Reducing settlement of soft clay using different grouting materials

1 Introduction

To recognize and classify soft soil, a variety of approaches can be used, such as ASTM or British standards. Clay is termed very soft if its unconfined compressive strength (UCS) is less than 25 kPa and soft if the value is between 25 and 50 kPa, as described in [1]. Soft soil is described as a geologically young clay or silty clay soil that has come to equilibrium under its own weight but has not had significant secondary or delayed consolidation after its development [2]. To consider the soil as a soft soil for construction purposes, the following criteria are adopted:

1. Very soft to soft clay with consistency index Ic < 0.75.

2. Fully saturated soil.

3. The “undrained shear strength” cu ≤ 40 kPa.

4. Has a tendency to flow.

5. Light to middle plastic behavior.

6. Very sensitive to vibration.

7. Thixotropic property.

Permeation grouting is a frequently used technique to improve soil qualities, such as foundation ground strength, permeability and the creation of impermeable zones behind dams, and deformability by filling around anchor bars. Permeation grouting is a popular geotechnical engineering technique for improving the soil in soil beds and reinforcing the underlying foundations of structures [3,4]. To compensate for tension relief and land loss due to tunnel excavation, grout is injected between the tunnel and the building foundations [5]. Injection materials, such as bentonite, cement, lime (L), and silica fume (SF), have been used in both laboratory and field applications. The mix used for slurry shield tunneling on phase 1 of the Cairo Metro, line 2, included hydrated L, SF, inert filler, rolled sand, crushed sand, and water. This mortar was non-cementitious but set to a strength of 0.82 MPa at 28 days. The strength development was due to the pozzolanic reaction between the L and the SF. The mortar was a very slow setting, with an initial set at 12–16 h. Despite this slow setting time, there was little evidence of the rings floating during construction, despite advance rates of up to 34.5 m/day [6]. The key to this was the rheology of the grout used. The sand and filler ingredients were chosen to ensure a well-graded blend. This gradation, together with the use of a combination of rounded and crushed sand, was intended to provide a high internal friction mix. Structures erected on soft soil may endure uncontrollable settlement and, as a result, may lose their load-bearing capacity. Soil improvement and stabilizations, as well as any combination of methods used to improve certain properties of natural soil deposits and geotechnical properties to increase strength and reduce settlement, or to change the permeability of existing soils, are the keys to solving such problems [7]. Furthermore, soil stability of both durability and strength is defined as attachment soil improvement through time. Soil can be enhanced mechanically through compaction control, preloading, soil replacement, and increasing insufficient particle sizes to improve grading, or chemically through the addition of chemical examples, such as cement, L, SF, and leycobond-h (LH). Finally, grouting can be improved by substituting the missing particle sizes to get a better grading while also causing a chemical reaction between the grout and the soil.

Fattah et al. [8] attempted to stabilize sand dunes by grouting with an L–SF material slurry. Samples of dune sand from northern Iraq were acquired. The soil density in situ was assessed using sand-cone tests for moisture content, grading, compaction, relative density, direct shear, and chemical testing. A 1,000 mL grout pump was utilized to steady a liquid container of the L–SF mix for grouting; the maximum pumping pressure was around 25 kPa. When dune sand was mixed with a slurry of L–SF, erosion was decreased by nearly 70% when the stabilizer grout was 33% (L-to-SF ratio, 3L:4SF) with 67% water of the overall mixture mass. The stability influence was enhanced by increasing the number and depth of grouting holes surrounding the stabilized area.

The goal of this research is to find out how injection affects soft clay consolidation settlement. Different injection materials are L, SF, and LH.

2 Experimental work

Soil, L, SF, LH, and water are the five materials used in this analysis. The material’s specifications are as follows.

Soil: Specimens of soil experienced in this study is a silty clay soil brought from Baladroz in the eastern region of Baghdad. The soil geotechnical properties used are listed in Table 1.

L: Al-Ahliyah Company for gypsum industries is used as a source of L and this product has been burnt to a temperature of 1,400°C to get rid of gypsum. Table 2 shows the chemical and physical properties of the L used.

SF: Micro-silica is a by-product of the reduction of high-purity quartz with coal in electric furnaces in the manufacturing of silicon and ferrosilicon alloys [9]. A gray-colored densified SF is used in this experiment. It is a pozzolanic material with a high amorphous silicon dioxide content. MEYCO®MS610 is the commodity used, and it contains extremely fine (0.1–0.2 m) latently reactive silicon dioxide, as shown in Table 3.

In order to maintain a bonding form between new to old, new to new concrete stronger than the concrete being bonded, LH is used. LH is a high polymer resin in water, non-settling, milk-white liquid, and a specific gravity of 1.08 with a highly adhesive bonding grout, which will adhere to most substrates.

3 Preparation of model test

The model tests are quite convenient to inject the slurry underneath and around the footing area using the setup shown in Figure 1, the test bed was prepared in a soil tank with dimensions (0.5 × 0.5 × 0.6) m, and the vertical load was applied to the circular footing with a diameter of 60 mm by steel plates put in the axial shaft with sensitivity 0.001 g. The displacement of the footing is measured by two deformation dial gages with 0.01 mm sensitivity. Trial tests were performed to determine the variation in the undrained shear strength with different liquidity indices as shown in Figure 2. The test was conducted at a liquidity index of 0.4 corresponding to an undrained shear strength cu = 9 kPa. The soil was then deposited in the soil tank in six layers, with each layer being carefully tamped to remove any trapped air with a metal hammer weighing 10 kg and having dimensions of (150 × 150) mm. This technique was repeated for all layers until the soil in the steel container reached a thickness of 400 mm. To prevent moisture loss, the soil surface was then covered with a polythene cover.

Figure 1

Setup of the laboratory footing model.

Figure 2

Variation in the undrained shear strength with liquidity index for the remolded clay after 48 h.

The silica and alumina in the clay become soluble and are released from the clay mineral when the pH exceeds 12.4. The silica and alumina are released to react with the calcium in the L to form cement. In the same way, fine pozzolanic component, such as alumina, in soil reacts with SF as mentioned by Fattah et al. [10]. However, the reaction of pozzolanic materials increases with subsistence bonding agent LH and reduces the bonding time required. Here, it is intended to investigate the grouting effect with (L, SF, L + SF, and L + SF + LH) after 1 day of the injection.

L grout: L slurry was made by mixing a dry L with water in three percentages (40, 50, and 60%) and for each percentage, 20 cm³ will be grouted in three CBR molds under a specific dead load of 80 N and the final settlement for each case was recorded.

SF grout: SF slurry was made by mixing a dry SF with water in three percentages (40, 50, and 60%) and for each percentage, 20 cm³ will be grouted in three CBR molds under a specific dead load of 80 N.

(L + SF) grout: This was made by mixing the same dry weight of L and SF with water in three percentages (40, 50, and 60%) and for each percentage, 20 cm³ will be grouted in three CBR molds under a specific dead load of 80 N.

(L + SF + LH) grout: This was made by mixing the same dry weight of L and SF with water in three percentages (40, 50, and 60%), the LH was added to the LSF slurry by 0.3% of the water content percentage previously, and then, 20 cm³ will be grout in three CBR molds under a specific dead load of 80 N. Table 4 presents the settlement recorded under a dead weight of 80 N.

4 Grouting apparatus

The technique of injecting a massive body of soft clay involves maintaining a control injecting pressure to ensure that a deep access to voids without destructing the soil structure by simple apparatus was designed for this study. The grouting device consists of a 50 mL needle that serves as a liquid tank for the grout slurry, and the highest pressure that can be delivered by hand is around 140 kN/m² when the needle is connected to a pressure gauge. Figure 3 shows how the grout is injected into the soil using an injection pipe with a diameter of 1 mm and a length of 100 mm.

Figure 3

Grouting apparatus.

5 Procedure of grouting the model

Based on the results shown in Table 4, it can be seen that the maximum reduction in the final settlement with slurry is 50% (L + SF + LH); thus, this proportion will be adopted as the optimum percentage used to reduce the footing model. A small quantity of slurry was prepared because the pozzolanic reaction between the components leads to aggregation and closes the pipe injection. So, 20 cm³ of slurry grout was mixed to be homogenous and the slurry was struck down by a plastic table spoon for a gel time of 20 s. The tests were carried out by penetrating the injection pipe through the soil to the desired depth and then, the injection was started with a hand-pressured needle. The control of grouting pressure is vital to the success of any grouting operation. In this study, two cases to stabilize soft clay by injecting the slurry underneath the footing area (before construction) and outside the footing area (after construction) were tried; for each case, the effectiveness of grouting spacing and the depth of grout were investigated.

Before footing construction, four different spacings were investigated for the depth of grout equal to d′ (footing diameter) (Figure 4).

Figure 4

Grouting spacing before footing construction with a depth of grouting equal to d′.

The optimum spacing of grouting points is investigated by using a spacing of 0.5 d′ and 1.0 d′ holes around the footing to a depth of 1.0 d′ (Figure 5).

Figure 5

Spacing of grouting points with a depth of grouting equal to d′.

To study the effect of the depth of grouting, models D, D1, and D2 are grouted to a depth of 1.5 d′. After footing construction, grouting can be used successfully to level an inclined building on the thick soft clay deposit with a final compensation efficiency of 10% [11]. Therefore, it is important to study the grouting effectiveness in reducing settlement in existing buildings. Three different spacings (90, 45, and 30°) were investigated to a depth equal to d′ (Figure 6).

Figure 6

Grouting points grid after footing construction, grouting to a depth d.

In the same way, the optimum spacing will be investigated by inserting holes around the footing at the distances of 0.5 and 1.0 d′ to a depth of d′ (Figure 7). To study the effect of the depth of grouting, models c, c1, and c2 were also grouted to a depth of 1.5 d′ 9 cm.

Figure 7

Grouting around the footing to a depth of d′.

6 Results and discussion

Different categories were done on a shallow foundation sitting on soft clay under static vertical load to evaluate the behavior of the settling soft clay modified by injection with (L + SF + LH) grout. The focus of the study is on the effect of grouting hole spacing and grouting depth. The failure is defined as the stress required to cause settlement equal to 10% of the model footing width (d′) in all model tests. The results of all model experiments involving the applied stress and the ensuing settlement are assessed in terms of load–settlement curves. There are four different types of model tests.

1. Grouting beneath the footing area: Experiments were carried out in this category to investigate the hole spacing inside the footing area, which consists of a rigid steel circular plate with a diameter of 60 mm, as shown in Figure 8, and the load–settlement curves for footing loaded in category 1 are shown in Figure 9.

Figure 8

Rigid steel footing resting on the soil.

Figure 9

Load–settlement relations for footings resting on soil grouted before footing construction.

The best grid spacing that demonstrated the maximum bearing capacity according to the failure criterion of (10% d′) corresponds to shapes C and D, according to the results of the load–settlement test. Grid C and D soil treatments comprised nine holes of grout. Grid C can be chosen to preserve the shape that requires the least amount of hole grout (minimum L + SF + LH) for economic reasons.

1. Grouting underneath and around the footing: In this spectrum, and to investigate the reduction in the settlement with the increasing in the grouting slurry around the foundation compared with previous category, the grout will be extended around the foundation. So the optimum grid shape C (Figure 9) will be grouted around the footing to a distance of 3 and 6 cm and the result of the load–settlement curves for this category is shown in Figure 10. It can be seen that the grouting near the footing at a distance of 0.5 d′ is more effective than at 1.0 d′ due to the shape of shear failure of soft clay around the footing. It is also intended to investigate the increase of grouting depth in category 2 from 1.0 to 1.5 d′. Figure 11 shows the load–settlement curves for footing resting on soil grouted according to category 2, to a depth of 1.5 d′.

Figure 10

Load–settlement relations for footings resting on soil grouted underneath and around the footing.

Figure 11

Load–settlement relations of footings resting on soil grouted underneath and around the footing to a depth of 1.5 d′ 9 cm.

It can be noticed that there is an increase in the bearing capacity of soft clay grouted to a depth of 1.5 d′ for all cases in Figure 11, and the bearing capacity increases because the soil is strengthened to a depth of 1.5 d′ where the stress induced by the applied loads is expected to spread through.

1. Grouting along the footing perimeter: This category includes experiments directed to investigate the effect of the injection of grouts into holes located along the perimeter of the footing area. Figure 12 shows the load–settlement curves for footing in this category. It was observed that the shape (c), which included eight holes of grout is substantially in favor of the reduction of the settlement compared to shapes b and d. However, the soil treated by grid (d) included more holes of grout but exhibited lower bearing capacity. This may be due to the small distance between pipes of injection in the critical zone (punching zone), which decreases the bearing capacity.

2. Grouting outside the footing area: This category simulates the condition of grouting the soil when the foundation is constructed and it is not possible to inject the grout below the footing area. Therefore, the location of the injection holes will be marked with a suitable distance extended to 0.5 and 1.0 d′. Figure 13 shows the load–settlement curves for footings on soil grouted according to this category.

Figure 12

Load–settlement relations for footings resting on soil grouted at the perimeter of the footing area.

Figure 13

Load–settlement relations for footings resting on soil grouted outside the footing area to a depth of 6 cm.

It can be seen that there is increase in the bearing capacity with increasing the number of holes, which were grouted around the footing and increasing in the grouting slurry. Figure 14 shows the load–settlement relations for footings resting on soil grouted outside the footing area to a depth of 9 cm.

The present results are compatible with the findings of Fattah et al. [12,13] who concluded that the treatment with L–SF shows a general decrease in the maximum dry unit weight from 17.1 to 15.8 kN/m³ at 6% L + 5% SF. The optimum percentage for UCS of L–soil mix is 4% due to pozzolanic reactions (the released silica and alumina react with the calcium from the L to form cement). The grouting near the footing to a distance of 0.5 B is more effective than grouting at a distance of 1.0 B due to the shape of the shear failure of soft clay around the footing.

Figure 14

Load–settlement relation of footing resting on soil grouted outside the footing to a depth of 9 cm.

7 Conclusions

In this study, it has dealt with the evaluation of the effect of the fine pozzolanic component as an injection slurry grouted in soft clay soil as well as the effectiveness of geometrical grouting holes on the reduction of the soft soil settlement.

1. The applied fine pozzolanic component grouts composed of L, SF, and LH are utilized to improve the bearing capacity of soft soil, and the mix of pozzolanic material in the slurry reduces the settlement. From the present experimental study, it was observed that the grout treatment can reduce the settlement even after footing construction due to grouting separation under the footing, so it is suitable for the field infrastructure ventures.

2. As regards the spacing of grouting points, there is a reduction in the settlement by 18.3, 50, 63.3, and 66.7% with different spacing shapes A, B, C, and D, respectively. In the same way, when the slurry is grouted at the perimeter of the footing area, the optimum spacing was the shape d with a reduction of settlement by 50%.

3. It was concluded that, as increasing the number of grouting holes around the footing area, there is a reduction of settlement due to the increase in the L–SF–LH slurry and the optimum shape was D2 with a reduction of settlement of 71.3% for hole depth equal to d′ (footing diameter) and a reduction of settlement 90% for hole depth equal to 1.5 d′.