Tag: foundation

The principal types of foundations

The principal types of foundations

 

Isolated Footings

 

Footings are understood formed by a rigid rectangular base of stone or concrete of dimensions : width B and length L, in which the ratio of L/B will not exceed 1.5.

The foundation structure will support the column load. The bearing capacity of the footing may be stimated, and its dimensions selected ; thereafter, a forcast of the settlement is made.

To illustrate the case pf footing foundations, consider a building with nine columns (Fig 1) supported on isolated footings. In this case, the footings will work independently of each other. Therefore, it is required that the differential settlements between footings will not exceed the allowable total and differential settlement requirements.

Figure 1: Isolated Footings

The differential settlements may be reduced selecting properly the area of the footings, and at times, using the stiffness of the superstructure.

From the structural point of view, however, the superstructure should not be allowed to take high secondary stresses induced bu the differential settlements of the footinhs, except in very special casees.

Singles footing foundations, in general, will be user only in soils of low compressibility and in structures where the differential settlements between columns may be controlled by the superstructure flexibility, or including in the design og the building joints or hinges that will take the differential settlements and/or rotations, respectively, without damaging the construction.

 

Continuous Footings

 

When it is necessary to control within certain limits the magnitude of differential settlements between columns supported on footings, and when soil deposits of medium or low compressibility atre encountered, it is recommended to use continuous footings. They may be defined as resisting elements joining columns together by foundation beams.

Continious footings are arraged by joining two or more columns together with beams.

The vertical differential displacements may be controlled via beam stiffness (Fig 2). The selection of the foundation beams, either running in one direction or the other along columns rows, depends largely on the layout of the column loads, and othe functional requirements concerning the structural and architectural design of the project.

Figure 2: Continuous Footings

For heavier loads, and when the project calls foor stiffness in both directions (namely, along column rows A, B and C also rows 1,2 and 3), the foundations is given stiffness with beams in both directions (Fig 3).

Figure 3: Continuous Footings

In this case, it may be observed that the footing slabs will cover practically all the foundation. This type of foundation using  continuous footings is advantageous in soils of medium compressibility, where it is necessary to control differential movements between colomns.

The foundation beams are designed with the necessary stiffness to fulfill the differential settlements requirements.

 

Raft Foundation

 

When the loads are so largee that continious footings will occupy close to 50% of projected area of the building, it is more economical to use a continuous mat covering the entire area, as shown in Fig 4. The total load in this case may be assumed uniformly distributed in the area covered by the building.

The soil reaction is determined on the basis of a safe bearing capacity. The total and differential settlements may be investigated considering the stiffness of tthe raft or foundation slab is a matter of economy, compatible with the allowable differential settlements.

Figure .4 : Mat Foundations

Flexibility is important to obtain economy ; however, restrictions in differential vertical displacements between colmuns may call for certain slab stiffness, either by making it thicker or by placing foundation beams joining columns rows.

The beams can be designed with the required stiffness to reduce differential displacements.

This type of foundation may be used generally in soil deposits of medium compressibility ; however, in certain instances, the surface raft foundation may be used in soils of high and very high compressibility, where large total settlements may be allowed.

This type of foundation may be used efficiently in reduucing differential settlement.

 

Compensated Foundations

 

In soil deposits of medium, high and very high compressibility and low bearing capacity, compensated foundations are indicated.

This type of foundation requires a monolithic box foundation, as shown in Fig 5. When the water table is close to the ground surface, water proofing is necessary to use the buoyancy effect in designing the foundation.

Figure .5 : Compensated Foundation

In the design of compensated foundations,  it should be borne in mind that the soil should be consedered as a material of two phases, namely : a solid and liquid phase.

Therefore, in a compensated foundation, the compensation is made by adding two effects :

  • Substitution of the submerged weight of solids.
  • The buoyancy effect by weight of liquid displaced.

Both effects are used to equalize the total weight of the building. The volume of the concrete box forming the foundation strructure annd basements will displace a weight of liquid that, according to Archimedes’ principal, will contribute in floating the foundation up to this value, reducing the load applied to the solid phase.

The load taken by the solid phase  will, however, deform the soil because of the change in effective stresses induced in the soil structure. It shoul be investigated from the point of view of bearing capacity of the soil and total and differentioal settlements, as previously discussed for other foundations.

 

Compensated Foundations with Friction Piles

 

When a compensated foundation as described is not sufficient to support the load with the allowable total settlement, in spite of desigin the foundation with sufficient stiffness to avoid detrimental differential settlements within the foundation itself, friction piles may be used in addition to the concept of compensation.

This case may be present in deposits of high or very high compressibility extending to great depth. The piles will reinforce the upper part of the soil where a higher comopressibility is encountered.

The applicability of this foundation calls for a soil that varies from very high compressibility at the upper part of the deposit, to medium or low compressibility at the bottom (Fig 6).

Figure 6: Compensated friction pile Foundation

 

Point Bearing Pile Foundations

 

When the loads to be supported are higher than those a compensated friction pile foindation can take, the nit will be required to find a deep-seated hard stratum of low to very low compressibility and high shear strenth, where piles can be driven to point bearing. One can distinguish two cases of point bearing foundations (Fig 7 and Fig 8).

The first case is recognized when the hard stratum of convenient thickness is found underlain by materials of medium compressibility. In these cases the piles should be evenly distributed as shown in Fig 7. After solving the problem of point bearing of the piles in the hard stratum, there still existas the problem of finding if the lower compressible soil stratum will have a safe bearing value, and also if the total and differential settlements will be within the allowable values specified for the foundation in question.

This type of foundation should be designed with sufficient stiffness to control differential settlements.

The second type of pile fooundation is recognized wwhen the point bearing piles rest in a firm deposit of low compressibility extending to great despth (Fig 8). In this case, it is economical to use groups of piles to solve the foundation problem.

Figure 7 : Point Bearing Piles

Figure 8 : Point Bearing Piles in groups

The columns will rest on single footings supported on the piles. The piles driven in the firm stratum develop laterad friction contributing to the mechanical properties on shear strengh of the depostigs in whttich the are driven, on the spacing of the piles, on the length of penetration into the bearing stratum, and on the state of density and confinement of such stratum.

Pier Foundations

Pier foundations are used to support very heavy loads in buried soil deposits of very low compressibility (Fir 9). Their load capacity is a function of the mechanical properties of the soil under the base of the pier, and of the confining stress of the bearing stratum. Actuelly, the bearing capacity of such an element is determined as a deep-seated isolated footing.

Figure 9 : Pier Foundation

The piers, columns-like elements cast in place, in most cases carry high loads of 500 ton or moore ; therefore, the compressibility of the deposit on which the are resting should be very low, in order that they may be recommended.

Pier shafts may be used from dimensiosn are also a function of the procedure used to perform the excavation, and of the way the hydraulic conditions are handled. The density of the material wherre these elements aare bearing may be altered during excavations if an upward water flow is produced. Specially important is the case when the material is a cohesionless fine sediment or when the cohesion is small, in which case it is necessary to perform the excavation using a pneumatic system, introducing air under sufficient high pressure to balance the flow of water toward the bottom of the excavation, preserving the natural confining and density connditions of the bearing stratum.

Usually, if precautions are taken in the installation of these elements, the settlements will be very small. The settlement, however, may be estimated knowing the stress-strain characterictics of the strata encouontered under the base of the piers. The nagative friction on these elements may take large proportions : hence, it shoul be estimated.

When these rigid elements are used in seismic regions to support loads through deposits of high and very high compressibility, it is necessary to investigate the effect ot the horizontal motion of the soil mass during earthquakes. The horizontal drift  forces against the piears because of soil displacement should not be overlooked. In occasions, rigid elements have been damaged because of the strong horizontal motions produced bu the earthquakes.

Sand Pier Foundations

The solution of foundations using sand pier or sand piles is shown in Fig 10.

Figure 10 : Sand Piers

This type of foundation is used to increase the load capacity of the soil by reducing its compressibility and increasing its shear strength capacity properties. This type of pile may be used in loose or medium dense sand deposits.

The improvement of the subsoil is a function of the volume of sand introduced at the time thses elements are installed. Usually first a hole is driven in the ground, then sand is introduced and highly compacted in layers, using a heavy ram.

The sand element will take the load because of the lateral confinement given by the subsoil. The deformation of these elements may be estimated by means of the stress-strain properties of the sand ussed, considering the pier as a long sand cylinder laterally confined bu the soil. This type of foundation is only recommended in places where the cost of cement is very high, and good aggregates to fabricate concrete are difficult to obtain.

 

Analysis of a Pole Foundation spreadsheet

Analysis of a Pole Foundation spreadsheet

 

POLEFDN” is a spreadsheet program written in MS-Excel for the purpose of analysis of a pole foundation assuming the use of a rigid round pier which is assumed free (unrestrained) at the top and subjected to lateral and vertical loads.

Specifically, the required embedment depth, the maximum moment and shear, the plain concrete stresses, and the soil bearing pressures are calculated.This program is a workbook consisting of six (6) worksheets, described as follows:

Doc – Documentation sheet

Pole Fdn (Czerniak) – Pole foundation analysis for free-top round piers using PCA/Czerniak method

Pole Fdn (UBC-IBC) – Pole foundation analysis for free-top round piers using UBC/IBC method

Pole Fdn (OAAA) – Pole foundation analysis for free-top round piers using OAAA method

Granular Soil (Teng) – Pole foundation analysis in granular soil using USS/Teng method

Cohesive Soil (Teng) – Pole foundation analysis in cohesive soil using USS/Teng method

Program Assumptions and Limitations:

1. Since there is not a universally accepted method for pole foundation analysis, this program offers up five (5) different methods of determining embedment length for pole foundations. The “Pole Fdn(Czerniak)” worksheet is the primary method emphasized in this program, since it provides the most detail in overall analysis. However, it does yield the most conservative embedment depth results of all the methods presented.

2. The references used in the different analysis methods in this program are as follows:

a.”Design of Concrete Foundation Piers” – by Frank Randall Portland Cement Association (PCA) – Skokie, IL, May 1968

b.”Resistance to Overturning of Single, Short Piles” – by Eli Czerniak ASCE Journal of the Structural Division, Vol. 83, No. ST2, Paper 1188, March 1957

c.1997 Uniform Building Code (UBC), Section 1806.8, page 2-45

d.Outdoor Advertising Association of America (OAAA) – New York, NY

e.”Tapered Steel Poles – Caisson Foundation Design” Prepared for United States Steel Corporation by Teng and Associates, July 1969

f.AASHTO Publication LTS-5 – Standard Specifications for Structural Supports for Highway Signs, Luminaries, and Traffic Signals (Fifth Edition, 2009)

Note: references “a” and “b” refer to the “Pole Fdn(Czerniak)” worksheet, while references “e” and “f” refer to both the “Granular Soil(Teng)” and “Cohesive Soil(Teng)” worksheets.

3. The “Pole Fdn(Czerniak)” worksheet assumes that the foundation is short, rigid, meeting the criteria that the foundation embedment length divided by the foundation diameter
4. This program will handle both horizontally as well as vertically applied loads. The vertical load may have an associated eccentricity which results in an additional overturning moment which is always assumed to add directly to the overturning moment produced by the horizontal load.

5. This program assumes that the top of the pier is at or above the top of the ground surface level.

6. This program assumes that the actual resisting surface is at or below the ground surface level. This accounts for any weak soil or any soil which may be removed at the top.

7. The “Pole Fdn(Czerniak)” worksheet assumes that the rigid pier rotates about a point located at a distance, ‘a’, below the resisting the surface. The maximum shear in pier is assumed to be at that ‘a’ distance, while the maximum moment in the pier is assume to be at a distance = ‘a/2′.

8. The “Pole Fdn(Czerniak)” worksheet calculates the “plain” (unreinforced) concrete stresses, compression, tension, and shear in the pier. The respective allowable stresses are also determined based on the strength (f’c) of the concrete. This is done to determine if steel reinforcing is actually required. However, whether minimum reinforcing is to be used or not is left up to the user. The allowable tension stress in “plain” concrete is assumed to be equal to 10% of the value of the allowable compressive stress.

9. The “Pole Fdn(Czerniak)” worksheet calculates the actual soil bearing pressures along the side of the pier at distances equal to ‘a/2′ and ‘L’. The respective allowable passive pressures at those locations are determined for comparison. However, it is left up to the user to determine the adequacy.

10. Since all overturning loads are resisted by the passive pressure against the embedment of the pier, this program assumes that the pier acts in direct end bearing to resist only the vertical loading. The bottom of pier bearing pressure is calculated, which includes the self-weight of the pier, assumed at 0.150 kcf for the concrete

 

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How to Test Compaction of Soil?

How to Test Compaction of Soil?

 

Compaction test is conducted in the laboratory to determine the relation between the dry density and the water content of the given soil compacted with standard compaction energy and to determine the OMC corresponding to the MDD.

The OMC obtained from the laboratory compaction test will help in deciding the amount of water to be used for compaction in the field. The MDD obtained from the laboratory compaction test helps in knowing the dry density achievable in the field compaction and also as a check for quality control.

Based on the compacted energy used in compacting the soil in the laboratory test, the laboratory compaction tests are of two types:

1. Standard Proctor test.

2. Modified Proctor test.

 

1. Standard Proctor Test:

In this test, the soil is compacted in three layers, each layer subjected to blows of a rammer with falling weight of 5.5 pounds (2.6 kgf) falling through a height of 12 in. in a cylindrical mold of internal diameter of 4 in. and effective height of 4.6 in.

The compaction parameters in a standard Proctor test are – W is the weight of rammer blow = 5.5 pounds, h is the height of fall = 12 in. = 1 ft, n is the number of blows per layer = 25, l is the number of layers = 3, and Vm is the volume of the mold = 1/30 cubic ft. The total compaction energy imparted on the soil per unit volume in this test is –

IS – 2720 (Part 7) – 1980 recommends the Indian Standard light compaction method based on standard Proctor test in metric system. The standard Proctor test or IS light compaction can be used as a criteria for the compaction of subgrades on highways and earth dams, where light rollers are used.

 

2. Modified Proctor Test:

 

Advances in construction technology resulted in the development of heavier rollers, which impart higher compaction energy during field compaction. To provide a laboratory control criterion for the higher compaction energy, the modified Proctor test was developed and standardized by the American Association of State Highway (and Transportation) Officials (AASHO or AASHTO) and by US Army Corps of Engineers for airfield construction.

In modified Proctor test, the soil is compacted in the same mold as in standard Proctor test, which has internal diameter of 4 in. and affective height of 4.6 in. giving a total internal volume of 57.805 cubic in. or 1/30 cubic ft. The rammer is bigger with a hammer of weight 10 pounds (4.5 kgf) falling through a height of 18 in. The soil is compacted in five layers, each layer being given 25 blows.

The compaction parameters in modified Proctor test are as follows – W is the weight of hammer blow = 10 pounds, h is the height of fall = 18 in. = 1.5 ft, n is the number of blows per layer = 25,l is the number of layers = 5, and Vm is the volume of the mold = 1/30 cubic ft.

The compaction energy imparted to the soil, per unit volume, in the modified Proctor test is:

Thus, the compaction energy in the modified Proctor test is (56250/12375) 4.55 times that in standard Proctor test.

If the soil fraction retained on 20 mm sieve is more than 5%, a bigger mold of 5.9 in. (15 cm) internal diameter and 5 in. (12.73 cm) internal height giving a total volume of 137.04 cubic in. or 1/12.611 cubic ft (2250 cm3) is used. When the bigger mold is used, the soil is compacted with 56 blows for each layer.

IS – 2720 (Part VII) – 1980 and Part VIII-1983 have recommended procedures corresponding to these two tests as follows:

1. IS light compaction test.

2. IS heavy compaction test.

1. IS Light Compaction Test:

The objective of the IS light compaction test is to determine the relation between the water content and the dry density of compacted soil and to determine the MDD and OMC from this test. The compaction energy used to compact the soil corresponds to that of standard Proctor test.

The compaction parameters in IS light compaction test are as follows – W is the weight of hammer blow = 2.6 kgf, h is the height of fall = 31 cm, n is the number of blows per layer = 25, and I is the number of layers = 3, and Vm is the volume of the mold = 1000 cubic centimeter (cc). The total compaction energy imparted on the soil in this test is –

E = Whnl = 2.6 × 31 × 25 × 3 = 6045 kgf cm

The total compaction energy imparted on the soil per unit volume in this test is –

E = 6045/1000 = 6.045 kgf cm/cm3

2. IS Heavy Compaction Test:

The IS heavy compaction test is similar to IS light compaction text except for the following differences:

1. A heavy rammer of 4.9 kgf falling weight that falls through a height of 45 cm is used for compacting the soil in IS heavy compaction.

2. The soil is compacted in five layers of equal thickness in the compaction mold.

3. The initial water content to be used in the first trial is 3%-5% for sandy and gravelly soils and 12%-16% below plastic limit for cohesive soils.

To increase the accuracy of the test results, it is desirable to reduce the increment of water in the region of OMC.

Shifting and Tilting of Well Foundations

Shifting and Tilting of Well Foundations

 

Shifting and tilting problems occurs generally during sinking process of well foundations. If proper care is not taken, they will cause serious problems and weaken the stability of foundations. Precautions to avoid shifting and tilting, limitations and rectifying methods are discussed below.

Shifting and Tilting of Well Foundations

  • When the well is moved away horizontally from the desired position, then it is called shifting of well foundation.
  • When the well is sloped against vertical alignment,it is called tilting of well foundation.

Precautions to Prevent Shifting and Tilting

It is safer and economical to avoid tilting and shifting of wells by adopting the following preventive measures:

  • The outer surface of the well curb and steining should be level, straight, and smooth.
  • The radius of the well curb should be kept 2-4 cm more than the outer radius of the well steining.
  • The cutting edge should be sharp and of uniform thickness.
  • The steining should be built in lifts and the entire steining height should be built in one straight line from bottom to top at right angles to the plane of the curb.
  • Dredging should be uniform on all sides of the well. For a twin well, dredging should be uniform in both the wells.
  • The well should be constructed in stages of small height increments.
  • The magnitude and direction of sinking of wells should be properly and carefully monitored on a continu­ous basis to identify any tilt or shift and adopt appropriate corrective measures immediately to rectify the same.
  • If the well shows a tendency for tilting, dredging should be done on the higher side. If this does not bring required improvement, sinking should be stopped and should be resumed only after the tilting is corrected.
  • Dredged material should not be deposited unevenly around the well.
  • When a kentledge is used to provide additional sinking effort, it should be placed evenly on the loading platform.

Limitations

  • The maximum tilt allowed in case of well foundation is 1 in 60.
  • The shift in well foundation should not be more than 1 % of depth of sunk.
  • Beyond the above limits, well foundation is considered as dangerous and in such a case, remedial measures to rectify shifting and tilting should be followed.

Rectifying Methods

Rectifying methods for Rectification of shifting and tilting problems in well foundations are as follows:

  1. Eccentric loading
  2. Excavation on higher side
  3. Water jetting
  4. Pulling the well
  5. Using hydraulic jacks
  6. Using struts
  7. Excavation under cutting edge
  8. Wood sleeper under cutting edge

1. Eccentric loading

  • The well tilt can be rectified by placing eccentric loading on the higher side. Higher side is nothing but the opposite side of tilt or lower side.
  • A loading platform is constructed on the higher side and load is placed on it.
  • This eccentric load will increase downward pressure on higher side and correct the tilt.
  • The amount of load and eccentricity is decided based on the depth of sinking.
  • Greater is the depth of sinking of well, larger will be the eccentricity and load.
Fig 1: Eccentric Loading on Well Foundation

2. Excavation on Higher Side

  • When well is tilted to one side, excavation should be increased on the other side which is opposite to tilted side.
  • This technique is useful only in the initial stages of well sinking.
Fig 2: Excavation on Higher Side

3. Water Jetting

  • Water jetting on external surface of well on the higher side is another remedial measure for rectifying tilt.
  • When water jet is forced towards surface of well, the friction between soil and well surface gets reduced and the higher side of well becomes lowered to make well vertical.
Fig 3: Water Jetting on Higher Side

4. Pulling the Well

  • The well can be pulled towards higher side using steel ropes.
  • One or more steel ropes are wound around the well with wooden sleepers packed in between well and ropes to prevent damage to the well steining by distributing load over to larger area of steining.
  • Pull should be carefully done otherwise,shifting of well foundation may occur.
Fig 4: Pulling the Well Foundation

5. Pushing using Jacks

  • Another method to rectify tilting and shifting of well foundation is using hydraulic jacks or mechanical jacks, the tilted well can be pushed from lower side to higher side.
  • Neighbor vertical well foundations or suitable arrangements made will give support to the jack system.
  • Care should be taken while pushing the well otherwise the well may shifts.
Fig 5: Pushing using Jacks

6. Using Struts

  • By providing struts as supports on the lower side or tilted side of well, further tilting can be prevented.
  • Wooden sleepers are provided between struts and well steining to prevent damage to well steining and to distribute pressure to larger area.
  • Struts are rested on firm base having driven piles.
Fig 6: Strutting the Well From Lower Side

7. Excavation under Cutting Edge

  • This technique is used for hard strata soils. In this method, the well is de-watered first and open excavation is carried out exactly under the cutting edge on the higher side.
  • If de-watering is not possible, soil strata is loosened using suitable equipment with the help of professional divers.

8. Wood Sleeper under Cutting Edge

  • If tilting towards lower side is increasing,then wooden sleepers are placed under cutting edge on lower side to control the tilting temporarily.
  • When well is corrected to vertical level, this sleepers can be removed.
Fig 7: Wood Sleeper under Cutting Edge
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