Soil investigation for building construction

Soil investigation for building construction

Soil investigation for building construction: Soil investigations are carried out to understand properties of soil and types of foundation suitable for the structure under consideration. In this article various types of soil investigations, their reports, various types of foundations for various types of soil are discussed. Sub Surface Soil Investigation

Sub-Soil conditions are examined using test borings provided by soil engineer(geotechnical). Number of borings and location of borings depend on building type and site conditions and as required by structural consultants.

Locations of borings samples are indicated on drawing. These are not taken directly under proposed columns. Borings indicate depth, soil classification (according to the unified soil system) and moisture content and sometimes ground water level is shown as well (Physical Properties), particle size, moisture content, density etc.)

Soil investigation for building construction
Soil investigation for building construction

Sub-Surface soil investigation Report

Recommendation should be based on testing of materials obtained from onsite borings and to include:

Also Read: High rise building for residential use

Bearing Capacity of Soil

Foundation design recommendations

Paving design recommendations

Compaction of soil

Lateral strength (active, passive and co-efficient of friction)

Permeability

Frost depth

Surface Soil Investigation

Surface soil investigations are required for construction for following cases:

High water table

Presence of trouble soils: Peat, soft clay, loose site or fine water bearing sands

Rock close to the surface (require Blasting for excavation.)

Dumps or fills

Evidence of slides or subsidence

Above ground indicator of soil conditions:

Near buildings -require shoring

Rock outcropping- indicate bed rock, good for bearing and frost resistance, bad for excavation.

Water (lake)-indicate high water table; some waterproofing of foundation will be required.

Level terrain-easy site work, fair bearing but poor

drainage

Gentle slopes – easy to work and excellent drainage.

• Convex terrain (ridge)-dry solid place to build.

Concave terrain(valley)-wet soft place to build Steep terrain-costly excavations, potential erosion and

sliding soil.

Foliage- some trees indicate moist soil; large trees indicate

solid ground.

Soil Classifications Engineers dealing with soil mechanics devised a simple classification system that will tell the engineers properties of a given soil. The unified soil classification system is based on identifying soils according to their textural and plasticity qualities and on their grouping with respect to behavior. Soils are usually found in nature as mixtures with varying proportion of particles of different sizes, each of these components contribute to the soil mixture.

Soil is classified on the basis of:

Percentage of gravel, sand and fines Shape of grain,

Plasticity and compressibility characteristics of soil In the unified soil classification system, the soil is given a descriptive name and a letter symbol indicating its principal characteristics. Placement of solid into its respective group is accomplished by visual examination and laboratory tests. In the unified soil classification, the terms cobbles, gravel, sand and fines (silt or clay) are used to designate the size ranges of soil particles.

Soil particle size ranges from largest to smallest: 1.0 Cobbles

2.0 Gravel (Coarse + Fine)

3.0 Sand (Coarse + Medium + Fine)

4.0 Fines consisting of clay or silt

Soil shear strength is made up of cohesion (water content, how sticky it is) and internal friction (based on size of grains) This is determined by triaxle compression testing.

Coarse Grained – are soils which composed of gravel or sands and which contain a wide variety of particles. These are most suitable for foundations when well drained and well confined. They are soils with good bearing value. Particularly The G series (GW, GP, GM GC) Identified on the basis of the percentage amount of gravel and sand. Fine Grained- are soil that are silt and clays (L, H) contain smaller particles of silt and clay. These are suitable for foundation but require compactions. The most suitable of this series (L) is the CL. These soils are identified based in their cohesive properties and permeability.

Highly Organic – are soil that are usually very compressible and are not suitable for construction. They contain particles of leaves, grass and branches peat, humus and swamp soil with highly organic texture are typical of this group (Pt). these are identified readily on the basis of color, texture and odor. Moisture content is also very high in this type of soil. Soil names shown on the unified soil classification system are associated with certain grain size and textural properties. This is the case for the course-grained soils. For silts and clay, the names are based on plasticity basis of the soil.

Relevant information of sample taken by borings which can aid the geotechnical engineer in determination of foundation includes 1.0 For coarse grain soil -The size of the particles mineralogical composition, shape of grains, and character of the binder

2.0 For fine grained soils -strength, moisture and plasticity In the preliminary stages a visual inspection can determine the behavior of the soil when used as component in the construction of a proposed building, Soil can be classified according to the classification of the unified soil classification system (later on laboratory testing can be performed)

Soil Problems

The problem of uplift pressure in soil can be reduced by having well drained and free draining gravel (GW, GP). Uplift pressure can occur in fine grained soils consisting of silts and clays, such soils can cause heaving of foundations and formation of boils.

Strength and consolidation which make up the compaction characteristic of the soil determines its stability for building foundations.

Due To Potential Frost Action

Regardless of the frost susceptibility of the various soil groups, two conditions must be present simultaneously before frost action will be a consideration – a source of water during the freezing period and a sufficient period of the freezing temperature to penetrate the ground.

In general silts and clay (ML, CL, OL) are most susceptible to freezing (as they contain moisture). Well drained granular soils are less susceptible to freezing and creating foundation problems.

Due To Drainage Characteristics

The drainage characteristics of soil are a direct reflection of their permeability. The presence of moisture in base, sub-base and sub grade materials may cause the development of pore water pressure and loss of strength.

The gravelly and sandy soils with little or no fines (GW, GP, SW, SP) have excellent drainage characteristics

Fine grained soils and highly organic soils have poor drainage characteristics

Compaction OF soil

The sheep’s foot and rubber-tired rollers are common pieces of equipment used to compact soils. Some advantage is claimed for the sheep’s foot roller in that it leaves a rough surface that afford better

bond between layers.

Granular soils consisting of well graded materials (GW, SW) furnish better compaction results than the poorly soil (GP, SP) Fine grained Soils can also be compacted For most construction projects of any magnitude, it is highly desirable to investigate the compaction characteristics of the soil be means of a field test section.

Suitability of soils for foundations depends preliminary on the strength, cohesion and consolidation characteristics of the soils. The type of structure load and its use will largely govern the adaptability of a soil as a satisfactory foundation material.

A soil might be entirely satisfactory for one type of construction but might require special treatment for other building.

• In general gravel and gravelly soils (GW, GP, GM, GC) have good bearing capacity and undergo little consolidation under load.

• Well graded sands (SW) usually also have good bearing capacity. • Poorly graded sands and silty sands (SP, SM) have variable capacity based on their density.

Some soils containing silts and clays (ML, CL, OL) are subject to liquefaction and may have poor bearing capacity and large settlements when subject to loads. Of the fine-grained soil group

CL is probably the better for foundations.

Organic soils (OL and OH) and highly organic soil (Pt) have poor bearing capacity and usually exhibit large settlement under load. The soil investigation report with recommendation for foundation type are given to structure consultant by soil engineer (Geo technical) for the design of various structures in a particular project.

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