped up with water.
Groundwater is one element that affects in the stability and foundation analyses. The groundwater Leuel was measured 24 hours after completion of the borehole.
However, the Low permeability of the soil will mean that the water level in the borehole is controlled more by drilling fluid rather than by the ground water itself. Significant fluctuations in the location of ground water table should be anticipated throughout the year, depending upon the amount of precipitation, evaporation, and surface runoff.
Three in situ deferment permeability tests can be conducted in the field tomeasure the soil and roe~ permeability. These three methods are:
– In Situ Permeability Test by Constant Head Test (ASTM D 2434)
– In Situ Permeability Test – Pacller Test (ASTM D 1586)
– In Situ Permeability Test – Falling Head Test (ASTM D 5093)
After reaching the level of a pervious stratum in which a test is to be made continue boring until penetration into stratum is not less than fiue times internal diameter of casing being used. Clean out borehole just to bottom of casing. If casing is below level of groundwater keep it full of water during cleaning and withdrawal of tools so that soil does not squeeze into it.
In gravity tests adjust rate of flow of water supplied to borehole so that water level remains constant against a mark inside borehole casing near top. If it is impossible to maintain water leuel a constant rate of flow producing a fluctuation of no more than 75 mm aboce and below the mark during a period of five minutes will be accepted. When steady state has been reached, the quantity of water entering borehole ouer a fixed interval of not less than five minutes will be recorded. Repeat until consistent results are obtained.
Tests may be carried out using a single or double Packer. Use only clean water with a temperature not less than of groundwater. Carry out tests at three pressure increments and two decrements equivalent to P/2 ,3P/3 and P, where P is the vertical overburden pressure at level of test. Normally the flow rate at each pressure will be recorded at ten minute intervals to a maximum of three periods or less if similar results are obtained in two consecutiue periods. Pressure difference between each increment/decrement should not normally be less than 35 KN/m2 and pressure in Packer units should be at least 300 KN/ m2 greater than test water pressure loss at uarying flow rates before use.
Where conditions in borehole indicate that a permeable stratum has been reached bale or pump out borehole to lower leرel of the water. Record water level until it returns to equilibrium. If it is impossible to lower water level in borehole either because soil is too permeable or because it flows into casing carry out a pouring in test instead. In this test pour water into borehole until casing is full. Record water level at internals until equilibrium position is reached.
Normally record the water level in the borehole at five minute internals for the first hour, fifteen minute internals for the second hour and a final reading after a suitable time to be decided by the Engineer but not less than four hours. Measure water level with a battery operated indicator or similar approued instrument. Take reasonable precautions to prevent ingress of water into hole during test period.
The purpose of this test is to investigate for the need of cathode protection and to have data necessary for the design of an adequate grounding system.
The soil Resistivity measurement shall be carried out in accordance with IEEE 81 standard. Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System. The measurement shall be done using Wenner Four Points Method with equal test rods spacing.
The area to be measured shall be the power block area of power plant, terminal substation, and switchyard. Before carrying out the measurement, the rectangular grid shall be drawn for the testing areas with mesh spacing at approximately 10-5 m. The measurement shall be made at euery intersection point of grid lines. The measurement at any point shall be done for two directions, one from the measured point along the direction from east to west and another shall be from the measured point along the direction from north to south. The measurement at any point shall consist of the measured data at the uarying space between test rods for the following distance; 5.0 ,4.0 ,3.0 ,2.0 ,1.0 ,0.5 m. For each area of measurement, the results of measurement shall be shown in the table for each point of measurement for each direction and euery designated space of measurement. The measured resistiuity data shall be aueraged for each of the same spacing of measured data. The ouerall aueraged resistiuity of each area shall also be reported.
This method is also lmown as open excauation method. It is one of the most satisfactory methods of soil inspection by uisual obseruation and it also prouides both disturbed and undisturbed sampling. It can be done manually by hand, using labor, or by using some equipment like backhoe, trencher or the dozer etc. test pits are used for exploring sites where greater sampling is required such as highways and airfields.
Bulk sample taken from the test pits of not less than 50 kg each shall be sent to test at laboratory for compaction and CBR test
A piezorneter is either a deuice used to measure liquid pressure in a system by measuring the height to which a column of the liquid rises against grauity, or a deuice which measures the pressure (more precisely, the piezometric head) of groundwater at a specific point. A piezometer is designed to measure static pressures, and thus differs from a pitottube by not being pointed into the fluid flow.
Obseruation wells giue some information on the water leuel in a formation, but must be read manually.
The piezometers in geotechnical engineering are either open wells or PVC standpipes (sometimes called Casagrande piezometers) installed into the borehole. A Casagrande piezometer will typically haue a solid casing down to the depth of interest, and a slotted or screened casing within the zone where water pressure is being measured. The casing is sealed into the borehole with clay, bentonite or concrete to preuent surface water from contaminating the groundwater supply. In an unconfined aquifer, the water leuel in the piezometer would not be exactly coincident with the water table especially when uertical component of flow uelocity is significant. In a confined aquifer under artesian conditions, the water leuel in the piezometer indicates the pressure in the aquifer, but not necessarily the water table. A -5cm diameter standpipe is common.
Piezometers in durable casings can be buried or pushed into the ground to measure the groundwater pressure at the point of installation.
Plate Load Test is a field test for determining the ultimate bearing capacity of soil and the likely settlement under a given load. The Plate Load Test basically consists of loading a steel plate placed at the foundation level and recording the settlements corresponding to each load increment. The test load is gradually increased till the plate starts to sink at a rapid rate. The total ualue of load on the plate in such a stage – Reaction diuided by the area of the steel plate gives the value of the ultimate bearing capacity of soil.
Plate load test is most suited for sands and clay›s. In sand, they are usually carried out when the project is big and large numbers of footings are available. In clay, here unconfined compression tests are not feasible due to presence of ssures and cracks, plate load tests are used to determine the ultimate bearing capacity.
Plate load test is helpful for the selection and design the foundation. To calculate safe bearing capacity suitable factor of safety is applied.