1. Foundation & Timbering of Trenches
Lahiru Rajapakshe
33136
Diploma in QS
Technical college-Maradhana

2. Content Foundation Function of foundation Types of foundation
To where each type of foundation can apply
De wattering of foundation trenches
Damp proofing course
Timbering of trenches

3. 01.Foundation Definition of foundation
Foundation is the lower portion of the building usually located below ground level, which transmits the loads of the super structure to the supporting soil. A foundation is therefore that part of the structure which is in direct contact with the ground to which loads are transmitted.

4. Foundations mainly can devided into two. Shallow Foundation
Shallow foundation are those foundations in which the depth at which the foundation is placed is less than the width of the foundation (D < B). Shallow foundations are generally termed as spread footing as they transmit the load of the super structure laterally into the ground.
Deep Foundation
Deep Foundation are those foundations in which the depth of the foundation is greater than its width (D>B). The D/B ratio is usually 4-5 for deep foundation. Unlike shallow foundation, the deep foundation transmits the load of the superstructure vertically to the rock strata lying deep. Deep foundations are used when the shallow foundation cannot support the load of the structure.

5. 2.Function of foundation
1. Reduction of load intensity
Foundation distributes the loads of the super structure, to a larger area so that the intensity of the load at its base (i.e. total load divided by the total area) does not exceed the safe bearing capacity of the sub-soil.
2. Even distribution of load
Foundations distribute the non-uniform load of the super structure evenly to the sub soil. For example, two columns carrying unequal loads can have a combined footing which may transmit the load to sub soil evenly with uniform soil pressure. Due to this, unequal or differential settlements are minimized.
3. Provision of level surface
Foundation provide leveled and hard surface over which the super structure can be built.
4. Lateral stability
It anchors the super structure to the ground, thus imparting lateral stability to the super structure. The stability of the building, against sliding and overturning, due to horizontal forces (such as wind, earthquake etc.) is increased due to foundations.
5. Safety against undermining
It provides the structural safety against undermining or scouring due to burrowing animals and flood water.
6. Protection against soil movements
Special foundation measures prevents or minimizes the distress (or cracks) in the super structure, due to expansion or contraction of the sub soil because of moisture movement in some problematic soils.

6. 3.Types of foundation Strip foundation Rubble foundation
Column foundation
Pad foundation
Short bored foundation
Pile & beam foundation
Raft foundation
Step foundation

7. Strip Foundation
Strip foundations are a type of shallow foundation that are used to provide a continuous, level strip of support to a linear structure such as a wall or closely-spaced rows of columns built centrally above them.Strip foundations can be used for most subsoils, but are most suitable for soil which is of relatively good bearing capacity. They are particularly suited to light structural loadings such as those found in many low-to-medium rise domestic buildings - where mass concrete strip foundations can be used. In other situations, reinforced concrete may be required.
Typical strip foundation measurements are as follows:
Single storey building: 450 mm wide and at least 200 mm deep.
Two storey building: 600 mm wide and 200 mm deep.
The underside of strip foundations should be deep enough to avoid frost action; for example, at least 450mm unless they are bearing on rock, and at least 1m on high shrinkage clays.
Deep strip foundations may be necessary where soil with a suitable bearing capacity is deeper.
Wide strip foundations may be required where the soil is soft or of a low bearing capacity, so as to spread the load over a larger area. Wide strip foundations will typically require reinforcement

8. .

9. Reinforced beam and concrete strip foundation
Plain concrete strip foundation
Reinforced concrete strip foundation
Reinforced beam and concrete strip foundation

10. 2.Rubble foundation.
The rubble trench foundation, an ancient construction ,is a type of foundation that uses loose stone or rubble to minimize the use of concrete and improve drainage. It is considered more environmentally friendly than other types of foundation because cement manufacturing requires the use of enormous amounts of energy. However, some soil environments are not suitable for this kind of foundation; particularly expansive or poor load-bearing (< 1 ton/sf) soils. A rubble trench foundation with a concrete grade beam is not recommended for earthquake prone areas
A foundation must bear the structural loads imposed upon it and allow proper drainage of ground water to prevent expansion or weakening of soils and frost heaving. While the far more common concrete foundation requires separate measures to insure good soil drainage, the rubble trench foundation serves both foundation functions at once.
To construct a rubble trench foundation a narrow trench is dug down below the frost line. The bottom of the trench would ideally be gently sloped to an outlet. Drainage tile, graded 1":8' to daylight, is then placed at the bottom of the trench in a bed of washed stone protected by filter fabric. The trench is then filled with either screened stone (typically 1-1/2") or recycled rubble. A steel-reinforced concrete grade beam may be poured at the surface to provide ground clearance for the structure.

11. The rubble-trench foundation is a relatively simple, inexpensive, and environment-friendly alternative to a conventional foundation, but may require an engineer's approval if building officials are not familiar with it. Frank Lloyd Wright used them successfully for more than 50 years in the first half of the 20th century, and there is a revival of this style of foundation with the increased interest in green building.
If an insulated slab is to be poured inside the grade beam, then the outer surface of the grade beam and the rubble trench should be insulated with rigid XPS foam board, which must be protected above grade from mechanical and UV degradation.

12. 3.Column foundation
Column Footing is also famous as independent footing. An independent footing is one which is provided under a column or either similar member for distributing concentrated load in the form of uniformly loads on the soil below. The footing may be square, rectangular or circular in plan. Depending upon the load to be carried and the bearing capacity of soil, this may be of brick masonry, stone, R.C.C., steel grill-age etc. On account of low bending strength the footings constructed with brick, stone or plain concrete require considerable depth to be safe to carry heavy loads. The depth of plain concrete footing can be appreciably reduced by providing reinforcements at its base to take up tensile stresses. R.C.C. column footings may be circular, rectangular or square in plan. The footing is reinforced both-ways by means of mild steel ribbed bars placed at right angles to one another at equal distances apart.

14. 4.Pad foundation
Pad foundations are used to support an individual point load such as that due to a structural column. They may be circular, square or reactangular. They usually consist of a block or slab of uniform thickness, but they may be stepped or haunched if they are required to spread the load from a heavy column. Pad foundations are usually shallow, but deep pad foundations can also be used.

15. 5. Short bored foundation
These are a form of foundation which are suitable for domestic loadings and clay subsoils where ground movements can occur below the 1„000 depth associated with traditional strip and trench fill foundations. They can be used where trees are planted close to a new building since the trees may eventually cause damaging ground movements due to extracting water from the subsoil and root growth. Conversely where trees have been removed this may lead to ground swelling.

16. 6. Pile & beam foundation
A pile is basically a long cylinder of a strong material such as concrete that is pushed into the ground to act as a steady support for structures built on top of it.
Piles may be classified by their basic design function (end-bearing, friction or a combination) or by their method of construction (replacement (driven) or displacement (bored)).
End-bearing piles develop most of their friction at the toe of the pile, bearing on a hard layer. The pile transmits load direct to firm strata, and also receives lateral restraint from subsoil.
Friction (or floating) piles develop most of the pile-bearing capacity by shear stresses along the sides of the pile, and are suitable where harder layers are too deep. The pile transmits the load to surrounding soil by friction between the surface of the pile and soil, which in effect lowers the bulb of pressure.

17. 7. Raft foundation
Raft foundations (sometimes known as Mat Foundations) are a large concrete slab which can support a number of columns and walls.The slab is spread out under the entire building or at least a large part of it which lowers the contact pressure compared to the traditionally used strip or trench footings.Because of the speed and volume of houses required after the second world war, the raft foundation was widely used. The raft foundation was cheaper, easier to install and most importantly, did not require as much excavation as the usual strip foundations.
When the Building Regulations were introduced in 1965 there were no generic rules for raft foundations as there were for strip foundations.This meant that to use a raft foundation, it had to be designed and approved by Building Control. This made the entire operation much more difficult and time consuming so raft foundations became less widely used almost overnight.

18. When Raft Foundations Are Used?
A raft foundation is usually preferred under a number of circumstances:
it is used for large loads, which is why they are so common in commercial building which tend to be much larger, and therefore heavier, than domestic homes
The soil has a low bearing capacity so the weight of the building needs to be spread out over a large area to create a stable foundation
The ratio of individual footings to total floor space is high. Typically if the footings would cover over half of the construction area then raft foundation would be used
If the walls of the building are so close that it would cause the individual footings to overlap, then raft foundations should be used

19. 8.Step foundation
A foundation constructed in a series of steps that approximate the slope of the bearing stratum. The purpose is to avoid horizontal force vectors that might cause sliding.

20. 4. To where each type of foundation can apply
To selection of a particulaly types of foundation often based on a number of factors, such as:
1. Adequate depth
The foundation must have an adequate depth to prevent frost damage. For such foundations as bridge piers, the depth of the foundation must be sufficient to prevent undermining by scour.
2. Bearing capacity failure
The foundation must be safe against a bearing capacity failure.
3. Settlement
The foundation must not settle to such an extent that it damages the structure.
4. Quality
The foundation must be of adequate quality so that it is not subjected to deterioration, such as from sulfate attack.
5. Adequate strength
The foundation must be designed with sufficient strength that it does not fracture or break apart under the applied superstructure loads. The foundation must also be properly constructed in conformance with the design specifications.
6. Adverse soil changes
The foundation must be able to resist long-term adverse soil changes. An example is expansive soil, which could expand or shrink causing movement of the foundation and damage to the structure.

21. 7. Seismic forces
The foundation must be able to support the structure during an earthquake without excessive settlement or lateral movement.
Based on an analysis of all of the factors listed above, a specific type of foundation (i.e., shallow versus deep) would be recommended by the geotechnical engineer.
The image given below can be used as a guide for selection of an appropriate type of foundation based on different soil conditions.

24. 5. De wattering of foundation trenches
Purposes for Dewatering
For construction excavations or permanent structures that are below the water table andare not waterproof or are waterproof but are not designed to resist the hydrostatic pressure
Permanent dewatering systems are far less commonly used than temporary or construction dewatering systems
Common Dewatering Methods
1. Horizontal drainage
2. Well points
3. Deep wells
1.Horizontal drainage
The installation of horizontal dewatering systems is relatively easy. A trencher installs an unperforated pipe followed by a synthetic or organic wrapped perforated pipe. The drain length is determined by the drain diameter, soilconditions and the water table. In general drain lengths of 50 meters is common. After installation of the drainpipe a pump is connected to the drain. After the water table has been lowered, the intended construction can start. After the construction is finished the pumps are stopped, and the water table will rise again. Installation depths up to 6 meters are common.

25. 2.Wellpoints
Wellpoints are small-diameter (about 50 mm) tubes with slots near the bottom that are inserted into the ground from which water is drawn by a vacuum generated by a dewatering pump. Wellpoints are typically installed at close centers in a line along or around the edge of an excavation. As a vacuum is limited to 0 bar, the height to which water can be drawn is limited to about 6 meters (in practice).Wellpoints can be installed in stages, with the first reducing the water level by up to five meters, and a second stage, installed at a lower level, lowering it further.The water trickling between the deep wells may be collected by a single row of well point at the toe. This method ensures a much thicker width free from seepage forces.
Wellpoint spears are generally used to draw out groundwater in sandy soil conditions and are not as effective in clay or rock conditions. Open pumps are sometimes employed instead of spears if the ground conditions contain significant clay or rock content.

26. 3.Deep wells
A deep well typically consists of a borehole fitted with a slotted liner and an electric submersible pump. As water is pumped from a deep well, a hydraulic gradient is formed and water flows into the well forming a cone of depression around the well in which there is little or no water remaining in the pore spaces of the surrounding soil. Deep wells work best in soils with a permeability of k = 10−3 m/s to 10−5 m/s; the amount of drawdown that a well can achieve is limited only by the size of the fish pump.
Deep wells can be installed in a ring around an excavation to lower the water level and maintain a safe, dry site. Several equations can be used to design deep well dewatering systems, however many of these are based on empirical data and occasionally fail. Practice and experience, along with a firm understanding of the underlying principles of dewatering, are the best tools for designing a successful system.Some dewatering situations "are so common that they can be designed almost by rule of thumb".
Deep wells are also used for aquifer testing and for groundwater drainage by wells

27. 6.Damp proofing course
Damp proofing in construction is a type of moisture control applied to building walls and floors to prevent moisture from passing into the interior spaces. Damp problems are among the most frequent problems encountered in homes.
Damp proofing is accomplished several ways including:
A damp-proof course (DPC) is a barrier through the structure designed to prevent moisture rising by capillary action such as through a phenomenon known as rising damp. Rising damp is the effect of water rising from the ground into property.The damp proof course may be horizontal or verticalA DPC layer is usually laid below all masonry walls, regardless if the wall is a load bearing wall or a partition wall.
A damp-proof membrane (DPM) is a membrane material applied to prevent moisture transmission. A common example is polyethylene sheeting laid under a concrete slab to prevent the concrete from gaining moisture through capillary action. A DPM may be used for the DPC.
Integral damp proofing in concrete involves adding materials to the concrete mix to make the concrete itself impermeable.

28. Surface coating with thin water proof materials for resistance to non-pressurized moisture such as rain water or a coating of cement sprayed on such as shotcrete which can resist water under pressure.
Cavity wall construction, such as rainscreen construction, is where the interior walls are separated from the exterior walls by a cavity.
Pressure grouting cracks and joints in masonry materials.
Materials widely used for damp proofing include
Flexible materials like butyl rubber, hot bitumen, plastic sheets, bituminous felts, sheets of lead, copper, etc.
Semi-rigid materials like mastic asphalt
Rigid materials like impervious bricks, stones, slates, cement mortar or cement concrete painted with bitumen, etc.
Stones
Mortar with waterproofing compounds
Coarse sand layers under floors
Continuous plastic sheets under floors

29. Masonry construction
A DPC is a durable, impermeable material such as slate, felt paper, metal, plastic or special engineered bricks bedded into the mortar between two courses of bricks or blocks. It can often be seen as a thin line in the mortar near ground level. To create a continuous barrier, pieces of DPC or DPM may be sealed together. In addition, the DPC may be sealed to the DPM around the outside edges of the ground floor, completely sealing the inside of the building from the damp ground around it.
In a masonry cavity wall, there is usually a DPC in both the outer and inner wall. In the outer wall it is normally 150 millimetres (5.9 in) to 200 millimetres (7.9 in) above ground level (the height of 2-3 brick courses). This allows rain to form puddles and splash up off the ground, without saturating the wall above DPC level. The wall below the DPC may become saturated in rainy weather. The DPC in the inner wall is usually below floor level, (under a suspended timber floor structure), or, with a solid concrete floor, it is usually found immediately above the floor slab so that it can be linked to the DPM under the floor slab. This enables installation of skirting boards above floor level without fear of puncturing it. Alternatively, instead of fitting separate inner and outer DPCs, it is common in commercial housebuilding to use a one-piece length of rigid plastic, (albeit an angled section), which fits neatly across the cavity and slots into both walls (a cavity tray). This method requires the need for weep vents to enable rainwater ingress to drain from the cavities otherwise rising dampness could occur from above the DPC.

31. 7. Timbering of trenches
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.

32. 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.

33. 2. Box sheeting.
This method is adopted in loose soils, when the depth of excavation does not exceed 4 metres. Fig (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 (b)]. If the height is more, braces are also provided along with struts.

34. 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 (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).

35. 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

36. 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.

37. References https://www.designingbuildings.co.uk/wiki/Strip_foundation