Engineering geology

Horizontal and Vertical Geotechnical Variations of Soils using GIS

According to USCS Classification for the City of An-Najaf, Iraq

The unified soil classification system (USCS) first proposed by Casagrande and subsequently developed by the Army Corps of Engineers (Geotechnical). It widely used in many building codes and books. AN-Najaf city is an expanding city due to its religious and spiritual status as the location of the Shrine of Imam Ali and many others holy shrines. The USCS was applied to the soil of AN-Najaf city to assess the soil types and any changes with depth. The data used was obtained from the National Center for Construction Laboratories & Researches (NCCLR)/Babylon laboratory reports and were collected from 464 boreholes across the study area for depths of 0-26 meters.

The data contains sieve analysis for boreholes and the plastic and liquid limits among many other soil properties. This research aims to create a geodatabase for the study area as part of a comprehensive database of all the soil properties in this area. The ArcGIS 10.5 software was used to interpolate the non-spatial data and produce the geotechnical maps for USCS. For numerical soil data, the best method for interpolation is Ordinary Kriging (OK), and it was used for Fine and Coarse percentage data for each depth. For nonnumerical soil data (USCS class), the Indicator Kriging (IK) method used because it is the only useful method for interpolating categorical data.

The USCS of the AN-Najaf city soil shows that the coarse soil occupied 85-95% in the zones 0-16 meters depth, while fine soil 5-15% subsequently this soil when compacted has a permeability of pervious to semi impervious, good shearing strength, low to very low compressibility and acceptable workability as a construction material. At depths between 16-26 meters, the fine soil percentage increased to 40% with a coarse soil percentage of 60%, indicating changes in soil characteristics as the permeability became semi-pervious to impervious, fair shearing strength, medium compressibility and fair workability as a construction material.

Geotechnical

Introduction

Classifying soil is a way to arrange it into groups or subgroups to describe its characteristics concisely (Das, 2008; 2013) (Das and Sobhan 2013; Das 2013). It is essential to clarify the soil classes before designing and constructing any project as the engineering characteristics of soil (stiffness, permeability, and strength) are influenced by the soil particles’ shape, size, arrangement and microscopic structure (Budhu 2015).

Generally, soils classified into (fine-grained) or (granular or coarse-grained) soils depending on the distributions of particles of the same size. Fine soils are determined by the percentage of the soil mass passing through a 0.075mm sieve, while granular soils are the soil mass that retained in a 0.075mm sieve, including sand, gravel, cobbles, and boulders.

If the percentage of fine soil passes through the sieve at a predefined proportion, usually 50% (but this could be less according to the soil classification system used), the soil is considered as Fine-grained. Fine-grained soils are furthermore classified into clay or silt using a hydrometer test.

Finally, soils subclassified according to consistency (Reale et al. 2018). There are many soil classification systems used by engineers, and they mostly use the same criteria for classification, such as the distribution of particles and plasticity (Das and Sobhan 2013).

However, the two main systems used by engineers are the unified system 2 and the AASHTO system, and they are both almost similar in using simple index properties like grain-size and atterberg limits (Das and Sobhan 2013; ASTM 2000). Sundry studies have conducted regarding the geotechnical properties of soil in different Iraqi regions, (Al-Baghdadi 2016), (Ali and Fakhraldin 2016), (Al-Shakerchy and Al-Khuzaie 2011), (Al-Maliki et al. 2018), (Al-Mamoori 2017; Al-Mamoori et al. 2018; AlMamoori et al. 2019). Geographic information systems (GIS) widely used for spatial data handling and manipulation.

Geotechnical assessment

A geotechnical assessment usually requires a large amount of spatial data. It is a robust and useful tool for analysing large quantities of data for geotechnical assessments and the undertaking of similar analyses on very large areas in a short period of time. A paramount feature of the GIS is its capability to create new data by combining current varied data that share a compatible spatial referencing system (Dai et al. 2001). This paper is part of a series of research papers aiming to create an extensive geodatabase for soil chemical and physical properties for part of the Najaf governorate using GIS.

The objective of this paper is to produce the geotechnical maps for the unified soil classification system of the study area and assess the geotechnical suitability of the foundations of residential areas. A GIS (ArcMap 10.5) software was used. For determining the geotechnical properties of the study area, data from 464 boreholes used.

Study area description

An-Najaf city is a double city (An-Najaf and Kufa) and is considered the capital of An-Najaf province, which is one of the eighteen provinces of Iraq. The city is situated 161 km to the southwest of the Iraqi capital, Baghdad on the edge of Mesopotamia (Tigris and Euphrates flood plain) in the east of the city, and of the southern desert (Western Plateau) in the west, and the ground slopes gently toward the flood plain (Al-Mamoori et al. 2019).

Study area location

The climate of An-Najaf city characterised as arid and semi-arid, with long, hot and dry summers and short winters. (Mail et al. 2016; Beg and Al-Sulttani 2020). For soil characteristics of the study area, the internal friction angle Ø of An-Najaf soil varies between 26.3 and 41.2 in most of the region (Ali and Fakhraldin 2016). The bearing capacity ranges between 5 and 20 Ton/m2 in this region (Al-Maliki et al. 2018), while the percentage sulfate content ranges between 0.36 and 14% for soil, and varies between 84 – 239% in groundwater (Al-Mamoori et al. 2018).

The gypsum content ranges between 10 and 25%, values that are considered very high (Al-Mamoori 2017). The liquid limit (LL) and plastic limit (PL) vary from 21 to 29% and 11 to 15%, respectively. The low values of LL and PL for the soil in western locations increases towards the eastern locations. The maximum dry density and optimum moisture content vary from 17 to 19 KN/m3 and 8 to 14%, respectively (Ali and Fakhraldin 2016).

Materials and methods

The study draws on data from 464 boreholes, with 13 soil tests for each borehole starting at a depth of 0-2 m and increasing to 24-26m. Two approaches have been utilised to calculate the SPT-N-value. The first approach involved collecting the geotechnical data, and the second data set arranged by using Excel to make it familiar with the ArcGIS 10.5 environment. The coordinates of the spatial boreholes designated by using the GPS device. The geotechnical maps were created using the ArcGIS 10.5 software.

Flowchart of the work

Materials:
The study data obtained from the reports of the National Center for Construction Laboratories & Researches (NCCLR)/Babylon laboratory, the data used was collected from 464 boreholes spread throughout An-Najaf and Kufa cities at depths of 0- 26 meters. The data contain the sieve analysis for boreholes and the plastic and liquid limits among many other soil properties.

Methods
USCS Classification:
The Unified Soil Classification System first proposed by Casagrande in 1942 and developed in 1952 by the Army Corps of Engineers (Das and Sobhan 2013). It widely used in many building codes and books (Reale et al. 2018; Robertson, 2016). The soil in this classification system divided into two master divisions: coarse soil (gravel and sand) and fine soil (clay and silt). If the retained soil in a No. 200 sieve is more than 50%, then the soil is coarse but if the soil passes through a No. 200 sieve, then the soil is fine (Reese et al. 2006)

GIS Mapping:
A geographic information system (GIS) is a set of rules and tasks for data analysis and processing using a computer. It is used to link information to its geographical location according to the coordinates, to arrange data into layers and then to transform it into maps for the selected area and thus show the geographic or other attributes of that area.

As each borehole has its spatial data and geotechnical data, this data has been arranged and horizontally tabulated in the Excel software in a way that is convenient for the ArcGIS 10.5 environment. The interpolation is an estimation of a value within two known values in a sequence of values; in other words, it is a
procedure used to predict the values of cells at specific locations that have missing sample data (Childs 2004).

Results and discussion – Geotechnical maps

This study is the first of its kind in Iraq to apply the Unified Soil Classification System to the soil of the study area to produce geotechnical maps for soil classes and soil types using the ArcGIS software. The data used is from 464 boreholes for depths of up to 26 meters. Geotechnical maps for soil classes and soil types produced, as seen figures below:

Geotechnical maps for (USCS), coarse and fine soils percentage for depths (0-10) m
Geotechnical maps for (USCS), coarse and fine soils percentage for depths (10-20) m
Geotechnical maps for (USCS), coarse and fine soils percentage for depths (20-26) m

The results maps show the followings:
a. Coarse soils: the classes present are (SP, SP-SM, SM), distributed as shown in. The study area lacks the classes (Gravels: GW, GP, GM, GC) or (SW, SC), which indicates that the soil is poorly graded silty sands. The percentage for gravels was less than 15%, so it was not considered.

b. Fine soils: the classes present are (OH, OL, CH, CL, ML), distributed as shown in. The study area lacks the classes (MH, PT), which indicates that the soil is silty, clay, or mixed organic soils with low or high plasticity.

c. SP soil class distribution is combined with the distribution of SP-SM class almost on all depths. Also noticed, that the distribution area of SP and SP-SM shrinks with depth to the north and east and small area in the middle.

d. SM class is dominant in the study area in all depths and its area increases with depth.

e. ML and CL classes occupy spotted small areas in the middle, east and south and spread with depth to the north of the study area.

f. OL and OH classes mostly are diapered in the first three depths levels (0-6) m, but they have a considerable area with depth.

They cover a small area in the southern part at depth (6-8) m and expands with depth in the middle, west and north of the study area.

Soil types used for geotechnical calculation

The Trend linear line and R-square for soil class were drawn and calculated to illustrate the change in the class percentage with depth as follows:

a. Silty Sands (SM): This class comprises the greater percentage of the soil for all depths. Its percentage was 62% at 2 meters, and 52% at 26 meters. Its percentage is nearly constant with depth, which is why its R2 is approximately 0.00009.

b. Poorly graded sands and silty sand (SP-SM): this soil class occupies the second rank, with a percentage of 39.6% up to a depth of 16 meters, after which its values reduce to 6.6%. The (R2) between the percentage and the depth was 0.826, and the correlation relationship is a strong inverse correlation.

c. Poorly Graded Sands (SP): this class is the third large percentage (62%) in the soil from 0-16 meters depth. After 16 meters, its values begin to reduce with the depth until it reaches 0%. The (R2) between the percentage and the depth was 0.78, and the correlation relationship is a strong inverse correlation.

d. Silts of Low Plasticity (ML): this class of soil, which describes fine soils, is the fourth rank in percentage until 16 meters in depth. After 16 meters in-depth, this class comprises the second-largest soil percentage as its values increase with depth until reaching 20%. The (R2) between the percentage and the depth was 0.76, and the correlation relationship is a strong extrusive correlation.

e. The clay of Low Plasticity (CL) and Clay of High Plasticity (CH): these classes are present in small percentages for depths of 0-16 meters, after which their values start to increase. The (R2) between the percentage and the depth for CL and CH was 0.68 and 0.88, respectively. The correlation relationship was a medium extrusive correlation for CL, and a strong extrusive correlation for CH.

f. Organic Silt, Clay of High Plasticity (OH) and Organic Silt, Clay of Low Plasticity (OL): these classes of fine soil were present in the study area at a very small percentage (OH=0.3% and OL=0%) until a depth of 16 meters. After this depth, their percentages increase to reach (OL=2.5 & OL= 12.9). The (R2) between the percentage and the depth was 0.19 for OL and 0.57
for OL.

In the charts shown below, each class has drawn against its percentage in two depths ranges: first, from 0-16 meters and, second, from 16-26 meters. This is done to analysis the change in the soil type before and after the 16 meters depth. The figure shows that the coarse soil classes (SP, SP-SM) decrease with a constant percentage of SM class, while the fine soil classes (OL, CH, ML) increase. The soil after 16 meters depth becomes a mixed soil of sand, clay, high-elastic clay, and organic matter.

Changes in the percentage of soil types before and after the depth of 16 m

The following chart indicates that the coarse soil (Sand) percentage was very high in the upper depths level, where it was 95% at 2 meters. These percentages decrease gradually with depth, and this change in the soil became obvious after 16 meters as the coarse soil percentage became 71% at 18 meters and reached 64% at 26 meters, while the fine soil is opposite as in coarse soil, its percentage increases with depth.

It can be noticed that the coarse soil percentage drops while fine soil percentage increases at about 18m depth, and this depth could be the contact between Dibdiba and Injana formations.

Changes in soil content percentage (Sand and Fine) with depth

Geotechnical engineers have created charts based on experience to help designers in selecting the appropriate soil for a particular construction. These charts results is used only as a guide and for making a preliminary assessment of the soil suitability for specific use (Budhu 2015). After applying the Unified Soil Classification System, the soil is evaluated depending on the table below. In the depths between 0 and 16 meters, coarse soil is dominant, with an 85-95% percentage. The coarse soil classes present are (SM, SP, and SP-SM).

The fine soil percentage was about 5-15%, so when the soil for these depths is compacted and saturated, it will have a permeability of previous to semi- previous, good shearing strength, low to very low compressibility and acceptable workability as a construction material.

At depths of between 16 and 26 meters, the percentage of fine soil classes increases to 40%, with 60% coarse classes, and this will result in remarkable changes in soil characteristics as the permeability becomes semi-pervious to impervious, fair shearing strength, medium compressibility and fair workability as a construction material.

Geotechnical

Conclusion

  • This study used the GIS software to produce geotechnical maps, which will help to prepare a database for the city and can utilise for primary designs.
  • Indicator Kriging give significant interpolated categorical (nominal) data map for soil USCN classes.
  • The result geotechnical maps of soil classification show that the coarse soil classes occupy most of the study area in all depths while the fine soil appears with depth especially after the depth 6 m and in the south, middle and north of study area.
  • The final geotechnical maps are very easy to use and help save money and time. They also provide a useful database for the city.
  • The soil of An-Najaf city for depths of 0-16 meters consists of the classes SP, SM, SP-SM at a percentage of 85%. Subsequently, when compacted, this soil has a permeability of pervious to semi-pervious, good shearing strength, low to very low compressibility and acceptable workability as a construction material.
  • At depths of 16-26 meters, the percentage of fine soil classes increases to 40%, with 60% coarse classes, and this will result in remarkable changes in soil characteristics as the permeability became semi-pervious to impervious, fair shearing strength, medium compressibility and fair workability as a construction material.

Adapted from an article of: Sohaib Kareem Al-Mamoori; L. A. J. Al-Maliki; A. H. Al-Sulttani; K. El-Tawil; H.M. Hussain; N. Al-Ansar

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