Innovative Shallow and Deep Foundations and Treatment in Soft Ground

Eun Chul Shin Prof. Incheon National University, Republic of Korea Vice president of ISSMGE for Asia

Abstract

It is a method to improve the soft ground and to separate the shallow foundation from the , and introduces each case separately for honeycell, point foundation, and support pile. Part 1 is about hexagonal hollow block with a straw shape called honeycell. Replace the soft ground with to fill the honeycell and install a shallow foundation. It can be said that it is the basic method which reduces the settlement amount and increases the of foundation. We analyze the support characteristics of honeycells by carrying out the plate loading test using the laboratory model , and introduce actual case examples. The point foundation of Part 2 is a method of shaping the top foundation, injecting the mortar mixed with and in the soft soil, rotating the mixer, and mixing with stirring to form an upgrading body having a predetermined uniform strength in the soil layer. It is a method to secure the bearing capacity of low-rise structures. We analyzed the bearing capacity by field test, also introduce case studies. Part 3 analyzes the support performance of the soft ground by means of field pilot test with thegeosynthetic- reinforced supported pile of embankment

1. Honeycell (Shallow Foundation)

1.1 Introduction

The hollow block is shaped like a hexagonal honeycomb shown is Fig.1.1. The honeycomb structure is the most economical structure for securing the maximum space with minimum material, and it is widely used in our daily life as a stable structure that distributes the force in a balanced manner. The hollow block foundation method is soft ground underneath the shallow foundation is replaced with a combination of mixed crushed stone. It can be said that it is a foundation improvement method which increases the bearing capacity by reducing the settling amount of foundation by forming artificial layered ground. The shallow foundation plate load test (KS F 2444) was carried out by using the sandstone as the soft ground and the crushed stone as the replacement material. The honeycomb shape is known to distribute the load, when this type is applied to the ground, the relationship between the stress distribution angle generated from the bottom of the hollow block and the internal angle of the ground increases the bearing capacity and decreases the settlement amount. It is necessary to analyze the stress distribution by installing the earth pressure system.

Figure 1.1 Modeling of honeycell shallow foundation

1.2 Plate Load Test in Laboratory

1.2.1 Setting of plate load test in laboratory Fig.1.2 shows a schematic diagram of the model earthenware, and Fig.1.3 shows the planar and lateral photographs of the model earthenware. The size of the model earthenware is 100cm wide and 120cm high and is cylindrical steel earthenware. The rebound beam consists of two H beams and is secured with a full force gauge. The plate load test equipment is shown in Fig. 1.4, with a maximum load 50tonf and a torque of 40mm, Electronic data collectors are installed on plate load test equipment to record loads. Plate decks(D:250mm) are used and shown in Fig.1.5. Two LVDT is were also installed on the plate to check the amount of sediment. The earth-pressure system is installed on the construction site to measure the vertical and horizontal pressure of the load. and Crushed Stone have been subjected to the materiality test as described in Table 1.1.

H-beam Jack

LVDT Measuring plate

Hhoney cell

H Honey cell Steel circular box

H2

Data logger Computer Pump Figure 1.2 Schematic diagram of plate load test in laboratory

Figure 1.3 View of circular steel box

(a) hyraulic jack (b) data logger

Figure 1.4 Equipment of plate load test

(a) Honeycell (b) hexagon plate

Figure 1.5 Shape of Honeycell and plate

Table 1.1 Properties of soil (sand, crushed stone)

Maximum Angle of Apparent Specific OMC dry unit internal USCS

gravity (%) weight friction (KPa) (g/cm³) (°)

Sand 2.65 1.91 0.99 9.4 1.69 4.92 33.44 SP

Crushed 2.68 50 0.91 5.53 2.37 138.96 47.83 GW stone

1.2.1 Experimental conditions

For the test conditions of a plate load test, the sand ground(1-a) was installed on the sand ground(1-b), the sand ground replaced by a crushed stone(1-c), and the hollow block were installed as shown in Table 1.2. Fig 1.6 shows the model diagram by experimental condition. The depth of the replacement (crushed stone) is 150mm, and the height of the hollow block is set at a 1:1 ratio. The width of the change was set at 400mm, wider than the lower plate. The relative density is 40%, which is the normal level of density. R`s level of relative strength is 95.9%. The hexagonal lower plate was used for the lower decks, While for the ground where the hollow block was installed, the hexagonal lower plate was identical to the hollow block shape. The relative density of the sand ground was 40 percent and a plate load test was conducted. The re-download method was performed in accordance with KS F 2444, Korea`s industry standard, by means of the plate load test on the shallow foundation. The end of the test stopped if the test load was more than three times the allowable load or if cumulative settlement exceeded 10% of the diameter of the lower plate

(1-a) (1-b)

(1-c) (1-d)

Figure 1.6 Laboratory model test conditions for honeycell foundation

Table 1.2 Cases of plate load test in laboratory model tests

Depth of Relative Test Caes Method Load replacement density( ), % 1-a Sand - 5kN, 8kN

Sand 1-b + - 5kN, 8kN Honeycell

Snad + 1-c 150mm 5kN, 8kN Replacement (crushed stone) KS F 2444 40%

Snad + Replacement 1-d 150mm 5kN, 8kN (crushed stone) + Honeycell

1.2.2 Results of plate load test

When hollow blocks are installed, The P-S graph of the plate load test results on the sand ground or on the crushed rock face shows linear behavior up to a certain load strength, and then shows the P-S curves for sharp deposits. The initial load strength according to the shallow foundation plate load test method is shown in Fig 1.7, showing the P-S graph by the condition of the plate load test performed at 92.4 kN/m2. The conditions of installing hollow blocks on the replacement (crushed stone) ring and filling the interior with the interior of it showed the lowest relative to the ultimate load strength, and the alignment of the P-S curve was constant. While the sand ground hollow blocks showed linear motion up to the extreme load strength, it was clear that the heavy air blocks on the ground were replaced by the crushed rock, the difference in deposits and unfilled conditions was significant. As a result of filling the inside of the hollow block with a blanking test, the initial load was subjected to the linear support force generated by the hollow block concrete, but then the support was lost in the form of penetration breakage. As a result of a plate load test under 5 kN and 8 kN due to difference in load, the load strength of 5 kN was reduced by a point of 134 kN/m2. A similar P-S curve was seen in the sand ground, while in the case of a replacement (crushed stone) ring, it is believed that the load bearing on the replacement (crushed stone) ring is greater as the load increases. The difference between sand and clasts, the mouth, the modulus of elasticity, and the , is apparently working. In particular, the location of sand and clasts, the concrete thickness of hollow blocks and the internal hollow width of the hollow block, are expected to have a significant impact on the ground level. In the case of a replacement (crushed stone), the ratio of 1:2 relative to 50mm thick of hollow block concrete is less than 25mm. When the upper load is delivered to the lower part of hollow block concrete at a certain rate, the higher shear strength of the higher application can be assumed to increase the support force and reduce the amount of sediment.

Load per unit area (kN/m2) 0 200 400 600 800 1000 1200 1400 1600 0 250 500 750 1000 1250 1500

(1/100 mm) (1/100 1750 2000 2250

Settlement 2500 2750 3000 Ultimate bearing capacity 3250 3500 Sand Replacement crushed stone Sand + Honeycell(unfilled hollow) Sand + Honeycell Sand + replacement + Honeycell(unfilled hollow) Sand + replacement + Honeycell (filled crushed stone) Figure 1.7 Total results of plate load test (Initial load = 5kN)

Load per unit area (kN/m2) 0 500 1000 1500 2000 2500 0

500 Ultimate bearing capacity

1000 (1/100mm)

1500 Settlmement

2000

2500 Sand Replacement Sand + Honeycell Sand + replacement + Honeycell(filled) Sand + replacement + Honeycell(unfilled)

Figure 1.8 Total results of plate load test (Initial load = 8kN)

1.2.3 Strengthening efficiency analysis

An experiment was conducted indoors in accordance with the shallow foundation plate load test method (KS F 2444). To analyze the results of the sand ground, hollow block installation, and the replacement (crushed stone) zone, the reinforced ground was reinforced against the sand ground using Equation 1.1.

(1.1)

Where, shows he support strength of the reinforced hollow block against the non-reinforced ground and the replacement ground. In addition, silver indicates the ultimate support force of the ground when applied with hollow blocks, and silver indicates the ultimate support force in the event of no reinforcement or replacement. Table 1.3 and Fig. 1.9 show the strength and strength of the test results. In this case, the support strength reinforcement ratio was shown to be effective against the non-reinforced ground by comparing the ultimate support force of the non-reinforced ground with the hollow block reinforcement ground.

Table 1.3 Strengthening efficiency for each test condition

Reinforcement Reinforcement Ultimate Permissible efficiency to the efficiency Case support support ground of the compared to (kN/m2) (kN/m2) replacement sand ground (crushed stone)

Sand ground A 277 92.3 1.0 - (original ground)

Sand ground+hollow B 831 277 3.0 - block(filled) Sand ground+hollow C 255 85 0.92 - block(blank) Replacement D 647 215.7 2.34 1.0 (crushed stone) Replacement (crushed E 831 277 3.0 1.28 stone)+hollow block(filled) Replacement (crushed F 831 277 3.0 1.28 stone)+hollow block(blank)

3.5

3 3 3 3

2.5

2.34 2

1.5

1.28 1.28 1 Strengthening efficiecy Strengthening 1 1 0.92 0.5

0 A B C D E F Strengthening Effeciency against sand Strengthening Effeciency against crushed stone

Figure 1.9 Strengthening efficiency for various test conditions

1.3 Field Case History

1.3.1 Honeycell construction procedure

The construction order of the honey cell foundation is as follows in Fig 1.10. To replace the soft ground with a stone, we will lay a stone after cutting it and make a pledge. Install Honey Cells and fill the area with Crushed Stone. The honey cell is made in units of 4 cells and is installed at low base.

Figure 1.10 Procedure of Honeycell construction

1.3.2 Field case history

As shown in Fig 1.11, when constructing a medium and low-rise building on a soft ground, it introduces the construction site by applying honeycell instead of a pile foundation. The drilling image is shown in Fig 1.12. The standard penetration test results in a point - 1.3 m depth from the surface of the ground, with an N value of 3 and an N of -3.5 m being a soft ground with an N value of 6. As shown in Fig. 1.13, the initial design was the same as in (a) at the base of the pile and the honeycomb was installed with (b). About -0.64 m from the surface was excavated to construct a honeycomb foundation. Plate load test was performed as shown in Fig. 1.14. As shown in Fig. 1.15, the P-S curve showed a constant linear behavior, and the allowable support force met the design support force (200 kPa).

Figure 1.11 Case history Figure 1.12 Drill boring

(a) Original design (b) Honeycell (Piled foundation) Figure 1.13 Foundation plan

Figure 1.14 View of field plate load test

70.0

60.0

Ultimate bearing capacity : 50.0 63.66 ton/㎡

) 40.0 ㎡

30.0 (Load,ton/

20.0

10.0

0.0 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 (Settlement, ㎜)

Figure 1.15 Result of field plate load test

2. Point Bearing Pile Foundation (Point Foundation, PF)

2.1 Introduction

The Point foundation method is a method of injecting and mixing soil cement (binders) to the natural ground. It is applied to the foundation of medium/low-rise structure, and it can secure the stable foundation through the replacement of foundation on the foundation of soft ground. Also, this technique is used to secure the living force of the upper layer and control the amount of sediment of the lower layer simultaneously by using special equipment to form the foundation The shape of the Point foundation (PF) forms a large diameter body (Head+Cone) as the primary support layer to the depth where the increase of the stress is effectively reduced as shown in Fig 2.1. Below the depth that reduces the stress range by more than 75%, form a small diameter sphere(Tail) with a secondary support layer. The Head+Cone part can secure support from the upper floor, and the tail section can be solidified to a dense ground to prevent residual settlement. Point foundation techniques can reduce the thickness of the slab to deliver the bottom floor load compared to the pile foundation. As shown in Fig 2.1, the girder is unnecessary because it is directly supported from the bottom of the slab, and residual settlement can be prevented. In addition, it is advantageous to secure the bearing capacity by applying the high replacement ratio only through the upper expansion of the foundation, and it is advantageous economically by applying a proper replacement ratio to the relatively good bottom layer. The Point foundation method estimates the bearing capacity of the entire ground based on the replacement rate, unlike the pile foundation to which the pile bearing capacity is applied.

Figure 2.1 Concept of point bearing foundation

The Point foundation method is a basic method which simultaneously forms head, cone, and tail. It is a method to form a primary supporting layer on the upper part and secondary settlement preventing layer soil cured body on the lower part while injecting the soil layer solidification agent(binders) into the soft ground. It is a basic construction method for structures that requiring low bearing capacity (allowable bearing capacity of 300kN/m2 or less) such as middle and low-rise structures, underground parking lots for apartments, warehouses and factory buildings. It can be applied instead of pile foundation. Mechanically, a large diameter ( 1400) body is constructed as a primary support layer to a depth where the increase in stress can be effectively reduced (75% reduction) so as to support the load acting on the foundation of the building. Then a small diameter (500~800) tails are formed from the depth of the stress range to 25% and the secondary support layer from the N-value 20~30 to the strong support layer. The ground is improved to secure the allowable ground force and to control the settlement.

2.2 Field Load Test

2.2.1 Static load test

Field load test is analyzed by measuring the displacement according to the load loading step by step using measuring instruments installed inside the upgrade body during the static load test. Therefore, it is carried out in accordance with a general pile load test (KS F 2444, 2015; KS F 2445, 2016; ASTM D 1944, 2014). To analyze the load – settlement behavior of the Point foundation, the load test of the Point foundation was carried out at the Songdo site in Incheon Metropolitan City. The ground structure where the test was carried out is a silty sand buried layer of about 20 m in depth and a thickness of 1.0 m. The buried layer has a N value of 10 to 30, which is relatively soft The silty clay layer is deeply distributed. As shown in Fig. 2.2, the head part of the point foundation is 1.4 m in diameter, and the tail part is placed at a value of N = 30 at a diameter of 0.5 m and 10.5 m from the surface of the ground.

Figure 2.2 Static load test in field In order to carry out the test, a stock load of load corresponding to the test load is first required. In the method of loading by load, there is a method in which a reaction force beam is installed around the test position and a load is made by using a reinforcing bar, and H- beam. And there is a way to use such as back hoe or dozer which is relatively easy to provide load in the field. In this test, it is carried out by stacking concrete blocks and using dead load. The general method of loading is to load less than 100kN / m2 in the first stage of loading or less than 1/5 of the expected allowable bearing capacity. The subsequent loading steps are generally increased by 25 kN / m2 for loose soil, 50 kN / m2 for dense , and 100 kN / m2 for very dense soils. The duration of the load is assumed to be the stop of settlement if the settlement for 1 minute is less than 1% of the settlement generated at the current load stage. The measurement time is measured in units of 1, 2, 3, 5, 10, and 15 minutes. The loading method in this test is carried out by the constant-load and time interval loading method (ASTM D 1194-94, 2014), and the load step is carried out in a total of 12 steps, as shown in Table 2.1, In order to confirm the elastic settlement and residual settlement, the unloading stage of 150kN is composed of 4 steps. The maximum load is 600kN.

Table 2.1 Loading steps

Step Load (kN) Load Strength (kN/m2) Load Duration (min) No. Sort 0 - 0 0 Initial Load 1 Loading 75 75 15min 2 Loading 150 150 15min 3 Loading 225 225 15min 4 Loading 300 300 15min 5 Loading 375 375 15min 6 Loading 450 450 15min 7 Loading 525 525 15min 8 Loading 600 600 15min 9 Loading 450 450 5min 10 Loading 300 300 5min 11 Loading 150 150 5min 12 Loading 0 0 5min

For the test method, a load plate of 1m × 1m is installed at the test position as shown in Fig. 2.3 (a), and the hydraulic jack is installed at the center of the load plate on the load plate. After the loading plate and the hydraulic jack were installed, a load test was carried out by placing a reaction force beam and a load bearing to support the concrete block and placing a concrete block corresponding to the test load thereon.

(a) Plate (b) Concrete blocks (c) Loading

Figure 2.3 Process of static load test 2.2.2 Results of field loading test

In general, the bearing capacity of the foundation is divided into the long-term allowable bearing capacity and the short-term permissible bearing capacity. Usually, the short term allowable bearing capacity is the yielding load strength. The long-term allowable bearing capacity is obtained by dividing the yield load strength by the safety factor of 2.0 and the value obtained by dividing the ultimate bearing capacity by the safety factor of 3.0, whichever is smaller. Also, based on the amount of settlement, the value obtained by dividing the load corresponding to 10% of the width of the loaded plate by the safety factor of 3.0 is set as the long-term allowable supporting force. In this test, the analytical criteria were examined, and the allowable bearing capacity was evaluated by analyzing the settlement pattern, settlement amount, and so on.

(1) Method using settlement criteria

When the safety factor of 2.0 was applied to the load carrying capacity of a load less than 25.0mm by using the settlement criteria, the support force was shown to be 300 kN/m2 (Dongyang Ground, 2016).

Table 2.2 Bearing capacity by settlement

Maximum Load Total Settlement Bearing Capacity (kN/m2) (mm) with Safety Factor (F.S=2.0) 600.0 12.46 300.0 kN/m2

(2) Method using P-S curves

The bearing capacity by the method using the P-S curve was evaluated. The results of the load test of the point foundation modifier are shown in the arithmetic coordinates and shown in Fig. 2.4. The maximum yielding load of 600 kN/m2 was found to yield the yield load at 350 kN/m2. Considering these results, the allowable bearing capacity considering the safety factor (F.S = 2.0) was estimated to be 175kN/m2.

Figure 2.4 P-S curve (3) Method using the log P – log S curve

The log P-log S curve of the point foundation modifier test results is shown in Figure 2.5. The yield load was not evaluated due to the constant increase in the amount of sediment during each loading phase up to 600 kN/m2 under load. As a result, the yield load of the point base modifier can be determined as 600kN/m2, and the allowable bearing capacity considering the safety factor (F.S = 2.0) can be calculated as 300kN/m2.

Figure 2.5 Log P – Log S curve

(4) Method using the S-log T curve

The point-based modifier test results are shown in Figure 4.3 as S-log T curves. As a result of testing up to 600 kN/m2 under maximum load, yield load appeared at 450 kN/m2. Considering these results, the allowable bearing capacity considering safety factor (F.S = 2.0) can be calculated as 225kN/m2.

Figure 2.6 S – log P

2.2.3 Summary of load test results

In general, the maximum curvature point of the load-settlement curve should be determined as the ultimate bearing capacity. In most tests, the maximum curvature point does not appear easily, so there are many difficulties in finding the yield point. Considering these conditions, the result can be judged by the yield load analysis method and the settlement amount standard method (the minimum value is allowed to be supported). Table 2.3 summarizes the results of the test results.

Table 2.3 Total results of static load test in field

Safety Yield/Ultimate Analysis Allowable Bearing Criteria Factor Remarks Load (kN/m2) Result Capacity (kN/m2) (F.S) Total 10%B Settlement - - 3.0 - (=100mm) Criteria Standard 25mm 600.0 600.0 2.0 300.0 of Settlement P – S 350.0 Yield Load log P – log S 600.0 2.0 175.0 Criteria S – log t 450.0 350.0 Reaction Modulus 102.0 (kN/m3) Deformation Modulus (kN/m2) 8,168.16 Maximum Load (kN/m2) 600 Total Settlement (mm) 12.46 Allowable Bearing Capacity 175.0 Based on Load Test (kN/m2)

2.3 Field Case History

The following is an example of the construction of a Point Foundation at the Seoul-Munhan Expressway in Korea. The highway route shown in Fig. 2.7 and Fig. 2.8 is the point where the point foundation is designed. The point foundation is designed on the basis of embankment and box girder, and the design is shown in Fig. 2.9.

Project L=35.6kM

Yellow Seoul Sea

Figure 2.7 Total length of highway project

Yellow Sea

Figure 2.8 Construction route of point foundation

Soil Conditions

PF Box Structure Road Embankment

PF soft soil layer

PF

Figure 2.9 Design of point foundation

Fig. 2.10 shows the strength of the modified clay and cement with the uniaxial strength test in the laboratory before applying the point foundation. Fig. 2.11 is the foreground for constructing the point foundation, and Fig. 2.12 is the figure after the point foundation formation.

(a) Modified clay (b) Mix with cement

Figure 2.10 Pre-mixing test in laboratory

(a) mixing work

(b) After mixing work

Figure 2.10 Installing PF in field

Figure 2.10 Visual Inspection of completed PF

3. Geogrid-reinforced and Pile-Supported embankment

3.1 Introduction

Construction of structures such as buildings, tanks, walls and embankments on soft clay soils raises several concerns that relate to bearing capacity failure and/or excessive settlement. Preloading, staged construction, over excavation, and replacement are some of the techniques commonly used to address these concerns. Another possible soil improvement technique that is utilized in some instances is the construction of geosynthetic- reinforced pile-supported earth platforms. Fig. 3.1(a) shows the construction of a bridge approach as reported by Reid and Buchanan (1984). Fig. 3.1(b) and 3.1(c), respectively, show the cases for widening of an existing road and subgrade improvement. Tsukada et al. (1993) described the condition shown in Fig. 3.1(c). Han and Gabr (2002) also briefly elaborated upon the construction technique cited above.

Bridge

Geosynthetic Fill

Concrete Piles Piles Pile

(a) bridge approach

New Existing Geosynthetic

Piles (b) widening existing raods

Centerline Pavement

Geosynthetic

Soil-cement Column Soft alluvium

(c) subgrade improvement

Figure 3.1 Examples of geosythetic-reinforced pile-supported earth platforms The concept of the construction of earth embankment supported by geogrid on piles is shown in Fig. 3.2 Piles are driven into a soft soil layer until firm ground is reached. A layer of geogrid (or geosynthetic) is placed over the pile caps and the soft soil. Then an earth platform is constructed over the layer of geogrid (Fig.3.2a). Since piles are much stiffer than the , differential settlement occurs. The geosynthetic layer placed at the soft soil-embankment interface will take the shape shown in Fig. 3.2(b) then goes into tension. These differential movements generate shear stresses within the embankment that increases the load on the piles and decreases the load on the subsoil. This mechanism is called arching.

Embankment

Pile cap Geogrid Soft layer Pile

Firm layer

(a) Pile cap Geogrid

(b) Figure 3.2 Fundamental concept of geosynthetic-reinforced pile supported earth platform

The mechanism of load transfer between the pile cap, embankment material, and the geosynthetic can be explained by referring to Fig.3.3. Let the height of the embankment fill be H. The fill material has a unit weight of γ. Let the magnitude of the uniform surcharge on the fill be qo. When the soil located between the pile caps moves downward, a shear stress τ develops in the fill resisting the movement. The shear stress reduces the pressure on the geosynthetic that is in tension. However, the load is transferred to the pile cap and thus to the firm soil layer below via the pile. The load transfer mechanism is essentially a ‘soil arching’ effect. Thus the load per unit area, σc, on the pile cap at this time will be greater than γH + qo and the load on the soft soil, σs, will be less than γH + qo. The degree of soil arching, ρ, can be given by the expression (McNulty, 1965),

ρ = qb/(qo + γH) (3.1) where qb = applied pressure on the top of the trap door.

During the second half of the nineties several studies relating to the geosynthetic-reinforced pile-supported earth platforms have been reported in the literature (Rogbeck et al., 1998; Jenner et al., 1998; Kempton et al.,1998; Maddison et al., 1996; Topolnicki,1996; and Hans and Gabr, 2000). British Stand-ard BS 8005 (1995) also provides some guidelines for design of over piles (initially developed by Jones et al, 1990). Further field observations and quantifications are necessary to develop better guidelines and standards. This paper presents the observations of a pilot scale test conducted in the field that relate to the settlement of an embankment built on a soft clay layer. The embankment was supported by end-bearing concrete piles that had been driven into the clay layer over which a layer of geogrid was placed.

H

Embankment

Geogrid

Soft soil

Figure 3.3 Load transfer mechanism due to arching earthy platform

3.2 Field Pilot Scale Test

3.2.1 Field Pilot scale test

The pilot scale field tests were conducted at the geotechnical experiment site at the University of Incheon, Incheon, South Korea. The test site measured 13m x 3m x 1.6m (depth). The test site was initially filled with marine clay obtained from the Bay of Incheon. The top of the marine clay was covered and left for three months to consolidate. The initial moisture content of the clay in place varied between 32% to 35 %; the liquid limit and plasticity index were determined to be 31.8% and 7.4%, respectively. Based on the unified system, the soil is classified as CL. At the end of the consolidation period, the variation of the undrained shear strength, cu, with depth was determined at three different locations using a hand vane shear test device. The magnitude of cu remained practically constant with depth and had an average value of 1.25 kN/m2. After the undrained shear strength was measured, precast piles were driven in desired locations into the clay layers by a standard penetration test (SPT) ham-mer. These 3 x 3 pile groups had varying center-to-center spacing. The diameters of the piles, d, and pile caps, b, were 0.1 m and 0.15 m, respectively. Following the driving of the piles, settlement plates were in-stalled on the clay layer. A layer of biaxial geogrid was then spread over the pile cap and clay. The physical properties of the geogrid are given in Table 3.1.

Table 3.1 Physical properties of geogrid used for soft soil reinforcement

Property Geogrid Polymertype Polypropylene Manufacturing type Biaxial Aperture size (mm) 25(MD), 33(CD) Rib thickness (mm) 0.75 Maximum tensile strength (kN/m) 250(MD), 400(CD) Tensile strength at 2% elongation(kN/m) 4.1(MD), 6.6(CD) Tensile strength at 5% elongation(kN/m) 8.5(MD), 13.4(CD) Note: MD-machine direction: CD-cross machine diretion

Surcharge on the clay-pile-geogrid system was applied by a three-stage construction of an embankment. The final height of the embankment was 2.7 m. The embankment material was weathered granite soil (uniformity coefficient = 2.29, coefficient of gradation = 1.23, Unified soil classification - SP-SM). The measurement of the settlement of the clay layer was continued with time. Fig. 3.4(a) shows the cross section of the test site with the embankment and piles in place. Fig. 3.4(b) shows the plan of the pile groups in the clay layer along with the location of the settlement plates. It needs to be pointed out that settlement measurements were continued in four sections—three sections with group piles and geogrid reinforcement (D/b = 3, 4, 5.33) and one unreinforced section.

3m

Embankment (decomposed granite)

2.7m

Geogrid Pile cap (0.15-m dia.)

Pile Soft marine clay 1.6m (0.1-m dia.)

Firm decomposed granite 3m

(a) Unreinforced Pile+geogrid

D=60cm D=45cm D=80cm

13m

(b)

Figure 3.4 (a) Cross section of test site; (b) Plan of Pile groups in clay layer

3.2.2 Results of filed pilot scale test

The variations in the height of the embankment and settlement of the clay layer with time for the four test sections are shown in Fig.5. The settlement measurements shown in Fig 5(a) are for the midspan in the horizontal direction. Similarly, the settlement variations at midspan in the diagonal direction are shown in Fig 5(b). Approximately 30 days from the beginning of embankment construction, the settlement in all four test sections reached maximum value and remained constant thereafter.

(a) midspan in horizontal direction (b) midspan in diagonal direction

Figure 3.5 Variation of settlement with time

The effectiveness of the pile-geogrid reinforcement in reducing the settlement of the soft clay layer can be observed from the relationship.

E = 1 - [S(PG) / S(UR)], (3.2)

where E = reinforcement effectiveness; S(PG) = settlement with pile and geogrid reinforcement; S(UR) = settlement of reinforcement section. The variation of E with D/b were calculated from the maximum settlement shown in Figs. 3.5(a) and (b), and they are shown in Fig. 3.6 From this figure it can be observed that; a. With D/b = 3, the settlement is reduced by about 40% or more due to reinforcement. b. The magnitude of E decreases with the increase in D/b. By extrapolation it can be estimated that the geogrid reinforcement is parctically ineffective at D/b ≈ 6 to 7.

Figure 3.6 Variation of strain in geogrid at the top of the pile cap

Fig. 3.7 shows the variation of strain with time and D/b in the geogrid at the top of the pile cap. For any given value of D/b, the maximum strain the geogrid was realized immediately after the completion of the embankment construction. The magnitude of strain for the case of D/b = 3 was found to be about 3.75 times the strain observed for the case of D/b = 5.33.

Figure 3.7 Variation of E with D/b

4. Concluding Remark

1. Economical solutions for both shallow foundation and deep foundation are presented with the laboratory test and filed case histories. Modular concrete block type shallow foundation called “Honeycell” method gives relatively high bearing capacity to support the load applied by medium height buildings though the stiffness of modular concrete block filled with the material.

2. Point bearing pile foundation called “Point Foundation” can be applied in soft clay and medium stiffer soil layer with enlarged pile top foundation and longer tip foundations through a method of injecting and mixing cement to the natural ground. It can support the load applied by the medium and low-rise buildings and factory building as as underground parking lot in soft clay soil.

3. Geosynthetic-reinforced pile-supported earth embankment is very useful foundation system in soft clay for the construction of highway and railway as well. The arching effect and load transfer mechanism from geosynthetic in between piles to the pile cap are greatly contributed to increase the bearing capacity of soil and reduce the probable settlement of soft ground.

REFERENCES

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