Improving The Engineering Properties of The Gypseous Soil Using Dynamic Compaction Method

The aim of this study is to improve the engineering properties of gypseous soils, located in Bajii city (160 km South of Mosul city) using the Field Dynamic Compaction (FDC) techniques. This technique consist of dropping a reinforced concrete block (1x1x1 m) weighing (2.4 Tons) from different heights on a gypseous soil of thickness varaing from (0.5-2.4 m) and underlain by sub-base material. The effect of FDC on the gypseous soil was investigated using SPT in the field and using the conventional laboratory apparatus in the laboratory tests. The results showed that the unit weight increased (14-32%) for one drop of the weight. The effective angle of internal friction (φˉ ) increased from (32 o ) to about (36 o ) while the compression index (Cc) decreased from (0.24) to about (0.07-0.1). The results of this study indicated that about (60-80 %) of the improvement was achieved from the first blow in loose to meduime soils. However about (25 %) of the improvement was reached in dense soils. The FDC is a promising technique for improving the engineering properties of gypseous soil and overcome most of the gypseous soil problems.


Introduction
It is a fact that soils are extremely complicated engineering materials whose constitutive response depends on many factors, including soil type, density, water content, structure, particles size, drainage conditions, duration of loading, stress history, confining pressure, and stress path [1,14].
The gypseous soil forms approximately (27-36) percentage of Iraqi soils and this type of soil is the source of many problems in the engineering structures which are built on it.
Soil improvement techniques fall under several categories, the improvement can takes place by densification, reinforcement, removal and replacement and physico-chemical alterations.Each category has its specific application in which it is more competitive than the others.Densification in general is the most commonly method used because it is relatively cheaper than other techniques especially when large area of construction is to be improved [3,12].
Dynamic compaction is one of the soil improvement method that is used to improve the soil properties to support buildings, roadways, and other heavy constructions [1,3].The dynamic compaction method was used to save cost and time relative to the other methods [4].By this method the soil densification can be achieved to significant depths by applying high energy impacts at the surface of the soils to ensure that the required characteristics are obtained.These impacts generate improvement from vertical and lateral displacement in the soil surrounding the impact point as well as from powerful shock waves occure in short duration which promote the re-arrangement of the soil particles into the denser configuration [9].
Dynamic Compaction is a cost effective method of ground improvement for sand densification.Furtherrmore, this method is appropriate for application to non-cohesive soils generally, the effectiveness of the this teqnique decreases as the cohesion of the soil increases [14,5,13].
In this research , the field dynamic compaction (FDC) was used to improve the engineering properties of the gypseous soil near Bajii (160 km South of Mosul city) where an electrical power station is to be constructed.

Dynamic Compaction
Dynamic compaction consists of dropping a heavy mass repeatedly from a certain height to impact the ground.The subsoil improvement is a function of the applied energy which relates to the weight, hight of dropping, grid spacing and numbers of drops at each point as well as it depends on distribution of the drop locations, the soil strength, and the degree of the soil saturation [14].
Sketch (1) shows the principles of the this technique used in this study.The selection of weight (W) and drop height (H) depends on the required depth (D) of improvement.Several studies investigated the effect of applied energy per drop on the depth of improvement and various relations were established [11,8,15].The most popular formula which is used in this study to predict the depth of improvement is Menard formula [2].
This formula is : where: D max = The maximum depth of improvement in meters, W = is the mass in tons H = is the fall height in meters , and n = is an empirical constant.
Lukas [7] studied the factors which affect n-value.These factors are the total amount of applied energy, the type of soil, the presence of soil layers, and the efficiency of the drop.Van [16] suggested (n =0.5) for clayey soils and (n =0.65) for silty soils while, Mayne [9,10] and Lukas [6] suggested n-vale ( 0.3-0.8)and (0.65-0.8) respectively for all soils.In this study a value of (n = 0.55) is used, and this value was obtained from the field test.
In this research the maximum thickness of the gypseous soil layer (D) that is to be improved is (2.4 m) which is located under tower (F-4) as shown in table (1).The FDC was executed by free dropping of reinforced concrete mass of 2.4 tons.
From equation ( 1): The fall height of (8.0 m) was used in FDC under all towers foundations and a parametric study on soil below or near tower (F-4) was conducted using field penetration test (SPT) and the results are presented in table (2).

Scope of the Study
In order to achieve the objectives of this study, the following field and laboratory tests were performed: i. Field works : In the site twenty two boreholes (one borehole under each tower's foundation) were made down to the sub-base materials.Ten boreholes were selected for this study.The locations of all boreholes are shown in figure (1) where the shaded area represents the selected tower's foundation.Thickness of gypseous soil layers from the bottom of foundation level down to the sub-base materials are listed in table (1).Disturbed and undisturbed samples were obtaind from the selected boreholes before and after the Field Dynamic Compaction (FDC).All the samples were wrapped, sealed tightly in plastic bags, labeled and transported to soil mechanics laboratory / Engineerin College-University of Mosul for the required tests.
The standared penetration test (SPT) was performd before and after the FDC and the results are presented in table (3).No water table was observed down to the investigated depths.ii.Labrotary Tests: In the labrotary, the properties of the soil before and after FDC were determined following the procedures given by the ASTM and the results are presented in tables (4-8) and figures (4)(5)(6)(7)(8).

Results and Discussion
This study as stated before, was conducted on gypseous soil found in Bajii city (160 km South of Mosul city).Generally the soil is rather uniform, it consists of gypseous silty soil down to about (0.5 m) above the sub-base material, then changed to either silty sand or fine sand until reached the sub-base material as shown in figs (2 & 3).The thickness of soil layer above the sub-base material is shown in table (1).It varies from (0.5 m) to (2.4 m).
The results of field penetration test are shown in table (2) and according to these results most of the soil is either loose or medium in its relative density and with depth the soil becomes dense.
The Unified Classification System indicates that most of the soil is either CL or CL-ML.The dry unit weight varies from (13.7 kN/m3 ) to (15.2 kN/m 3 ) before FDC and increased to (16.7-18.2kN/m 3 ) after FDC with an increases of (14-32%).
The effective angle of internal friction (ϕˉ ) varies from (32 o to 34 o ) while, the effective cohesion intercept (cˉ ) is zero.After FDC the internal friction (ϕˉ ) increased to (35 o -36 o ).
The results of the consolidation test showed that the compression index (Cc) varies from (0.13 to 0.24).According to these results, the expected settlement is about (4.0 cm) for each one meter thick of the soil.After FDC the compression index (Cc) become (0.07-0.1).
The results of this study indicate that about (60-80 %) of the improvement was achieved from the first blow in loose to meduime soils, while about (25 %) of the improvement was reached in dense soils.

CONCLUSION
Fig. (1): Site plane with location of towers' foundation

Figure ( 2 )
represents a typical log of boring while, figure (3) represents a typical soil profile at section F-F( see Fig.1).

Fig. ( 3 )Fig
Fig. (3): Typical soil profile Section (F-F) No. of blows are the average of the three values for each depth.