Store array of objects in DB column. Laravel

Trying to store array of objects in DB and getting an error : array_merge(): Argument #1 is not an array

$tmp = new Projects;
$tmp->items = '{"id":"108124505876479","name":"Wakeboarding"},{"id":"112003352149145","name":"Bouldering"},{"id":"110008522357035","name":"Handball"}';
$tmp->save();

Found a lot of answers, but no one is useful.

Update (model file and migration added):

Model file:

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class Projects extends Model
{
    protected $casts = [
        'items' => 'array',
    ];

}

Migration file:

<?php

use Illuminate\Support\Facades\Schema;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Database\Migrations\Migration;

class CreateProjectsTable extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::create('projects', function (Blueprint $table) {
            $table->increments('id');
            $table->string('gID', 100);
            $table->string('name', 100);
            $table->text('items');
            $table->timestamps();
        });
    }

    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
        Schema::dropIfExists('projects');
    }
}

and the function:

$tmp = new Projects;
$tmp->name = "aaa";
$tmp->gID = "FLSKJDF3R23R";
$tmp->items = [["id" => "108124505876479", "name" => "Wakeboarding"],["id" =>  "aa", "name" => "volkswagen" ]];
$tmp->save();