Building Foundation

Foundations / footing are the structures supporting the wall or columns to transmit or distribute the load of the super structure to the soil / stratum below to it.

Types of foundations:

A. Shallow Foundations
Shallow foundation transmits the weight on to immediate subsoil. Shallow foundation may be of following types.
1. Isolated / pad / spread / single column footing: 

2. Combined column footing:

3. Strap footings or Cantilever

4. Continuous foundation

5. Mat foundation or Raft foundation

6. Strip wall foundation                                               
B. Deep Foundations

A deep foundation will transmit the load on to inner hard soil or bedrock. It is usually at depths of 3 meters below finished ground-level. The deep foundation may be classified following types.
1.Pile foundation

2.Pier foundation

3.Caissons or Well foundation

Regarding back filling:

As per IS : 1904 - 1986: CODE OF PRACTICE FOR DESIGN AND CONSTRUCTION OF FOUNDATIONS IN SOILS : GENERAL REQUIREMENTS  : The refilling of the excavation shall he done with care so as not to disturb the constructed foundation, and shall be compacted in layers not exceeding 15cm thick with sprinkling of minimum quantity of water necessary for proper. compaction.

As per IS 1200:
a. Compact each layer of earth not exceeding 200mm.
b. Earth / soil should be free of vegetation/twigs if any.
c. Rolling should be done with 5 ton mechanical roller after sprinkling enough water and core cutter samples taken for every 150Smtr.
d. Proceed to next layer after achieving minimum of 95% of MDD(maximum dry density).

As per Hilton Head Public Service District’s, http://www.hhpsd.com/

BACKFILLING FOR UTILITIES: 

1. Backfill trenches and excavations immediately after the pipe are laid, unless other protection is directed or indicated. 

2. Select and deposit backfill materials with special reference to the future safety of the pipes. 

3. In lower portion of trench, deposit approved backfill and bedding material in layers of 6" maximum thickness, and compact with suitable tampers to the density of the adjacent soil until there is a cover of not less than 12" using special care not to damage pipe and pipe coatings. 

4. Except for special materials for pavements, backfill the remainder of the trench with material free from stones larger than 6" or ½ the layered thickness, whichever is smaller, in any dimension. 

5. Under roads, streets and other paved area: 

a. Mechanically tamp in 6" layers using heavy duty pneumatic tampers or equal. 

b. Tamp each layer to a density equivalent of not less than 100% of an ASTM D698 Proctor Curve. 

c. Provide additional compaction by leaving the backfilled trench open to traffic while maintaining the surface with crushed stone. 

d. Refill any settlement with crushed stone and continue such maintenance until replacement of pavement is authorized by the Engineer.

BACKFILLING AND COMPACTION FOR STRUCTURES

1. Use suitable material for all filling and backfilling operations:  Provide suitable material free from organic matter and deleterious substances, containing no rocks or lumps over 6" in greatest dimension, and with not more than 15% of the rocks or lumps larger than 2½” in their greatest dimension.

2. Fill under structures: Deposit suitable materials in layers not exceeding 8" in depth and compact each layer using proper equipment.

a. Do not place rock that will not pass through a 6" diameter ring within the top 12" of the surface of the completed fill or rock that will not pass through a 3" diameter ring within the top 6" of the completed fill.

b. Do not place broken concrete, bricks, or asphaltic pavement in fills.

3. Backfill excavations as promptly as progress of the Work permits, but not until completion of the following:

a. Inspection and acceptance of construction below finish grade including, where applicable, dampproofing and waterproofing.

b. Inspecting, testing, approving and recording locations of underground utilities.

c. Removing concrete form work.

d. Removing shoring and bracing, and backfilling of voids with satisfactory materials. e. Removing trash and debris.

f. Cast in place foundation walls have been in place seven days

4. Placing and compacting:

a. Place backfill and fill materials in layers not more than 8" in loose depth.

b. Before compacting, moisten or aerate each layer as necessary to provide the optimum moisture content with ±2%.

c. Compact each layer to required percentage of maximum density for area.

d. Do not place backfill or fill material on surfaces that are muddy, frozen, or containing frost or ice. e. Place backfill and fill materials evenly adjacent to structures, to required elevations.

f. Take care to prevent wedging action of backfill against structures by carrying the material uniformly around the structure to approximately the same elevation in each lift.

g. Do not operate heavy equipment closer to foundation or retaining walls than a distance equal to height to backfill above the footing. Compact remaining area using power driven hand tampers.

h. Where the construction includes basement or other underground walls having structural floors over them, do not backfill such walls until the structural floors are in place and have attained sufficient strength to support the walls.

5. Soil Compaction Requirements:

a. Compact soils to not less than the following percentages of maximum dry density as determined in accordance with ASTM D698, Method A (Standard Proctor).

b. Existing in place subgrade below structures where subgrade has been disturbed by water, improper dewatering, or construction traffic: Top 12" of subgrade - 100% and Below top 12" of subgrade 98%

c. Fill beneath structures and beneath an area extending 10 feet beyond the limits of the foundation: Top 12" of subgrade - 100%

d. Compaction of suitable material used to replace unsuitable material below foundation subgrades: Top 12" of subgrade - 100% and Below top 12" of subgrade 98%.

Many thanks to google.com and http://www.hhpsd.com/, Doc88.com, from where the details were gathered.