SUPREME KNOWLEDGE FOUNDATION GROUP OF INSTITUTIONS MANKUNDU, HOOGHLY RECENT TRENDS IN CIVIL ENGINEERING PRACTICES – A GEOTECHNICAL OVERVIEW 25th September 2014 Suvendu Dey Chief Engineer (Civil) M. N. Dastur & Company (P) Ltd., Kolkata
WHY INVESTIGATE ? Bearing capacities may be better than anticipated and a different foundation design could have saved a few lakhs of rupees. Often, a thorough soil investigation may show that moving a unit just a few metres on the site can avoid faulty underlying material. Bearing capacity and settlement potential of the soil needs to be ascertained to determine whether the stability of foundations can be obtained. This is required because foundation failures in structures are practically irreparable. A client would especially want to know what types of foundations and other construction details are likely to be required so that he can figure his cost to build. The client would also like to know the additional cost for land development, if any, due to any problem or condition of the sub-soil.
MAJOR COMPONENTS OF SOIL INVESTIGATION For an industrial plant, the field work for a typical soil investigation may consist of the following major components : • Boreholes/Drillholes • Standard Penetration tests in boreholes • Trial Pits • Plate Load Tests – Routine/Cyclic • Block Vibration Tests • Collecting samples of soil/rock/water • Static (Dutch) Cone Penetration Tests • Dynamic Cone Penetration Tests • Electrical Resistivity Tests • Pressuremeter Tests
SOME RECENT ADVANCEMENTS Although the testing methodology remains largely unchanged, huge advancement has been brought about in equipment and machinery, leading to more efficiency and accuracy as well as saving in time. Some traditional tests have been replaced by state-of-the-art testing methods that are more scientific and universally acceptable. • Downhole and crosshole seismic tests • Thermal Resistivity Tests • Mackintosh Probe • Hand/Pocket Penetrometers • Piezocones • Vibrating Wire Piezometers • Vacuum sampling of water/soil-water solutions
Some Recent Advancements (cont’d…) Piezocone Mackintosh Probe Pocket Penetrometer VW Piezometer
TYPES OF FOUNDATIONS Deep Foundation Shallow Foundation Resting on soil at a shallow level with adequate bearing capacity Piles Any foundation system must satisfy certain essential criteria such as strength, settlement and serviceability. Hard Stratum Taken down to deeper level to rest on stratum with higher bearing capacity
SHALLOW FOUNDATIONS Foundations are designed to have an adequate load carrying (bearing) capacity with limited settlement. Failure of a shallow foundation may occur in two ways : (a) by shear failure of the soil supporting the foundation (b) by excessive settlement of the soil supporting the foundation Other design considerations include sliding, overturning, scour etc. Foundation failure may occur due to error in design, faulty construction, low grade materials, poor workmanship, force majeur etc.
Shallow Foundations (cont’d…) Typical Bearing Capacity Failure Settlement Crack
Shallow Foundations (cont’d…) Typical Foundation Settlements Overturning Failure
PILE FOUNDATIONS A pile is a relatively small diameter shaft which is driven or installed into the ground by suitable means. • Piles may be classified into different categories based on the following : • Load Transfer Mechanism and Functional Behaviour • Method of Installation • Pile Material • Orientation • Shape • Cross-section • Special Types
Pile Classification Pile Types Load Load Installation Installation Cross Cross - - Special Special Material Material Orientation Orientation Shape Shape Transmission Transmission Method Method Section Section Types Types End Bearing End Bearing Bored Bored Timber Timber Vertical Vertical Cylindrical Cylindrical Circular Circular Micropiles Micropiles Batter/ Batter/ Driven Driven Steel Steel Tapered Tapered Square Square Suction Suction Friction Friction Raker Raker Bearing & Bearing & Under Under Hexagonal/ Hexagonal/ Others Others Concrete Concrete Friction Friction Reamed Reamed Octagonal Octagonal Tension & Tension & Prestressed Prestressed H or I H or I Anchor Anchor Precast Precast Section Section Annular or Annular or Cast Cast - - in in - - situ situ Pipe Pipe Composite Composite Pile Foundations (cont’d…) Pile Classification
Imposed Load (P) Pile Cap Pile Skin Friction (Pf) End bearing (Pb) P = Pf + Pb Pile Foundations (cont’d…) Load Transfer Mechanism
SOME RECENT ADVANCEMENTS Use of high end software to analyse and design foundations with precision. Use of state-of-the art equipment, machinery and construction techniques to ensure quality and minimise human errors. Use of high grade Portland/Pozzolonic cements, with or without plasticisers and high grade reinforcing steel. Introduction of customised courses and change in curriculum in engineering institutions to better equip the modern day engineer.
Some Recent Advancements (cont’d…) Piling equipment and process have seen a sea change over the years. The once popular conventional tripod rig is fast giving way to high powered, mechanised, hydraulic rotary rigs. These rigs are powerful, versatile and speedy. They are self erecting and can sink pile bores of even upto 100 m. They have telescopic kellies, are easily retractable and can be quickly ferried from one point to the other. They come with a host of attachments to suit the suit conditions and piling methodology.
Some Recent Advancements (cont’d…) Driven Pile Rig Bored Cast-in-Situ Piling Rig
Some Recent Advancements (cont’d…) Cutting Tools
Some Recent Advancements (cont’d…) Another fairly recent development has been the advent of Continuous Flight Auger (CFA) piles. The process involves one continuous operation, without alternate lowering or raising, with the help of an auger of the required length. These are quick to install and are very useful in favourable sub-soil conditions and within reasonable depths (say 30 m). This method is gradually gaining popularity in our country.
COMPRESSION TO VERIFY TENSION SHEAR PILE TESTS • No matter how accurately soil investigation and pile design are done, the ground never ceases to throw up surprises. Pile foundations installed at site must be tested to verify their load carrying capacities . The most common pile load tests that are carried out, include : • Initial Load Tests • Routine Load Tests • Load tests, initial or routine, are conducted to verify the following load carrying capacities of a pile : • Vertical Compression • Vertical Tension • Horizontal Shear
Pile Tests (cont’d…) Static Load Test by Maintained Load Method Kentledge for Pile Load Test
SOME RECENT ADVANCEMENTS Although carrying out load tests on piles are quite common, it has become imperative these days to check the pile health parameters too. The advent of non-destructive, integrity tests have given the engineers a tool to assess parameters such as condition of the pile shaft, soundness of pile concrete, proper base contact, diameter and length of pile etc. The most common is the Sonic Integrity Test (SIT).
Some Recent Advancements (cont’d…) An even more recent advancement has been the introduction of dynamic or high strain tests on piles. An impact force is applied axially by a pile driving hammer resulting in a large strain impact to the top of the pile. The test method is used to provide data on strain or force and acceleration, velocity or displacement of a pile under impact force. The data are used to estimate the bearing capacity and integrity of a pile as well as hammer performance, pile stresses and soil dynamic characteristics.
SOIL IMPROVEMENT – WHAT & WHY Vast areas of India, particularly in the marine and alluvial environment, consist of weak and problematic soils. These have low shear strength and high compressibility, often accompanied with swelling properties. Important/settlement sensitive structures cannot be placed on them without pile foundations or appropriate ground treatment measures. Although piles are an easy solution, the cost often becomes prohibitive. Also, for large area loading like material stockpiles, storage tanks etc, that can tolerate more displacements, providing piles do hurt the pocket of the client and the credibility of the designer. Thus, there is ample scope and need to utilise alternative solutions that are technically and economically viable. An appropriate improvement technique not only serves the purpose, but, often works out economical too.
SOIL IMPROVEMENT – LOOKING BACK The earliest known improvement adopted by man was to excavate a portion of the existing loose/soft soil and refill it with compaction. With time, he learnt to reject undesirable soil and replace it with competent borrowed earth from nearby sources. Over the ages, man learnt to reinforce the ground beneath his dwelling house by driving relatively stiffer members into the ground. From tree trunks/logs to holes filled with stones, and later, inserting metal pipes/rods into the ground, man has used them all! He even learnt to apply different techniques to suit different sub-soil conditions etc. Today, man has at his disposal state-of-the-art methods for shallow and deep soil improvement along with natural or synthetic fibres to reinforce the ground.
SOME RECENT ADVANCEMENTS Soil Nailing is an in-situ soil reinforcement technique that is used to stabilise natural slopes, cut or excavation, walls in stiff cliffs, granular soils and also soft rocks. The nails are installed perpendicular to the surface of the slope in such a way as to create a stable mass of soil. This process creates a single block of earth that is stable and able to hold back the soil behind. Soil nailing works in tension as well as with bending and shearing forces. Lateral earth forces are picked up in both tension and bending stress in the nail. Generally, the soil nails increase the bonding of the soils through their ability to carry tensile loads.
Some Recent Advancements (cont’d…) Rapid Impact Compaction (RIC),also known as Dynamic Compaction, imparts energy by dropping a 5.0 ton weight from a height of about 1 m onto a foot. Energy is delivered at a rate of 35 to 65 blows per minute. The foot, measuring about 1.5 m in diameter, is maintained in contact with the ground to allow efficient and safe transfer of energy. Potential applications for RIC include compaction of loose soils (cohesionless and semi-cohesive) to improve bearing capacity and reduce settlements.
Some Recent Advancements (cont’d…) Geosynthetics constitute a group of fabrics made from synthetic polymers such as polypropylene, polyester, polyethylene, polyamide and PVC, or natural fibres including cotton, jute, coir and bamboo. They find wide application in separation, reinforcement, drainage and filtration To be effective in reinforcement of soil, sufficient deformation has to be allowed so as to enable the tensile strength of the fabric to come into play. This makes them particularly suitable in soft clay deposits that are by nature susceptible to large deformations. These days, the major application areas of geosynthetics are roads, railways, embankments in soft soils, drainage, retaining walls, erosion control etc.
Some Recent Advancements (cont’d…) Compaction Grouting involves injection of thick consistency soil-cement grout, under pressure, into the soil mass, consolidating, and thereby densifying the surrounding soils in-place by pressure densification. Improved compaction of displaced soils and greater uniformity of the treated soil mass is achieved. A secondary benefit is that the grout columns add strength along the vertical axis, as grout compressive strengths exceed those of the surrounding soils. Chemical Grouting is done for soils with medium to low permeability (k<10-4 m/sec). Sodium silicates are commonly used, which reacts to form sodium carbonate and silica, that remain in the voids as grout.
Some Recent Advancements (cont’d…) Vibro-compaction is a deep compaction ground treatment technique for in-situ densification of granular soils by means of a vibrating probe, or vibroflot. The vibroflot consists of a vibrator, connected to an upper follow tube. The vibrator houses an electric motor in the upper part, to drive an eccentric weight in the lower compartment. Capable of generating 1500 to 1800 rpm, the vibrator develops a centrifugal force of 30 to 50 t, creating vibrations in a horizontal plane. Under the influence of vibration, loose particles are rearranged into a more compact state, improving the engineering properties of the treated profile. Consequently, the bearing capacity of the treated soil is increased and the settlement potential reduces.
Some Recent Advancements (cont’d…) Stone Columns are being increasingly used for improvement of soft ground to increase bearing capacity and reduce settlement. The technique has been successfully applied to foundations of buildings, embankments, storage tanks and other light/medium loaded structures. Stone columns are essentially a system of soil reinforcement with additional advantage of providing a drainage path. Hence the ground treated by stone columns can be improved further by preloading. Two approaches are commonly used, a wet top feed method and a dry bottom feed method.
Some Recent Advancements (cont’d…) Prefabricated Vertical Drains (PVD) with preloading is one of the most effective means to improve bearing capacity and reduce settlements in soft clay deposits. This method facilitates accelerated consolidation of soft and saturated clay deposits employing a superimposed surcharge with the help of predetermined artificial drainage paths. The basic phenomenon involved is one of three-dimensional consolidation. The introduction of synthetic drains has replaced the existing system of using sand drains and sand wicks. This technique is widely used for large area loading like material stockpiles, tanks, silos, bunkers etc and is also known to have been applied for airstrips and runways.