Soil Investigations REPORT

Running head: SOIL 1

FOUNDATION, SITE AND SOIL INVESTIGATION

Student’s Name

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Date

SOIL 2

Introduction

Soil is investigated to comprehend different properties that makes up the soil this is

important when one is about to start constructions (Ramachandran, 2010). This article is

purposed to examine soils and determine the suitable foundations.

Types of Soil’s Investigation for Different Foundation Selection

Subsurface Soil Investigations

This is often done by a soil engineer using Test-boring method. The type and site for

these borings often depend on the kind of building and soil condition in the area. Normally, the

borings are often spaced from 100 to 150′ apart. However, they may be spaced 50’ apart for any

detailed job where soil footing is closely spaced, and soil condition is not even. This means that

less boring sample are required where widely spaced columns would be located (Ramachandran,

2010). Borings indicate the soil’s depth, classification, moisture content, water from the ground

level and other physical property. In summary, the report recommendation includes:

• Comportment capacity of soil.

• Paving designs

• Recommended foundation design

• Soil compaction

• Lateral strength etc.

Surface Investigations

This kind of analysis is required to ascertain cases like:

A) Water table that is too high: This is indicated by the presence of soil know as trouble (e.g.

peat and soft-clay); close-to-surface rocks; fills or dumps; and slides.

B) Above ground indicators of soil conditions

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The indicators include:

 The Foliage – some trees indicate moist soil. Large trees indicate solid ground.

 Near the Buildings – require shoring or earth and existing foundations.

 The Concave Terrain (Valley) – the soft wet place to build.

 The Convex Terrain (Ridge) – the dry solid place to build.

 The Steep Terrain – costly excavations, potential erosion, and sliding soils.

 The Rock Outcropping – indicate bedrock, good for bearing and frost resistance, bad for

excavations.

 Level of Terrain – easy site work, fair bearing, but poor drainage.

 The Water (lake) – this normally shows the high water table, on this case waterproofing

of foundations is a must while laying a foundation.

 The Gentle Slopes – this is the best site and easiest to work on, the drainage is very

good.

 Terrain that is steep – potential erosion, costly excavations, and sliding soils.

How can the Soil be classified

Soil may be classified based on their plasticity and their texture, and behavior according

to the unified system of the classifications.

Compressibility and Plasticity

In the combined soil grouping, the terms like sand gravel, cobbles, and fines (silt or clay)

are used to estimate the sizes and range of soil particles in a particular area. Soil is typically

arranged from the smallest to biggest. Cobbles is the largest followed by gravel and coarse then

clay or silt; and they are highly-organic (Kang & Tripathi, 2009). Soil strength is determined

through the cohesion of its particles and internal friction which is determined by the size of

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grains (Holmes, 2016). The triaxle compression method subdivides soils into fine-grained, highly

organic, and coarse-grained,

Coarse-gained soils are made up of gravel and sands thus making them most appropriate

for foundations when the place is well drained and confined appropriately. This is a soil with

good bearing value because they carry large percentage of gravel and sand. The fine-grained

types are also suitable in foundational constructions due to their cohesive properties and

permeability. In general, the fine-grained are preferred for foundation due to their strength,

moisture and plasticity while the coarse-grained are considered based on the size of the particles,

mineralogical composition, shape of grains, and character of the binder. Visual inspection

determines the behavior of the soil when used as the component in the construction of the

proposed building (Holmes, 2016). Strength and consolidation which make up the compaction

characteristics of the soil determine its suitability for building foundations.

Soil Issues

The soil uplift pressures can be reduced by having well drained and free draining gravels

(GW, GP). This phenomenon is common in fine-grained soils and may lead to heaving of

foundations and formation of boils. Frost action will often occur if and when a source of water

exists during the freezing period and enough time for the freezing temperature to penetrate the

ground. Silts and clays are more susceptible to freezing since they contain moisture (Upadhyaya,

Patel, & Patel, 2014). However, well-drained granular soils are less susceptible to freezing and

creating foundation problems.

In general therefore, the gravelly and sandy soils have excellent drainage characteristics

compared to the fine-grained soils due to high permeability.

Foundation types

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The simple spread footing foundation is suitable for the fine-grained soils because it

mostly depends on the magnitude of the load. If the soil is poor and structural loads are relatively

heavy, then alternate methods are required. Pile foundations might be required in some cases

where fine cohesive silt and clay soil is present (Understand Construction Newsletter, 2017).

Sometimes it might be desirable and economically advantageous to over-excavate soils that are

not offering any bearing capacity then they might be refilled and compacted again. A suitable

foundation system, beating capacity, minimum depth and design of the foundation is determined

through the boring experiment.

A safe bearing capacity of soil equals to the ultimate bearing capacity divided by a safety

factor (usually 2-4). Ultimate bearing capacity is defined as the maximum unit pressure a soil can

sustain without permitting large amounts of settlements (Holmes, 2016). Bedrock has the highest

safe bearing capacity. Well, graded gravel and sand that are confined and drained have a safe

bearing capacity of 3,000 – 12,000 PSF. Silts and clays have the lowest safe bearing capacity of

1,000 – 4,000 PSF.

Role of Foundations

 Transfer the building load to the ground.

 Anchor building against wind and seismic load.

 Isolate building from frost heaving.

 Isolate building from expansive soils.

 Holds building from moisture.

 Provide living spaces (basement, storage).

 Houses mechanical systems.

Foundation configurations are Slab on Grade, Crawl Space, and Basement.

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Foundation Types

The Bridge-foundation type is used for most buildings where the loads are light and there

are strong shallow soils. At columns, there are single spot square pads where bearing walls have

an elongation form. These are almost always reinforced. These footings deliver the load directly

to the supporting soils. Area of Spread footing is obtained by dividing the applied force by the

soils safe bearing capacity (f=P/A). Low-rise buildings (1-4 Stories) require firm soil conditions

that are capable of supporting the building in the area of the spread footings. When needed,

columns can be connected with grade beams to provide more lateral stability against earth-

quakes. These

Spread footings should be above the water table. Concrete spread footings are at least as

thick as the width of the stem (Ramachandran, 2010). As the weight of the building increases the

bearing capacity or depth of good bearing soil will decrease hence the need for constant

reinforcement using drilled piers and/ or caissons.

Pile Foundations is applicable for expansive compressive soils with heavy loads where

deep soils cannot take the building load and where the soil is in better capacity if excavated deep

below. Friction piles rely on resistance from skin pile against the soil and would be used where

there is no reasonable bearing stratum. On the other hand, end-bearings transfer directly to the

soil of good bearing capacity, which depends on either the structural strength of the pile itself or

the strength of the soil (Understand Construction Newsletter, 2017). These piles could be wood,

steel, reinforced concrete, or cast in place of concrete piles.

Mat Foundations consists of reinforced concrete raft or mats and it is used for small light

load buildings on very weak or expansive soils such as clays. The mats allow the building to

float on or in the soil like a raft. These kinds of concretes could be where the soil requires a large

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bearing area. The feet of the buildings are usually large and expansive. It is normally preferable

for buildings 10-20 stories tall where it provides resistance against overturning (Holmes, 2016).

It is used instead of piles because it can be less expensive, less obtrusive and can usually be used

over expansive clays, silts to let foundation settle without great differences.

General Summary of Soil Investigation and Types of Foundation

Ranking of Soil for foundations:

 Sand & Gravel – Best

 Medium & Hard Clays – Good

 Silts & Soft Clays – Poor

 Organic Silt and Clays – Undesirable

 Peat – Unsuitable

The higher the Plasticity Index Cohesiveness of the soil, the higher the potential for

shrinkage and swelling characteristic of the soil (e.g. clay soils). Coarse soils are granular soils

consisting of gravel and sands. Cohesive soils are usually organic in nature made up of silts and

clays. Differential settlements in concrete foundations should be limited to ¼ to ½” maximum.

The cost of foundations is usually 5% of total construction cost. Foundational is most

economical where safe bearing capacity is at least 3000 PSF. The common types of foundations

include Spread Footings, Mat foundation, Bridge and Pile foundation (Ramachandran, 2010).

Piles are most expensive, 2 or 3 times the price than Spread Footings. However, Mat foundation

would be suitable for small to medium buildings comprising of 10-20 storey high.

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References

Holmes, M. (2016, July 2016). Foundation Systems and Soil Types. Retrieved May 01, 2018,

from Homebuilding and Renovation: https://www.homebuilding.co.uk/foundation-

systems-and-soil-types/

Kang, B., & Tripathi, P. (2009). Technical paper 1: Soil classification and characterization.

Retrieved May 01, 2018, from FAO Corporate Document Repository:

http://www.fao.org/wairdocs/ilri/x5546e/x5546e04.htm

Ramachandran, V. (2010, June 27). Soil Investigations for Foundations. (L. Stonecyphe, Editor)

Retrieved May 01, 2018, from Bright Hub Engineering:

https://www.brighthubengineering.com/geotechnical-engineering/75575-soil-

investigations-for-foundations/

Understand Construction Newsletter. (2017). Pile Foundations. Retrieved May 01, 2018, from

Understand Building Foundation: http://www.understandconstruction.com/pile-

foundations.html

Upadhyaya, D., Patel, J., & Patel, D. (2014). Preventation of Hydrostatic Uplift Pressure

underneath of Basement Floor Slab in High Water Table Area. Retrieved May 01, 2018,

from Academia.edu:

http://www.academia.edu/14337524/Preventation_of_Hydrostatic_Uplift_Pressure_under

neath_of_Basement_Floor_Slab_in_High_Water_Table_Area

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