EXCAVATIONS IN GROUND WITH SUB-SOIL WATER.

Excavations of foundation trenches in ground having high water table, or in water-logged area pose great problems because of water oozing in the trench from sides, bringing with it the soil from the sides. The timbering, if provided, would become loose and collapse. Excavatíons can be carried out by dewatering the sub-soil water. Foundation dewatering can be done by the following methods:

(i) Ditches and sumps 
(ii) Well point system
(iii) Shallow well system
(iv) Deep well system
(v) Vacuum method.
and (vi) Electro-osmosis method.

TIMBERING OF TRENCHES - SOILS.

When the depth of trench is large, or when the sub-soil is loose, the sides of the trench may cave in. The problem can be solved by adopting a suitable method of timbeting. Timbering of trenches, sometimes also known as shoring consists of providing timber planks or boards and struts to give temporary support to the sides of the trench. Timbering of deep trenches can be done with the help of the following methods:

1. Stay bracing.
2. Box sheeting
3. Vertical sheeting
4. Runner system
5. Sheet piling.

1. Stay bracing. This method (Fig. 2.31) is used for supporting the sides or a bench excavated in fairly firm soil, when the depth of excavation does not exceed about 2 metres. The method consists of placing vertical sheets (called sheathing) or polling boards opposite each other against the two walls of the trench and holding them in position by one or two rows of struts. The sheets are placed at an interval of 2 to 4 metres and generally, they extend to the full height of the trench. The polling boards may have width of about 200 mm and thickness of 44 to 50 mm. The struts may have size 1OO x 100 mm for trench upto 2 m wídth and 200 x 200 mm for trench upto 4 m width.

 FIG. 2.31  STAY BRACING.

2. Box sheeting. This method is adopted in loose soils, when the depth of excavation does not exceed 4 metres. Fig. 2.32 (a) shows the box like structure, consisting of vertical sheets placed very near to each other (sorne times touching each other) and keeping them in position by longitudinal rows (usually two) of wales. Struts are then provided across the wales.

Another system of box sheeting, shown in Fig. 2.32(b), is adopted for very loose soils. In this system, the sheeting is provided longitudinally, and they are supported by vertical wales and horizontal  struts [Fig. 2.32 (b)]. If the height is more, braces are also provided along with struts.

FIG. 2.32 BOX SHEETING.

3. Vertical sheeting. This system is adopted for deep trenches (upto 10 m depth) in soft ground. The method is similar to the box sheeting [Fig. 2.32 (a)] except that the excavation is carried out in stages and at the end of each stage, an offset iS provided, so that the width of the trench goes on decreasing as the depth increases. Each stage is limited to about 3 m in height and the offset may vary from 25 to 50 cm per stage. For each stage, separate vertical sheeting, supported by horizontal wailings and struts are provided (Fig. 2.33).

4. Runner system. This system is used in extremely loose and soft ground, which needs immediate support as excavation progresses. The system is similar to vertical sheeting of box system, except that in the place of vertical sheeting, runners, made of long thick wooden sheets or planks with iron shoe at the ends, are provided. Wales and struts are provided as usual (Fig. 2.34). These runners are driven about 30 cm in advance of the progress of the work, by hammering

                     FIG. 2.33 VERTICAL SHEETING.             FIG. 2.34 RUNNER SYSTEM.


5. Sheet piling. This method is adopted when (i) soil to be excavated is soft or loose (ii) depth of excavation is large (iii) width of trench is also large and (iv) there is sub-soil water. Sheet piles are designed to resist lateral earth pressure. These are driven in the ground by mechanical means (pile driving equipment). They can be used for excavating to a very large depth.

EXCAVATION AND TIMBERING OF FOUNDATION.

Excavation of foundation trenches can be done either manually with the help of conventional implements, shown in Fig. 2.29 or with the help of special mechanical equipment. Fig. 2.30 (a) shows a drag shovel which can excavate the foundation trench upto a width of 1.7 m. Fig. 2.30 (b) shows a multi-bucket trencher or a itcher, which can excavate treches upto 1.5 m width and 5 m deep. The boom iS raised and Iowered as required by the driver moving a lever and can be locked in any position. The spoil is carried up from the trench by buckets (having cutting teeth) attached to a continuous steel chain and tipped on to a belt conveyor at the top the rise, from where it is deposited to either left or right hand side of the trench.

⇒  TIMBERING OF TRENCHES.

 FIG. 2.29 IMPLEMENTS FOR FOUNDATION EXCAVATION.

FIG. 2.30 EXCAVATIONG EQUIPMENT.

SETTING OUT FOUNDATION TRENCHES.

Setting out or ground tracing is the process of laying down the excavation lines and centre lines etc. on the ground, before excavation is started. After the foundation design is done, a setting out: plan, sometimes also known as foundation layout plan, is prepared to some suitable scale (usually 1:50). The plan is fully dimensioned.

For setting out the foundations of small buildings, the centre line of the longest outer wall of the building is first marked on the ground by stretching a string between wooden or mild steel pegs driven at the ends. This line serves as reference line. For accurate work, nails can be fixed al the centre of the pegs. Two pegs, one on either side of the central peg, are driven at each end of the line. Each peg is equidistant from the central peg, and the distance between the outer pegs corresponds to the width of foundation trench to be excavated.

Each peg may project about 25 to 50 mm above ground level and may be driven at a distance of about  2 m from the edge of excavation so that they are not disturbed.

When string is stretched joining the corresponding pegs (say 2-2) al the two extremities of the line, the boundary of the trench to be excavated can be marked on the ground with dry lime powder. The centre lines of other walls, which are perpendicular to the long wall, are then marked by setting out right angles. A right angle can be set out by forming a triangle with 3, 4 and 5 units long. These dimensions should be measured with the help of a steel tape. Alternatively, a theodolite or prismatic compass may be used for setting out right angles. Similarly, outer lines of the foundation trench of each cross-wall can be set out, as shown in Fig. 2.27.

 FIG. 2.27 SETTING OUT WITH THE HELP OF PEGS.

For a big project, reference pillars of masonry may be constructed as shown in Fig. 2.28. These pillars may be about 20 cm thick, and about 15 cm wider than the width of the foundation trench. The top of the pillars is plastered, and is set at the same level, preferably at the plinth level. Pegs are embedded in these pillars and nails are then driven in the pegs to represent the centre line and the outer lines of the trench. Sometimes, additional walls are provided to represent plinth lines.

 FIG. 2.28 SETTINGS OUT USING MASONRY PILLARS.

CAUSES OF FAILURES OF FOUNDATIONS AND REMEDIAL MEASURES.

The foundations may fail due to the following reasons:

1. Unequal settlement of sub-soil. Unequal settlement of the sub-soil may lead to cracks in the structural components and rotation thereof. Unequal settlement of sub-soil may be due to (i) non-uniform nature of sub-soil throughout the foundation, (ii) unequal load distribution of the soil strata, and (iii) eccentric loading. The failures of foundation due to unequal settlement can be checked by : (i) resting the foundation on rigid strata, such as rock or hard moorum, (ii) proper design of the base of footing, so that it can resist cracking, (iii) limiting the pressure in the soil, and (iv)avoiding eccentric loading.

2. Unequal settlement of masonry. As stated earlier, foundation includes the portion of the structure which is below ground level. This portion of masonry, situated between the ground level and concrete footing(base) has mortar joints which may either shrink or compress, leading to unequal settlement of masoray. Due to this, the superstructure will also have cracks. This could be checked by (i) using mortar of proper strength, (ii) using thin mortar joints, (iii) restricting the height of masonry to 1 m per day if lime mortar is used and 1.5 m per day if cement mortar is used, and (iv) properly watering the masonry.

3. Sub-soil moisture movement. This is one of the major causes of failures of footings on cohesive soil, where the sub-soil water level fluctuates. When water table drops down, shrinkage of sub-soil takes place. Due to this, there is lack of sub-soil support to the footings which crack, resulting in the cracks in the building.

During upward movement of moisture, the soil (specially if it is expansive) swells resulting in high swelling pressure. If the foundation and superstructure is unable to resist the swelling pressure, cracks are induced.

4. Lateral pressure on the walts. The walls transmitling the load to the foundation may be subjected to lateral pressure or thrust from a pitched roof or an arch or wind action. Due to this, the foundation will be subjected to a moment (or resultant eccentric load). If the foundation has not been designed for such a situation, it may fail by either overturning or by generation of tensile stresses on one side and high compressive stresses on the other side of the footing.

5. Lateral Movement of sub-soil This is applicable to very soft soil which are liable to move out or squeeze out laterally under vertical loads, specially at locations where the ground is sloping. Such a situation may also arise in granular soils where a big pit is excavated in the near vicinity of the foundation. Due to such movement, excessive settlements take place, or the structure may even collapse. If such a situation exists, sheet piles should be driven to prevent the lateral movement or escape of the soil.

6. Weathering of sub-soil due to trees and shrubs. Sometimes, small trees, shrubs or hedge is grown very near to the wall. The roots of these shrubs absorb moisture from the foundation soil, resulting in reduction of their voids and even weathering. Due to this the ground near the wall depresses down. If the roots penetrates below the level of footing, settlements may increase, resulting in foundation cracks.

7. Atmospheric action. The behaviour of foundation may be adversely affected due to atmospheric agents such as sun, wind, and rains. If the depth of foundaion is shallow, moisture movements due to rains or drought may cause trouble. If  the building lies in a low lying area, foundation may even be scoured. If the water remains stagnant near the foundation, it will remain constantly damp, resulting in the decrease in the strength of footing or foundation wall. Hence it is always recommended to provide suitable plinth protection along the external walls by (i) filling back the foundation trenches with good soil and compacting it, (ii) providing gentle ground slope away from the wall and (iii) providing a narrow, sloping strip of impervious material (such as of lime or lean cement concrete) along the exterior walls.

METHODS OF IMPROVING SAFE BEARING PRESSURE OF SOILS.

Sometimes, the safe bearing pressure of soil is so low that the dimensions of the footings work out to be very large and uneconomical. In such a circumstance, it becomes essential to improve the safe bearing pressure, which can be done by the following methods : (i) increasing depth of foundation (ii) compacting the soil (iii) draining the soil (iv) confining the soil (v) grouting and (vi) chemical treatment.

1. Increasing depth of foundation. It has been found that in granular soil, the bearing capacity increases with the depth due to the confining weight of overlying material. However, this is not economical since the cost of construction increases with the depth. Also, the load on the foundation also increases with the increase in the depth. The method is useful only when better bearing stratum is encountered at greater depth.

2. Compaction of soil. It has found that compaction of natural soil deposits (loose) or man-made fills results in the improvement of bearing capacity and reduction in the resulting settlements. Compaction of soil can be effectively achieved by the following means:

(a) Ranmining moist soil. The foundation, soil is moistened and then compacted with the help of hand rammers or mechanically operated frog rammers or vibratory rollers. The voids of the soil are very much reduced, resulting in the reduction in settlements.

(b) Rubble Compaction into the soil. A Iayer of 30 to 45 cm thick well graded rubble is spread over the foundation level (Fig. 2.26) and weIlrammed. If this layer of rubble gets buried in the soil (specially when it is very loose) another Iayer of 15 cm thick rubble is spread and well rammed manually. This results in an increase in the bearing value of the soil

FIG. 2.26  RUBBLE COMPACTION INTO THE SOIL.

(c) Flooding the soil. The bearing pressure of very loose sands can be increased by flooding the soil. The method is very effecilve in improving the safe bearing pressure of dune sands, which cannot otherwise be effectivety compacted. The Author has an experience of improving the bearing power of desert soils by this method at  many locations where it was required to support heavy loads.

(d) Vibration. Heavy vibratory rollers and compactors may compact a Iayer of granular soils to a depth of 1 to 3 m. If the method of flooding and then vibration is used, sandy soil can be very effectively compacted, resulting in increased safe bearing power and decreased settlements when super-structure loads come on the soil. After flooding the soil, so that moisture penetration is al least 1 to 2 m, form vibrators or platform vibrators (about 1 m x 1.5 m base area, with a pair of eccentrically loaded motors) can be silded on the sand surface with the help of two labourers. A large area can be covered by this process, without the help of sophlsticated vibrating equipment.

(e) Vibroflotation. It is a commercial method which combine the effect of vibration and jetting. A heavy cylinder, known a vibroflot is inserted in the ground (soil) while the cylinder vibrates due to a rotary eccentric weight. A water jet on the tip of the vibro flot supplies a large amount of water under pressure. As the vibro flot sinks, clean sand is added into a crater that develops on the surface. The method is very useful when foundation is required to support heavy loads spread over a greater area.

(f) Compaction by pre-loading. This method is useful when he footing is founded on clayey solis which result in long term settlements. Pre-loading results in accelerated consolidation, so that settlements are achieved well before the actual footing is laid. The load used for this process is removed before the construction of the footing.

(g) Using sand piles. This method is very useful in sandy soil or soft soils. Hollow pipes are driven in the ground, at close interval. This results in the compaction of soil enclosed between the adjacent pipes. These pipes are then gradually removed, filling and ramming sand in the hole, resulting in the formation of sand piles.

3. Drainage of soil. It is a weIl known fact that presence of water decreases the bearing power of soil, specially when it is saturated. This is because of low shearing strength of soil in presence of excess water. Drainage results in decrease in the voids ratio, and improvement of bearing power.

4. Confining the soil. Sometimcs the safe bearing pressurc of the soil is low because of settlements resulting due to the lateral movement of loose granular soil. Such a tendency of lateral movement can be checked by confining the soil, outside ihe peritneter of foundation area, by driving sheet piles, thus forming an enclosure and confining the soil.

5. Grouting. This method is useful in loose gravels and fissured rocky strata. Bores holes in sufficient numbers are driven in the ground and cement grout is forced through these under pressure. The cracks, voids and fissurcs of the strata are thus filled with the grout, resulting in the increase in the bearing value.

6. Chemical treatment. In this method, certain chemicals are grouted in the place of cement grout. The chemical should be such that it can solidify and gain early strength.

SETTLEMENT OF FOUNDATIONS.

The vertical downward movement of the base of a structure is calied settlement and its effect upon the structure depends on its magnitude, its uniformity, the length of the time over which it takes place, and the nature of the structure itself.

Foundation settlement may be caused by some or a combination of the following reasons:

1. Elastic compression of the foundation and the underlying soil.

2. Inelastic (or plastic) compression of the undertying soils, which is much larger tban the elastic compression. The inelastic compression can be predicted by the theory of consolidation.

3. Ground waser lowering. Repeated lowering and raising of water level in loose granular soil tends to compact the soil and cause settlement of the ground surface. Lowering of water level in fine grained soils cause consolidation settlement. The major settlements in the city of Maxico has been due to ground water lowering, and due to this, the city has been called as the ‘sinking city of Maxico’.

4. Vibrations due to pile driving, blasting and oscillating machineries may cause settlement in deposits of granular soils.

5. Seasonal swelling and shrinkage of expansive clays.

6. Ground movement on earth slopes, such as surface erosion, slow creep or landslide.

7. Other causes such as adjacent excavation, mining subsidence, underground erosion, etc.

A certain amount of elastic and inelastic settlement of foundations is unavoidable, and it should be taken into account in design. Provided the settlement is uniform over the whole area of the building and is not excessive, it does little damage. If, however, the amount of settlement varies at different points under the building, giving rise to what is known as relative or differential settlement, stresses will be set up in the structure. These may be relived in the case of brick structure, for example, by the setting up of a large number of cracks at the joints, but in more rigid structures, overstressing of some structural members might occur.

It is suggesed that the allowable pressure should be selected such that the maximum settlement of any individual foundation is 2.5 cm. It has also been suggested that the differential settlement of uniformly loaded continuous foundation and of equally loaded spread foundations of approximately the same size, is unlikely to exceed half the maximum settlement, and that normal structures such as office buildings and flats can satisfactorily withstand differential settlements of about 18 mm between adjacent columns spaced 6 to 8 m apart.

The recommendations of American Codes are based upon the simple logic that if the maximum total settlement is kept within a reasonable limit, the differential settlement will be only a fraction (generally about three-quarters of this limit), depending upon the type of structure and pattern of Ioading. The allowable maximum settlement values are given below:

According to Polshin and Tokar, brick masonry will crack( due to differntial settlement) when the unit elongation amounts to 0.0005. Based on this criterion, the permissible differential settlement of brick walls is shown in Fig. 2.25, and is as follows

Where L is the wall length an H si the height of wall measured above the base of footing. The rate of differential settlement is defined as the slope or the relative settlement between two points divided by the horizontal distance.

FIG. 2.25  PERMISSIBLE DIFFERENTIAL SETTLEMENT FO BRICK WALLS.

DESIGN OF FOUNDATIONS - PRESUMPTIVE BEARING CAPACITY VALUES FROM CODES.

For the design of foundations of lightly loaded structures and for a preliminary design of any structure the presumptive safe bearing capacity may be used. The presumptive safe bearing capacities of various types of soils are given in Table 2.2.

TABLE 2.2  PRESUMPTIVE SAFE BEARING CAPACITY ( IS: 1904-1961 )

Note 1. Compactness or loosenEss of non-cohesive matenais may be determined by driving a wooden picket of dimension 5cm x 5 cm x lOcm with a sharp point. The picket shall be pushed vertically into the soil by full weight of a person and if the penetration of the picket exceeds 20 cm, the loose state shall be asaumed to exist.

Note 2. No generalised values for presumptive safe bearing capacities can be given for these types of soils. In such area, adequate site investigation shall be carried out and expert advice shall be sought.

Note 3. Peat may occur in a very soft spongy condition or may be quite firm and compact. While ultimate bearing capacity may be high in the compact cases very large consolidation settlements occur even under small presures and the movements continue for decades.

Note 4. The strength of made-up ground depends on the nature of the material, its depth and age, and the method used for consolidating it.

Note 5. The presumptive safe bearing values may be increased by an amount equal to weight of the material (virgin soil) removed from above the bearing level, that is, the base of the foundation.

Note 6. For non-coheisve soils, the presumptive safe bearing values shall be reduced by 50 percent if the water table is above or near the bearing surface of the soil. If the water table is below tje bearing surface of the soil at a distance at least equal to the width of the foundation, no such reduction shall apply. For intermediate depths of the water table, proportional reduction of presumptive safe bearing value may be made.

Dutch cone test - SITE EXPLORATION.

This test is used for getting a continuous record of the resistance of soil by penetrating steadily under static pressurc a cone with a base of 10cm^2 (3.6 cm in dia.) and an angle of 60º  at vertex. The cone is carried at the lower end of a steel driving rod which passes through a steel tube (mantle) with external diameter equal to the base of the cone. Either the cone or the tube, or both together can be forced into the soil by means of jack. To know the cone resistance, the cone along is first forced down for a distance of 8 cm and the maximum value of resistance Is recorded. The steel tube is then pushed down upto the cone, and both together are further penetrated through a depth of 20 cm to give the total of cone resistance and the frictional resistance along the tube.

The Cone test is considered very useful in determining the bearing capacity of pits in cohesionless soils, particularly in fine sands of varying density. The cone resistance qc (kg/cm^2) is approximately equal to 5 to 10 times the penetration resistance N.

Standard Penetration Test - SITE EXPLORATION.

The test (IS : 2131—1963) is performed in a clean hole, 55 to 150 mm in diameter. A casing or drilling mud may be used to support the sides of the hole. A thick wall split tube sampler, 50.8 mm outer dia. and 35 mm internal dia. is driven into the undisturbed soil at the bottom of the hole under the blows of 65 kg drive weight with 75 cm free fall: The minimum open Iength of the sampler should be 60 cm. The sampler is first driven through 15 cm as a seating drive. It is further driven through 30 cm and the number of blows required for this are counted. This number of blows is termed as penetrarion resistance N.

In very fine, or sitly, saturated sand, an apparent increase in resistance occurs. Terzaghi and Peck have recommended the use an equivalent penetration resistance Ne, in place of the actually observed value of N, when N is greater than 15. Ne is given by the following relation:

Terzaghi and Peck’s empirical charts for determining nct bearing pressure qp for footing on sand depend on B and N value, to limit maximum settlement of individual footing to 2.5 cm and differential settlement of 2 cm, assuming that a differential settlement of 2 cm can be tolerated by most of the ordinary structures. The empirical relations are represented by the following equation:

The standard penetration test is very useful for the design of rafts. The safe bearing value for rafts may be taken as smaller of the values of q1 and q2 given bellow:

PENETRATION TESTS - SITE EXPLORATION.

These tests involve the measurements of the rcsistance to penetration of a sampling spoon, a cone or other shaped tool under dynamic or static loadings. The resistance is empirically correlated with sorne of the engineering properties of soil, such as density index, bearing capacity etc. Two commonly used penetration tests are

(i) Standard penetration test, The test (IS : 2131—1963) is performed in a clean hole, 55 to 150 mm in diameter. A casing or drilling mud may be used to support the sides of the hole....

(ii) Dutch cone test. This test is used for getting a continuous record of the resistance of soil by penetrating steadily under static pressurc a cone with a base of 10cm^2 (3.6 cm in dia.) and an angle of 60º  at vertex....

Limitations of plate load test - FOUNDATION SITE EXPLORATION.

The plate load test has the following limitations:

1. The test results reflect only the character of the soil located within the depth Iess than twice the width of bearing plate (corresponding to a pressure bulb of one-tenth of the loading intensity at the test plate). Since the foundations are generally larger, the settlement and resistance against shear failure will depend on the properties of a much thicker stratum.

2. It is essentially a short duration test, and hence the test does not give the ultimate settlement, particularly in the case of cohesive soils.

3. Another limitation is the effect of the size of foundation. For clayey soils the ultimate pressure for a large foundation is the same as that for the test plale. But in dense sandy soils, the bearing capacity increases with the size of the foundation, and the tests on smaller size bearing plates tend to give conservative values.

PLATE LOAD: Safe bearing pressure on permissible settlement - FOUNDATION SITE EXPLORATION.

The safe bearing capacity determined above is on the basis of shear failure. The settlement of the footing also governs the bearing capacity of soil. Such a bearing pressure can be obtained from the load settlement curve, corresponding to the desired settlement of the test plate. Generally, the permissible settlements of footings are specified in the Codes. The corresponding settlement of the test plate can be found from the following relationship applicable for granular soil:

The net loading intensity corresponding to settlement ρp, is then determined from Fig. 2.24. The safe bearing pressure is then lesser of the following two values: (i) safe bearing capacity found on the basis of shear failure, and (ii) net loading intensity corresponding to settlement ρp, of the plate.

 FIG. 2.24  LOAD SETTLEMENT CURVES.

PLATE LOAD: Test procedure - FOUNDATION SITE EXPLORATION.

The plate is firmly seated in the hole, and if the ground is slightiy uneven a thin layer of sand is spread underneath the plate. The load is applied with the help of a hydraulic jack (preferably with the remote control pumping unit), in convenient increments, say of about one-fifth of the expected safe bearing capacity or one-tenth of the ultimate bearing capacity. Settlement of the plate is observed by 2 dial gauges fixed at diametrically opposite ends and supported on a suitable datum bar. The dial gauges should have a sensitivity of 0.02 mm. Settlement should be observed for each increment of load after an interval of 1, 4, 10, 20, 40 and 60 minutes and thereafter at hourly intervals until the rate of settlement becomes less than 0.02 mm per hour. After this, next load increment is applied. The maximum load that is to be applied corresponds to 1 1/2 times the eslimated ultimate load or to 3 times the proposed allowable bearing pressure.

The water table has marked influence on the bearing capacity of sandy or gravelly soil. If the water table is already above the, level of footing, it should be lowered by pumping and the bearing plate seated after the water table has been lowered just below the footing level. Even if the water table is located above 1 m below the base level of the footing, the load test should be made at the level of water table itself.

The load intensity and settlement observations of the plate load are plotted as shown in Fig. 2.24 (a). Curve I corresponds to general shear failure and curve II corresponds to local shear failure. Curve III is a typical of dense cohesionless soils which do not show any marked shear failure under the loading intensities of the test.
When the load settlement curve [Fig. 2.24 (a)] does not indicate any marked breaking point, failure may alternatively be assumed corresponding to a settlement equal to one-fifth of the width of the test plate. In order to determine the safe bearing capacity it would be normally sufficient to use a factor of safety of 2 or 2.5 on utlimate bearing capacity.

 FIG. 2.24  LOAD SETTLEMENT CURVES.

PLATE LOAD TEST - FOUNDATION SITE EXPLORATION.

Plate Load Test is a field test to determine the ultimate bearing capacity of soil, and the probable settlement under a given loading. The test esssentially consists in loading a rigid plate (usually of steel) at the foundation level, and determining the settlements corresponding to each load increment. The ultimate bearing capacity is then taken as the load at which the plate starts sinking at a rapid rate. The method assumes that down to the depth of influence of stresses, the soil strata is reasonably uniform.

FIG.  2.21  TEST PIT.

The beanng plate is square, of minimum recommended size 30 cm square, and maximum size, 75 cm square. The plate is machined on sidcs and edges, and should have a thickness sufficient to withstand effectively any bending stresses that would be caused by the maximum anticipated load. The thickness of steel plate should not be less than 25 mm.

FIG. 2.22 PLATE LOAD TEST: REACTION BY GRAVITY LOADING.

The test pit width is made five times the width of plate (Bp). At the centre of the pit, a small square hole is dug whose size is equal to the size of the plate and the bottom level of which corresponds to the level of actual foundation (Fig. 2.21). The depth Dp, of the hole should be such that

The loading to the test plate may be applied with the help of a hydraulic jack. The reaction of the hydraulic jack may be borne by either of the following two methods:

(a) gravity loading platform method,
(b) reaction truss method.

In case of gravity loading method a platform is constructed over a vertical column resting on the plat form, and the loading is done with the help of sand bags, stones or concrete blocks. The general arrangement of test set-up for this method as shown in Fig. 2.22. When load is applied to the test plate, it sinks or settles. The settlement of the plate is measured with the help of sensitive dial gauges,. For square plate, two dial gauges are used. The dial gauges are mounted on independently supported datum bar. As the plate settles, the ram of the dial gauge moves down and settlement is recorded. The load is indicated on the load-gauge of the hydraulic jack.


Fig. 2.23 shows the arrangement when the reaction of the jack is borne by a reaction truss. The truss is held to the ground through soil anchors. These anchors are firmly driven in the soil with the help of hammers. The reaction truss is usually made of mild steel sections. Guy ropes are used for the lateral stability of the truss.

Note. In olden days, the loading on the plate was made with the help of gravity loading consisting of weighed sand bags on a platform constructed over the central loading column. The settlement of the plate was measured with the help of a dumpy level. Such an arrangement is crude since the settlements are not measured upto the desired accuracy and the arrangement gets disturbed during the incremental loading. Certain mishaps have also been reported due to the tilting of the loading platform (Fig.  2.22) or by reaction truss (Fig. 2.23).

FIG.  2.23  PLATE LOAD TEST: REACTION BY TRUSS.

⇒ Test procedure.
⇒ Safe bearing pressure on permissible settlement.
⇒ Limitations of plate load test.

ANALYTICAL METHODS - FOUNDATIONS SITE EXPLORATION.

A number of analytical methods have been developed To determine the ultimate bearing capacity of soil. These methods use two important shear parameters of soil: (i) angle of internal fricction Ø and (ii) cohesion c. These parameters are determined in the laboratory, by conducting shear tests on soil samples (preferably, undisturbed samples) collected from the bore holes or test pits. Out of the various theories developed, only two are briefty given here: (i) Rankine’s analysís and (ii) Terzaghi’s analysis.

(a)  Rankine’s Analysis

Rankine considered the equilibrium of two soil elements, one immediately below the foundation (element I) and the other just beyond the edge of the footing (element II), but adjacent to element

I. When the load on the footing increases, and approaches a value qf, a state of plastic equilibrium is reached under the footing. For the shear failure of element I, element II must also fail by lateral thrust from element I.

 FIG. 2.19

Now, for element I, the major principal stress p1 from vertical direction is

 

According to Rankine’s active earth pressure theory the resulting stress p2 (called the minor stress) in the horizontal direction is given by

(i.e. minor principal stress=major principal stress x ka)
where ka = co-efficient of active earth pressure

where  Ø is the angle of repose for the soil.

For element II, the vertical stress p3 is evidently equal to the weight of overburden =  γ D. However, the stress p2 in the horizontal directfon is the same as found in (i) above. However, since p2 is much more than p3, major stress on element II is p2 and minor stress is p3. From Rankine’s earth pressure theory, minor principal stress = ka x (major principal stress)

Substituting the values of p2 and p3, we get

Eq. (2.7) gives the bearing capacity of cohesionlcss solis as zero at the ground surface. This is not consisten with the general experience. However, Eq. 2.6 may be used in the following form to get the minimum depth of foundation.:

where q= intensity of loading.

(b) Terzaghi’s Analysis*

An analysis of the condition of complete bearing capacity failure, usually termed as general shear failure was made by Terzaghi by assumlng that the soil behaves like an ideally plastic material. Fig. 2.20 (a) shows a shallow footing in which the depth D iS equal to or less than the width B of the footing. The loaded soil fails along a composite surface ABCB1A1.

FIG.  2.20  TERZAGHI'S ANALYSIS

Terzaghi gave the following equations:

where Nc,  Nq and Nγ are the dimensionless numbers, called the bearing capacily factors, the values of whích can be obtained from Table 2.1. The above analysis corresponds to general shear failure in which the soil properties are such that a slight downward movement of footing develops fully plastic zones and the soil bulges out [Fig. 2.20 (c)]. In case of fairly soft or loose and compressíble soil, large deformation may occur below the footing before the failure zones are fully developed. Such a failure is known as local shear failure [Fig. 2.20 (d)] which is associated with considerable vertical soil movement before soil bulging takes place. The bearing capacity factors corresponding to the local shear failure are indicated with dashes, i.e. Nc’ , Nq’ and Nγ’ (Table 2.1). Terzaghi gave the following equation for local shear failure:

 TABLE 2.1  TERZAGHI'S BEARING CAPACITY FACTORS.

BEARING CAPACITY OF SOILS - SITE EXPLORATION.

As stated earlier, a foundation should be designed to satisfy two essential conditions:

(i) It must have sorne specificd safety against ultimate failure.
(ii) The settlements under working loads should not exceed the allowable limits for the super-structure.

The bearing capacity of the soil, used for the design of foundations (i.e.  for determining the dimensions of the foundations) is determined on the basis of the above two criteria.

In general, the supporting power of a soil or rock is referred to as its bearing capacity. The term bearing capacity is defined after attaching certain qualifying prefixes, as defined below:

1. Gross pressure intensity (q). The gross pressure intensity q as the total pressure at the base of the footing due to the weight of the super-structure, self weíght of the footing and the weight of the earth fill, íf any.

2. Net Pressure intensity (qn). It is defined as the excess pressure, or the difference in intensities of the gross pressure after the construction of the structure and the original overburden pressure. Thus, lf D is the depth of the footing

where γ is the unit weight of soil aboge the level of footíng.

3. Ultimate bearing capacity (qf). The ultimate bearing capacny is defined as the minimum gross pressure intensity at the base of the foundation at which the soil fails in shear.

4. Net ultimate bearing capacity (qnf). It is the minimum net pressure intensity causing shear failure of the soil. The ultimate bearing capacity qf, and net ultimate bearing capacity (qnf) are evidently connected by the relation



5. Net safe bearing capacity (qns). The net safe bearing capacity is the net ultimate bearing capacity divided by a factor of safty F:

6. Safe bearing capacity (qs). The maximum pressure which the soil can carry safely without risk of shear failure is called the safe bearing capacity. It is equal to the net safe bearing capacity plus original overburden pressure



Sometirnes, the safe bearing capacuty is also referred to as the ultimte bearing capacity qf divided by a factor of safety F.

7. Allowable bearing pressure (qa). It is the net loading intensity at which neither the soil fails in shear nor there is excessive settlement detrimental to the structure in question. The allowable bearing pressure thus depends both on the sub-soil and the type of building concerned, and is generally less than, and never exceeds, the safe bearing capacity.

Methods ot Estimating Bearing capacity

The bearing capacity of soil can be determined by the following methods:

(a) Analytical methods involving the use of soil parameters
(b) Plate load test on the soil
(c) Penetration test
(d) Presumptive bearing capacity values from codes.

SOIL SAMPLES AND SAMPLERS - FOUNDATIONS SITE EXPLORATION.

Soil samples can be of two types:

(i) Disturbed samples.
(ii) Undisturbed samples.

A disturbed sample is that in which the natural structure of soil gets partly or fully modified and destroyed although with suitable precautions the natural water content may be preserved. Such a soil sample should, however, be representative of the natural soil by maintainlng the original proportion of the various particles intact. An undisturbed sample is that in which the natural structure and properties remain preserved.

The sample disturbance depends upon the design of the samplers and the method of sampling. To take undisturbed samples from bore holes properly designed sampling tools are required. The sampling tube when forced into the ground should cause as little remoulding and disturbance as possible. The design features of the sampler, that govern the degree of disturbance are (i) cutting edge (ii) inside wall friction and (iii) non-return valve.

Fig. 2.18 shows a typical cutting edge of a sampler, with the lower end of the sampler, with the lower end of the sampler tube. The following terms are defined with respect to the diameters marked in Fig. 2.18.

FIG. 2.18 LOWER END OF A SAMPLER.

The area ratio should be as low as possibie. It should not be greater than 25 percent; for soft sensitive soil, it should preferably not exceed to porcent. The inside clearance should lie between 1 to 3 percent and the outside clearance should not be much greater than the inside clearance. The walls of the sampler should be smooth and should be kept properly oiled so that wall friction is minimum. Lower value of inside clearance allows the elastic expansion of soil and reduces the frictional drag. The non-retum valve, invariably provided in samplers, should permit easy and quick escape of water and air when driving the sampler.


Types of Samplers

The samplers are classified as thick wall or thin wall samplers depending upon the area ratio. Thick wall samplers are those having the area ratio greater than 10 percent. Depending upon the mode of operation, samplers may be classified in the following three common types : (i) open drive sampler (including split spoon samplers), (ii) stationary piston sampler and (iii) rotary sampler.

The open drive sampler is a tube open at its lower end. The sampler head is provided with vents (valve) to permit water and air to escape during driving. The check valve helps to retain sample when the sampler is lifted up. The tube may be seamless or it may be split in two parts; in the latter case it is known as split spoon sampler.

The stationary piston sampler consists of a Sample cylinder and the piston system. During lowering of the sampler through the hole, the lower end of the sampler is kept closed with the piston. When the desired sampling elevation is reached, ihe piston rod is clamped, thereby keeping the piston stationary, and the sampler tube is advanced down into the soil. The sampler is then Iifted up, with piston rod clamped in position. The sampler is more suitable for sampling soft soils saturated sands.

Rotatory samplers are the core barrel type having an outer tube provided with cutting teeth and a removable thin wall liner inside. It is used for firm to hard cohesive soils and cemented soils.

CHOICE OF EXPLORATION METHOD - FOUNDATIONS.

The choice of a particular exploration method depends on the following factors: (a) nature of ground (b) topography and (c) cost.


1. Nature of ground

In clayey soils, borings are suitable for deep exploration and pits for shallow exploration. In sandy soils, boring is easy but special equipment should be used for taking representative samples below the water table. Such samples can however, be readily taken in trial pits provided that, where necessary, some forrn of ground water lowering is used.

Borings are suitable in hard rocks while pits are preferred in soft rocks. Core borings are suitable for the identification of types of rock but they cannot supply data on joints and fissures which can only be examined in pits and large diameter borings.

When the depth of exploration is large, and where the area of construction site is large, geophysical methods (specially the electrical resistivity method) can be used with advantage. However, borings at one or two locations should be carried out, for calibration purposes. In soft soil, sounding method may also be used to cover large area in relatively shorter duration.


2. Topography.

In hilly country, the choice between vertical openings (for example, boring sand trial pits) and horizontal openings (for example, headings) may depend on the geological structure, since steeply inclined strata are most effectively explored by headings and horizontal strata by trial pits or borings. Swamps and areas overlain by water are best explored by borings which may have to be put down from a floating craft.


3.Cost.

For deep exploration, borings are usual, as deep shafts are costly. However, if the area is vast, geophysical methods or sounding methods may be used in conjunction with borings. For shallow exploration in soil, the choice between pit and borings will depend on the nature of the ground and the information required for shallow exploration in rock; the cost of boring a core drill to the site will only be justified if several holes are required; otherwise trial pits will be more economical.

GEOPHYSIAL METHODS - SITE EXPLORATION.

Geo-physical methods are used when the depth of exploration is very large, and also when the speed of investigation is of primary importance. Geo-physical investigations involve the detecttion of significant differences in the physical properties of geological formations. These mehods were developed in connection with prospecting of useful minerals and oils. The major method of geo-physical investigations are: gravitational methods, magnetic methods, seismic refraction method, and electrical resistivity method. Out of these, seismic refraction method and electrical resistivity methods are the most commonly used for Civil Engineering purposes.



Seismic refraction method

In this method, shock waves are created into the soil at their ground level or a certain depth below it by exploding small charge in the soil or by striking a plate on the soil with a hammer. The radiating shock waves are picked up by the vibrarion detector (also called geophone or seismometer) where the time of travel of the shock waves gets recorded. A number of geophones are arranged along a fine (Fig. 2.16). Sorne of ihe waves, known as dírect or pruna?y waves nave! directly from the shock point along the ground surface and are picked first by the geophone. The other waves which travel through the soil get refracted at the interface of two soil strata. The refracted rays are also picked up by the geophone. If the underlying layer is denser, the refracted waves travel much faster. As the distance between the shock point and the geophone increases, the refracted waves are able to reach the geophone earlier than the direct waves. By knowing the time of travel primary and refracted waves at various geophones, the depth ofv arious strata can be evaluated, by preparing distance-time graphs and using analytical methods.

Seismic refraction method is fast and reliable in establishing profiles of different strata provided the deeper layer have increasingly greater density and thus higher velocities and also increasingly greater thickness.

Different kinds of materlais such as gravel, clay hardpan, or rock have characteristic seismic velocities and hence they may be identified by the distance-time graphs. The exact type of material cannot, however, be recognised and the exploration should be supplemented by boring or soundings and sampling.

FIG. 2.16  SEISMIC REFRACTION METHOD.

Electrical Resistivity Method

The electrical resistivity method is based on the measurement and recordíng of changes in the mean resistivity of various solis.

Each soil has its  own resistivity depending upon its water content, compaction and composition; for example, it is low for saturated silt and high for loose dry gravel or solid rock.

The test is conducted by driving four metal spikes to serve as electrodes into the ground along a straight line at equal distance. A direct voltage is imposed between the two outer electrodes, and the potential drop is measured between the inner electrodes. The mean resistivity Q (ohm-cm) is computed from the expression

The depth of exploration is roughly proportional to the electrode 

FIG. 2.17 RESISTIVITY METHOD.

spacing. For studying vertical changes in the strata, the electrode system is expanded, about a fixed central point, by increasing the spacing gradually from an initial small value to a distance roughly equal to the depth of exploration required. The method is known as resistivity sounding.

To correctly interpret the resistivity data for knowing the nature and distribution of soil formation, it is necessary to make preliminary trial or calibration tests on known formations.

SUB-SURFACE SOUNDINGS - SITE EXPLORATION.

The sounding methods consist of measuring the resistance of the soil with depth by means of penetrometer under static or dynamic loading. The penetrorneter may consist of a sampling spoon, a cone or other shaped tool. The resistance to penetration is empirically correlated with some of the engineering properties of soil, such as density index, consistency, bearing capacity etc. The value of these tests lie in the amount of experience behind them. These tests are useful for general exploration of erratic soil profiles, for finding depth to bed rock or stratum, and to have an approximate induction of the strength and other properties of solis, particularly for cohesionless soils, from which it is difficult to obtain undisturbed samples. The two commonly used tests are standard penetration test and the cone penetration test.

BORING METHODS - SITE EXPLORATION.

The following are the various boring methods commonly used:

(i) Auger boring.
(ii) Auger and shell boring.
(iii) Wash boring.
(iv) Percussion boring.
(v) Rotary boring.

(i) Auger boring
Augers are used in cohesive and other soft soils aboye water table. They may either be operated manually or mechanically. Hand augers are used upto a depth upto 6 m. Mechanically operated augers are used for greater depths and they can also be used in gravelly soils. Augers are of two types: (a) spiral auger and (b) post-hole auger.

FIG. 2.12  AUGER.

Samples recovered from the soil brought up by the augers are badly disturbed and are useful for identification purposes only. Auger boring is fairly satisfactory br explorations at shallow depths and for exploratory borrow pits.

(ii) Auger and shell boring
Cylindrical augers and shells with cutting edge or teeth at Iower end can be used for making deep borings. Hand operated rigs are used for depths upto 25 m and mechanised rigs up to 50 m. Augers are suitable for soft to stiff clays, shells for very stiff and hard clays, and shells or sand pumps for sandy soils. Small boulders, thin soft strata or rock or cemented gravel can be broken by chisel bits attached to drill rods. The hole usually requires a casing. Fig. 2.13 shows a typical sand pump.

FIG. 2.13  SAND PUMP

(iii) Wash boring
Wash boring is a fast and simple method for advancing holes in all types of soils. Boulders and rock cannot be penetrated by this method. The method consists of first driving a casing through which a hollow drilled rod with a sharp chisel or chopping bit at the lower end is inserted. Water is forced under pressure through the dril rod which is alternativety raised and dropped, and also rotated. The resulting chopping and jetting action of the bit and water disintegrates the soil. The cuttings are forced upto the ground surface in the form of soil-water slurry through the annular space between the drill rod and the casing. The change in soil stratification could be guessed from the rate of progress and colour of wash water. The samples recovered from the wash water are almost valueless for interpreting the correct geo-technical properties of soil.

(iv) Percussion boring
In this method, soil and rock formations are broken by repeated blows of heavy chiesel or bit suspended by a cable or drill rod. Water is added to the hole during boring,  if not already present and the slurry of pulverised material is bailed out at intervals. The method is suitable for advancing a hole in all types of solis, boulders and rock. The formations, however, get disturbed by the impact.

(v) Rotary boring
Rotary boring or rotay drilling is a very fast method of advancing hole in both rocks and soils. A . drill bit, fixed to the lower end of the drill rods, is rotated by a suitable chuck, and is always kept in firm contact with the bottom of the hole. A drilling mud, usually a water solution of bentonite, with or without other admixtures, is continuously forced down to the hollow dril rods. The mud returning upwards brings the cuttings to the surface. The method is also known as mud rotary drilling and the hole usually requires no casing.

Rotary core barrels, provided with commercial diamond-studded bits or a steel bit with shots, are also used for rotary drilhng and simultaneously obtaining the rock cores or samples. The method is them also known as core boring or core drilling. Water 15 circulated down drill rods during boring.

FIG. 2.14  WASH BORING.

Record of borings

In all exploration work it is very important to maintain an accurate and explicit record of borings. Soil/rock samples are collected at various depths, during boring. These samples are tested in the laboratory for identification and classification. The samples are suitabty preserved and arranged serially according to the depth at which they are found. A boring chart, similar to the one shown in Fig. 2.15 Is prepared for each bore hole. A site plan should be prepared, showing the disposition of various bore holes on it.



Number and disposition of trial pits and borings

The number and disposition of the test pits and borings should be such as to reveal any major changes in the thickness, depth or properties of the strata affected by the works, and the immediate surroundings.

(a) for a compact buliding site covering an area of about 0.4 hectares, one bore hole or trial pit in each comer and one in the centre should be adequate.
(b) For small and less important buildings, even one bore hole or trial pit in the centre will suffice.
(c) For very large arcas covering industrial and residential colonies, the geological nature of the terrain will help in deciding the number of bore holes or trial pits. Dynamic or static cone penetration tests may be performed at every 100 metres by dividing the area into grid patterns and number of bore holes or trial pits decided by examining the variation in the penetration curves.

                                                    FIG. 2.15  DETAILS OF BORING.

OPEN EXCAVATION (OPEN TRIAL PITS) - SITE EXPLORATION.

Trial pits are tho cheapest method of exploration in shallow deposits, since these can be used in all types of soils. In this method, pits are excavated at the site, exposing the sub-soil surface thoroughly.

Soil samples are collected at various levels. The biggest advantage of this method is that soil strata can be inspected in their natural condition and samples (disturbed or undisturbed) can be conveniently taken. A typical trial pit is shown in Fig. 2.11.

FIG. 2.11. TRIAL PIT

The method is generally considered suitable for shallow depths, say upto 3 m. The cost of open excavation increases rapidly with depth. For greater depths and for excavation below ground water table, specially in pervious soils, measures for lateral support and ground water lowering becomes necessary.